Mohamed Raghibi

Baokou Xiong

Satyajit Phadke

Mérièm Anouti
email: meriem.anouti@univ-tours.fr

.

Keywords: Electrolyte, Lithium-ion, batteries, gas volume, pouch cell swelling, safety

Introduction

The generation and management of gases in Li-ion batteries is drawing the attention of both industrialists [START_REF] Diaz | Gas generation measurement and evaluation during mechanical processing and thermal treatment of spent Li-ion batteries[END_REF][START_REF] Seo | Gas-Evolution Induced Volume Fraction Changes and Their Effect on the Performance Degradation of Li-Ion Batteries[END_REF] and the research community in this field [START_REF] Wang | Stretchable Carbon Nanotube Dilatometer for In Situ Swelling Detection of Lithium-Ion Batteries[END_REF][START_REF] Fan | Sustainable Recycling Technology for Li-Ion Batteries and Beyond: Challenges and Future Prospects[END_REF]. The objective is to identify and quantify the gases to understand the origin of their formation and to remedy the harmful consequences of their accumulation during battery operation [START_REF] Aiken | A Survey of In Situ Gas Evolution during High Voltage Formation in Li-Ion Pouch Cells[END_REF]. Gas generation is a complex phenomenon depending on various parameters such as the composition of the electrolyte, the nature of the electrodes, the cycling regime or even operating conditions such as temperature and the cutoff voltage [START_REF] Diaz | Gas generation measurement and evaluation during mechanical processing and thermal treatment of spent Li-ion batteries[END_REF][START_REF] Woody | Strategies to limit degradation and maximize Li-ion battery service lifetime -Critical review and guidance for stakeholders[END_REF][START_REF] Raijmakers | A review on various temperatureindication methods for Li-ion batteries[END_REF]. It can therefore be difficult to find the ideal conditions to minimize the amount of gas generated and its effect on aging and safety. Consequently, this problem cannot be overlooked because, beyond optimizing battery performance, safety is a particularly sensitive aspect for consumer use of batteries [START_REF] Zhang | Computational identification of the safety regime of Li-ion battery thermal runaway[END_REF][START_REF] Xie | Influence of cycling aging and ambient pressure on the thermal safety features of lithium-ion battery[END_REF][START_REF] Liao | Experimental evaluation of thermolysis-driven gas emissions from LiPF6-carbonate electrolyte used in lithium-ion batteries[END_REF]. The generation of gases in lithium-ion batteries poses above all a technological challenge [START_REF] Kharbachi | Exploits, advances and challenges benefiting beyond Li-ion battery technologies[END_REF][START_REF] Ferg | The challenges of a Li-ion starter lighting and ignition battery: A review from cradle to grave[END_REF]. Their accumulation in sealed batteries prevents their evacuation, which increases the danger. The generation and evolution of gases during battery operation and storage, that is to say without electrochemical cycling, significantly reduces the battery lifespan and cyclability [START_REF] Xie | Influence of cycling aging and ambient pressure on the thermal safety features of lithium-ion battery[END_REF][START_REF] Gandoman | Concept of reliability and safety assessment of lithium-ion batteries in electric vehicles: Basics, progress, and challenges[END_REF].

The majority of the reactions that produce gases in accumulators are localized at the electrode / electrolyte interfaces, which are particularly sensitive areas [START_REF] Liao | Experimental evaluation of thermolysis-driven gas emissions from LiPF6-carbonate electrolyte used in lithium-ion batteries[END_REF]. The gases create vacuum zones which cause a displacement of the electrolyte to fill this vacuum, causing a discontinuity in the diffusion of the species in solution, in particular the lithium ions. In general, the phenomena of gas generation and their accumulation at the interfaces result in an increase in the electrical resistance of the cell [START_REF] Jiang | Rapid prediction method for thermal runaway propagation in battery pack based on lumped thermal resistance network and electric circuit analogy[END_REF], but also in an increase in internal pressure, leading to mechanical stress of the electrode materials. Swelling ranging from 6 to 20% is observed on average after storage at high temperature for commercial pouch cells [START_REF] Zhang | Dissecting anode swelling in commercial lithium-ion batteries[END_REF]. In addition, other intrinsic aging parameters of the batteries due to the cycles of intercalation / deintercalation of lithium ions further reduce the lifespan of the battery.

For all these reasons, it is important to identify the electrolyte components that promote the generation of gas in lithium-ion batteries and those that on the contrary reduce it. The accumulation of gases in cells cannot be dissociated from their solubility in electrolytes. Gas solubility measurements have already been reported in the literature [START_REF] Blanchard | Solubility of carbon dioxide in alkylcarbonates and lactones[END_REF][START_REF] Hong | Effect of Acetonitrile on the Solubility of Carbon Dioxide in 1-Ethyl-3methylimidazolium Bis(trifluoromethylsulfonyl)amide[END_REF] and by our group [START_REF] Dougassa | Low pressure methane solubility in lithium-ion batteries based solvents and electrolytes as a function of temperature. Measurement and prediction[END_REF][START_REF] Dougassa | Viscosity and carbon dioxide solubility for LiPF6, LiTFSI, and LiFAP in alkyl carbonates: Lithium salt nature and concentration effect[END_REF][START_REF] Dougassa | Low pressure carbon dioxide solubility in lithium-ion batteries based electrolytes as a function of temperature. Measurement and prediction[END_REF][START_REF] Anouti | Low pressure carbon dioxide solubility in pure electrolyte solvents for lithium-ion batteries as a function of temperature. Measurement and prediction[END_REF] which has reported data concerning the ex situ measurement of solubility as a function of the composition of the electrolyte and the temperature. However, the solubility of gas alone is not sufficient to understand the overpressure mechanisms in lithium-ion batteries because these measurements do not take into account real conditions during cycling, including the catalytic activity of the charged electrode material interfaces. Few studies have quantified the volume of gas generated and its correlation to operating conditions and the electrolyte composition. Dahn et al., for example, focused on these measures through the study of the effect of additives on gas generation [START_REF] Xia | Improving Linear Alkyl Carbonate Electrolytes with Electrolyte Additives[END_REF][START_REF] Ma | 1/3Mn1/3Co1/3]O2/graphite, Li[Ni0.5Mn0.3Co0.2/graphite and Li[Ni0.6Mn0.2Co0.2/graphite pouch cells[END_REF]. They reported pressure measurements in the presence of additives such as flame retardants (TTFP, TTSPi) [START_REF] Sinha | Comparative Study of Tris(trimethylsilyl) Phosphate and Tris(trimethylsilyl) Phosphite as Electrolyte Additives for Li-Ion Cells[END_REF], or polymerization initiators [START_REF] Xia | Some Fluorinated Carbonates as Electrolyte Additives for Li(Ni0.4Mn0.4Co0.2)O2/Graphite Pouch Cells[END_REF][START_REF] Liu | Gas Evolution during Unwanted Lithium Plating in Li-Ion Cells with EC-Based or EC-Free Electrolytes[END_REF]. The paucity of data on the quantification of gases generated during cycling no doubt resides in the difficulty in implementing these measures without disturbing cell cycling. In the present study, we report the development of an original protocol allowing the measurement of the volume of gas generated during cycling of Gr//NMC in pouch-cell systems. We compare the values obtained at two temperatures with different electrolyte formulations. The identification of the gases generated by GC/MS as well as ex-situ measurement of the gas solubility (O2, H2) carried out in this study made it possible to discuss the results obtained. The aim was to discern the respective roles of salts, solvents and two ionic liquids at two concentrations (5% and 30% wt) to understand their impact on battery swelling.

