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Fermilab

Fermi National Accelerator Laboratory (Fermilab), located in Batavia, Illinois, near Chicago, is a United States Department of Energy national laboratory specializing in high-energy particle physics.

Fermilab's Main Injector, two miles (3.3 km) in circumference, is the laboratory's most powerful particle accelerator. The accelerator complex that feeds the Main Injector is under upgrade, and construction of the first building for the new PIP-II linear accelerator began in 2020. Until 2011, Fermilab was the home of the 6.28 km (3.90 mi) circumference Tevatron accelerator. The ring-shaped tunnels of the Tevatron and the Main Injector are visible from the air and by satellite.

Fermilab aims to become a world center in neutrino physics. It is the host of the multi-billion dollar Deep Underground Neutrino Experiment (DUNE) now under construction. The project has suffered delays and, in 2022, the journals Science and Scientific American each published articles describing the project as "troubled". Ongoing neutrino experiments are ICARUS (Imaging Cosmic and Rare Underground Signals) and NOνA (NuMI Off-Axis νe Appearance). Completed neutrino experiments include MINOS (Main Injector Neutrino Oscillation Search), MINOS+, MiniBooNE and SciBooNE (SciBar Booster Neutrino Experiment) and MicroBooNE (Micro Booster Neutrino Experiment).

Since 2007, Fermilab has been operated by the Fermi Research Alliance (FRA), a joint venture of the University of Chicago, and the Universities Research Association (URA); although in 2023, the Department of Energy (DOE) opened bidding for a new contractor due to concerns about the FRA performance. Fermilab is a part of the Illinois Technology and Research Corridor.

On-site experiments outside of the neutrino program include the SeaQuest fixed-target experiment and Muon g-2. Fermilab continues to participate in the work at the Large Hadron Collider (LHC); it serves as a Tier 1 site in the Worldwide LHC Computing Grid. Fermilab also pursues research in quantum information science. It founded the Fermilab Quantum Institute in 2019. Since 2020, it also is home to the SQMS (Superconducting Quantum Materials and Systems) Center.

Asteroid 11998 Fermilab is named in honor of the laboratory.

History
Weston, Illinois, was a community next to Batavia voted out of existence by its village board in 1966 to provide a site for Fermilab.

The laboratory was founded in 1969 as the National Accelerator Laboratory; it was renamed in honor of Enrico Fermi in 1974. The laboratory's first director was Robert Rathbun Wilson, under whom the laboratory opened ahead of time and under budget. Many of the sculptures on the site are of his creation. He is the namesake of the site's high-rise laboratory building, whose unique shape has become the symbol for Fermilab and which is the center of activity on the campus.

After Wilson stepped down in 1978 to protest the lack of funding for the lab, Leon M. Lederman took on the job. It was under his guidance that the original accelerator was replaced with the Tevatron, an accelerator capable of colliding protons and antiprotons at a combined energy of 1.96 TeV. Lederman stepped down in 1989 and remained Director Emeritus until his death. The science education center at the site was named in his honor.

The later directors are:
 * John Peoples, 1989 to 1996
 * Michael S. Witherell, July 1999 to June 2005
 * Piermaria Oddone, July 2005 to July 2013
 * Nigel Lockyer, September 2013 to April 2022
 * Lia Merminga, April 2022 to present

The Tevatron
Prior to the startup in 2008 of the Large Hadron Collider (LHC) near Geneva, Switzerland, the Tevatron was the most powerful particle accelerator in the world, accelerating protons and antiprotons to energies of 980 GeV, and producing proton-antiproton collisions with energies of up to 1.96 TeV, the first accelerator to reach one "tera-electron-volt" energy. At 3.9 mi, it was the world's fourth-largest particle accelerator in circumference. One of its most important achievements was the 1995 discovery of the top quark, announced by research teams using the Tevatron's CDF and DØ detectors. It was shut down in 2011.

Fermilab Accelerator Complex
Since 2013, the first stage in the acceleration process (pre-accelerator injector) in the Fermilab chain of accelerators takes place in two ion sources which ionize hydrogen gas. The gas is introduced into a container lined with molybdenum electrodes, each a matchbox-sized, oval-shaped cathode and a surrounding anode, separated by 1 mm and held in place by glass ceramic insulators. A magnetron generates a plasma to form the ions near the metal surface. The ions are accelerated by the source to 35 keV and matched by low energy beam transport (LEBT) into the radio-frequency quadrupole (RFQ) which applies a 750 keV electrostatic field giving the ions their second acceleration. At the exit of RFQ, the beam is matched by medium energy beam transport (MEBT) into the entrance of the linear accelerator (linac).

The next stage of acceleration is linear particle accelerator (linac). This stage consists of two segments. The first segment has five drift tube cavities, operating at 201 MHz. The second stage has seven side-coupled cavities, operating at 805 MHz. At the end of linac, the particles are accelerated to 400 MeV, or about 70% of the speed of light. Immediately before entering the next accelerator, the H− ions pass through a carbon foil, becoming H+ ions (protons).

The resulting protons then enter the booster ring, a 468 m circumference circular accelerator whose magnets bend beams of protons around a circular path. The protons travel around the Booster about 20,000 times in 33 milliseconds, adding energy with each revolution until they leave the Booster accelerated to 8 GeV. In 2021, the lab announced that its latest superconducting YBCO magnet could increase field strength at a rate of 290 tesla per second, reaching a peak magnetic field strength of around 0.5 tesla.

The final acceleration is applied by the Main Injector [circumference 3319.4 m], which is the smaller of the two rings in the last picture below (foreground). Completed in 1999, it has become Fermilab's "particle switchyard" in that it can route protons to any of the experiments installed along the beam lines after accelerating them to 120 GeV. Until 2011, the Main Injector provided protons to the antiproton ring [circumference 6283.2 m] and the Tevatron for further acceleration but now provides the last push before the particles reach the beam line experiments.

Proton improvement plan
Recognizing higher demands of proton beams to support new experiments, Fermilab began to improve their accelerators in 2011. Expected to continue for many years, the project has two phases: Proton Improvement Plan (PIP) and Proton Improvement Plan-II (PIP-II).

