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Space Transport and Engineering Methods/Photon Engines

There are a variety of propulsion techniques that use either internally generated or externally supplied photons for propulsion. Stars, of course, are a natural supply of photons, but a purpose-made light source of sufficient beam intensity can also be used. Photons, being massless, travel at the maximum possible speed. Thus they are theoretically the highest possible exhaust velocity. By the same token, being massless, they do not transfer much momentum.

A. Photon Sails
As noted above, light does not transfer much momentum. So the following methods reach useful thrust level by maximizing area to mass ratio. The large area increases thrust relative to mass. The resulting structure resembles a terrestrial sail.

66 Solar Sail
Alternate Names: Lightsail

Type: Solar Flux by Photon Deflection

Description: Starlight, for example from the Sun, reflecting off a large area sail produces a force because the momentum of the photons is reversed. The force for perpendicular (normal) reflection is

 * (1+r)(E/c) ,

where r is the reflectivity of the sail, E is the incident light power, and c is the speed of light. Thus even perfectly absorbing surfaces see light pressure, but reflecting ones perform up to twice as well. No real surface is perfectly reflecting, so analysis of sails needs to take into account the actual optical properties. At sail angles other than perpendicular to the light source the net force has a direct component from the incident light, and a reflected or diffuse component depending on the surface properties of the sail. For reflection the incident and reflected angles are the same amount relative to the plane of reflection. The diffuse component is light scattered by a rough surface into a wide range of angles. The force is reduces by a cosine factor due to the spread of directions.

The absorbed component of light is eventually re-radiated by blackbody radiation. If this happens equally from both sides of the sail, it produces no net thrust. Coatings can modify the emissivity of each side and therefore give a net thrust. For high performance sails coatings add too much mass. It ends up being better to use bare metal foil of high reflectivity and just ignore the small part not reflected. Thickness so low as to be partially transparent, and micro-perforations smaller than typical light wavelengths can lower weight even further without significantly reducing reflectivity. Manufacturing, installation, and operation with extremely thin foil sails will be a challenge. The best mass-to-reflectivity material for visible light is an Aluminum/Magnesium alloy. Despite higher mass, refractory metal foils like Tungsten can be used at much higher temperatures. Therefore they can be used much closer to stars where light intensity is higher, and develop more thrust. This can be used for initial kick to outer system missions. The sail shape can be maintained by struts and tension wires, or by the balance of rotation and light pressure, or some combination.

At the distance of the Earth from the Sun, the incident power is 1370 MW per square kilometer. This produces about 8 Newtons/square kilometer for high-reflectivity sails. The attractiveness of solar sails is they use no fuel, and in principle the reflector can be made of local metals in the case of asteroids. The disadvantages are low thrust unless you build them very large, limited angular control, and inverse square dependence of thrust with distance from the source star.

Status: As of 2012 a few test flights of sails have been attempted, with varying success. Any object exposed to sunlight will see a light pressure force, but most spacecraft do not have a low enough mass per area for the force to be more than a minor correction to its motion.

Variations:

 * 66a Gravity Tractor - In this variation, the sail is not attached to a cargo such as an asteroid you are trying to move. The mutual gravity of the objects anchors the sail.  Reasons to do this are not having to devise methods to attach the sail, and allowing the sail to maneuver itself with less restrictions.

References:

 * Wikipedia article - Solar sail
 * Marchal, C., Solar Sails and the ARSAT Satellite - Scientific Applications and Techniques, L'Aeronautique et L'Astronautique, no 127, pp 53-7, 1987.
 * Friedman, Louis, Starsailing: Solar Sails and Interstellar Travel, Wiley, New York, 1988, 146 pp.

67 Laser Lightsail
Alternate Names:

Type: Laser by Photon Deflection

Description: A high powered laser is aimed at a target sail. The beam of photons are reflected off the sail material. Reflection of the photons reverses their momentum vectors' component which is normal to the sail. By conservation law, the sail gains momentum. Laser sails can have higher performance than solar sails because the laser beam intensity is not as limited as the local star light, and can be focused over longer distances. The sail can be designed to optimize reflectivity at the laser wavelength. This method is still by the overheating of the sail and lack of sufficiently powerful lasers for useful missions.

Advanced interstellar mission concepts have proposed very large phase plate type lenses for long range focus. Another advanced concept is to use two sails to slow down a vehicle, by releasing the first sail and using that to reflect light back to the second, smaller one.

Status: Not tested as of 2012 mainly due to lack of powerful lasers. Megawatt class lasers are not powerful in this context.

Variations:

References:

 * Forward, Robert L., Roundtrip Interstellar Travel Using Laser-Pushed Lightsails Journal of Spacecraft and Rockets, vol. 21, pp. 187-95, Jan.-Feb. 1984
 * "LightSail Project Documents"
 * "Laser-pushed Lightsail"
 * Fresnel plate lens for spacecraft propulsion. Gregory L. Matloff. "Deep Space Probes: To the Outer Solar System and Beyond". Section 7.5 The Fresnel Lens: A method of improving laser collimation. p. 103.
 * "Laser Technologies for Starflight"; "Key Issues for Interstellar Sails".

68 Microwave Sail
Alternate Names: Starwisp

Type: Beamed Microwave by Photon Deflection

Description: In this method microwaves are reflected off a very thin, open wire mesh. The momentum change of microwave photons bouncing off the mesh provides thrust. Because an open mesh of thin wires can have a very low weight, in theory this propulsion method can give high accelerations. With feasible power levels, the cargo mass will still be small. A good use for such would be delivering nanofactories, which can then build larger scale infrastructure. The beam can supply power to operate the factory at the destination. This is related to the non-transport energy delivery by microwave concept.

Status: Generating lots of microwave power is well understood. Keeping it focused over useful distances in space, and building the lightweight sail material are not.

Variations:

References:

 * Wikipedia article - Starwisp

B. Photon Rockets
This group of methods involve emitting photons from inside the vehicle, rather than reflecting photons from an external source. Like photon reflection, photons do not carry much momentum, so it is a low thrust set of concepts, although the exhaust velocity is the maximum possible. It can be considered an addition to other propulsion methods by directional emission of radiator waste heat.

69 Thermal Photon Reflector
Alternate Names: Nuclear Photonic Rocket

Type: Nuclear Reactor via Photon Emission

Description: A heat generating device, such as a nuclear reactor, is at the focus of a parabolic reflector. The thermal photons are focused into a near parallel beam, which propels the vehicle. Another high-energy source is a matter-antimatter reaction, which is absorbed by a blanket of heavy metals and converted to heat. This method is probably not practical, since emitting photons gives less momentum than emitting the reaction products of the energy source directly. It might be worth using directional radiators to get rid of surplus heat in addition to the reaction products. In that case it increases the total efficiency.

Status: Not tested as of 2012

Variations:

References:

 * Wikipedia Article: Nuclear photonic rocket

70 Stellar Photon Engine
Alternate Names: Stardrive

Type: Solar Flux by Photon Emission

Description: This is similar to the previous Thermal Photon Reflector method, except an entire star is at the focus of a cloud of lightsails which are balanced by gravity vs light pressure. Since both gravity and light intensity vary as the inverse square of distance from the star, a specific thickness of sail will be balanced at any distance. The ratio of the star's luminosity to its mass will determine the thickness. The star's spectrum and desired sail distance will determine what to make the sail out of. By directing the light in one particular direction rather than symmetric in all directions as natural stars do, the sails convert the star into an unbalanced light emitter, and so create a net thrust for the combined star + sail combination.

Sails with a slight dihedral angle oriented perpendicular to the star will direct the light to miss the star itself, and will be stable in orientation. Flat sails which do not reflect the light directly back to the star are slightly more efficient since none will be absorbed by the star, but will accelerate themselves sideways. So that type of sail must change its orientation periodically to maintain position. Sails which are not flat, such as a shallow cone, can direct the light to miss the star without drifting.

To give a numerical example, a sail at the Earth's distance from the Sun produces about 8 newtons/km^2. Solar acceleration at that distance is 0.006 m/s^2. Therefore if your sail mass is 1330 kg/km^2, light pressure will balance gravity. If made of aluminum, then it needs to be 0.5 microns thick. Foils that thick are commercially sold today. A sail cloud reflecting 10% of the Sun's output will have an area of 28 x 10^15 km^2 and a mass of 37.5 x 10^15 tons (3.7% of the mass of the largest asteroid, 1 Ceres), Such a sail cloud would produce a thrust of 225 x 10^15 Newtons, and accelerate the Sun by 3.5 meters per second per million years.

Uses for this type of engine are adjusting orbits of binary or multiple star systems, escaping future supernovae, and generally moving stars within a galaxy or between galaxies. The physics of this method are simple. The challenge is one of scale, and the extremely slow accelerations it produces because stars are very massive. Note that moving a star does not generally bring along objects in orbit about the star, they would need their own propulsion.

 * Moving Planets - A more practical (relatively) use for this method is moving planets, because of their much lower mass. The sails would be anchored by the planet's rather than the star's gravity. A larger secondary cloud of sails would direct more sunlight to the anchored sails than they would collect on their own.  A reason to do this for the Earth is the Sun is increasing its brightness by 10% per billion years due to increasing fusion rate in the core.  Thus if you want to keep the Earth habitable, you would want to move it slowly outwards.  Alternately, if you wanted to make Venus more habitable, you could move it into the Asteroid belt, where the excess greenhouse effect would be an advantage rather than a detriment.  This would still likely be a slow method.  Using gravity assist maneuvers with large asteroids or outer Solar System bodies would be faster.  Unlike gravity assist for spacecraft, in this case the object doing the flyby is massive enough to affect the orbit of the planet.  If a secondary body such as a gas giant is used to absorb the orbit changes, the flyby object could be used multiple times to shift the small planet without expending too much energy.

Status: Very far from testing. Possibly not even used in science fiction stories.