Methodology and procedure

Pouch cell preparation and gas volume measurements

The Gr//NMC pouch cells used in this study were purchased empty, to be filled with electrolyte, from the company Li-Fun corporation (China) and sealed under vacuum. They can deliver a capacity of 200 mAh for a cutoff voltage of 4.2 V. The positive electrode dimensions are 200 mm x 26 mm while the negative electrode measures 204 mm x 28 mm.

The two electrodes are coated on both sides; a free strip at the end of the roll allows sealing without the risk of short circuit. The size of the active surface is between 90 and 100 cm².

The electrodes are wound with a Celgard type separator. Before filling them with electrolyte, the pouch cells, initially sealed under vacuum without electrolyte, are opened and dried under vacuum at 80 ° C for 12 hours to remove traces of water. The cells are then transferred to a glove box to be filled with the electrolyte without contamination. The bags are sized for 0.9 g (approximately 1 mL) of electrolyte (supplier recommendation).

The sealing of the filled pouch cells is carried out using a vacuum heat sealer with controlled pressure and temperature. To improve the wettability of the electrodes and the separator, the pouch cells are charged at 1.5 V v.s Li + /Li and maintained at this voltage for 24 hours at room temperature in order to polarize the electrodes without initiating intercalation processes.

The volume measurement of the pouch cells was carried out before the first cycle, (to determine the volume of the pouch cells before gas generation), and after each end of charging and discharging. This means that two volume measurements were made per cycle.

The volume of the pouch cell was determined by means of two successive weighings using a device (Figure 1a) adapted to the precision balance from Sertorius. The first weighing was carried out with the basket (Figure 1b) placed on the support (in the air), and the second was performed with the basket plunged into distilled water. The volume of the pouch cell according to their swelling state (Figures 1 c andd) is then calculated using the following equation:

V

with ∆V 10 mL The repeatability of the measurements was ensured by the meticulous preparation of the pouch cells in an opening, drying, filling and then sealing process to avoid problems of electrolyte and / or gas leakage due to poor sealing. To prevent the released gases from being inserted at the jelly-roll level (Figure 2a), which leads to an increase in the cell resistance by separation of the battery elements (electrodes, separator), we pressed the pouch cells as shown in (Figure 2c). Four pouch cells were pressed at 2 N.m -1 using a torque wrench (Figure 2b) and tested simultaneously to ensure the reliability of the measurements. The applied pressure maintains sufficient optimum stress for the cohesion of the battery without causing the pouch cells to open at the jelly-roll level and therefore the leakage of the electrolyte or the gases.

Repeatability and uncertainty

The electrochemical and volume measurements showed good repeatability both for galvanostatic profiles and for gas volumes. Figure 3 shows the galvanostatic profiles of the first cycle obtained during reproducibility tests on four different pouch cells (Figure 3a), and the repetition of five volume measurements for each of them (Figure 3b). The pouch cell characterization methodology was validated for the rest of this work with a volume measurement error between 2% and 4%.

Gas analysis

The composition of the gases in the pouch cells was analyzed by gas chromatography coupled with mass spectroscopy (GC-MS) in order to detect the gases described as mainly responsible for battery swelling, i.e. CO2, CO, CH4, C2H4, H2, and O2. The gas mixture was taken directly from the pouch cells at the end of the cycle using a "gas-tight" type chromatography syringe, and the collected gas mixture was injected directly into the column.

The oven was maintained at 50 ° C for a carrier gas flow rate set at 0.5 mL.min-1. The optimal volume for detection was 2 µL to analyze even slightly swollen pouch cells.

Electrolyte vapor pressure and gas solubility

The measurement of gas solubility was carried out by bringing a known quantity of gas into contact with a quantity of degassed liquid of measured volume and density. The quantity of solubilized gas was obtained by the difference between the initial pressure before addition of gas and the final pressure after thermodynamic equilibrium. The measurement takes into account the free volume occupied by the vapor phase of the solution. The pressure was measured with an accuracy of 0.0 1mBar. The experimental device described by Dougassa et al. [START_REF] Dougassa | Low pressure methane solubility in lithium-ion batteries based solvents and electrolytes as a function of temperature. Measurement and prediction[END_REF][START_REF] Dougassa | Low pressure carbon dioxide solubility in lithium-ion batteries based electrolytes as a function of temperature. Measurement and prediction[END_REF][START_REF] Anouti | Low pressure carbon dioxide solubility in pure electrolyte solvents for lithium-ion batteries as a function of temperature. Measurement and prediction[END_REF] allows both vapor pressure and gas solubility measurements. In order to ensure repeatability of the results, the experiments were carried out in two identical devices in parallel. The device has a double wall to circulate a thermostatically controlled bath to control the temperature. The electrolyte is introduced in a glove box and is then degassed several times by repeated crystallization cycles under vacuum. When the thermodynamic equilibrium is established, at T given, the vapor pressure of the electrolyte is read.

As the determination of the solubility of gas in the electrolyte is detailed in one of our previous studies [START_REF] Dougassa | Low pressure methane solubility in lithium-ion batteries based solvents and electrolytes as a function of temperature. Measurement and prediction[END_REF][START_REF] Dougassa | Low pressure carbon dioxide solubility in lithium-ion batteries based electrolytes as a function of temperature. Measurement and prediction[END_REF][START_REF] Anouti | Low pressure carbon dioxide solubility in pure electrolyte solvents for lithium-ion batteries as a function of temperature. Measurement and prediction[END_REF], we will just summarize it here. The quantity of gas dissolved in the liquid phase is calculated from 2 successive pVT (pressure volume temperature) measurements. During the first pVT measurement, only the electrolyte is present. The device is kept at constant temperature Tini, then a quantity of gas is introduced into the equilibrium cell containing the electrolyte. The solubility of the gases in the studied mixture can be expressed as a fraction of their molar fractions in solution, x1:

, where is the amount of gas dissolved in the liquid solution and is the amount of solvent in the liquid phase introduced into the cell. corresponds to the quantity of alkyl carbonate solvents, and to the quantity of lithium salt in solution. The Henry constant can then be calculated from the molar fraction of solubilized gas according to the equation:

liq 1 liq solv liq 1 1 n n n x + = liq 1 n liq 3 liq 2 liq solv n n n + = liq 2 n liq 3 n
, where f1 is the fugacity of the gas, and is its coefficient of fugacity at the equilibrium pressure peq and equilibrium temperature Teq.