The overall goals of PIP are to increase the repetition rate of the Booster beam from 7 Hz to 15 Hz and replace old hardware to increase reliability of the operation. Before the start of the PIP project, a replacement of the pre-accelerator injector was underway. The replacement of almost 40 year-old Cockcroft–Walton generators to RFQ started in 2009 and completed in 2012. At the Linac stage, the analog beam position monitor (BPM) modules were replaced with digital boards in 2013. A replacement of Linac vacuum pumps and related hardware is expected to be completed in 2015. A study on the replacement of 201 MHz drift tubes is still ongoing. At the boosting stage, a major component of the PIP is to upgrade the Booster ring to 15 Hz operation. The Booster has 19 radio frequency stations. Originally, the Booster stations were operating without solid-state drive system which was acceptable for 7 Hz but not 15 Hz operation. A demonstration project in 2004 converted one of the stations to solid state drive before the PIP project. As part of the project, the remaining stations were converted to solid state in 2013. Another major part of the PIP project is to refurbish and replace 40 year-old Booster cavities. Many cavities have been refurbished and tested to operate at 15 Hz. The completion of cavity refurbishment is expected in 2015, after which the repetition rate can be gradually increased to 15 Hz operation. A longer term upgrade is to replace the Booster cavities with a new design. The research and development of the new cavities is underway, with replacement expected in 2018.
 * PIP (2011–2018)

 * PIP-II

The goals of PIP-II include a plan to delivery 1.2 MW of proton beam power from the Main Injector to the Deep Underground Neutrino Experiment target at 120 GeV and the power near 1 MW at 60 GeV with a possibility to extend the power to 2 MW in the future. The plan should also support the current 8 GeV experiments including Mu2e, Muon g−2, and other short-baseline neutrino experiments. These require an upgrade to the Linac to inject to the Booster with 800 MeV. The first option considered was to add 400 MeV "afterburner" superconducting Linac at the tail end of the existing 400 MeV. This would have required moving the existing Linac up 50 m. However, there were many technical issues with this approach. Instead, Fermilab is building a new 800 MeV superconducting Linac to inject to the Booster ring.

Construction of the first building for the PIP-II accelerator began in 2020. The new Linac site will be located on top of a small portion of Tevatron near the Booster ring in order to take advantage of existing electrical and water, and cryogenic infrastructure. The PIP-II Linac will have low energy beam transport line (LEBT), radio frequency quadrupole (RFQ), and medium energy beam transport line (MEBT) operated at the room temperature at with a 162.5 MHz and energy increasing from 0.03 MeV. The first segment of Linac will be operated at 162.5 MHz and energy increased up to 11 MeV. The second segment of Linac will be operated at 325 MHz and energy increased up to 177 MeV. The last segment of linac will be operated at 650 MHz and will have the final energy level of 800 MeV.

As of 2022, the estimated PIP-II accelerator start date for the accelerator is 2028. The project was approved for construction in April 2022 with an expected cost to the Department of Energy of $978M and with an additional $330M in contributions from international partners.

Discoveries by Fermilab experiments
The following particles were first directly observed at Fermilab:
 * The top quark announced in 1995 by the DØ experiment and CDF experiment.
 * The bottom quark, which was observed as a quark-antiquark pair called the Upsilon meson announced in 1977 by Experiment 228.
 * The tau neutrino, announced in July 2000 by the DONUT collaboration.
 * The bottom Omega baryon, announced by the DØ experiment of Fermilab in 2008.

In 1999, physicists at on the KTeV experiment were also the first to observe direct CP violation in kaon decays.

The DØ experiment and CDF experiment each made important contributions to the observation of the Higgs Boson, announced in 2012.

Evolution of the post-Tevatron experimental program
Fermilab dismantled the CDF (Collider Detector at Fermilab) experiment to make the space available for IARC (Illinois Accelerator Research Center).

Fermilab physicists continue to play a key role in the world-wide collider program. The LHC Physics Center (LPC) at Fermilab is a regional center of the Compact Muon Solenoid Collaboration (the experiment is housed at CERN). The LPC offers a vibrant community of CMS scientists from the US and plays a major role in the CMS detector commissioning, and in the design and development of the detector upgrade. Fermilab is the host laboratory for USCMS, which includes researchers from 50 U.S. universities including 715 students. Fermilab hosts the largest CMS Tier 1 computing center, handling approximately 40% of global CMS Tier 1 computing requests. On February 9, 2022, Fermilab's Patricia McBride (physicist) was elected spokesperson of the CMS collaboration.

During this time-frame, the laboratory also established a new program in research in cutting-edge information science, including the development of quantum teleportation technology for the quantum internet and increasing the lifetime of superconducting resonators for use in quantum computers.

On-site program in the 2020s
The on-site program in the 2020s is largely focused on the ``Intensity Frontier'' of particle physics, especially neutrino physics and rare physics searches using muons. A program exploring nucleon structure is also continuing.

List of recent past, ongoing, and planned experiments running on-site

 * ANNIE: The Accelerator Neutrino Neutron Interaction Experiment (Status, June 2023: completed run, planned future run)
 * Deep Underground Neutrino Experiment (DUNE), formerly known as Long Baseline Neutrino Experiment (LBNE) (Status, June 2023:  planned future run)
 * ICARUS experiment: Originally located at the Laboratori Nazionali del Gran Sasso (LNGS) and moved to Fermilab. (Status, June 2023: running)
 * MiniBooNE: Mini Booster Neutrino Experiment (Status, June 2023: completed run)
 * MicroBooNE: Micro Booster Neutrino Experiment (Status, June 2023: completed run)
 * MINERνA: Main INjector ExpeRiment with νs on As (Status, June 2023:  completed run)
 * Mu2e: Muon-to-Electron Conversion Experiment (Status, June 2023: planned future run)
 * Muon g−2: Measurement of the anomalous magnetic dipole moment of the muon (Status, June 2023: completed run)
 * NOνA: NuMI Off-axis νe Appearance (Status, June 2023: running)
 * SeaQuest (Status, June 2023: completed run)
 * SBND: Short-Baseline Neutrino Detector (Status, June 2023: planned future run)
 * SpinQuest (Status, June 2023: planned future run)

LBNF/DUNE
Fermilab strives to become the world leader in neutrino physics through the Deep Underground Neutrino Experiment at the Long Baseline Neutrino Facility. Other leaders are CERN, which leads in Accelerator physics with the Large Hadron Collider (LHC), and Japan, which has been approved to build and lead the International Linear Collider (ILC). Fermilab will be the site of LBNF's future beamline, and the Sanford Underground Research Facility (SURF), in Lead, SD, is the site selected to house the massive far detector. The term "baseline" refers to the distance between the neutrino source and the detector. The far detector current design is for four modules of instrumented liquid argon with a fiducial volume of 10 kilotons each.

According to the 2016 Conceptual Design Report, the first two modules were expected to be complete in 2024, with the beam operational in 2026. The final modules were planned to be operational in 2027. In 2022, the cost for two far detector modules and the beam, alone, had risen to $3B. This led to a decision by the Department of Energy Office of Science to phase the experiment. Phase I would consist of two modules, to be completed in 2028-29, and the beamline, to be completed in 2032. The installation of phase II, the remaining two far detector modules, is not yet planned and will be at a cost above the $3B estimate for phase I.

A large prototype detector constructed at CERN took data with a test beam from 2018-2020. The results show that ProtoDUNE performed with greater than 99% efficiency.

LBNF/DUNE program in neutrino physics plans to measure fundamental physical parameters with high precision and to explore physics beyond the Standard Model. The measurements DUNE will make are expected to greatly increase the physics community's understanding of neutrinos and their role in the universe, thereby better elucidating the nature of matter and anti-matter. It will send the world's highest-intensity neutrino beam to a near detector on the Fermilab site and the far detector 800 miles (1300 km) away at SURF.