Variations:

References:

71 Gamma Ray Thruster
Alternate Names: redshift rocket

Type: Antimatter by Photon Emission

Description: Gamma rays produced by antimatter annihilation behind a vehicle can be absorbed by a thick layer of heavy metals. The momentum of the gamma ray photons produces thrust. Like other photon engines, it is likely more effective to use the decay particles directly, since they have more mass and can be directed better, but using the gamma rays as a supplement makes sense.

Status: Currently theoretical, as antimatter is only produced in tiny amounts in particle accelerators.

Variations:

References:

C. Amplified Photon Pressure
In this approach, a reflecting cavity is used to multiply the photon pressure. In one arrangement, a laser emits a particular frequency, and the walls of an enclosing cavity and the vehicle back end are designed to have a high reflectivity for that frequency. In another arrangement, there is no enclosing cavity, and a laser gain medium is placed in front of one mirror. The vehicle carries a second mirror, and the pair of mirrors plus gain medium form a laser amplifier which happens to have a large vacuum gap between the energy source and vehicle. In both cases the light will bounce multiple times until finally absorbed or dispersed, each bounce adding a momentum increment. The wavelength can be adapted to the physical dimensions, for example using microwaves instead of a laser if it creates an efficient reflection. The multiple reflections will extract energy from the photons as they repeatedly bounce off the moving vehicle by redshift. If sufficient bounces at sufficient projectile velocity happen, the light would be redshifted to a wavelength not efficiently reflected, or merely lose the majority of their energy, thus ending the momentum transfer.

References:

 * Young K. Bae Corporation, List of Articles, 2006-2012. The paper on Ultrahigh Precision Formation Flying has a good illustration of the concept.


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Space Transport and Engineering Methods/External Interaction Methods

External interactions are transport methods which create a force via fields or physical interaction with objects outside the vehicle itself.

72 Ionospheric Current Engine
Alternate Names: Electrodynamic Engine, Alfven Engine

Type: Electrical via Magnetic Field

Description: A current-carrying wire in a planetary magnetic field will experience an IxB force (current cross product with magnetic field, which is perpendicular to both). This is the same type of force created in electric motors. Currents which only flow in one direction cannot be sustained, electric charges would accumulate at one end. Therefore for this method the current loop is closed through an Ionosphere, the portion of a body's atmosphere which is ionized and can carry a current. The wire accelerates in one direction (pulling a vehicle along), and the ionosphere accelerates in the other direction. The ionosphere motion eventually dissipates within itself. When current is driven through the wire by a power supply, this functions as a motor, generating positive thrust. When current is allowed to flow in the other direction unopposed, it functions as a generator, producing power which can be used on the vehicle, and creating drag. Both power and drag can be useful in come circumstances, but any power generated will be at the expense of orbital kinetic energy.

A current loop thruster produces more thrust/watt than an ion or plasma engine. No propellant is consumed directly, although some material is consumed by a Plasma Contactor to produce a plasma that enables good electrical contact with the ionosphere. Effectively this gives an equivalent exhaust velocity of 250 km/s. Advantages are this is a relatively simple device, and relatively high thrust for an electric thruster. Limitations of this method are the planet must have both a substantial magnetic field and sufficient ion density to operate effectively, and the direction of thrust is governed by the local magnetic field direction. Planetary magnetic fields are often offset and tilted with respect to their poles, and in any case only point in one direction at any given point in orbit. The thrust direction is restricted to a circle perpendicular to the field direction. Earth orbit up to around 1000 km, and some areas around Jupiter are good candidates, other places less so. Uses include orbit maintenance for low orbit vehicles and Skyhooks, and apogee raising for reaching higher orbits. This method may be combined with other electric thrusters and share the same power source, changing which thrust method is used as appropriate for altitude or thrust direction.

The motion of a long wire in a magnetic field will generate a static voltage even when not used for propulsion. Such voltage build-ups happen to all spacecraft, a topic known as Spacecraft Charging, but are much larger with a long conducting wire. Therefore proper insulation and equipment grounding is needed to avoid damage. Even without the plasma contactor running, spark discharges to the surrounding ionosphere can happen.

Status: An experiment called the Tethered Satellite System was flown on the Space Shuttle in the 1980's.

Variations:

References:

 * Belcher, J. W. "The Jupiter-Io Connection: an Alfven Engine in Space", Science vol 238 no 4824 pp 170-6, 9 Oct 1987.

73 Gravity Assist
Alternate Names: Planetary Flyby, Celestial Billiards

Type: Kinetic Energy by Gravity Field

Description: Momentum exchange between a planetary body or large satellite and a vehicle allows changing the vehicle's direction and velocity in other reference frames. When considering just the vehicle and large body, a hyperbolic orbit has the same velocity on approach and departure, but gravity forces change the direction. When the large body is in turn in orbit around another larger planet or star, the change in vehicle direction can increase or decrease it's velocity relative to the larger velocity. By conservation of momentum, such gravity assist flybys affect the object you fly past, but typically the vehicle is so much smaller than the body it is flying past, so the change in the body's orbit is too small to measure.

The amount of velocity change is theoretically limited to twice the escape velocity from the body, by changing the vehicle direction 180 degrees. Thus larger bodies can produce larger velocity changes. As a practical matter gravity assist usually results in significantly less than this because the vehicle arrives with excess velocity and the desired final direction limits the flyby parameters. Excess velocity reduces the time the gravity of the body can act. More typical values are 0.5 to 1.0 times escape velocity measured from 1.5 body radii from it's center. The velocity changes are still large enough that they are extensively used in planetary exploration missions, often using multiple flybys of different planets, or even the same planet multiple times. The fuel cost of lining up for a gravity assist in these cases is much lower than doing the equivalent velocity change directly. The disadvantage is longer mission times consumed by the gravity assists compared to a direct transfer orbit, and needing to select mission dates according to when the planets are in the right positions. Lunar gravity assist to escape from or return to Earth orbit is particularly useful, as the Moon's orbit is short enough to give frequent opportunities.

Status: Used extensively in planetary missions, often multiple times for a single spacecraft.

Variations:

References:

74 Gravity Counterweight
Alternate Names: Dumb-Waiter System

Type: Potential Energy by Gravity Field

Description: In this method mass falling down a gravity well can be an energy source to power payloads going up the gravity well. Most typically this would be via a space elevator, using the falling mass to directly lift a cargo via cable, or to generate power to lift the cargo electrically. This is most efficient when cargo is going in both directions and are the same mass. In that case cargo delivery only consumes the inefficiency of the motors, which can be just a few percent. Another variation is to use braking energy of cargo above synchronous altitude, which sees a centrifugal force higher than gravity, to power lifting cargo from the body surface to synchronous altitude. The energy for this comes at the expense of rotation of the body by slowing it down. Braking will induce sideways forces on the elevator.

Status: Counterweights are commonly used in elevators on Earth. Space applications have not been tried.

Variations:

References:

Aerodynamic Interactions
Interaction with the atmosphere of large bodies which have one, such as the Earth, can produce significant forces. These forces are distinct from propulsion using an atmosphere which was discussed under Air-Breathing Engines, which are principally thrust forces.

75 Aerodynamic Forces
Alternate Names: Aerobrake, Airfoils

Type: Kinetic Energy by Aerodynamic Forces

Description: This method uses aerodynamic interactions with an atmosphere to provide forces perpendicular to forward motion (lift), intentionally oppose forward motion (drag) or change orbit parameters (using lift, with some drag). Buoyancy, or lift by displacement without forward motion, is covered by method 3 Aerostat. There is a wide range of conditions and applications for aerodynamic forces and it is a well developed field of engineering.

The range of velocities is from well below the local speed of sound, or Subsonic, to many times the speed of sound, or Hypersonic, even beyond escape velocities. All bodies which are not symmetric or pointing directly parallel with the velocity direction will generate some lift, and all moving bodies generate drag. The ratio of lift to drag, known as L/D Ratio ranges from less than 1.0 for blunt re-entry bodies to 25 or more for good subsonic Airfoil shapes designed to maximize lift. Structures whose main purpose is to generate lift are called Wings. All lifting bodies induce some drag by virtue of the lift force not being entirely perpendicular, as well as body drag from other parts of the vehicle besides the wings.

Lift forces can be used to gain altitude as part of a climb to orbit. Drag forces would be minimized as they counteract thrust to reach orbital velocity. On return from orbit, purposely designed devices to slow down include parachutes, heat shields, and High Q Aerobrakes. While already in orbit, pure drag generating Low Q aerobraking can be done without special devices as long as the drag and heating are within what the structure can withstand. Orbital speed lift devices can change orbit direction, at the cost of some drag loss.

Status: Powered aircraft have operated for over 100 years. Jet aircraft have been used as the carriers for rockets. They use a combination of 25 Fanjet for forward thrust and wings for lift.

Variations:

 * 75a Parachute - Relatively low Mach number drag device to come to a complete stop or low enough velocity for a terminal landing device.
 * 75b High Q Aerobrake - Q is dynamic pressure, the pressure caused by motion through an atmosphere. High Q aerobrakes generate a lot of drag at high Mach numbers. They are often designed as inflatable devices, called Ballutes (from balloon and parachute), but extendable flaps or panels would also fall into this category.
 * 75c Heat Shields - They are drag devices integrated into a vehicle structure, but primarily designed to dissipate the extreme heat of re-entry.
 * 75d Low Q Aerobrake - These are often re-purposed existing parts of a vehicle which are used for braking. By keeping the braking drag low, and using multiple orbits, often the braking can be done without special components on the spacecraft.  Obviously that will take longer.

References:

Mechanical Interactions
This category involves direct physical contact with a natural or human-made body in space.