Experimental section

Electrolyte formulation

The electrolyte components were battery-grade ethylene carbonate, EC, (Sigma-Aldrich), ethyl methyl carbonate, EMC, (Sigma Aldrich, 99.9), dimethyl carbonate, DMC, (Sigma Aldrich, 99.9) and propylene carbonate, PC, (Sigma Aldrich, 99.9). The ionic liquids Methyl butyl pyrrolidinium bis (trifluoro-sulfonyl) imide, (Pyrr14 TFSI), (Sigma Aldrich, 99), and Methyl tributyl-phosphonium bis (trifluoro-sulfonyl) imide, (Phos1444 TFSI), (MTI, 99), were used without prior treatment. For the electrolyte, a binary solvent mixture with a mass ratio of 3/7 or 1/1, and a ternary solvent mixture with a mass ratio of 1/1/3 were formulated in the glove box and mixed with 1.0 mol.L -1 of lithium hexafluorophosphate (LiPF6) salt (Sigma Aldrich, battery grade) or 1.0 mol.L -1 of lithium bis (trifluoro-sulfonyl) imide (LiTFSI) salt (Sigma Aldrich, 99.95%). The electrolytes were prepared using a Sartorius 1602 MP balance with ±10 -4 g accuracy inside an argon filled MBraun glovebox, at 25 °C, with less than 10 ppm of moisture content. The water content of each electrolyte was measured using an 831 Karl-Fisher Coulometer (Metrohm), with values lower than 20 ppm.

Electrochemical Testing

The N/P ratio of the Gr//NMC pouch cells with a nominal capacity of 200 mAh for a cutoff voltage of 4.2 V was between 1.1 and 1.4. This ratio corresponds to the ratio of the total anode capacity to the total cathode capacity. The pouch cells were dried under vacuum at 80°C for 12 h and placed in the glovebox without exposure to ambient air. For electrochemical testing, the pouch cells were filled with 1 mL of electrolyte (carefully measured with a micropipette). All the steps involving the pouch cell preparation were performed inside an atmosphere-controlled glovebox where the H2O and O2 concentration was maintained below 0.1 ppm. Potential limits of 4.2 V -2.9 V were used for all galvanostatic testing. Galvanostatic cycling was carried out using a VMP multichannel potentiostatic galvanostatic system (Biologic Science Instrument, France).

( ) ( )

2 eq eq eq 2 2 2 2 0 H , , , lim 2 x p T p x x T p f K x φ ≅ ≡ → 1 ϕ

Results and Discussion

The electrolyte components chosen in this study are four solvents (EC, PC, EMC, and DMC) and two salts (LiPF6 and LiTFSI) among those most frequently used in batteries. We also selected two ionic liquids comprising the same TFSI -anion associated to the two homologous cations: the ammonium (methyl butyl-pyrrolidinium) and the phosphonium (methyl tributylphosphonium) group. Table 1 andTable 2 give the name, structure, and charge distribution of the solvents and salts selected for this study, respectively. The electrolyte formulations with a binary mixture of EC/EMC or EC/DMC, and a ternary mixture of EC/PC/DMC or EC/PC/EMC enabled comparison of the measurements by revealing the role of each solvent.

The concentration of the tested salts was kept at 1mol/L and the ionic liquids were added to (EC/EMC + 1mol/L) in mass proportions of 5% and 30%. The gases that form in batteries have several origins, such as the materials themselves, and the electrolytes. The use of graphite requires the formation of an SEI (Solid Electrolyte Interphase) which causes the formation of several gases, resulting from low potential chemical and electrochemical decomposition reactions of alkylcarbonates, catalyzed by graphite. These reactions lead to the formation of several gases including ethene (C2H4), carbon dioxide (CO2), or dioxygen (O2), according to the following mechanisms:

2C3H4O3 + 2 Li + + 2e - C2H4 + [CH2OCO2Li]2 (eq1) 2C3H4O3 + 2 Li + + 2e - CO2 + [C2H4OLi]2 (eq2) [CH2OCO2Li]2 CO2 + C2H4 + Li2CO3 + O2 (eq3)
The NMC cathode is a metal oxide. These materials are known to release dioxygen by high potential oxidation. The dioxygen produced can then react with the solvents to give CO2:

2,5 O2 + C3H4O3 3 CO2 + 2 H2O (eq4)
To compare the effects of the solvent salts and the ionic liquids, the same program was applied to the Gr//NMC pouch cells, and is summarized in Table 3. Whatever the electrolyte composition, the first cycle for SEI formation was carried out at 60 °C, at C/20, corresponding to a mass current density of J = 8.5 mA.g -1 NMC. These conditions were chosen in order to favor the formation of a good SEI which ensures efficient cycling thereafter. The first cycle at C/20 shows an irreversible capacity of approximately 22 mAh.g -1 which represents 13% to 14% of the total initial charge capacity, 170 mAh.g -1 for (EC / EMC: 3 /7 + 1M LiPF6) and 160 mAh.g -1 for (EC / DMC: 1/1 + 1M LiPF6), respectively. These values are consistent with the irreversible and initial capacities reported in the literature, i.e. between 10% and 20% [START_REF] Komaba | Alkali carbonate-coated graphite electrode for lithium-ion batteries[END_REF][START_REF] Verma | Chemical surface treatments for decreasing irreversible charge loss and preventing exfoliation of graphite in Li-ion batteries[END_REF]. For both electrolytes, the retention capacity evaluated when returning

Effect of PC in alkylcarbonate ternary mixture on gas generation

The comparison of the performances during the first 27 cycles and their correlation with the gases generated, expressed in cumulative volume (v in mL), is collated in figure 5 (a-f). As in the case of binary mixtures, the performances of the two ternaries were similar, with initial capacities of 155 mAh.g -1 and 165 mAh.g -1 in the presence of DMC and EMC, respectively.

After 27 cycles, at different regime, 98% of the initial capacity is recovered. The difference with the performances observed with the binary mixture is the behavior with strong current density. This result is logical given that PC is viscous (η = 2.93 cP at 20°C) [START_REF] Pires | Density, conductivity, viscosity, and excess properties of (pyrrolidinium nitrate-based Protic Ionic Liquid + propylene carbonate) binary mixture[END_REF] and disfavors the rapid diffusion of Li + ions. Concerning gas formation, the addition of PC does not drastically modify the observations noted in the case of binary mixtures: (i) the DMC generates more gas (+ 25%) than EMC, (ii) and the cycles carried out at 40 °C do not increase the formation of gas. In addition, the absolute quantity of gas measured with and without PC is very close. This result means that PC does not participate in the decomposition reactions of alkylcarbonates which form gases (eq. 1-4). In summary, among the 4 solvents tested here, it is essentially EC which is at the origin of the quantities of gas measured. We can therefore establish the following classification of the 4 solvents with respect to their respective gas-forming roles: EC> DMC> EMC> PC. This result was expected as EC is responsible for the formation of the SEI and it is at this stage of battery life that the majority of gases are formed. The SEI formation and the gas generated in significant quantities generally leave traces on the layer. After cycling, the surface of the materials undergoes a transformation which carries