About other neutrino experiments
The MiniBooNE detector was a 40 ft diameter sphere containing 800 tons of mineral oil lined with 1,520 phototube detectors. An estimated 1 million neutrino events were recorded each year. SciBooNE sat in the same neutrino beam as MiniBooNE but had fine-grained tracking capabilities. The NOνA experiment uses, and the MINOS experiment used, Fermilab's NuMI (Neutrinos at the Main Injector) beam, which is an intense beam of neutrinos that travels 455 mi through the Earth to the Soudan Mine in Minnesota and the Ash River, Minnesota, site of the NOνA far detector. In 2017, the ICARUS neutrino experiment was moved from CERN to Fermilab.

Muon g−2
Muon g−2: (pronounced “gee minus two”) is a particle physics experiment to measure the anomaly of the magnetic moment of a muon to a precision of 0.14 ppm, which will be a sensitive test of the Standard Model.

Fermilab is continuing an experiment conducted at Brookhaven National Laboratory to measure the anomalous magnetic dipole moment of the muon.

The magnetic dipole moment (g) of a charged lepton (electron, muon, or tau) is very nearly 2. The difference from 2 (the "anomalous" part) depends on the lepton, and can be computed quite exactly based on the current Standard Model of particle physics. Measurements of the electron are in excellent agreement with this computation. The Brookhaven experiment did this measurement for muons, a much more technically difficult measurement due to their short lifetime, and detected a tantalizing, but not definitive, 3 σ discrepancy between the measured value and the computed one.

The Brookhaven experiment ended in 2001, but 10 years later Fermilab acquired the equipment, and is working to make a more accurate measurement (smaller σ) which will either eliminate the discrepancy or, hopefully, confirm it as an experimentally observable example of physics beyond the Standard Model.

Central to the experiment is a 50 foot-diameter superconducting magnet with an exceptionally uniform magnetic field. This was transported, in one piece, from Brookhaven in Long Island, New York, to Fermilab in the summer of 2013. The move traversed 3,200 miles over 35 days, mostly on a barge down the East Coast and up the Mississippi.

The magnet was refurbished and powered on in September 2015, and has been confirmed to have the same $1,300 ppm$ (0.13%) p-p basic magnetic field uniformity that it had before the move.

The project worked on shimming the magnet to improve its magnetic field uniformity. This had been done at Brookhaven, but was disturbed by the move and had to be re-done at Fermilab.

In 2018, the experiment started taking data at Fermilab. In 2021, the laboratory reported that results from initial study involving the particle challenged the Standard Model, with the potential for discovery of new forces and particles.

In August 2023, the Fermilab group said they may be getting closer to proving the existence of a new force of nature. They have found more evidence that sub-atomic particles, called muons, are not behaving in the way predicted by the current theory of sub-atomic physics.

Delays, cost overruns, safety failures and management turmoil in the 2010s and 2020s
Starting in the 2010s, delays and cost over-runs led to substantial concerns about mismanagement of the laboratory.

Oversight issues that developed from 2014–2022
In 2014, the Particle Physics Project Prioritization Panel ("P5") recommended three major initiatives for construction on the Fermilab site. Two were particle physics experiments: the Deep Underground Neutrino Experiment and Mu2e. The third was the PIPII accelerator upgrade described above. Also, P5 recommended Fermilab participation in LHC at CERN.

As of 2022, two P5-recommended Fermilab projects had suffered substantial delays:
 * The Deep Underground Neutrino Experiment with the enabling Long Baseline Neutrino Facility was proposed to P5 as a $1B project; the cost estimate in 2021 dollars was more than $3B, with far detector operations beginning 2029 and full operation by 2032.
 * The Mu2e experiment was to produce preliminary results in 2020, but this is now delayed until 2026.

Even smaller experiments, below the cost-level of individual P5 approval, that were proposed at the time of the 2014 P5 suffered considerable delay. The Short-Baseline Near Detector (SBND) that was proposed in 2014 with a $10M cost scale was originally scheduled for data taking in spring 2018, but is now scheduled to begin in autumn 2023.

The Department of Energy raised flags as early as Fiscal Year (FY) 2019. Each year, the US Department of Energy Office of Science reviews and grades the national laboratories in its portfolio on eight performance metrics. Fermilab has received the lowest grades among the national laboratories in FY2019, 2020, 2021 and 2022. A rare C grade was assigned for project management in 2021, reflective of the delays and cost overruns. In an article in the journal Science, James Decker, who was principal deputy director of DOE’s Office of Science from 1973 to 2007, stated that the performance evaluation for 2021 was "one of the most scathing I have seen".

Also, in 2020, the high-energy physics community expressed concern that the cost of major projects at Fermilab have led to diversion of funds from the high-energy physics core research program, harming the health of the field. Congress increased the annual HEP budget from less than $800 million by about $250M to more than $1 billion—a 30% increase that went mainly to support large projects at Fermilab.

Oversight issues continuing since 2022
The Fermilab project delays led to substantial change in leadership in 2022. In September, 2021, Nigel Lockyer, Director of Fermilab, resigned. Lockyer was replaced by Lia Merminga, head of the PIP II project. On March 31, 2022, James Siegrist, Associate Director for High Energy Physics in the Department of Energy Office of Science, who had overseen the response to the P5 report, stepped down. In September 2022, it was announced that Deputy Director for Research Joseph Lykken would step down, to be replaced by Yale Professor Bonnie Fleming, who previously served as Deputy Chief Research Officer for the neutrino program at Fermilab. Regina (Gina) Rameika joined the DOE’s Office of Science as the Associate Director for the Office of High Energy Physics in his place on November 7, 2022, moving from her role as spokesperson of the DUNE Experiment.

Although these replacements represent a substantial turn-over of the top ranks, the new management was drawn from the 2014-2022 management team and  Fermilab has continued to suffer turmoil since this change-over. In particular, new issues arose with regard to safety and access of employees, visitors and contractors.

On May 25, 2023, a contractor fell 23 ft while attempting to secure reinforcing bars on a wall for the new PIP II project site. The contactor was air-lifted to hospital in an accident considered the worst on site in decades. A DOE-appointed Accident Investigation Board concluded that the incident was preventable and "recommended a long list of managerial and safety controls needed to prevent a recurrence of such an accident." As a result of the accident, the $1B PIP II project that is crucial for the success of the DUNE Experiment has been delayed.

On Sept. 1, 2023, Chief Research Officer Bonnie Fleming announced that the Fermilab accelerator system was temporarily shut down for safety reasons. On Sept. 9, 2022, DOE issued order DOE O 420.2D entitled “Safety of Accelerators”. This document establishes the accelerator-specific safety requirements for DOE-funded accelerators and their operations. Fermilab management had one year to comply with the order, but did not meet the requirements in that timeframe. The Fermilab Main Accelerator will resume running when the DOE O 420.2D implementation is complete, with the earliest estimate being March, 2024.

In view of the many issues facing the laboratory, in January 2023, the DOE announced a process to rebid the contract for the management of the laboratory due to the performance issues. The DOE announcement of the competition explained: "the purpose of this contract competition is to solicit and award a new M&O contract that will result in improved contractor performance and efficiencies at FNAL. DOE expects that this competition will elicit new and innovative approaches for planning the Laboratory’s future." At an informational meeting for potential bidders on March 1, 2023, the presentation slides expanded upon issues under "Major Challenges/Risks" including highlighting the concern: "The Laboratory continues to have challenges in Financial Management and Acquisition Management. Audits repeatedly highlight the same deficiencies and control failures year after year. Any corrective actions implemented have resulted in little to no progress. Significant procurement issues have hindered the laboratory’s ability to successfully deliver efficient and effective business systems/resources to enable the Science Mission. Substantial concerns remain regarding the ability to expend Government funds in an effective, efficient, and compliant manner."