76 Rheobrake
Alternate Names: Lithobrake, Crashportation

Type: Lower Kinetic Energy by Friction

Description: This methods uses mechanical friction against a planetary surface to slow down. For example, imagine a rail made of cast basalt on the lunar surface. It is laid level to the ground, and is shaped like the concrete highway dividers. A vehicle wanting to land is in a low grazing orbit. It aligns with the rail, just above it, then extends some clamps over the rail. By applying clamping pressure, the vehicle can brake from lunar orbit to a stop. Obviously the brake will be dissipating a lot of heat, and will therefore have to be made of high temperature material such as graphite. Another approach is to have a 'runway' which is a smoothed area on the lunar surface. The arriving vehicle slows down to below orbital speed, then gravity pulls it down to the runway, and friction with skids on the bottom of the vehicle slows it down.

In order to not melt the brake, it should maximize surface area and possibly have cooling systems like heat pipes. The rail will not see such concentrated heating, so cooling is not as much of a challenge. The main advantage is not requiring fuel to land on a body. The main disadvantage is the large size and mechanical accuracy required for the landing rail or runway. When the velocity is greater than the speed of sound in the brake materials, irregularities in the landing surface can create shock waves in the device. This method is easier to implement on smaller bodies where the amount of orbital kinetic energy to dissipate is less.

Status: Mechanical brakes are used in many vehicles on Earth. High performance ones are used in passenger aircraft. Use for space transport is theoretical at this point.

Variations:

 * 76a Rock Cloud - This involves creating an artificial 'atmosphere' of particles to slow down against. A cloud of lunar dust could be raised by electrostatic forces and an arriving vehicle slows by impact of the dust particles, or deflection by charged surfaces on the vehicle.

References:

76a Space Harpoon
Description: In 2015, NASA proposed using a harpoon and cable to hook to a passing object, and use braking of the cable to accelerate and match velocity. Redeploying the cable and using the objects spin to sling the vehicle would add additional velocity.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Space Transport and Engineering Methods/Comparisons Among Methods

Transport methods can be sorted by performance, status, and cost measures. The following sections describe the measures and make a first attempt to rank the various methods. Technology is constantly progressing, so any selection of methods needs to be updated with the most current data, or projections for future planning purposes.

Exhaust Velocity
Any propulsion method that expels matter or energy to produce thrust obeys conservation of momentum, which is defined as mass times velocity. The exhaust momentum is equal to and opposite change in vehicle momentum. Given a finite supply of materials for the exhaust, mass is fixed. Therefore higher velocity leads to higher total momentum, and thus better performance for the vehicle. Exhaust velocity is measured in meters/second. Some systems, like 72 Ionospheric Current Engine, do not use exhaust for momentum exchange, but have a consumed supply - plasma generated for contact to the ionosphere in this case. The effective exhaust velocity is then calculated from the thrust force and mass flow rate.

Specific Impulse
Specific impulse is a derived measure traditionally used for launch from Earth, and then extended to space. It is defined as pounds thrust per pounds/second of fuel consumed, which simplifies to units of seconds. When multiplied by one Earth gravity (9.80665 m/s^2) it converts to exhaust velocity, which is the preferred measure since it is in SI units.

Transport Optimization
Optimizing Vehicles

In past rockets, this has been done by using different type of fuel for different stages in a rocket. In the early part of the flight, air drag is important, so a dense fuel is preferred. A dense fuel means smaller fuel tanks, and hence less area to create drag. Thus the Saturn V used liquid oxygen/kerosene and the Shuttle uses solid rockets for the first stage, both being dense fuels. Both use liquid oxygen/ liquid hydrogen for the second stage. This has the highest performance in use for a chemical rocket fuel.

The Pegasus rocket uses an aircraft to get above the bulk of the atmosphere. A sub-sonic jet engine has about ten times the performance of a chemical rocket, mostly because it does not have to carry oxygen to burn.

Many, many propulsion combinations are possible in getting to Earth orbit and beyond. A large part of space propulsion design is choosing which methods to use and when to switch from one to another.


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Space Transport and Engineering Methods/References

A system designer should know the current state of knowledge in topics relevant to their work. There are several reasons. One is to not repeat work already done by someone else. Another is to stimulate new ideas and improvements. In addition to the references listed below and elsewhere in the book, it is very useful to know how to find additional information. Categories of information include:

 * Current News (Magazines, Newspapers, Blogs)
 * Archival Publications (Journals, Preprint Archives)
 * Books
 * Online Data (Web and other protocols)
 * Technical Reports
 * Product Data
 * Discussion Forums

Once information is located you should record where and how you found it, to save having to find it again. There are a number of ways to do that, depending on type of media: building a personal library in paper or electronic form, bookmarking in web browsers, etc. As long as you have the ability to find the data again, the particular methods can be left to personal preference.

Current News
As of 2012, obtaining current news via paper-based sources is nearly obsolete. Internet-based electronic delivery is faster and less expensive. There are innumerable online websites, magazines, and blogs covering every topic relevant to space systems. One distinction between sites is original reporting vs re-posting of news from elsewhere. You should also consider the quality and experience of the writers and any bias they might have. We cannot list all possible news sources, and any such list would rapidly become obsolete. Therefore we will list a few good examples and encourage looking for good sources in your fields of interest.

General News

 * Google News - General news aggregator. This means they do not write original stories, but rather link to stories from multiple other sources. Searchable and customizable, it is a good starting point to find other regular sources. Story selection for the front page is automated, but the full database indexes several thousand news sites.
 * Reddit Technology, Science, and Space Topics - Reader submitted daily links divided by topic, with active discussions. There are many topic headings, but the three listed are the largest related to space.  It is another news aggregator, but human-submitted and filtered by votes of the readers.

General Space

 * Space.com - General news website for anything related to space.

General Technology

 * Next Big Future - Blog for all kinds of new and future science and technology. Often covers space topics.

General Science

 * EurekAlert - Current science and technology press releases. Often a day earlier and unmodified by journalists.

Specific Industries

 * Aviation Week - Aerospace industry news, including space projects.
 * Automation World - Manufacturing automation.
 * EE TImes - Electronics engineering. This link is to the news summary.

Multiple Documents
The following sites index, search, or link to multiple documents:

Library Indexes
Two steps in finding the current state of knowledge are (1) finding what works exist on a given topic, and (2) locating a copy of the work to look at. Library indexes help with both steps.

 * Library of Congress Online Catalog - This is an index to the largest single library in the world, including books and periodicals. Basic search is by title, author, and subject.  Items are shelved and indexed by subject, using a cataloging Call Number system. If you use the "Call Number Browse" type of search, you can look through all the entries under that subject, which is useful for doing general research on a subject.  The LC Classification Outline has a summary of the call number system.  For space projects, the headings Q (Science) and T (Technology) are the most relevant, although other sections can be useful depending on the topic.

 * WorldCat - This is an online database of the collections of over 10,000 libraries worldwide. If a work is not available nearby or online, a process of Inter-Library Loan can be used from a local library to borrow it from another library.

Online Search
In addition to well-known search engines such as Google and Bing, there are more specialized searches such as:

 * Google Scholar - for searching scholarly works
 * Google Books - for searching the text of books in general
 *  Open Directory Specialized Search Engine List - Links to many other search websites.

Online Repositories
Repositories contain multiple online documents which usually can be downloaded. In some cases they only index the document online and you then have to request or find a physical copy.

 * Rocket Science Library - A collection of open source, public domain, and online works compiled in parallel with the development of this book. A list of additional works is included for ones where copyright restrictions prevent including the document itself.

 * NASA Technical Reports Server - Access to approximately 1 million documents in NASA databases.
 * Defense Technical Information Center - Public access to the unrestricted subset of approximately 2 million US DOD and other technical documents.
 * US Patent and Trademark Office - Search and download patent data. Patents often contain very useful technical data.
 * arXiv.org - Open access to over 750,000 electronic papers in Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance and Statistics. Operated by Cornell University Library.
 * NATO Research and Technology - FTP server of NATO technical and conference reports. Some have relevance to space projects. It is recommended to find titles and document numbers by other search methods, since the ones here are stored only by their document numbers.
 * National Space Society Library - Online library with multiple space references.
 * Defense Acquisition University ACC Practice Center - Multiple documents about how the US government manages projects, including engineering methods in general, and space projects as a topic.

Reading Lists

 * Encyclopedia Astronautica Recommended Books

Other Wikibooks

 * Astrodynamics
 * High School Earth Science/Early Space Exploration and High School Earth Science/Recent Space Exploration describes how far we've come --
 * Colonizing Outer Space and Conplanet speculates on how far we may go, and what living on other planets may be like after we get there.

General Space Reference

 * Jet Propulsion Laboratory, Basics of Space Flight, 2012.
 * USAF Air Command and Staff College, Space Primer, 2nd Ed., 2009.

General Space Systems Design

 * Griffen, Michael D. and French, James R., Space Vehicle Design, 2nd ed., AIAA Education Series, 2004.
 * Hammond, Walter E., Design Methodologies for Space Transportation Systems, AIAA Education Series, 2001.
 * Ley, Wittmann, and Hallmann, eds. Handbook of Space Technology, Wiley, 2009.

Propulsion

 * Glenn Research Center, Beginner's Guide to Rockets - website designed for young students and teachers.

 * Hunter, Maxwell, Thrust into Space, Holt, Rhinehard, and Winston, 1966. - An introductory textbook on space propulsion.

 * Sutton, George P. and Biblarz, Oscar, Rocket Propulsion Elements, 8th ed., Wiley, 2010, ISBN-13: 978-0470080245. - This has been considered the standard reference for classical rocket propulsion. It is mostly concerned with solid, liquid, and hybrid chemical rockets, and has one chapter on electric propulsion.

 * Bolonkin, Alexander "Review of new ideas, innovation of non-rocket propulsion systems for Space Launch and Flight" 3 parts, 2010, The Internet Archive:
 * Part 1, Part 2, Part 3

 * Mallove, Eugene F. and Matloff, Gregory L., The Starflight Handbook: A Pioneer's Guide To Interstellar Travel, 1989. - A semi-technical introduction to interstellar flight.

 * Morgenthaler, G. W.; Tobiska, W. K. "Aerospace Century XXI: Space Flight Technologies", Proceedings of the 33rd Annual AAS International Conference, Boulder, Colorado, 26-29 Oct. 1986. Published as Advances in the Astronautical Sciences, vol 64, pt 2, 1987.