(a) (b) (d) (c) (e) (f)

the stigmata of deposits, cracks due to mechanical stresses or gas diffusion, and agglomeration zones of particles following a migration of transition metals. Scanning electron microscopy associated with an energy dispersion spectrometry microanalysis (EDX) was carried out. The latter is based on the detection of X photons, offering the possibility of observing the SEI. The graphite used in this study is artificial. Before being cycled (Figure 6 a), it presents a striated appearance where the "scales" characteristic of graphite morphology can be seen [START_REF] Ghimbeu | Influence of graphite characteristics on the electrochemical performance in alkylcarbonate LiTFSI electrolyte for Li-ion capacitors and Li-ion batteries[END_REF]. These consist of stacks of graphene sheets which measure approximately 10 µm. There is no particular organization of these clusters. On the other hand, the graphene sheets are clearly observed. Quantification by EDX reveals a carbon content of 99.32% with a low proportion of oxygen, less than 1%. On lithiated graphite (Figure 6 b), it is no longer possible to see graphene sheets because they are covered by a homogeneous layer of SEI obtained by the reduction of solvents and salts during the first lithiation. Quantification by EDX analysis evaluated the level of visible carbon at the surface to be only 33.72%, oxygen 39.64%, fluorine 25.20% and phosphorus 1.44%. These elements account for the composition of the SEI, in particular by mineral compounds of the LiF, Li2CO3 type or the organic part (lithium alkoxides and polycarbonates) [START_REF] Myung | Electrochemical behavior and passivation of current collectors in lithium-ion batteries[END_REF]. On the other hand, on the lithiated graphite, discontinuities of the SEI in the form of cracks are visible. These cracks in the passivation layer are created by the evacuation of gases generated by the formation of the SEI [START_REF] Kalhoff | Enabling LiTFSI-based Electrolytes for Safer Lithium-Ion Batteries by Using Linear Fluorinated Carbonates as (Co)Solvent[END_REF].

On the graphite discharged during the first cycle (Figure 6 c), characteristic graphite grains can be seen because the SEI is partially dissolved. This was confirmed by the quantification by EDX which revealed a significant proportion of carbon (91.39%) but which remains lower than that measured on the virgin graphite electrode (> 99%), which means that there remains a thin film of SEI on the surface of the graphite. It is then clear from these SEM pictures and their corresponding EDX analysis that the passivation layer evolves throughout the battery lifetime as shown by XPS [START_REF] Self | Survey of Gas Expansion in Li-Ion NMC Pouch Cells[END_REF] during the charge-discharge cycles and that at least part of the formation / dissolution of the SEI continues throughout the cycling. LiTFSI formed 35% more gas than LiPF6, probably because its passivation layer is thicker.

Comparative effect of LiPF6 and LITFSI salts on gas generation with temperature

For the rest of the cycles, while at 25 °C, the quantity of gas changed little, at 40 °C, in the presence of LiTFSI the volume of gas generated decreased in particular during the rapid cycles at C /1. A rational explanation for these observations is the slow dissolution of the gas generated during the first charge. This dissolution will depend on two parameters: the nature of the salt and the temperature. The solubility of gases such as O2, H2, CO2, CH4 and C2H4 in battery electrolytes is an endothermic phenomenon which is favored by temperature [START_REF] Xia | Improving Linear Alkyl Carbonate Electrolytes with Electrolyte Additives[END_REF]. It is therefore logical to observe a decrease in the volume of gas with temperature. On the other hand, the nature of the lithium salt has a distinct effect on the solubility of gases in carbonatebased electrolytes.

We have shown in previous work [START_REF] Dougassa | Low pressure methane solubility in lithium-ion batteries based solvents and electrolytes as a function of temperature. Measurement and prediction[END_REF][START_REF] Dougassa | Viscosity and carbon dioxide solubility for LiPF6, LiTFSI, and LiFAP in alkyl carbonates: Lithium salt nature and concentration effect[END_REF][START_REF] Dougassa | Low pressure carbon dioxide solubility in lithium-ion batteries based electrolytes as a function of temperature. Measurement and prediction[END_REF][START_REF] Anouti | Low pressure carbon dioxide solubility in pure electrolyte solvents for lithium-ion batteries as a function of temperature. Measurement and prediction[END_REF] that salt plays a decisive role in the dissolution mechanism of CO2 in electrolytes. LiPF6 has a rather negative effect on the CO2 dissolution process, so-called salting-out, while LiTFSi has a rather positive effect, called "salting-in".

For example, at 80 °C in the DEC, the Henry constant, which is inversely proportional to the molar fraction of gas dissolved in the electrolyte, KH = 12.66 MPa without salt, becomes KH = 18.14 MPa in the presence of LiPF6 and remains unchanged KH = 12.28 MPa in the presence of LiTFSi [START_REF] Dougassa | Low pressure methane solubility in lithium-ion batteries based solvents and electrolytes as a function of temperature. Measurement and prediction[END_REF]. The effect of gas solubility in electrolytes will be discussed further in section 4.5.2. The explanation of this opposite effect on the solubility of gases according to the nature of the salt is the delocalized and elongated structure of the TFSI -anion which allows empty cavities in the structure of the mixture (solvents + salt) and thus allows the introduction of a quadrupole such as the CO2 molecule. This means that after the gases have been generated

during the initial stage of preparation of the SEI, they are slowly dissolved in the electrolyte, all the more so since the salt has a salting-in power as here LiTFSI. In conclusion, the effect of salt on the swelling of cells can be summarized as follows: (i) the consumption by

25°C 40°C

dissolution of the gases generated could be a positive point for the safety of the batteries by reducing the pressure; (ii) the solubility of the gases is very different according to their nature and it is the most soluble gases that are favored by the presence of TFSI-; (iii) despite the interest of TFSI in the solubility of gases, LiPF6 provides a higher SEI quality and consequently a higher performance.

Effect of ionic liquids on gas generation

Ionic liquids can be considered as the other salts of the electrolyte (LiPF6 and LiTFSI) except that their cation, which differs from Li + , does not participate in capacity. Their role is predominant at the polarized interface due to their ionic character. They also improve the transport properties of the electrolyte and its thermal characteristics depending on the quantities added. The solubility of gases can also be influenced by ionic liquids as shown by many studies [START_REF] Shaahmadi | The CO2/CH4 gas mixture solubility in ionic liquids[END_REF][START_REF] Faúndez | Solubility of hydrogen sulfide in ionic liquids for gas removal processes using artificial neural networks[END_REF][START_REF] Ellegaard | Solubilities of gases in ionic liquids using a corresponding-states approach to Kirkwood-Buff solution theory[END_REF][START_REF] Huang | Tuning ionic liquids for high gas solubility and reversible gas sorption[END_REF]. We chose here an ammonium and a phosphonium at two concentrations, 5% and 30%, to distinguish their effect as additives (5%) and co-solvent (30%).