Access
Fermi National Accelerator Laboratory (aka Fermilab) was founded in 1967 as an open science research laboratory, and, to this day, does not conduct classified research. The initial plans called for Fermilab to employ a guard force "...to control movement of personnel onto the site when tests are being conducted" as noted in the December 1971 Environmental Statement and "..to enforce the site boundary". Early leadership moved toward greater public openness allowing for ease of scientific collaboration and public enjoyment to include the relocated bison heard.

In the late 2010s and early 2020s, the management of Fermilab began to introduce severe restrictions on access to the Fermilab site by the public and by scientists. By spring 2023, the restrictions had become so onerous that more than 2500 physicists and visitors to the laboratory signed an “open petition to elected representatives to reopen Fermilab.”   The petition stated that:   “The access policy changes undermine critical aspects of the scientific process as well as the basic functioning of Fermilab. Hosting research meetings, interviewing prospective employees, collaborating with scientists outside the lab, and enacting our famously impactful education programs have all been hindered.“   With respect to the general public, the petition stated:  “Today, the general public is only permitted to access the main road, and with ID requirements that are becoming increasingly stringent, soon its doors will be closed to tourists and even to some immigrants. We can no longer drive or bike around the premises freely. The dog park, Wilson Hall with its exhibits on the top floor, and other areas are no longer generally accessible. Fishing and other activities open to the public have been canceled.”  The petition emphatically requested that access policies be reverted to the open laboratory model that governed the laboratory prior to 2020.

In May 2023, Director Lia Merminga posted a response to the petition on the Fermilab website, noting that some areas on site remain open to the public during specific hours with ID access requirements. Merminga's response justifies the new restrictions because the lab "manage[s] a large amount of non-public information"---reasoning that conflicts with the petition that points out that the lab is fully tax-payer funded, does no classified research, and has a government mandate to publish all of its scientific results. Further coverage of the petition and the management response appeared in the magazines Physics Today and Physics World.

In keeping with Real ID requirements for DOE facilities, all unescorted adult visitors entering the site must present a government-issued photo ID compliant with the Real ID Act. Up-to-date specifics about access can be found on the Fermilab website.

Architecture
Fermilab's first director, Robert Wilson, insisted that the site's aesthetic complexion not be marred by a collection of concrete block buildings. The design of the administrative building (Wilson Hall) was inspired by St. Pierre's Cathedral in Beauvais, France, though it was realized in a Brutalist style. Several of the buildings and sculptures within the Fermilab reservation represent various mathematical constructs as part of their structure.

The Archimedean Spiral is the defining shape of several pumping stations as well as the building housing the MINOS experiment. The reflecting pond at Wilson Hall also showcases a 32 ft hyperbolic obelisk, designed by Wilson. Some of the high-voltage transmission lines carrying power through the laboratory's land are built to echo the Greek letter π. One can also find structural examples of the DNA double-helix spiral and a nod to the geodesic sphere.

Wilson's sculptures on the site include Tractricious, a free-standing arrangement of steel tubes near the Industrial Complex constructed from parts and materials recycled from the Tevatron collider, and the soaring Broken Symmetry, which greets those entering the campus via the Pine Street entrance. Crowning the Ramsey Auditorium is a representation of the Möbius strip with a diameter of more than 8 ft. Also scattered about the access roads and village are a massive hydraulic press and old magnetic containment channels, all painted blue.

Wildlife
In 1967, Wilson brought five American bison to the site, a bull and four cows, and an additional 21 were provided by the Illinois Department of Conservation. Some fearful locals believed at first that the bison were introduced in order to serve as an alarm if and when radiation at the laboratory reached dangerous levels, but they were assured by Fermilab that this claim had no merit. Today, the Fermilab bison herd is a popular attraction that draws many visitors and the grounds are also a sanctuary for other local wildlife populations. A Christmas Bird Count has occurred at the lab every year since 1976.

Working with the Forest Preserve District of DuPage County, Fermilab has introduced barn owls to selected structures around the grounds.

Tritium on site
During running, particle beams produce tritium, an isotope of hydrogen consisting of a proton and two neutrons that is weakly radioactive with a half-life of 12.3 years. This can bind with oxygen to form tritiated water. Tritium levels measured on site are low compared to federal health and environmental standards. Fermilab monitors tritium leaving the site in surface and sewer water, and provides an FAQ sheet for those who want to learn more.

At an informational meeting for potential bidders for the management contract, held on March 1, 2023, the presentation slides indicated that although the rate of tritium leaving site is below the required standards, there is sufficient tritium contamination on site to represent a "challenge". In particular, tritium produced in the NuMI beamline that send neutrinos to experiments in Minnesota has been pumped into the industrial water cooling system that is used for equipment across the Fermilab campus. As a result, it is concluded that now "tritium contamination is largely throughout the research complex."

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Nuuk

Nuuk (Nuuk, formerly Godthåb ) is the capital of and most populous city in Greenland, an autonomous territory in the Kingdom of Denmark. Nuuk is the seat of government and the territory's largest cultural and economic center. The major cities from other countries closest to the capital are Iqaluit and St. John's in Canada and Reykjavík in Iceland. Nuuk contains a third of Greenland's population and its tallest building. Nuuk is also the seat of government for the Sermersooq municipality. In January 2024, it had a population of 19,872. Nuuk is considered a modernized city after the policy began in 1950.

The city was founded in 1728 by the Dano-Norwegian missionary Hans Egede when he relocated from the earlier Hope Colony (Haabets Koloni) where he arrived in 1721. The governor Claus Paarss was part of the relocation. The new colony was placed at the Inuit settlement of Nûk and was named Godthaab ("Good Hope"). "Nuuk" is the Greenlandic word for "cape" (næs) and is commonly found in Greenlandic place names. It is so named because of its position at the end of the Nuup Kangerlua fjord on the eastern shore of the Labrador Sea. Its latitude, at 64°11' N, makes it the world's northernmost capital, only a few kilometres farther north than the Icelandic capital Reykjavík. When home rule was established in 1979, the authorization of place names was transferred to Greenlandic authorities, who subsequently preferred Greenlandic names over Danish ones. The name Godthåb mostly went out of use over the next two decades.

The campus of the University of Greenland, hosting Statistics Greenland and the main holdings of the Public and National Library of Greenland, are at the northern end of the district, near the road to Nuuk Airport.

Nuuk receives its electric power mainly from the renewable energy-powered Buksefjord hydroelectric power plant by way of a 132 kV powerline crossing Ameralik fjord over a distance of 5376 m, the world's longest free span.