Early Works
Although technology has changed drastically since these were written, physics has not, and they serve as a guide to what technical problems need to be solved in any design:

 * Bureau of Naval Personnel, Principles of Guided Missiles and Nuclear Weapons, 1959.

 * Koelle, Heinz Hermann, ed., Handbook of Astronautical Engineering, New York, McGraw-Hill, 1961. - This is an excellent comprehensive reference handbook representing the state of the art as of 1961. Has a chapter covering nuclear, electric, and solar-thermal propulsion.

 * Merrill, Grayson, ed. Principles of Guided Missile Design, series, Van Nostrand, 1958. - This is a series of at least 9 volumes with different authors. Although written in the Cold War nuclear missile era, the technology is relevant to space projects.  Likely much of it will need updating due to being written 50 years ago:

 * Guidance - Locke, Arthur et. al.
 * Aerodynamics, Propulsion, Structures, and Design Practice - Bonney, E.A, Zucrow, M.J., and Besserer, C.W.
 * Operations Research, Armament, Launching - Merrill, G., Goldberg, H., and Helmholz, R.H.
 * Missile Engineering Handbook - Besserer, C.W.
 * Space Flight - Ehricke, Krafft A.
 * Systems Engineering - Jerger, J.J.
 * Range Testing - Freitag, R.F.
 * Airborne Radar - Povejsil, D.J., Raven, R.S., and Waterman, P.J.
 * Automatic Flight Control - Povejsil, D.J., Kelly, A.J., Mathews, C.W., and McCourt, A.W.

Resource Use

 * Faughnan, Barbara (ed.); Maryniak, Gregg (ed.) "Space Manufacturing 5: Engineering with Lunar and Asteroidal Materials", proceedings of the 7th Princeton/AIAA/SSI Conference, Princeton, New Jersey, 8-11 May 1985.

Websites
This category includes online sites where the data is in the site itself, rather than the site being a link to other documents.

General Space Websites

 * Permanent - Organization dedicated to space development. Their website has extensive discussion on relevant topics.
 * Rocket and Space Technology - Personal website with extensive coverage of the basics of space flight, hardware, and missions.
 * Yarchive - An archive of space related posts from the USENET discussion system, covering many topics.
 * Centauri Dreams - Long-running blog about deep space exploration.

Real Time Data

 * Real Time Satellite Tracking - Provides tracking simulation of Earth satellite orbits based on official tracking data.
 * Eyes on the Solar System - 3D Simulation of Solar System spacecraft and planetary bodies beyond Earth orbit, including current, and past and future positions.

Life Support

 * USDA Nutrient Data Laboratory - Includes a food and nutrition database, from which food requirements for space projects can be determined.

Multiple Propulsion Concepts

 * Diesposti, R. S.; Pelouch, J. J. "Performance and Economic Comparison of Externally Energized vs Chemically Energized Space Propulsion", AIAA paper number 81-0703 presented at 15th International Electric Propulsion Conference, Las Vegas, Nevada, 21-23 June 1981.
 * Andrews, Dr. Dana G, ed. Advanced Propulsion Systems Concepts for Orbital Tansfer: Final Report, Boeing document D180- 26680 produced under NASA contract NAS8-33935.
 * Forward, Robert "Alternate Propulsion Energy Sources", AFPRL TR-83-067. (NTIS AD-B088 771/1) Dec 1983, 138p. Keywords: Propulsion energy, metastable helium, free-radical hydrogen, solar-pumped plasmas, antiproton annihiliation, ionospheric lasers, solar sails, perforated sails, microwave sails, quantum fluctuations, antimatter rockets.
 * Wang, S.-Y.; Staiger P. J. "Primary Propulsion of Electro-Thermal, Ion and Chemical Systems for Space Based Radar Orbit Transfer", AIAA/SAE/ASME/ASEE 21st Joint Propulsion Conference, AIAA paper number 85-1477, 1985.
 * Forward, R. L. "Advanced Space Propulsion Study - Antiproton and Beamed Power Propulsion", Final Report, 1 May 1986 - 30 Jun 1987, Hughes Research Laboratories, report AFAL-TR-87-070, 1987.DefenseTechnical Information Center #AD-A189 218. National Technical Information Service # AD-A189 218/1 PC A10/MF A01 [Quote:, goes into detail on beamed power systems including " 1) pellet, microwave, and laser beamed power systems for intersteller transport; 2) a design for a near-relativistic laser-pushed lightsail using near-term laser technology; 3) a survey of laser thermal propulsion, tether transportation systems, antiproton annihilation propulsion, exotic applications of solar sails, and laser-pushed interstellar lightsails; 4) the status of antiproton annihilation propulsion as of 1986; and 5) the prospects for obtaining antimatter ions heavier than antiprotons." Again, there is an extensive bibliography.]
 * Forward, R. L. "Exotic Propulsion in the 21st Century", in Aerospace Century XXI (see Morgenthaler and Tobiska, under Propulsion above).
 * Forward, Dr. Robert L., Future Magic., Avon, 1988. ISBN 0-380-89814-4. "Nontechnical discussion of tethers, antimatter, gravity control, and even further-out topics."

More References to be sorted
These should eventually end up under the appropriate section of the book:

 * Kunz, K. E. "Orbit Transfer Propulsion and Large Space Systems", J. Spacecraft and Rockets vol 17 no 6 pp 495-500, Nov.-Dec. 1980.
 * Poeschel, R. L. "Comparison of Electric Propulsion Technologies", AIAA paper number 82-1243 presented at AIAA/SAE/ASME 18th Joint Propulsion Conference, Cleveland, Ohio, 21- 23 June 1982.
 * Jones, R. M.; Kaplan, D. I.; Nock, K. T. "Electric Propulsion Systems for Space Stations" AIAA/SAE/ASME 19th Joint Propulsion Conference, AIAA paper number 83-1208, 1983.
 * Jones, R. M. "Space Supertankers: Electric Propulsion Systems for the Transportation of Extraterrestrial Resources" AIAA/SAE/ASME 20th Joint Propulsion Conference, AIAA paper number 84-1323, 1984.
 * Phillips, P. G.; Redd, B. "Propulsion Options for Manned Missions to the Moon and Mars", in Aerospace Century XXI (see Morgenthaler and Tobiska, under Propulsion above).
 * Matloff, G. L. "Electric Propulsion and Interstellar Flight", 19th International Electric Propulsion Conference, Colorado Springs, Colorado, 11 May 1987.
 * Korobeinikov, V. P. "On the Use of Solar Energy for the Acceleration of Bodies to Cosmic Velocities", Acta Astronautica, v 15 no 11 p 937-40, November 1987.
 * Kerrebrock, J. L "Report of the National Commission on Space- One Commissioner's View", in Aerospace Century XXI (see Morgenthaler and Tobiska, under Propulsion above).
 * Harvego, E. A.; Sulmeisters, T. K. "A Comparison of Propulsion Systems for Potential Space Mission Applications", ASME Winter Meeting, Boston, Massachusetts, 13 December 1987, 1987.
 * Byers, David C.; Wasel, Robert A. "NASA Electric Propulsion Program", NASA Technical Memorandum 89856, May 1987.

 * Lorrey, M; High Density Fuels, web page, March 2006.
 * Licht, S; Solar Driven Synthesis, in Advanced Materials, 2011.

Discussion Forums
Although online forums are informal, the collective knowledge of the participants is sometimes impressive. The discussion history can be searched or new questions asked to make use of it.


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Software Engineers Handbook/Language Dictionary/Pascal

Pascal
Here is the wikipedia page.

Type
Pascal is a full procedural programming language.

Execution Entry Point
Program Name (input, output) begin end.

General Syntax
a:=b;

Comments
(* this is a comment *)

or

{ This is a comment }

or

//This is a comment

Variable Declarations
Variables declarations are necessary: var i: integer ; k: boolean; d:Real;

Method Declaration/Implementation

Scope

Conditional Statements

Looping Statements

Output Statements
 úΜÈŔŔŶ—

Error Handling/Recovery

Containers

Algorithms

Garbage collection

Tips

 * i have found it helpful when i switched from qbasic to pascal to just sit down and write 10 small or large programs for each of the most needed things in pascal example,*for loops,variable declaration,while loops*may be called something else*

Web References

 * Free Pascal

Books and Articles


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Guide to Schools/Alphabetical/King George School

=The King George School=

Grades and Ages
The King George School served students from grades 9-12.

Special Characteristics of Students
King George School served students with a history of at-risk behaviors, but with a some willingness to work with the problems and attempt to address them. Many students entered King George School after completing a wilderness program, more restrictive therapeutic or emotional growth school or other more intense intervention. Others arrived at the school directly from home.

All students needed to be academically capable, as academic standards were comparable to many conventional boarding schools.

Size of School
In May 2010, King George School had a student body of about 35. It later closed.

Description of Physical Plant
King George School is on the grounds of the former King George Farm. It is a few miles from the rural village of Sutton, and about 25 miles north of St. Johnsbury, USA. If you drive to the campus, you will leave the paved road then continue a mile or two on gravel. As you approach the King George property, you will see the main classroom building on your left and the administration building on the right.

The classroom building includes a dance studio, a theater and an art studio, in addition to an adequate array of classrooms. This building was designed to blend with the ubiquitous white frame buildings so common in northern New England.

Across the road is the building that houses the office of the head of school, admission and financial services. Behind the administration building is a photo lab and darkroom, and a small barn that houses various unexpected projects.

Just beyond the classroom building is the boys' dormitory. Rooms are designed for about four boys per room. Design is simple, practical and clean. The girl's dorms are located down the road with the Horse Barn, a refurbished barn, that has become a very comfortable place for assemblies, meetings and recreation. Nearby is a small chapel, strongly resembling an old Quaker meeting house.