Effect of Phos1444 TFSI on gas formation

It was found that phosphonium ILs exhibited lower viscosities, higher conductivities and thermal stabilities than those of the corresponding ammonium [START_REF] Hilder | Physicochemical characterization of a new family of small alkyl phosphonium imide ionic liquids[END_REF][START_REF] Tsunashima | Physical and electrochemical characterization of ionic liquids based on quaternary phosphonium cations containing a carbon-carbon double bond[END_REF][START_REF] Tsunashima | Low viscous and highly conductive phosphonium ionic liquids based on bis(fluorosulfonyl)amide anion as potential electrolytes[END_REF]. However, most of the work reporting the introduction of these ionic liquids concerns half-batteries with a lithium anode [START_REF] Hilder | Physicochemical characterization of a new family of small alkyl phosphonium imide ionic liquids[END_REF][START_REF] Tsunashima | Physical and electrochemical characterization of ionic liquids based on quaternary phosphonium cations containing a carbon-carbon double bond[END_REF]. Graphite, with the formation of SEI, a key and delicate stage, is more sensitive to the introduction of additives comprising groups capable of reducing at low potential such as phosphonium. Figure 8 shows the electrochemical performances as well as the gas volumes measured with a binary mixture (EC / EMC: 3/7 + 1M LiPF6) comprising (0%, 5% and 30%, by mass) of Phos1444 TFSI (named IL1). A significant increase in irreversible capacity is observed with the addition of ionic liquids (Figure 8 a,b) which goes from 13% (section 4.1) to 20% then 37% for 0%, 5% and 30 % of Phos1444 TFSI, respectively. The consumption of the electrolyte during the stage of SEI formation results in the generation of larger quantities of gas. For 5%, the quantity of gas measured increases by 30% compared to the reference without IL (Figure 8 The performance of pouch cells at different current densities is also correlated with the high generation of gas when ionic liquid is added (Figure 8 e). In fact, adding 30% of ionic liquid reduces the capacity by 50% at low C-rate (C / 1). This result is expected due to the size of the alkyl phosphonium cation and its diffusion in the electrolyte. In addition, its polarity promotes its adsorption on the surface of the graphite under load and catalyzes the reduction reactions.

(a) (

(f) 20

Effect of Pyrr14 TFSI on gas formation

Figure 9 shows the performance of Pyrr14 TFSI (denoted IL2) in the same compositions as Phos1444 TFSI (5% and 30%, by mass) and compares its performance with that of the reference, without ionic liquid. First of all, the electrochemical performances for the three compositions (0, 5 and 30%) are close to or slightly higher in favor of 5% Pyrr14 TFSI (Figure 9 e) with initial capacities of 160 and 140 mAh.g -1 at C/20 for 5 and 30% respectively (Figure 9 a, b), although the reversible capacity is greater (28 mAh.g -1 for 5%, and 47 mAh.g -1 for 30%). The addition of Pyrr14 TFSI 5% seems to limit the formation of gas (Figure 9 d,f) by 16% by reducing the volumes measured under the same conditions. For the largest quantity of ionic liquids, 30%, the gases formed at the first charge are 25% greater than those of the reference without ionic liquid, and this quantity of gas does not vary throughout the cycling (Figure 9 c). The addition of ionic liquids to the electrolytes is appreciated in the field of batteries because it helps to decrease the flammability of the electrolytes and increases the working temperature window, provided that it does not prevent cycling. From this point of view, Pyrr1444 TFSI plays its role perfectly for the two proportions tested, and Phos1444 TFSI to a lesser extent when 5% by mass is added in the electrolyte. In order to complete these quantitative results of the volumes of gas generated, an analysis of the composition of the gases collected in the pouch cells was carried out by gas chromatography coupled with mass spectroscopy (GC-MS). This analysis was performed several weeks after the cycling of the pouch cells had stopped, so we consider that the solubility equilibrium of the gases had been reached. The results for the two systems (EC/PC/3DMC + 1M LiPF6) and (EC/PC/3DMC + 1M LiTFSI) at three temperatures are collated in Figure 10.

Figure 10 GC-MS analysis of Gr//NMC pouch cells with (EC/PC/3DMC + 1M LiPF6) and (EC/PC/3DMC + 1M LiTFSI) cycled at different temperatures.

Composition analysis of the gas taken from a Gr//NMC pouch cell, cycled at 25 ° C. with (EC/PC/3DMC + 1M LiPF6), shows (Figure 10 a) that the most intense peak is at m / z = 44, which corresponds to carbon dioxide CO2. The ratio of the peaks at 27.9 and 31.9 is higher than the N2/O2 proportion in the calibration (air). The CH4 peak is also visible with a relative abundance of around 3% here. By comparison, the results obtained with LiTFSI show very little CO2 and CH4 even though the overall quantity of gas measured at the end of the cycling was equivalent for the two salts (2.4 to 2.9 ml). This confirms the hypothesis that we proposed previously, namely that a large part of the gas generated in the presence of LiTFSI salt is dissolved in the electrolyte and is therefore not detectable by GC-MS.

At 40 °C, the peaks found with LiPF6 are identical to those observed at 25 °C with higher intensities, in particular the peaks at m / z = 27.9 relative to C2H4 and / or (CO). The relative abundance of CO2 here is 65% compared to C2H4/CO. In the presence of LiTFSI, characteristic peaks at m / z = 16, 44, 51 and 69 are observed; they correspond to the presence of trifluoroacetamide originating from the degradation of the TFSI-anion. At 60 °C, the relative abundances increase. In the case of LiPF6, the most intense peak remains that of CO2, while for LiTFSI, the intensities of the CO2 peaks are lower than at 25 ° C or 40 ° C. This confirms the previous hypothesis that the solubility of gases in the presence of LiTFSI is favored by temperature. obtained in DMC were compared with those obtained by Kolar et al. [START_REF] Kolář | Prediction of gas solubility in battery formulations[END_REF], while the measurements obtained for CO2 and CH4 were carried out in our previous work [START_REF] Dougassa | Low pressure methane solubility in lithium-ion batteries based solvents and electrolytes as a function of temperature. Measurement and prediction[END_REF].

Gas solubility in electrolyte as a function of temperature

The very low solubilities of O2 and H2 compared to that of CO2 are similar in the two solvents. The results obtained in propylene carbonate show that the solubility of H2 and O2 decreases with temperature, as do CO2 and CH4. The solubility in both solvents increases in the following order: O2 <H2 <CH4 <CO2. In DMC, unlike CO2 and CH4, whose solubility decreases in this solvent, H2 and O2 show an opposite tendency. For CO2 and CH4, solubility is an exothermic process which decreases with temperature [START_REF] Self | Survey of Gas Expansion in Li-Ion NMC Pouch Cells[END_REF][START_REF] Shaahmadi | The CO2/CH4 gas mixture solubility in ionic liquids[END_REF][START_REF] Faúndez | Solubility of hydrogen sulfide in ionic liquids for gas removal processes using artificial neural networks[END_REF], whereas in the case of O2 and H2, dissolution in the alkylcarbonate corresponds to an endothermic process. These observations on the solubility of gases lead to two important conclusions for our study:

(i) the two gases responsible for the swelling of the salts are those which are the least soluble, and not necessarily the most abundant, i.e. O2 and H2; ( ii) the temperature affects the solubility of gases differently depending on the nature of the alkylcarbonate (PC or DMC).