History
The site has a long history of habitation. The area around Nuuk was first occupied by the ancient, pre-Inuit, Paleo-Eskimo people of the Saqqaq culture as far back as 2200 BC when they lived in the area around the now abandoned settlement of Qoornoq. For a long time, it was occupied by the Dorset culture around the former settlement of Kangeq, but they disappeared from the Nuuk district before AD 1000. The Nuuk area was later inhabited by Viking explorers in the 10th century (Western Settlement), and shortly thereafter by Inuit peoples. Inuit and Norsemen both lived with little interaction in this area from about 1000 until the disappearance of the Norse settlement for uncertain reasons during the 15th century.

The city proper was founded as the fort of Godt-Haab in 1728 by the royal governor Claus Paarss, when he relocated the missionary and merchant Hans Egede's earlier Hope Colony (Haabets Koloni) from Kangeq Island to the mainland. At that time, Greenland was formally still a Norwegian colony (until 1814) under the united Dano-Norwegian Crown, but the colony had not had any contact for over three centuries. Paarss's colonists consisted of mutinous soldiers, convicts, and prostitutes and most died within the first year of scurvy and other ailments. In 1733 and 1734, a smallpox epidemic killed most of the native population as well as Egede's wife. Hans Egede went back to Denmark in 1736 after 15 years in Greenland, leaving his son Poul to continue his work. Godthaab became the seat of government for the Danish colony of South Greenland, while Godhavn (modern Qeqertarsuaq) was the capital of North Greenland until 1940, when the administration was unified in Godthaab.

In 1733, Moravian missionaries received permission to begin a mission on the island; in 1747, there were enough converts to prompt the construction of the Moravian Brethren Mission House and the formal establishment of the mission as New Herrnhut (Nye-Hernhut). This became the nucleus for present-day Nuuk as many Greenlanders from the southeastern coast left their territory to live at the mission station. From this base, further missions were established at Lichtenfels (1748), Lichtenau (1774), Friedrichsthal (1824), Umanak (1861), and Idlorpait (1864), before they were discontinued in 1900 and folded into the Lutheran Church of Denmark.

Around 1850, Greenland, and especially the area around Nuuk, were in crisis. The Europeans had brought diseases and a culture that conflicted with the ways of the native Greenlanders. Many Greenlanders were living in poverty. In 1853, Hinrich Johannes Rink came to Greenland and was surprised at how local Greenlandic culture and identity had been suppressed under Danish influence. In response, in 1861, he started the Atuagagdliutt, Greenland's first newspaper, with a native Greenlander as editor. This newspaper based in Nuuk later became an important token of Greenlandic identity.

During World War II, there was a reawakening of Greenlandic national identity. The use of written Greenlandic grew, a council was assembled under Eske Brun's leadership in Nuuk. In 1940, an American and a Canadian Consulate were established in Nuuk.

Under new regulations in 1950, two councils amalgamated into one. This Countryside Council was abolished on 1 May 1979, when the city of Godthåb was renamed Nuuk by the Greenland Home Rule government. The city boomed during the 1950s when Denmark began to modernize Greenland. As in Greenland as a whole, Nuuk is populated today by both Inuit and Danes. Over a third of Greenland's total population lives in the Nuuk Greater Metropolitan area.

An article examining indigenous influences on cities worldwide suggested, "One city... stands out. Nuuk... has probably the highest percentage of aboriginal people of any city: almost 90% of Greenland's population of 58,000 is Inuit, and at least eight in 10 live in urban settlements. Nuuk also celebrates Inuit culture and history to an extent that is unprecedented in many cities with higher total aboriginal populations. By proportion and by cultural authority and impact, it may well be tiny Nuuk that is the most indigenous city in the world."

Geography
Nuuk is located at approximately 64.16667°N, -51.73333°W at the mouth of Nuup Kangerlua (formerly Baal's River ), some 10 km from the shores of the Labrador Sea on the southwestern coast of Greenland, and about 240 km south of the Arctic Circle. Initially, the fjord flows to the northwest, to then turn southwest at 64.71667°N, -50.61667°W, splitting into three arms in its lower run, with three big islands in between the arms: Sermitsiaq Island, Qeqertarsuaq Island, and Qoornuup Qeqertarsua. The fjord widens into a bay dotted with skerries near its mouth, opening into Labrador Sea at approximately 64.05°N, -51.96667°W. Some 20 km to the northeast, reaching a height of 1210 m, Sermitsiaq can be seen from almost everywhere in Nuuk. The mountain has given its name to the nationwide newspaper Sermitsiaq. Closer to the town are the peaks of Store Malene, 790 m, and Lille Malene, 420 m. The magnetic declination at Nuuk is extreme.

Climate
Nuuk has a maritime-influenced tundra climate (Köppen ET) with cold, long, snowy winters and cool, short summers. Although the winters in Nuuk are relatively cold, they are milder compared to other tundra climates, such as in Alaska in the United States or parts of Eastern Siberia. Instead, peak winter is similar to identical latitudes in the Nordic countries. On 21 December, the shortest day and longest night of the year, the sun rises at 11:22 am and sets at 3:28 pm. By contrast, on the longest day and shortest night of the year, 21 June, the sun rises at 3:53 am and does not set until 1:03 am, giving a Midnight Sun. Nuuk can have mild temperatures on brief occasions year-round, with each month having recorded 13 C or warmer, although only June, July, August, and September have recorded what could be considered hot weather (defined as 22.5 C or higher). The monthly averages range from -9 C to 7 C, whereas all-time extremes range from -32.5 C on 14 January 1984 to 26.3 C on 6 July 2008. The record wind in Nuuk is 68 km/h.

The average monthly temperature (7.4 °C in July) is colder than what is considered the limit for trees (10 °C during the warmest month). There are a few planted trees which do not sustain well.

Demographics
With 19,872 inhabitants as of January 2024, Nuuk is by far the largest town in Greenland. The population of Nuuk has doubled since 1977, increased by over a third since 1990, and risen by almost 21% since 2000. In addition to those born in Greenland, data from 2015 showed 3,826 were born outside the country. Attracted by good employment opportunities with high wages, Danes have continued to settle in the town. Today, Nuuk has the highest proportion of Danes of any town in Greenland. Half of Greenland's immigrants live in Nuuk, which also accounts for a quarter of the country's native population.

Government
As the capital of Greenland, Nuuk is the administrative center of the country, containing all of the important government buildings and institutions. The public sector bodies are also the town's largest employer.

As of January 2021, the mayor of Nuuk is Charlotte Ludvigsen. She replaced former mayor Asii Chemnitz Narup in 2019 following a social media scandal involving posts criticizing her party. Like Narup, Ludvigsen is a member of the Inuit Ataqatigiit party.

Greenland's self-government parliament, the Inatsisartut, is in Nuuk. It has 31 seats and its members are elected by popular vote on the basis of proportional representation to serve four-year terms. All of Greenland's major political parties have their headquarters in Nuuk, including the Inuit Ataqatigiit, Siumut, Democrats, Atassut, Association of Candidates and the Women's Party.