The campus goes together to form an integrated whole, that is comfortable, secluded, peaceful, practical, and very attractive.

Special Programs
King George School is proud of its academic college preparatory curriculum, supplemented by available qualified therapists, licensed in the state of Vermont. The King George School is a recognized independent school by the Vermont Department of Education and is a member of the National Association of Therapeutic Schools and Programs.

It is also well known for its diverse visual and performing arts program.

The best descriptions of these programs are on King George School's own web site. Please click on links above.

Extra-Curricular Activities other than Athletics
The student life program at the King George School provides students with many opportunities to enjoy activities both on and off campus. Activities include (but are not limited to) mountain biking, skiing, snowboarding, hiking, swimming, rock and ice climbing, rock band, weightlifting, drama, local concerts and lectures, and more.

Ownership
Universal Health Services acquired King George School in August 2005, along with a several of the other former Cedu/Brown schools. Some of the previous acquisitions by Universal use the name "school" but are licensed as treatment facilities (for example, Provo Canyon School.) The mission of the school quietly changed to that of a treatment center. Enrollment declined, and the former school eventually closed.

General Description of School
King George School was a private, college-preparatory therapeutic boarding school for grades 9-12. It was a fine academic institution during its early years, with a strong visual and performing arts program to encourage student self expression and self disclosure.

The school suffered during the later days of operating as one of the Brown Schools/Cedu Schools during the time the Brown Schools were in decline, culminating in bankruptcy in April 2005. The school continued operating during the bankruptcy period. In August 2005, the school and most other Brown Schools properties were acquired by Universal Health Services. After that change of ownership, the school continued its decline, and eventually closed.


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Template:Guide to Schools/School outline

=|Name of school=

Contact Information
Address City State Zip/pc phone

Administrative head
name, title

Admission Contact
name,title

Head of Academics
name,title

Head of counseling or therapauetics
name,title

Other key staff
name,title

Ownership
| Parent Company


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Celestia/Celx Scripting

CELX Scripting
CELX scripts can be used to interact with the user, change Celestia's viewpoint and what it shows, and do sophisticated calculations.

CELX first appeared in Celestia v1.3.1. CELX consists of the Lua-programming language combined with a Celestia-specific API. That is, CELX uses Lua syntax and most of the basic Lua-libraries, and adds Celestia-specific functionality.

CELX's biggest advantage over CEL scripting is that it is a real programming language, including control structures like functions and loops, and offers a lot more possibilities, such as interactivity. However this power also makes CELX harder to learn and use, and provides the main disadvantage over CEL scripting.

 * Introduction to CELX Scripting

 * CELX Lua Methods: a list of CELX Methods (functions).

 * CELX Questions and Answers

 * CELX Development Suggestions

title: Celestia/Celx Scripting


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Celestia/Cel Scripting

CEL scripting
CEL scripts are short text-based programs that Celestia can read. They contain simple commands that take control of Celestia (once it is launched) and enable a script writer to design a specific journey or set of scenes that a user of the script will see and experience. For example, a script may take the user on a short tour of the solar system, pausing briefly at all of the planets. In that way, a CEL script is used to change the position of Celestia's viewpoint.

CEL scripting is basically a sequence of commands similar to key-presses, which are executed in specific timing and at a particular speed to give viewers a visual experience that the script writer wants to display. There is however no interactivity with the user during the execution of a CEL script.

Scripts can take users to very specific locations in the Celestia universe. For example, a script can be written to take them into orbit around Venus, or take them outside the Milky Way to view it from intergalactic space. What happens when they get there is controlled by the script as well. There may be a 30 second pause, followed by a movement to another location. There may also be text displayed, to teach the user some information about a particular place.

Scripts also have the ability to change specific Celestia (render) settings. For example the rendering of orbits can be turned on/off or planets can be given a marker or alternative surface texture. Note however, when a CEL script ends or is terminated by the user, there is NO ability to reset those settings to the values they were before running the script. It is advised for the user to have their own reset.cel script, so specific Celestia (render) settings can be easily restored after running other CEL scripts.

A script can be launched from the Celestia/File/Open script... or Celestia/File/scripts drop down menu and it will run until it is complete. Meanwhile it can be paused by the user by pressing the [Spacebar] key or it can be stopped at any time by pressing the [Esc] key. When the file extension ".cel" of a script is also correctly associated with the Celestia program, it is also possible to launch the script externally, e.g. from a hyperlink or by double clicking.

Scripts offer a Celestia user several key advantages:
 * 1) A Celestia script writer can give inexperienced users a beautiful visual tour of a particular Celestia environment.
 * 2) Important celestial events can be displayed for a user, controlled by script. For example, a script can take a user to 1957 to see Sputnik 1 orbiting Earth on its maiden flight. The script can put the viewer into a position just 10 meters from Sputnik, and pace it as it flies over the Soviet Union, then display a paragraph of text explaining the significance of Sputnik.
 * 3) If no script were available, it might take a user many minutes to set up Celestia to go to the same spot at the same time. Also, no text would be displayed.
 * 4) In the special Celestia-ED version of the Celestia program, CEL scripts have been given the ability to also command sound and music files to play. As such, scripts can be designed which choreograph complex movements of planets or spacecraft, while moving to music.

Writing Celestia Script Commands
Anyone can write a CEL script. Some knowledge of how to do so is required and is summarized below, but you do not have to be a very experienced Celestia user to write a CEL script.

The very first character in a CEL script must be an opening curly brace "{". The very last character in a script must be a closing curly brace "}".

Therefore, an empty, but valid Celestia CEL script file looks like the following:

{ }

Every script you create for Celestia will have at least one command line in it, which is a line of text instructing Celestia to set or do something.

A typical single Celestia script command line consists of the following pieces:
 * A command name, such as wait
 * A space (spacebar)
 * An opening curly brace "{"
 * Zero or more argument names, along with the value you want the argument set to.
 * A closing curly brace "}"

The "#" character can be used as the first character in a script line to make that line a comment line, which the script engine will ignore (not execute).

Examples:

Assign a  value of 5.5, to the argument duration, in the command wait, which tells Celestia to pause for the number of seconds you enter.

Note: there are no double quote marks (") around the  value (5.5) because double quote marks are used only to specify a  value.

wait { duration 5.5 }

Assign a  value of "Mars", to the argument object, in the select command:

Note: Here, double quote marks tell Celestia that a , or text value (Mars), is being sent to it. When sending a  to Celestia, the value must be enclosed in double quote marks.

select { object "Mars" }

Example of a comment line.

 * 1) This is a comment line

Some Celestia CEL script commands, such as print, send more than one argument. In these cases, you enclose all arguments and their associated values between the command's opening and closing curly braces, such as:

Note: All arguments for each command must be enclosed within the command's curly braces, such as the example above shows. You may include as much white space (spaces, blank lines, etc.) as you deem necessary, and you may even specify each argument and its associated value on a different line – all for clarity.

print { text "Hello universe." row -4 column 1  duration 5 }

-- OR --

print { text "Hello universe." row -4 column 1 duration 5 }

When you save a Celestia CEL script file to disk, the file extension must be ".cel", which allows your operating system to recognize the file as belonging to Celestia, and allows Celestia to know this file is one of its CEL scripts. Also, you must save the file as plain text. Rich Text Format files do not function in Celestia because they also contain embedded formatting information.

Index of available CEL commands
An index of the available CEL commands can be found at the CEL command index section of the CEL to CELX migration chapter.

By clicking on a specific command, you will be routed to the explanation of its functionality. The index also contains an explanation on how that CEL command can be migrated to equivalent CELX objects and methods. Before starting with CELX scripting however, you may need to have some programming experience with CEL scripting first.

CEL scripting versus CELX scripting
A main advantage of CEL scripting is that it's MUCH easier to use than CELX scripting. The syntax is easy due to the simple structure of the commands, and most of the time it's very easy to translate keyboard commands directly into their CEL counterparts.

The primary disadvantage in using CEL-scripting versus CELX-scripting is its limited flexibility, its inability to respond to keyboard commands, the absence of a possibility to reset changed user settings to the values they were before running the CEL script and a lack of support for many of the new Celestia features that appeared in Celestia version and newer Celestia versions/releases.