The solubility of gases follows the order: CO2> CH4> C2H4 >> O2 ~ H2.

We also measured the solubility of (O2 and H2) in the ternary mixture EC/PC/3DMC with and without salts (LiPF6 and LiTFSI). It should be noted that for the two gases, in the ternary mixture without salts, there is a decrease in solubility with temperature. In the case of H2, the two salts have a "salting in" effect and correspond to endothermic processes favored by temperature. With O2, at low temperature, the solubility is greater with LiPF6 than with LiTFSI, but this tendency is reversed with the temperature from 40 °C. These solubility measurements tell us about the ability of the electrolytes to dissolve part of the gases formed during cycling to avoid excessive swelling of the cell pouches. We can summarize this trend by concluding that

LiTFSI is interesting at high temperatures and for systems that generate large amounts of gases such as cathodes based on transition metal oxides.

Conclusion

In this study, we evaluated the gas volumes in the Gr//NMC system for various electrolyte compositions as a function of temperature. An original device allowing the volumes of gas to be quantified at each cycle, and the analysis of their nature at the end of cycling allowed us to compare the systems studied with good repeatability and reliability. We evaluated the effects of salts, solvents and ionic liquids on the amount of gas formed. The results made it possible to determine the quantity of gas produced and to show that it is essentially linked to the formation of the SEI at the first charge. It is greater in the case of LiTFSI compared to LiPF6

(2.8 mL vs 1.9 mL). These observations on the quantities of gas were correlated with the solubility of gases in the electrolyte. LiTFSI allows a "salting in" effect which physically results in the wilting of the pouch cells and a reduction in the volume of gas during cycling.

This phenomenon, attributed to a better solubility of the gases produced in the electrolyte in the presence of this salt, is accentuated with temperature. Analysis of the gas composition by GC-MS revealed a predominance of CO2 and CH4 gases and confirmed the effect of salt in the quantity and composition of the gas mixture. In conclusion, ILs and LiTFSI are interesting at high temperatures and for systems that generate large amounts of gases such as cathodes based on transition metal oxides.

Figure 1 .

 1 Figure 1. Device for measuring pouch cell volume (a), weighing basket (b), Photos showing two pouch cells according to their swelling state (c, d).

Figure 2 .

 2 Figure 2. Different elements of a pouch cell (a), Torque wrench and pressing system (b), photo of a 4 pouch cells system pressed between two plates at a pressure of 2N.m -1 (c)

Figure 3 .

 3 Figure 3. Comparison of galvanostatic profiles obtained for the first cycle in the case of four different Gr // NMC pouch cells (a) and repeatability of volume measurements (b).

Figures 4a and 4b

  Figures 4a and 4bcompare the galvanostatic profiles of pouch cells comprising the two binary mixtures (EC / DMC: 1 /1 + 1M LiPF6) and (EC / EMC: 3/7 + 1M LiPF6) at different regimes. For each electrolyte two parallel measurements were made and are represented by the different cell numbers, for example Cell 202 and 203 in the case of EC/DMC as shown in (Figure 4c).The electrochemical performances obtained for the two electrolytes based on a binary mixture of alkyl carbonates are very close.

Figure 4 .

 4 Figure 4. Comparative electrochemical and gas volume characterizations for Gr//NMC with (EC/DMC: 1/1 + 1M LiPF6) and (EC/EMC: 3/7 + 1M LiPF6).

7 +

 7 to a C/5 regime (cycle 23) was high (> 99%). At high current density, (C/1), the capacity remained stable around 120 mAh.g -1 . During the formation of the SEI on graphite in the first cycle, the average volume of gas released in the case of the cell based on (EC / DMC: 1/1 + 1M LiPF6) was 2.62 ± 0.03 mL ( Figure 4 e), 31% higher than in the case of (EC / EMC: 3/1M LiPF6) (Figure4 f). The gas is mainly formed during the first charge. The cumulative volume dropped slightly during the cycles. This phenomenon is more visible on the representation of the volume generated per cycle (Figure4 g, 4 h), and can be attributed to the slow dissolution of the gas released in the electrolyte as well as to the reactivity of less soluble gases such as O2 in favor of CO2, which is more soluble (eq. 4). No significant change was observed at 40 ° C for the two electrolytes. After 27 cycles, the pouch cell containing (EC / DMC: 1/1 + 1M LiPF6) was 51% more swollen than that which cycled with (EC / EMC: 3/7 + 1M LiPF6). There is a definite advantage in working with EMC instead of DMC for obvious safety reasons.

Figure 5 .

 5 Figure 5. Comparative electrochemical and gas volume characterizations for Gr//NMC with (EC/PC/3DMC + 1M LiPF6) and (EC/PC/3EMC + 1M LiPF6).

Figure 6 .

 6 Figure 6. SEM images at the same magnification (x 20.000) and associated EDX of a graphite electrode cut of a pouch cell of the Gr//NMC system at different stages of the first cycle in the presence of (EC/PC/3DMC + 1M LiPF6).

Figure 6

 6 presents the SEM and EDX (a) Before cycling (b) After the first charge (c) After the first discharge radiographs of the graphite surfaces cut on pouch cells (a) before cycling (b) at the end of the first charge and (c) at the end of the first discharge.

Figure 7

 7 Figure 7 compares the gases measured during the cycling of two ternary mixtures of EC/PC/ 3DMC solvents comprising either 1M LiPF6 or 1M LiTFSI at two temperatures, 25 °C and 40 °C. The first SEI training cycle was always carried out at 60 ° C. During this first cycle,

Figure 7 .

 7 Figure 7. Comparison of variations in the volume of Gr//NMC pouch cells with EC/ PC/3DMC + 1 mol.L -1 LiX (X = PF6 or TFSI) as a function of temperature.

  c, f), i.e. 2.65 mL and 2.02 mL. With 30% ionic liquid (Figure8 d), the quantity of gas formed is almost multiplied by 3, with 5.78 mL (Figure8 d).

Figure 8 .

 8 Figure 8. Comparative electrochemical and gas volume characterizations for Gr//NMC with (3EC/ 7EMC + 1M LiPF6) with 5 % and 30 % in mass of Phos1444TFSI (denoted IL1).

Figure 9 .

 9 Figure 9. Comparative electrochemical and gas volume characterizations for Gr//NMC pouch cells with 1 mol.L -1 LiPF6 in 3EC /7EMC with 5 % and 30 % in mass of Pyrr14TFSI (denoted IL2).

4. 5 .