KANUKOKA
KANUKOKA (Kalaallit Nunaanni Kommunit Kattuffiat) was based in Nuuk. It was an association of Greenland's municipalities, led by Enok Sandgreen. The aim of the organisation was to facilitate cooperation among all five municipalities of Greenland: Avannaata, Kujalleq, Qeqertalik, Qeqqata, and Sermersooq. However, Sermersooq and Qeqertalik both withdrew and KANUKOKA was dissolved as of Tuesday, 31 July 2018. The organisation ran the municipal elections every four years, with the last election taking place in 2016. All municipal authorities in Greenland were members of the organisation until its 2018 dissolution. The association was overseen by Maliina Abelsen, the Minister for Social Affairs in the Government of Greenland.

Economy
Although only a small town, Nuuk has developed trade, business, shipping and other industries. It began as a small fishing settlement with a harbor, but as the economy developed rapidly during the 1970s and 1980s, the fishing industry in the capital declined. The port is nevertheless still home to almost half of Greenland's fishing fleet. The local Royal Greenland processing plant absorbs landed seafood amounting to over DKK 50 million (US$7 million) per annum, mainly (80%) shrimp, but also cod, lumpfish and halibut. Seafood, including seal, is also sold in abundance in Nuuk's fish markets, the largest being Kalaaliaraq Market. Minerals including zinc and gold have contributed to the development of Nuuk's economy.

The city, like much of Greenland, is heavily dependent upon Danish investment and relies on Denmark for block funding.

Energy
All of Greenland's electricity is supplied by the government-owned company Nukissiorfiit, which has a monopoly on the electricity in Greenland. Since 1993, Nuuk has received its electric power mainly from Buksefjord hydroelectric power plant by way of a 132 kV powerline crossing Ameralik fjord over a distance of 5376 m, the world's longest free span.

Education
Nuuk has several educational institutions of higher learning. The University of Greenland (Ilisimatusarfik), the only university in Greenland, is in Nuuk. The university was founded in 1987 and expanded in 2007 with the new building, Ilimmarfik, housing departments of journalism, management and economics, language, literature and media, cultural and social history, theology and religion and social work. Nuuk is also home to the Department of Learning (Ilinniarfissuaq), the oldest educational facility in Greenland, in the old colonial part of Nuuk (Nuutoqaq: Old Nuuk). Other notable educational institutions include the Department of Nursing and Health Science, Nuuk Technical College and the Iron & Metal School.

Healthcare
The city is served by Queen Ingrid's Health Center. The health center serves as the regional health center for Region Sermersooq. It has an emergency room, and a central clinic with several general practitioners. Queen Ingrid's Health Center is the central hospital for Greenland.

Tourism
The Nuuk Tourist Office was built in 1992 to house the headquarters of the new National Tourist Board of Greenland.

Shopping
Shops in Nuuk offer local art and craftwork. In July 2012, Greenland's first shopping center, Nuuk Center (NC), opened. The center has Greenland's first underground parking. Several supermarkets exist, such as Nuuk Center, Pisiffik, Brugseni, and Spar.

Airport
Nuuk has an international airport 4 km to the northeast of the town center. Built in 1979, it is a hub for Air Greenland, which is also headquartered in Nuuk and operates its technical base at the airport. There are flights inside Greenland and to Iceland. A decision has been made to extend the runway to allow for flights to European destinations, such as Denmark.

Sea
As a result of the high cost of flying goods to Greenland, Nuuk and other towns in Greenland are connected to Denmark by cargo vessels which sail mainly from Aalborg during the warmer months after the winter ice has melted. They bring clothing, flour, medicine, timber and machinery and return with deep-frozen shrimp and fish. For most of the year, Nuuk is served twice-weekly by the coastal ferry of the Arctic Umiaq Line, which links the communities of the western coast.

Roadways
The majority of buses and cars owned in Greenland operate in Nuuk. There are no roads connecting Nuuk with other areas of Greenland. The main street in Nuuk is Aqqusinersuaq, with a number of shops and the 140-room Hotel Hans Egede.

Since 2009, the city bus service Nuup Bussii provides city transport services in Nuuk for the Sermersooq municipality, linking the town center with the airport, the outlying districts and neighborhoods of Nuussuaq, Qinngorput, as well as Qernertunnguit in Quassussuup Tungaa. In 2012, the buses transported more than 2 million passengers around the city of Nuuk.

Historical buildings
Hans Egede's House, built in 1721 by the Norwegian missionary Hans Egede, is the oldest building in Greenland. Standing close to the harbor among other old houses, it is now used for government receptions.
 * Hans Egede's House

The Church of Our Saviour of the Lutheran diocese of Greenland was built in 1849, and the tower was added in 1884. The red building with a clock tower and steeple is a prominent site on the landscape. The church received the status of Nuuk Cathedral in 1994, when the first bishop was Kristian Mørk, followed in 1995 by Sofie Petersen, a native of Greenland and the second woman in Denmark to become a bishop.
 * Nuuk Cathedral

The Herrnhut House was the center of the Moravian mission of New Herrnhut. Other landmarks include the Hans Egede Church and the Statue of Hans Egede.

Greenland National Museum is in Nuuk and was one of the first museums established in Greenland, inaugurated in the mid-1960s. The museum has many artifacts and exhibits related to Greenland's archaeology, history, art, and handicrafts, and contains the Qilakitsoq mummies.
 * National Museum

 * Modern Architecture

Examples of modern architecture include the Katuaq cultural center by Schmidt Hammer Lassen (1997), the campus of the University of Greenland by Tegnestuen Nuuk and KHR Arkitekter (2008), the Nuuk Center by KHR Arkitekter (2012) and the Anstalten Correctional Facility by Friis & Moltke and Schmidt Hammer Lassen (2019).

Cultural

Katuaq is a cultural center used for concerts, films, art exhibitions, and conferences. It was designed by Schmidt Hammer Lassen and inaugurated on 15 February 1997. Katuaq contains two auditoria, the larger seating 1,008 people and the smaller, 508. The complex also contains an art school, library, meeting facilities, administrative offices and a café.

The Nuuk Art Museum is the only private art and crafts museum in Greenland. The museum contains a notable collection of local paintings, watercolors, drawings, and graphics, some by Andy Warhol; and figures in soapstone, ivory, and wood, with many items collected by archaeologists.

Educational
Ilisimatusarfik, the University of Greenland, is in Nuuk and is the national university of Greenland. Most courses are taught in Danish, although a few are in Kalaallisut as well. , the university had approximately 150 students (almost all Greenlanders), around 14 academic staff, and five administrators. Its library holds approximately 30,000 volumes.

The National Library of Greenland in Nuuk is the largest reference library in the country, devoted to the preservation of Greenland's cultural heritage and history. The library holdings are split between the public library in the town center and Ilimmarfik, the campus of the University of Greenland. As of 1 January 2008, there are 83,324 items in the library database at Ilimmarfik.

Sports
Nuuk's sports clubs include Nuuk IL (established in 1934), B-67, and GSS Nuuk. Nuuk Stadium is a multi-purpose stadium, used mostly for football games. The stadium has a capacity of 2,000. The stadium can also be used as an entertainment venue: the Scottish rock band Nazareth performed at the venue. Nuuk also has the Godthåbhallen, a handball stadium. It is the home of the Greenland men's national handball team and has a capacity of 1,000. There is a hill for alpine skiing with an altitude difference around 300 meters on the mountain Lille Malene, with the valley station close to the airport terminal. There is also the Nuuk golf course, the only arctic golf course in the world.