The main differences between CEL scripting and CELX scripting are summarized below:
 * The extension of CEL and CELX scripts differ from each other:
 * CEL script: .CEL
 * CELX script: .CELX
 * More words and more commands make CELX scripts look more complex than CEL scripts, but it also gives your scripts more possibilities, including the usage of Lua logic (like building loops, make decisions on what to do, calculations, etc.), and interactive keyboard input. The CEL to CELX migration section will help you with the more complex CELX equivalents of the CEL commands, so you can use these equivalents in your own script. Besides the equivalents, there are also examples included. You can copy the equivalents into your own script and modify them according your own wishes. After a while you’ll become more familiar with CELX scripting and copying or typing a sequence of CELX methods and objects will become easier each time.
 * Keyboard interaction is possible in CELX, but not in CEL scripting. Within CELX you can use the callback for keyboard input, which must have the name celestia_keyboard_callback. After a script activates the handling keyboard-input by calling celestia:requestkeyboard(true), any keypress will result in a call to a method with this name.
 * Angles in CELX are defined in radians instead of degrees in CEL. 180 degrees (half a circle) is the same as π radians = 3.14159265 radians. You can use the Lua math.rad and math.deg functions to convert degrees to radians and [i]vice versa[/i]:
 * radians = math.rad(  )
 * or you can convert yourself: radians = (  / 180 * 3.14159265)
 * or use Lua math.pi: radians = (  / 180 * math.pi)
 * degrees = math.deg(  )
 * or you can convert yourself: degrees = (  * 180 / 3.14159265)
 * or use Lua math.pi: degrees = (  * 180 / math.pi)
 * The units of the components of a position object in CELX are millionths of a light-year. When you have position components defined in km (CEL scripting) or miles, you first have to convert those components to millionths of a light year. Therefore, you can use a constant, which must be initialized first within your CELX script:
 * From km to millionths of a light year, use constant: uly_to_km = 9460730.4725808.
 * From miles to millionths of a light year, use constant: uly_to_mls = 5912956.5453630.
 * Next you can convert km or miles to millionths of a light year as follows:
 * millionths_of_a_light_year =  / uly_to_km
 * millionths_of_a_light_year =  / uly_to_mls
 * Be aware that some default parameter values have changed in CELX compared with the equivalent defaults in CEL scripting.
 * Time in CELX is defined in TDB (Barycentric Dynamical Time) instead of UTC or Juliandate, because UTC includes "leap seconds" in order to stay aligned with the Earth's varying rotation. Leap seconds happen essentially randomly, when they are needed. Although Celestia does incorporate a table of leap second times, the use of UTC in versions of Celestia before v1.5.0 caused problems when used with ephemerides which are defined using TDB. Starting with version, although it still displays UTC on the screen, Celestia uses the TDB time scale internally for everything else. As a result, Celestia places objects much more accurately than before.
 * To convert from Juliandate and/or UTC to TDB is explained in the section: CEL: time.
 * Transparency of markers can be true or false in CELX (starting from Celestia version ). In CEL scripting there’s no possibility to enable or disable the transparency of markers. The Celestia releases /1.5.1 and older always displayed the markers, but from Celestia version (or Celestia-ED version 1.5.1), the default marker transparency is true, resulting in disappearing markers in case of Occultations, or partially show markers on the surface of celestial objects. To overcome this, the usage of the false parameter in the CELX: object:mark method is necessary, which also makes CELX scripting necessary, instead of CEL scripting!
 * Sometimes, timing can be more sensitive in CELX scripting than it is in CEL scripting, especially when lots of Lua commands and Celestia (related) methods are used within a short period of time with a high timerate value ([i]i.e.[/i] when time has been speeded up). Solving those timing issues on your own computer may not always give the same result on other computers with different CPU speeds. In those cases, the usage of a CEL equivalent part within your CELX script may give a more consistent result.
 * It is possible to insert CEL script parts within a CELX script by defining the following function at the beginning of your CELX script:

function CEL(source) local script = celestia:createcelscript(source) while script:tick do       wait(0) end end -- Within the subsequent part of your CELX script you can now use CEL script commands as follows: CEL({  })

 * CEL and CELX Scripts can be temporarily paused, by pressing the [Spacebar] key. Pressing the [Spacebar] key again will result in the continuation of the script. Note: When using CEL script parts and functions within a CELX script, this pause function will not work completely correctly. Although the scene you’re looking at will pause directly, the CEL script parts and functions will continue to run in the background, resulting in a part of the script being visually skipped when you press the [Spacebar] key again to continue the script.

title: Celestia/Cel Scripting


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Celestia/Celx Scripting/Q&A

Introduction
This page is intended to be a repository of practical questions and answers (Q&A) on the subject of CELX Scripting. There are two ways to get something listed here:

- Just list your general questions in the Q&A section below, and someone will post an answer to them. Please post discussions about a specific answer (e.g. requests for clarification) to that answer's discussion page. Unfortunately, however, few, if any, CELX programming experts peruse this WikiBook on a regular basis.

or (preferred)

- Post your question on the Celestia Scripting Forum. Once the answer is determined to your satisfaction there, please transcribe the question and summarise the final answer here so that we can build up a knowledge base in this WikiBook.

How do I calculate the angle between two objects?
Answer

How do I assign a CELX function to a particular key on the keyboard?
Answer

'The questions & answers below have been moved here from one of the "discussion" pages. Those pages are for discussing issues related to editing WikiBook pages, not for providing content for a WikiBook.'

If there's something that isn't clear, ask about it here.

Just click on the "edit this page" button immediately above and start typing. Or click on one of the [edit] buttons in this page and insert a new question or answer at its bottom.

Test Scripts and Questions
Using the very first, very simple CELX example script here, to wit:

celestia:flash("Hello, Universe!") wait(2.0) The "flash" function is defined within the object "Celestia." When this script is executed, I expected it to do nothing more than display the text "Hello Universe!" Prior to executing this script, I had centered the Earth in my Celestia window, and I was in the "Follow" mode. When I execute this script, the viewpoint changes so that the Earth gets moved to one side or the other prior to seeing the "Hello Universe!" message. Am I correct in thinking that this movement is caused by Celestia's definition of the "flash" function? In other words, the "flash" function in Celestia itself probably includes some movement function to be executed. Yes?

 * No. I don't see the earth move when I try this. How are you running the script?

After investigating the properties of the "flash" and "print" functions in Celestia, I tried the following script to center the message on my display. This also causes movement of the Earth, even though I thought I centered everything on the screen since I'm using 0 (zero) in the parameters list. Can anyone explain what's going on here? celestia:print("Hello Universe!", 5, 0, 0, 0, 0) wait (5)

 * I don't see this either. May be same problem as above.

 * Mystery explained... Open Scripts dialog and I double-clicked on the celx script in order to open it. If I simply use the dialogs provided by Celestia to open the script, the movement does not occur. Thanks.
 * In other words, do NOT double-click on the file name. Simply click on the OPEN dialog box.
 * One other note... If I do this using a CEL script, there is no apparent effect on the CEL script.

How do I execute celx scripts on Debian GNU/Linux (celestia 1.6.1)? I attempt with $ celestia --url test.celx and I greeted with invalid option -- '-'

 * Celestia on linux currently does not support the --url parameter. Celx scripts can be executed with the parameter "-f /path/to/script.celx". Note the script path must be the fully explicit path and not the relative path. See debian bug #682258

Test script from the BrainDead
flashRadius = function (bodyName) body = celestia:find(bodyName) -- finds object radius = body:radius -- gets radius celestia:flash("The radius of " .. body:name .. " is " .. radius .. " km.") wait(2.0) end flashRadius("Mercury") flashRadius("Venus") flashRadius("Earth") flashRadius("Mars") flashRadius("Jupiter") flashRadius("Saturn") flashRadius("Uranus") flashRadius("Neptune") flashRadius("Pluto") flashRadius("Charon") function flash(message) celestia:flash(message) wait(2.0) end message = "End of this test script" flash = celestia:flash(message,5)

 * Important point in the above... The variable "message" must be defined prior
 * to execution of the "flash" function or the script causes an error which says
 * that it expected a string. This makes sense. Still learning here.

 * I'm really confused by this though. How is it that this script shows the
 * text "End of this test script" if the flash function has been defined,
 * but it has NOT been called? In other words, why don't I need a line
 * which calls the function? i.e. - flash(message)

 * You didn't call "flash", but you did call "celestia:flash". The statement:

 * calls the "flash" method of the "celestia" object (that's what the right side of the assignment says to do) and then assigns the result (nil) to the variable "flash", redefining it. Probably not what you wanted. You only do an assignment to the "flash" variable when you define the function. Remember that:

 * is equivalent to:

 * The function name "flash" is just another variable. HankR 17:59, 18 October 2005 (UTC)

 * Okay, I think I understand what you mean. Appreciate the help.--BrainDead 22:45, 18 October 2005 (UTC)

Another test script

 * Okay, this works, but I have questions again... First, thanks to Malenfant for the idea here.
 * How do I round the values presented by tethysPosition and newdistance?
 * Use <
 * Why does the newdistance value not show either 10000km or the value shown on Celestia's display?
 * Celestia's display shows the distance to the surface, you calculated the distance center to center. Plus, Tethys has moved since the time when you got its position. You calculated the distance to its old position, not its current position. You need to get the position again. Anytime you call wait, time will pass and things will move. HankR 03:31, 24 October 2005 (UTC)
 * I'm still trying here. --BrainDead
 * Thanks again for the help, Hank. I understand this one now.--BrainDead 10:46, 24 October 2005 (UTC)

Corrected Script

 * This revision of the script corrects the errors I had earlier, and it reflects an integer value.
 * Can I ask though, HOW you knew about the integer correction? I can find that nowhere in any of
 * the documentation I have.

 * It's in the Lua 5.0 reference manual.

 * Where did you find "..string.format("%i").." and can I display, say two decimal places? Perhaps
 * using something like "..string.format("%2")???? --BrainDead 00:33, 25 October 2005 (UTC)

 * uses a C-style ouput format control string. Use "%.2f" for two decimal places.

 * PS - Am I okay with the format here? Should I NOT post so much script? Should I use different
 * headers? Sorry, just me... Confused ol' Bob

 * It might be helpful if this material were reformatted as a FAQ page for CELX. Put each question in a separate section with the question as the section title. This would make it more useful for others.HankR 01:05, 25 October 2005 (UTC)

title: Celestia/Celx Scripting/Q&A


text: User talk:24.21.101.225

title: User talk:24.21.101.225


text:
User talk:81.108.123.73

Please stop adding nonsense to Wikibooks. It is considered vandalism. If you would like to experiment, use the sandbox. Thank you.

title: User talk:81.108.123.73


text: User talk:201.200.69.70

title: User talk:201.200.69.70


text:
User talk:65.103.61.29

This is your last warning. The next time you vandalize a page you will be blocked from editing Wikibooks.

title: User talk:65.103.61.29


text: User talk:203.199.203.9

title: User talk:203.199.203.9


text:
User talk:83.89.250.153

Please stop adding nonsense to Wikibooks. It is considered vandalism. If you would like to experiment, use the sandbox. Thank you.

title: User talk:83.89.250.153


text: User talk:59.92.130.163

title: User talk:59.92.130.163


text:
User talk:Kokonutz~enwikibooks

Image:Junkdump.jpg, Image:Junk
There images lack license information (see Image copyright tags). I also don't think they follow Neutral Point Of View rule. --Derbeth 18:35, 30 October 2005 (UTC)

Your account will be renamed
Hello,

The developer team at Wikimedia is making some changes to how accounts work, as part of our on-going efforts to provide new and better tools for our users like cross-wiki notifications. These changes will mean you have the same account name everywhere. This will let us give you new features that will help you edit and discuss better, and allow more flexible user permissions for tools. One of the side-effects of this is that user accounts will now have to be unique across all 900 Wikimedia wikis. See the announcement for more information.