 5 Analysis of Gases on pouch cell and their solubility in the electrolyte 4.5.1. Nature of the gases as a function of temperature

Figure 11

 11 Figure 11 depicts the measured solubilities of oxygen and of dihydrogen expressed in the Henry constant, obtained in pure propylene carbonate (figure 11 a) and dimethyl carbonate (figure 11 b) as a function of temperature. From the four solvents used, we chose a cyclic alkylcarbonate (PC) and a second linear alkylcarbonate (DMC) for comparison. The results

Figure 11 .

 11 Figure 11. Solubilities of O2 and H2 measured in PC (a) and DMC (b), obtained in this work and compared with [41] (named ref) and comparison with the solubilities of other gases [18, 20, 21].

Figure 12 Figure 12 .

 1212 Figure 12. Effect of the lithium salt on the solubility of H2 (a) and O2 (b) in the ternary mixture EC / PC/3DMC 1 M LiX (X = PF6 or TFSI).

Table 1 .

 1 Name, structure, and charge distribution of the alkylcarbonate used in this study

	EC	PC
	Cyclic
	alkylcarbonates
	Linear
	alkylcarbonates

EMC DMC

Table 2 .

 2 Name, structure, and charge distribution of the ions for salts and ILs used in this

	study
	Salt's anions	PF6 -	TFSI -
	Name	Hexafluoro phosphate	bis (trifluoro-sulfonyl) imide
	Structure
	Charge
	distribution
	Liquid ionic's cations	Pyrr14 +	Phos1444 +
	Name	Methyl butyl-pyrrolidinium	Methyl tributyl-phosphonium
	Structure
	Charge
	distribution

Table 3 .

 3 Gr//NMC pouch cycling program

	Cycle	C-rate	Temperature	∆E range (V)
	1	C/20	60 °C
	2-6	C/5	25 °C
	7-16	C/1	25 °C	2.9 -4.2
	17-22	C/2	40 °C
	23-27	C/5	25 °C

4.1. Comparative effect of DMC and EMC on gas generation

2.39 2.41 2.46 2.53 2.47 2.60 2.49 2.62 2.59 2.80 2.63 2.62

gas volume (mL) state 302 (EC:PC:3DMC) 303 (EC:PC:3DMC)

	60°C	60°C					40°C
	2.48	2.52	2.56	2.52	2.53	2.44	2.53	2.41	2.56	2.45	2.55	.48
												0,9
												0,6
												0,3
												0,0
												C/20 C	C/20 D	C/5	C	C/2	C/5

40°C 60°C gas volume (mL) State 300 (EC:PC:3EMC) 301 (EC:PC:3EMC) 60°C

	2.02	1.97	1.96	1.95	1.88	1.84	1.78	1.76	1.82	1.83	1.77	.79

1.54 1.45 1.59 1.51 1.60 1.55 1.62 1.52 1.72 1.60 1.77 40°C 60°C gas volume (mL) State 500 (3EC:7EMC) 5% IL2 501 (3EC:7EMC) 5% IL2

		3,6 4,0			503 (3EC:7EMC) 30% IL2 502 (3EC:7EMC) 30% IL2		3,6 4,0
	gas volume (mL)	2,0 2,4 2,8 3,2	2.75 60°C 2.59	2.82 60°C 2.60	2.52	2.76	2.54	2.80	2.54 2.82 40°C	2.80 0% 2.55	1,2 1,6 2,0 2,4 2,8 3,2
											0,8
											0,4
											0,0
											C/20 C	C/20 D	C/5	C	C/2	C/5
						state

Acknowledgements

The authors would like to thank "La Région Centre Val de Loire" and "Le Studium Loire Valley Institute for Advanced Studies" for financial support through the Obama project under the Lavoisier regional program. We would also like to thank Dr. Pierre-Yvon Raynal at the


N V Emelyanov

A E Drozdov

V Emelyanov

Asteroid satellite ephemeride service. Mutual occultations and eclipses

Keywords:

A tool has been developed for computing the ephemerides of asteroid satellites, which are needed to plan and verify new observations, as well as for space missions.

The proposed ephemeris server is available online at http://www.sai.msu.ru/neb/nss/html/multisat/nssAsthe.htm and http://nsdb.imcce.fr/multisat/nssAsthe.htm Motion models for 62 asteroid satellites are included, which are computed based on all available observations. The proposed ephemeris server can provide relative satellite positions for a series of time instants specified by the user. The server displays the time intervals of mutual occultations and eclipses in asteroid systems with satellites, as well as times of satellite elongations, when the observing conditions are most favorable.

The circumstances of all mutual occultations and eclipses in the satellite systems are determined. During the years 2021-2025, events are to occur in 21 of the 62 systems considered. The plots show the time intervals of the phenomena.

The reliability of the computed ephemerides is verified by comparing their predictions with the results published by other authors.

We use our original method to estimate the accuracy of asteroid satellite ephemerides. Thus the asteroid satellite ephemeris server provides not only ephemerides but also the uncertainties of the predicted satellite positions. The low accuracy of the ephemerides of some satellites is due to the fact that their orbits are determined from a small number of observations spanning a short time interval.

Our tool can be used with more accurate orbital parameters and for new satellites to be discovered in the future. We hope to work on this initial release and to consider adding updates including refined astrometry and additional systems.

Introduction

The knowledge about the dynamics of asteroid satellites is in demand for space missions. It is also needed for the theory of the formation and evolution of the solar system. This knowledge is obtained by building satellite motion models based on observations. The models have the form of satellite ephemerides, which allow the relative position of objects to be computed for whatever times in the future and past; they are also needed to plan new observations and validate the results obtained. Furthermore, ephemerides allow the time intervals and circumstances of mutual occultations and eclipses in asteroid systems with satellites to be calculated in advance. Photometric observations of these events are needed to provide additional data on mutual satellite positions.

In this work we develop a tool for providing ephemerides of asteroid satellites to any user online via the Internet. The tool computes Keplerian orbits of satellites with the parameters of the ephemerides inferred from all available observations [START_REF] Emelyanov | Orbit determination for sixty two moons of asteroids based on astrometric observations[END_REF].

We explain the choice of satellites for ephemerides in the next section.

We compute the ephemerides of 62 asteroid satellites and provide them online via the ephemeris server. The server offers remarkable features including the detection of mutual occultations and eclipses of a satellite with their asteroid.

Our ephemeris is based on observations that have errors, which results in that the ephemeris have errors too. We pay special attention to obtaining estimates of the ephemeris errors. These estimates are obtained by generating 1000 samples of the orbital parameters by the Monte Carlo method using the covariance matrix of parameters as described in the paper [START_REF] Emelyanov | Precision of the ephemerides of outer planetary satellites[END_REF].

This paper has the following layout. In the first part we explain the selection of the satellites with motion models incorporated in our ephemeris server. In the second part we describe the tools that we provide for deriving the ephemerides of asteroid satellites. The third part is dedicated to the ephemerides of mutual occultations and eclipses in asteroid-satellite systems. The following sections describes the estimates of the uncertainties of the predicted satellite positions and how they can be obtained via the ephemeris server.