Notable people

 * Johan Carl Christian Petersen (1813–1880), a seaman and interpreter
 * Hans Lynge (1906-1988), writer, dramatist, painter, politician, printmaker and sculptor.
 * Finn Lynge (1933–2014), politician, Indigenous rights activist and priest; the sole MEP for Greenland, 1979 until 1984
 * Agnethe Davidsen (1947–2007), Greenland's first female government minister
 * Rasmus Lyberth (born 1951), musician and actor.
 * Sofie Petersen (born 1955), a Lutheran bishop & Bishop of Greenland from 1995 to 2020.
 * Minik Thorleif Rosing, (DK wiki) (born 1957), geologist.
 * Bo Lidegaard (born 1958), historian and journalist
 * brothers Otto Rosing (born 1967), film director & Lars Rosing (born 1972), actor.
 * Maliina Abelsen (born 1976), Greenland's Minister for Social Affairs
 * Aaja Chemnitz Larsen (born 1977), politician, member of the Danish Folketing
 * Sara Olsvig (born 1978), politician member of the Danish Folketing, 2011–2014.
 * Nive Nielsen (born 1979), singer-songwriter and actress
 * Julie Berthelsen (born 1979), pop singer and songwriter.
 * Bibi Chemnitz (born 1983), fashion designer
 * Múte Bourup Egede (born 1987), politician, seventh Prime Minister of Greenland

Sport

 * Nils Nielsen (born 1971), football manager, head coach for the Switzerland women's national football team, 2018-2022
 * Jesper Grønkjær (born 1977), footballer, played 400 games and 80 for Denmark
 * Mads Andersen (born 1995), a Danish chess grandmaster.

Twin towns and sister cities
Nuuk is twinned with:

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

title: Nuuk


text:
Natural satellite

A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are colloquially referred to as moons, a derivation from the Moon of Earth.

In the Solar System, there are six planetary satellite systems containing 288 known natural satellites altogether. Seven objects commonly considered dwarf planets by astronomers are also known to have natural satellites:, Pluto, Haumea, , Makemake, , and Eris. As of January 2022, there are 447 other minor planets known to have natural satellites.

A planet usually has at least around 10,000 times the mass of any natural satellites that orbit it, with a correspondingly much larger diameter. The Earth–Moon system is a unique exception in the Solar System; at 3,474 kilometres (2,158 miles) across, the Moon is 0.273 times the diameter of Earth and about $1/undefined$ of its mass. The next largest ratios are the Neptune–Triton system at 0.055 (with a mass ratio of about 1 to 4790), the Saturn–Titan system at 0.044 (with the second mass ratio next to the Earth–Moon system, 1 to 4220), the Jupiter–Ganymede system at 0.038, and the Uranus–Titania system at 0.031. For the category of dwarf planets, Charon has the largest ratio, being 0.52 the diameter and 12.2% the mass of Pluto.

Terminology
The first known natural satellite was the Moon, but it was considered a "planet" until Copernicus' introduction of De revolutionibus orbium coelestium in 1543. Until the discovery of the Galilean satellites in 1610 there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetæ ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited.

The first to use the term satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus ("Narration About Four Satellites of Jupiter Observed") in 1610. He derived the term from the Latin word satelles, meaning "guard", "attendant", or "companion", because the satellites accompanied their primary planet in their journey through the heavens.

The term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of "moon". In 1957, however, the launching of the artificial object Sputnik created a need for new terminology. The terms man-made satellite and artificial moon were very quickly abandoned in favor of the simpler satellite, and as a consequence, the term has become linked primarily with artificial objects flown in space.

Because of this shift in meaning, the term moon, which had continued to be used in a generic sense in works of popular science and fiction, has regained respectability and is now used interchangeably with natural satellite, even in scientific articles. When it is necessary to avoid both the ambiguity of confusion with Earth's natural satellite the Moon and the natural satellites of the other planets on the one hand, and artificial satellites on the other, the term natural satellite (using "natural" in a sense opposed to "artificial") is used. To further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earth's natural satellite (a proper noun), but not when referring to other natural satellites (common nouns).

Many authors define "satellite" or "natural satellite" as orbiting some planet or minor planet, synonymous with "moon" – by such a definition all natural satellites are moons, but Earth and other planets are not satellites. A few recent authors define "moon" as "a satellite of a planet or minor planet", and "planet" as "a satellite of a star" – such authors consider Earth as a "natural satellite of the Sun".

Definition of a moon

There is no established lower limit on what is considered a "moon". Every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been considered a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets. Small asteroid moons (natural satellites of asteroids), such as Dactyl, have also been called moonlets.

The upper limit is also vague. Two orbiting bodies are sometimes described as a double planet rather than primary and satellite. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon. Some authors consider the Pluto–Charon system to be a double (dwarf) planet. The most common dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger body, though this is somewhat arbitrary because it depends on distance as well as relative mass.

Origin and orbital characteristics
The natural satellites orbiting relatively close to the planet on prograde, uninclined circular orbits (regular satellites) are generally thought to have been formed out of the same collapsing region of the protoplanetary disk that created its primary. In contrast, irregular satellites (generally orbiting on distant, inclined, eccentric and/or retrograde orbits) are thought to be captured asteroids possibly further fragmented by collisions. Most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits. The Moon and the Moons of Pluto are exceptions among large bodies in that they are thought to have originated from the collision of two large protoplanetary objects early in the Solar System's history (see the giant impact hypothesis). The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to commonly form by this process. Triton is another exception; although large and in a close, circular orbit, its motion is retrograde and it is thought to be a captured dwarf planet.

Temporary satellites
The capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations, temporary satellites should be a common phenomenon. The only observed examples are, ,.

was a temporary satellite of Earth for nine months in 2006 and 2007.

Tidal locking
Most regular moons (natural satellites following relatively close and prograde orbits with small orbital inclination and eccentricity) in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. This phenomenon comes about through a loss of energy due to tidal forces raised by the planet, slowing the rotation of the satellite until it is negligible. Exceptions are known; one such exception is Saturn's natural satellite Hyperion, which rotates chaotically because of the gravitational influence of Titan. Pluto's four, circumbinary small moons also rotate chaotically due to Charon's influence.

In contrast, the outer natural satellites of the giant planets (irregular satellites) are too far away to have become locked. For example, Jupiter's Himalia, Saturn's Phoebe, and Neptune's Nereid have rotation periods in the range of ten hours, whereas their orbital periods are hundreds of days.

Satellites of satellites

No "moons of moons" or subsatellites (natural satellites that orbit a natural satellite of a planet) are currently known. In most cases, the tidal effects of the planet would make such a system unstable.

However, calculations performed after the 2008 detection of a possible ring system around Saturn's moon Rhea indicate that satellites orbiting Rhea could have stable orbits. Furthermore, the suspected rings are thought to be narrow, a phenomenon normally associated with shepherd moons. However, targeted images taken by the Cassini spacecraft failed to detect rings around Rhea.