Unfortunately, your account clashes with another account also called Kokonutz. To make sure that both of you can use all Wikimedia projects in future, we have reserved the name Kokonutz~enwikibooks that only you will have. If you like it, you don't have to do anything. If you do not like it, you can pick out a different name.

Your account will still work as before, and you will be credited for all your edits made so far, but you will have to use the new account name when you log in.

Sorry for the inconvenience.

Yours, Keegan Peterzell Community Liaison, Wikimedia Foundation 23:20, 17 March 2015 (UTC)

Renamed
 This account has been renamed as part of single-user login finalisation. If you own this account you can |log in using your previous username and password for more information. If you do not like this account's new name, you can choose your own using this form after logging in: . -- Keegan (WMF) (talk) 05:09, 19 April 2015 (UTC)

title: User talk:Kokonutz~enwikibooks


text:
User talk:Hstovring

Copyright problem with Image:TempC samplemean norm.jpg
BTW, this image is not used. --Derbeth 18:37, 30 October 2005 (UTC)

Tags corrected
Thanks for pointing out the omission. I have added the appropriate tag to this picture and the other three I have uploaded. And yes, they are being used, but in a book not advertised yet: Introduction to medical statistics. --hstovring 19:58, 7 November 2005 (UTC)

title: User talk:Hstovring


text:
User talk:Crd

Copyright problem with Image:Fig283 Page 1.png
BTW, this image is not used. --Derbeth 18:37, 30 October 2005 (UTC)

title: User talk:Crd


text:
User:Sugipula~enwikibooks

foreigner

title: User:Sugipula~enwikibooks


text:
User:Basejumper123

About Me
Hi, Im Basejumper! I work on wikibooks in spare time. I write all sorts of texts that I am knowlegeable about.

Current Projects
Wikijunior Solar System

Wikijunior Dinosaurs

Introduction to Physical Science

Reorganizing Bookshelves

You can also find me on...
 * Wikipedia
 * Wikisource
 * Wikiquote
 * Wiki Commons

title: User:Basejumper123


text:
Budget Watch Collecting/Removing the movement

The first step in most watch restoration is to remove the movement. You'll need to do this for clean and lube, and it is recommended for crystal polishing, especially for non-water-resistant watches. (The last thing you want inside your watch is abrasive polish)

1. For most watches, remove the back. Screw-backs need a special tool to remove without a strong chance of damage--Luckily Indian "Jaxa-Type" removers are inexpensive. Pick the proper tips--Most watches have notches, for those use the small rectangular tips. Holes use the round tips, flats use the wedge-shaped tips with a broad flat spot. The remaining tips are "Rolex style". Don't use these unless you want to gouge the case back, you need the round Rolex rings to actually remove these backs.

Adjust the top tips to fit the wrench areas. For 6 holes or notches, adjust the top tips to fit 2 notches with 1 between. For flats, you need to pick a pair of flats about 1/3 of the circumference apart, but selected so the bottom tip hits a flat and not a point. The upper tips should be adjusted slightly small, they will spread when you tighten the bottom.

Adjust the bottom pin to fit its slot, and tighten slightly. Tighten more for flats.

Twist the tool counterclockwise.

For stubborn cases, (which includes most flats) you may need to support the case in a case clamp, and maybe support the case clamp in a sturdy vise, so you can apply downward pressure while turning. You may also want to use penetrating oil around the case as a last resort.

The other common type of watch is the snap-on. A watchmaker's case knife is best, but any sturdy or slightly sharp knife will do. Most watches have a slight wide spot to fit the knife in. Use the knife as a wedge rather than prying.

Once the back is off, there's usually a movement ring that just lifts out.

A few watches are "remove through crystal", with either a one-piece case or a glued caseback. See ../Removing and replacing crystals/.

There's an extra complication on some of these, you need to remove the stem before you can get to the usual stem release. Some will have a two-piece stem. Most two-piece have a snap fit, just pull the crown hard and even--You may want to wedge sturdy tweezers between crown and case and wiggle them off. Other two-piece stems slide apart when the slot is aligned properly--For these you might be able to see between the dial and case to get the stem slot lined up, otherwise hold the watch upside down so that if the movement comes free it will fall into your palm. Jiggle the case while slowly turning the crown. Plastic cases may have a plug at the back to access the standard stem release. Finally, some Timex one-piece cases need a special wrench to hold a flat spot on the stem while unscrewing the crown. This wrench can be made by filing a flat screwdriver or piece of brass.

Once the back is off, you will probably need to remove the stem--A few have a slot for the stem rather than a tube, these will allow the movement to lift out immediately.

If it is necessary to remove the stem, most watches release best when the stem is in the set position. Mechanical watches will usually have a button or screw near the stem. Push the button while wiggling the stem free, or loosen the screw about 1 1/2 turns, then wiggle the stem. If the stem won't come loose, slowly loosen the screw while wiggling the stem--You want to avoid completely unscrewing the screw, or you will have to remove the dial and replace the set lever.

Quartz watches will usually have a push area, but it can be hard to find. Sometimes it will be marked "push here" other times you have to experiment.

At this point the movement should be free to fall out.


text:
Invertebrate Zoology/Protozoans

One of the earliest forms forms of life, protozoans have simple body plans and are always single-celled, like the protozoan depicted to the right.

Protozoans exist as individuals, but some are colonial.

All the metabolic activities of an organism like respiration, digestion, excretion and reproduction are carried out by these single celled organisms.

title: Invertebrate Zoology/Protozoans


text:
Invertebrate Zoology/Sponges

Phylum Porifera ("pori" = pores, "fera" = bearers) popularly known as sponges, a group simple organisms. Sponges are sessile organisms found attached to a substratum.

The majority of sponges are marine, living in seas and oceans. There is one family of fresh water sponges (Family Spongillidae).

Unlike Protozoans, the Poriferans are multicellular. However unlike higher metazoans the cells that make up a sponge are not organised into tissues. Primarily their body consists of a thin sheet of cells over a frame (skeleton).

As their name suggests, Poriferans are characterized by the presence of minute pores called ostia on their body and One large opening hole like structure called Oscullum. Poriferans body have consist many ostia in numbers but osculum are one.

title: Invertebrate Zoology/Sponges


text:
Talk:Blender 3D: Noob to Pro/Die Another Way

This page is a little confusing and doesn't follow conventions from other articles (most notably keynames, such as GKEY or WKEY). It also isn't part of the chain of next/previous pagelinks. Perhaps someone should fix this? If I have time I might revise it a little bit. 68.126.1.251 20:41, 30 October 2005 (UTC)

I agree. This tutorial is not as clear as the previous ones.

good stuff, a couple minor issues though
In addition to the above mentioned problems with keyboard naming convention, there's a lot of confusion between panel / window etc... this stuff could be cleared up a lot. In particular, the section on assigning materials is VERY confusing... clearing up the nomenclature and adding some screenshots would be very helpful.

Also, a suggestion... the modelling goes MUCH quicker if you extrude and merge the 'dots' BEFORE you spin-dupe the dice. (i.e., after applying your face loop cuts to one side of the die and deleting the other sides, extrude and merge the 9 faces that will become dots. Then spin dupe and you save a lot of time extruding and merging) ... note that this will change the correct number of vertices we should have after removing duplicates (since we add 9 vertices to our side * 6 sides = 54 new vertices, + the 384 originally... now we should 438 vertices after spin-duping and removing duplicate vertices).

I'd be happy to help edit this thing... not really sure how that works though :)

About the step after loop cut
The step 5 is not leading to any option to enter numbers directly. Or am I missing something? I have tried it many times before I have added the comment. car313

I have problems with section 18. I cann't assign the color to the dents. A few pictures of the process would help.

And here is where I quit the tutorial due to frustration.
I AM using blender 2.4, so this might be a part of the problem, I'm a total noob when it comes to blender. I can't get the "loop cut" exact using my mouse (I'm using a logitech g5 2000 dpi laser mouse), it jumps from .168 to .174, and from .338 to .344 (there has got to be a way to type these in right?). I went ahead and did "loop cut" form NUM1 and NUM3, then horizontally on NUM3 so I allready had all the grids I needed. After removing the bottom in section 8, and putting the 3d cursor at 0,0,0 "spin dup" following the directions DID NOT produce a cube the ends of the dupes protruded through the sides of each other in a # shape, so I figured it was due to my use of the cube blender creates by default, however I see no way to aligh the cube so the bottom is on the surface of the grid(like just typing the value, as in maya),which would allow "spin dup" to do what was intended. personally I would like to see the "simple person" and "mountain out of a mole hill" continued as I have a feeling of a loss of continuity with the "puppy from a cube" stub, and "gingerbread man" which seems to dupe "simple person". As I said I am a complete NOOB, today is the first day I have ever used blender, I would be more then happy to contribute to the tutorials once I figure this freaking program out. I would like to see the "simple person" and "mountains" continued untill there is a "detailed person" in a "lifelike setting" for now I have to search for other tutorials. Drn8 23:58, 23 December 2005 (UTC)

How to enter numbers at step 5
I use Blender 2.4, and I am also a beginner.

After several trials, I think I found the way to enter numbers at STEP 5.

You have to move the cursor to the bottom bar of 3D viewer window and click LMB(laft mouse button) once. Then press GKEY ⇒ XKEY(or Y, Z) ⇒ enter numbers you want--140.116.202.68 16:14, 17 January 2006 (UTC)

As of Blender 2.4 loop cut seems to allow you to enter the number of cuts you want to make. So if you select k->loop cut, select the face and then enter KEY9 you can get the described effect without the hasslo of measuring the sections.

how to turn on subdivision surfaces?
hi, i think this is a good tutorial, since i understood everything right, until point 12: "12. Tab out of editmode. If you haven't done this already, hit set smooth in the editing panel and turn on subdivision surfaces" i did everything exactly as it was said, but i can't find a button to turn on "subdivision surfaces" and the screenshot doesn't help me either. do i have to add a "subsurf modifier"? but when i do this it doesn't look like the following screenshot in the tutorial nor can i turn it on or off, as it is said. any help? i think i'm just missing something, but i can't get it. thanks

Shade

The tutorial says, "In 2.41, you will need to use "Add Modifier" on the Modifiers tab, to add a SubSurf modifier." --Davidfg4

colour
i cannot mak two differrent colours. why? if i make a new material it is the same like first, so all my materials are the same. what should i do? my version of blender is 2.41

[User Answer: I am also relatively new to Blender, using 2.41 and found this point particularly frustrating until I solved it. The key to using materials to make seperate colours is to step between the Edit: Link and Materials tab and the Shading: Material tab.

In summary: you must make new materials in the Edit panel using the 'Link and Materials' tab, seperate then colour the materials in the Shading panel using the Materials tab under the Materials button, then go back to the Edit panel and assign different materials to specific layers.

Make sure you are in Edit Mode and have the whole mesh selected before you start.

1. Go to the Edit panel, look under the Link and Materials tab (hit F9 to go to the edit pnael). Delete any materials you have created using the 'Delete' button. Create the number of materials you need - in this case two. Click 'new' twice - you will see '2 Mat 2' in the dialog box above.

2. Go to the Shading panel (hit F5), find the 'Materials' tab and click on the number '2' to the right of the OB and ME buttons. You'll be asked if you want to make these single user - comply with 'yes'. Use arrows either side of '2 Mat 2' to switch between the two materials. This is the bit where you assign the colours to the two materials - stick with red for the first and white for the second. After you have done this your dice will be entirely red.

3. Got back to the Edit panel, then make sure you have the faces (not just vertices) of the indents selected - there will be four triangular faces for each indent. Then select the second material, the white one, then assign.

If you find these steps useful go back to the Penguin you made in an earlier tutorial and apply the same process to give it a white chest, orange beaks and feet.

Blender is a very complex tool but even after only a few days I can see the sheer potential. If you get stuck whilst following a tutorial try searching google using search terms relating to the operation you are having difficulty with you are stuck on (e.g. "blender 3d materials howto"), or consulting the manual on the blender site. You will usually find others will have had the same difficulties as you. Good luck!]

A revised page
I have tried to get to grips with Blender for a long time and not until I came across this Noob to Pro set of tutorials have I made any progess. This, I hope, will be something to give back to the community.

I have nearly finished a revised page, based on 2.41, incorporating the suggestions and alternative paragraphs which have already been added to the existing page, with changes where 2.41 does things differently and additional images.

I don't know anything about modifying wikibooks — where can I put it so that you can “mercilessly” edit it?

wynnmc

The revised page has been added to the Noob to Pro Contents and created.

There are a few whimperings in the discussion page there...

--Wynnmc 11:11, 11 June 2006 (UTC)

Incorporating 2.42
This is a very good tutorial, but with the new version out it can be extremely simplyfied.

The note below the first point already explains how to skip point 3-9. Point 10 and 11 can be combined into one since you can now just select all the faces for one side of the dice, extrude and move them backwards and merge→collapse them.

I'm willing to make these changes myself. Just give me a go. -- DanMan 10:24, 17 July 2006 (UTC)

Lack of subsections
It would be fine to make a structure with subtitles. --N comme Nul 20:44, 1 August 2006 (UTC)

Die easy
I strongly favor the "Die easy" tutorial over this one. It's much clearer and easier. Anyone agree? ~ Yes, this tutorial seems like a waste of time.MDD4696 03:26, 16 August 2006 (UTC)

Die Easy (proposed revision)
Thanks to Mdd4696 for the compliment. Credit should be given to the author of Die Another Way for all the hard work put in finding out how to model dice: dividing the faces to get the pip locations, making the pips round instead of square, colouring them — not to mention the arrangement of the pips on the faces and their spacing. Hours of work — and then there's writing it all down so a Noob can follow.

After a time away from Blender I have come back and discovered that Die Easy has now been overtaken by Blender versions and could do with an update.

However, I don't want this to seem a competition between Die Another Way and Die Easy. Die Easy was written because it seemed to me that updating Die Another Way would end up in a mess and starting afresh would produce something that Noobs (like me) could follow.

Again it seems that there are so many things to remove and user suggestions to add to Die Easy that it should be rewritten from scratch.

But, before that, I would like to be sure that it is not seen as a competition and that a revised Die Easy would be welcome.

Please let me know what you think.

I would also like to hear what the author of Die Another Way thinks too.

--Wynnmc 10:54, 22 May 2007 (UTC)

Having problem with the pips.
Thanks for this great tutorial. I am only having a problem with the pips.Here is the screenshot of what I have so far: link title As you can see that the perimeter around the pips are not sharp. This happens when I click "set smooth" button. If I don't click 'set smooth' then the dice looks edgy, but when I click, this time the pips are smoothed out and doesn't look sharp. Can anyone tell me what should I do here? Thanks in advance.

Yes, I got the same problem (the pip edges are not sharp when "set smooth" is used). What I did, instead of using "set smooth" I bumped up the subserf level to 6. It slowed down everything considerably, but allowed me to get sharp edges.

I wonder what we did wrong.

Yeah. I have that problem. I'm using blender 2.48 and when I click Set Smooth the edges don't look sharp.

Is there any way to stop this happening?

Edge Creasing the Pips; regarding the Newbie Note
I'm using version 2.49a, and I found that you can indeed select all of the pip edges when using SHIFT+EKEY then mouse to crease all of the pip edges at once. If anyone else wants to verify that this works, perhaps the newbie note should be amended that this isn't an issue any longer from version 2.49a onwards.

Camera Setup. How to?
I've tried to setup camera with help of Fly mode (BTW it's very comfortable :)). But my problem is that: When I switch to edit mode and try to rotate view, it move around the camera, not dice. How to change it?

Easier way to locate the pips
It is very easy to find the centre of the pips simply by selecting the faces and using WKEY->Subdivide multi, and entering 9. With 9 divisions on each face, you will have one node at each pip centre.


text: User talk:68.94.14.40

title: User talk:68.94.14.40


text:
Celestia/Celx Scripting/Q&A/Angles

Finding the angles between two objects is a handy thing to be able to do in a script. It would mostly be useful for finding angular separations and phase angles (illustrated below) which can be used for a variety of other purposes.

The angular separation between two objects seen is a measure of how far apart they are in the sky as seen by the observer.

The phase angle is usually described for non-emissive objects orbiting stars (e.g. planets, asteroids, comets, moons, etc.) and is the angle of illumination of the object seen by an observer. It is the angle between the Star-Target line and the Target-Observer line. Using the image as an example, a phase angle of 0 degrees means that the observer is directly between the star and the planet - thus, the star is behind the observer, and the planet would appear "Full" to the viewer. A phase angle of 90 or 270 degrees means that the observer is over the terminator of the planet (the line dividing day and night), so he would see a 'Half-Full" planet. A phase angle of 180 degrees means that the planet is directly between the observer and the star, so the observer is in the planet's shadow and would see a "New" planet. At intermediate phase angles the observer would see a crescent or gibbous planet.

Theory
Celestia stores the positions of objects as a table consisting of an x-coordinate, y-coordinate, and z-coordinate. The line connecting two points is known as a vector, so basically we're finding the angle between two vectors. First we have to calculate each of the vectors. To do this we find the positions of the objects at either end of the line, and then simply subtract one from the other. So in order to calculate a Phase Angle, the vector of the Star-Planet line is (position of the planet - position of the star), and the vector of the Observer-Planet line is (position of the planet - position of the observer).

Once we have these vectors, we need to use a special mathematical function used for vectors called the Dot Product. This is reproduced below:

 * $$ \mathbf{a} \cdot \mathbf{b} = a \, b \cos \theta \;$$

We'd refer to this as "a dot b", where a and b are the vectors involved. The ab on the right-hand-side of the equation are the lengths of the two vectors multiplied together, and the angle theta is the angle between the two vectors. In this case, Celestia already knows what the vectors are (and how long they are and what the angle is between them) and so can calculate the dot product - we just need to extract the information we want in a script. We can also normalise the vectors so that the lengths equal 1 - this makes the calculation a little simpler (and conveniently enough there's a 'normalize' script command that does this). This reduces the equation we need in the script to:

 * $$ \theta = \arccos (\mathbf{a} \cdot \mathbf{b}) \;$$

The script below illustrates how to perform this calculation in Celx.

Download the script
This is a complete script that you can run as-is to display the phase angle to a target in the solar system. The target must already be selected before the script is run.

camera = celestia:getobserver			-- set camera object target = celestia:getselection	 		-- set target object sunpos = celestia:find("Sol"):getposition		-- find sun position campos = camera:getposition				-- find camera position planpos = target:getposition				-- find target position sun_target_vector = planpos - sunpos			-- determine sun-target vector obs_target_vector = planpos - campos			-- determine observer-target vector phase_angle = math.acos(sun_target_vector:normalize * obs_target_vector:normalize)	-- find phase angle (in radians) celestia:flash("Phase angle to target is "..string.format("%.2f", math.deg(phase_angle)).." degrees\n") wait(2)

title: Celestia/Celx Scripting/Q&A/Angles


text:
Talk:Guide to Schools/Alphabetical/King George School

This is the beginning of a sample of a school descripription page. The topics will be folded into a template available for other schools. I plan to do that myself, if no one else gets there first. I hope that contributors will maintain consistency in school listings. If you want to change the outline followed by other schools listing, please change it here, in addition to handling the other schools differently. Tom Croke Frog One 23:01, 30 October 2005 (UTC)

Progress
We currently have made some progress in getting this page to where we want it as demonstration. We do intend to add some photographs and some additional copy. Frog One 12:45, 4 November 2005 (UTC)