It has also been proposed that Saturn's moon Iapetus had a satellite in the past; this is one of several hypotheses that have been put forward to account for its equatorial ridge.

Light-curve analysis suggests that Saturn's irregular satellite Kiviuq is extremely prolate, and is likely a contact binary or even a binary moon.

Trojan satellites
Two natural satellites are known to have small companions at both their and  Lagrangian points, sixty degrees ahead and behind the body in its orbit. These companions are called trojan moons, as their orbits are analogous to the trojan asteroids of Jupiter. The trojan moons are Telesto and Calypso, which are the leading and following companions, respectively, of the Saturnian moon Tethys; and Helene and Polydeuces, the leading and following companions of the Saturnian moon Dione.

Asteroid satellites
The discovery of 243 Ida's natural satellite Dactyl in the early 1990s confirmed that some asteroids have natural satellites; indeed, 87 Sylvia has two. Some, such as 90 Antiope, are double asteroids with two comparably sized components.

Shape

Neptune's moon Proteus is the largest irregularly shaped natural satellite; the shapes of Eris' moon Dysnomia and ' moon Vanth are unknown. All other known natural satellites that are at least the size of Uranus's Miranda have lapsed into rounded ellipsoids under hydrostatic equilibrium, i.e. are "round/rounded satellites" and are sometimes categorized as planetary-mass moons. (Dysnomia's density is known to be high enough that it is probably a solid ellipsoid as well.) The larger natural satellites, being tidally locked, tend toward ovoid (egg-like) shapes: squat at their poles and with longer equatorial axes in the direction of their primaries (their planets) than in the direction of their motion. Saturn's moon Mimas, for example, has a major axis 9% greater than its polar axis and 5% greater than its other equatorial axis. Methone, another of Saturn's moons, is only around 3 km in diameter and visibly egg-shaped. The effect is smaller on the largest natural satellites, where their gravity is greater relative to the effects of tidal distortion, especially those that orbit less massive planets or, as in the case of the Moon, at greater distances.

Geological activity
Of the twenty known natural satellites in the Solar System that are large enough to be gravitationally rounded, several remain geologically active today. Io is the most volcanically active body in the Solar System, while Europa, Enceladus, Titan and Triton display evidence of ongoing tectonic activity and cryovolcanism. In the first three cases, the geological activity is powered by the tidal heating resulting from having eccentric orbits close to their giant-planet primaries. (This mechanism would have also operated on Triton in the past before its orbit was circularized.) Many other natural satellites, such as Earth's Moon, Ganymede, Tethys, and Miranda, show evidence of past geological activity, resulting from energy sources such as the decay of their primordial radioisotopes, greater past orbital eccentricities (due in some cases to past orbital resonances), or the differentiation or freezing of their interiors. Enceladus and Triton both have active features resembling geysers, although in the case of Triton solar heating appears to provide the energy. Titan and Triton have significant atmospheres; Titan also has hydrocarbon lakes. All four of the Galilean moons have atmospheres, though they are extremely thin. Four of the largest natural satellites, Europa, Ganymede, Callisto, and Titan, are thought to have subsurface oceans of liquid water, while smaller Enceladus also supports a global subsurface ocean of liquid water.

Occurrence in the Solar System
Besides planets and dwarf planets objects within our Solar System known to have natural satellites are 76 in the asteroid belt (five with two each), four Jupiter trojans, 39 near-Earth objects (two with two satellites each), and 14 Mars-crossers. There are also 84 known natural satellites of trans-Neptunian objects. Some 150 additional small bodies have been observed within the rings of Saturn, but only a few were tracked long enough to establish orbits. Planets around other stars are likely to have satellites as well, and although numerous candidates have been detected to date, none have yet been confirmed.

Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos. The giant planets have extensive systems of natural satellites, including half a dozen comparable in size to Earth's Moon: the four Galilean moons, Saturn's Titan, and Neptune's Triton. Saturn has an additional six mid-sized natural satellites massive enough to have achieved hydrostatic equilibrium, and Uranus has five. It has been suggested that some satellites may potentially harbour life.

Among the objects generally agreed by astronomers to be dwarf planets, Ceres and have no known natural satellites. Pluto has the relatively large natural satellite Charon and four smaller natural satellites; Styx, Nix, Kerberos, and Hydra. Haumea has two natural satellites;, , Makemake, , and have one each. The Pluto–Charon system is unusual in that the center of mass lies in open space between the two, a characteristic sometimes associated with a double-planet system.

The seven largest natural satellites in the Solar System (those bigger than 2,500 km across) are Jupiter's Galilean moons (Ganymede, Callisto, Io, and Europa), Saturn's moon Titan, Earth's moon, and Neptune's captured natural satellite Triton. Triton, the smallest of these, has more mass than all smaller natural satellites together. Similarly in the next size group of nine mid-sized natural satellites, between 1,000 km and 1,600 km across, Titania, Oberon, Rhea, Iapetus, Charon, Ariel, Umbriel, Dione, and Tethys, the smallest, Tethys, has more mass than all smaller natural satellites together. As well as the natural satellites of the various planets, there are also over 80 known natural satellites of the dwarf planets, minor planets and other small Solar System bodies. Some studies estimate that up to 15% of all trans-Neptunian objects could have satellites.

The following is a comparative table classifying the natural satellites in the Solar System by diameter. The column on the right includes some notable planets, dwarf planets, asteroids, and trans-Neptunian objects for comparison. The natural satellites of the planets are named after mythological figures. These are predominantly Greek, except for the Uranian natural satellites, which are named after Shakespearean characters. The twenty satellites massive enough to be round are in bold in the table below. Minor planets and satellites where there is disagreement in the literature on roundness are italicized in the table below.

Moons of planets
• The Moon, Earth's natural satellite

• Moons of Mars

• Moons of Jupiter

• Moons of Saturn

• Moons of Uranus

• Moons of Neptune

Moons of dwarf planets and small Solar System bodies
• Minor-planet moon

• Moons of Pluto

• Dysnomia, Eris's natural satellite

• Moons of Haumea

• S/2015 (136472) 1, Makemake's natural satellite

• Xiangliu, Gonggong's natural satellite

• Weywot, Quaoar's natural satellite

• Vanth, Orcus' binary natural satellite

All moons

 * Natural Satellite Physical Parameters (JPL-NASA, with refs – last updated July 2006)
 * Moons of the Solar System (The Planetary Society, as of March 2009)
 * The JPL's Solar System Dynamics page
 * Planetary Names: Planet and Satellite Names and Discoverers
 * "Upper size limit for moons explained" Kelly Young. Nature (vol 441, p. 834) 14 June 2006
 * Images of planets and major moons (not to scale)
 * The Planetary Society – Moon Montage(s)
 * Album of moon images by Kevin M. Gill
 * The Atlas of Moons by the National Geographic Society
 * The Atlas of Moons by the National Geographic Society

Jupiter's moons

 * * Scott S. Sheppard
 * Scott S. Sheppard

Saturn's moons

 * Satellite-hunters find four new moons of the planet Saturn David Brand | 26 October 2000
 * Saturn's New Satellite S/2003 S1 Scott S. Sheppard

title: Natural satellite