Asteroid satellites included in the ephemeride service

We selected the satellites for which suitable motion models are available and perturbing factors can be neglected. In our previous study [START_REF] Emelyanov | Orbit determination for sixty two moons of asteroids based on astrometric observations[END_REF], we analyzed perturbing factors and evaluated the suitability of the Keplerian model. We reproduce here some of the results. The perturbing factors considered are : the non-sphericity of the primary, the gravitational influence of the Sun and other satellites. Our estimates show that short-period perturbations due to these factors are completely small and negligible. Perturbations from the Sun may be significant only for several satellites. We published the values of the parameter characterizing these perturbations in [START_REF] Emelyanov | Orbit determination for sixty two moons of asteroids based on astrometric observations[END_REF]. Long-period perturbations due to the oblateness of bodies are absent. Only secular perturbations remain, which appear as the precession of the orbit around some fixed axis. The position of this axis is determined from observations. To determine the coordinates of this axis the period of observations has to be comparable with the precession period. These perturbations are significant only in the systems of 22 Kalliope, 45 Eugenia, and 87 Sylvia. Our analysis shows the orbital precession for the last two satellites to be too small to allow the determination of the coordinates of the symmetry axis of the primary. An analysis of 22 Kalliope [START_REF] Emelyanov | Dynamics of the binary asteroid system (22) Kalliope-Linus based on observations Mon[END_REF] shows that the axis of the precession cone lies in the range of 1.5 degrees and our attempts to determine the precession from observations were unsuccessful. Further details can be found in [START_REF] Emelyanov | Dynamics of the binary asteroid system (22) Kalliope-Linus based on observations Mon[END_REF].

Asteroids with two satellites having noticeable mutual attraction are excluded from consideration.

Our program for computing the orbits in the ephemeris server can adopt any model of precessing elliptical satellite motion with no special action required for this.

We ultimately selected only those satellites for which the adopted Keplerian model of motion is suitable and disturbing factors can be neglected. We actually selected the orbits of 62 asteroid satellites, 13 of which belong to the main belt, two asteroids are Trojans moving in the orbit of Jupiter, and the remaining ones are trans-Neptunian objects.

We calculate the ephemerides using the orbital parameters determined in our previous study and published by [START_REF] Emelyanov | Orbit determination for sixty two moons of asteroids based on astrometric observations[END_REF]. These parameters are available in electronic form and can be taken from the file attached to this cited paper as Supplementary material.

Features of the proposed asteroid satellite ephemeride server

The asteroid satellite ephemeris server is a a computer program that can be launched by a user from the Internet at http://www.sai.msu.ru/neb/nss/html/multisat/nssAsthe.htm or http://nsdb.imcce.fr/multisat/nssAsthe.htm

The Cartesian coordinates of the satellite relative to the asteroid at given times are calculated using the known parameters of the Keplerian orbit that we determined earlier from astrometric observations [START_REF] Emelyanov | Orbit determination for sixty two moons of asteroids based on astrometric observations[END_REF].

The heliocentric coordinates of the asteroid are calculated with our special program EMEASTER from the geocentric angular coordinates of the asteroid obtained with the Miriade ephemeris server (IMCCE, France) available at http: //vo.imcce.fr/webservices/miriade/? forms.

The user can choose one of the 62 satellites on the request page, set the necessary parameters, send the request and receive the satellite ephemeris on a separate page. The request is made by filling the appropriate data entry fields in several menus.

The user manual with a detailed description of the features of the server is available via the link given on the main page of the server. This paper is not a proper place for the user manual, but it would be good to learn some of the important features of the server before staring to use it.

The ephemeris is produced on a separate page that appears when the request is submitted. At the top of that page some data about the system is displayed, including the satellite orbit parameters followed by the ephemeris table.

The user selects one of four ephemeris types: -ephemeris with a constant time step, -ephemeris in 1 day step only for the times when the inclination of the orbital plane (the tilt) to the direction toward the Sun or observer is such that mutual occultations and eclipses are possible, -ephemeris with the time step selected by the program only for the times when mutual occultations and eclipse occur, -ephemeris for the times of maximum satellite elongation.

The user can choose a point on the Earth (IAU observatory code) for which ephemeris is displayed. The output table gives the right ascension and declination of the asteroid and various satellite coordinates (topocentric and heliocentric) relative to the primary component for the time instants of the ephemeris.

Two special features of the ephemeris server are most important. The first is that the server can out the times of the beginning, the maximum phase, and the end of each mutual occultation and eclipse in the asteroid system with the satellite. These times are necessary for organizing and performing photometric observations of the events.

The second is the output of the times of maximum separation between the satellite and asteroid (maximum elongation) when the conditions are most favorable for astrometric observations.

The purpose of obtaining asteroid satellite ephemeris can be to compare it with the results of new observations. Inevitable deviations of the relative satellite coordinates from its ephemeris provide very valuable information, which should be used to further adjust the satellite motion model.

Circumstances of mutual occultations and eclipses of asteroid and satellite

Photometry of asteroids with satellites during their mutual occultations and eclipses is an efficient source of positional data about the satellites. However, these events are rare and observers and researchers look for each such occasion.

It is clear that the geocentric and heliocentric directions pass through the plane of the satellite orbit twice per revolution of the asteroid around the Sun, and these configurations lead to the events considered. However, such cases are rare for trans-Neptunian objects (TNO) with long orbital periods.

Our newly developed ephemeris server provides the circumstances of all mutual occultations and eclipses in asteroid-satellite systems for the time interval 2021-2025. According to our calculations, mutual occultations and eclipses are to occur in 21 of the 62 systems included in the ephemeris server.

For observations of mutual occultations and eclipses of the asteroid with satellite it is important to know the times of the beginning and end of each phenomenon. It is this data that the ephemeride server provides for each phenomenon. The time intervals of a mutual occultation and mutual eclipse may overlap. The duration of each phenomenon can range from several minutes to 3 hours and no phenomena are to occur for many satellites in the time interval from 2021 to 2025 inclusive.

To determine the times of the phenomena our prorgam calculates the topocentric or heliocentric angular distance between the centers of the bodies and compares this distance with the sum of the radii of the disks of the bodies, which are assumed to be spherical.

It would be interesting to see an event schedule for each satellite as illustrated in Fig. 4.

During some events the asteroid may be located in the sky close the sun making observations difficult to perform. The thick and thin lines in Fig. shows the graphs of the time intervals, when phenomena occur without and with interference from the sun, respectively. The adopted criterion of the interference from the sun is that the topocentric angular distance between the asteroid and the sun is less than 50 degrees.

The graphs show that for some satellites mutual occultations and eclipses occur almost every year during 10 months. For other satellites, these phenomena are rare. Graphs allow you to select an object and plan observations.

The ephemeris tool used for predicting the events gives us the most probable satellite positions corresponding to the observations used to derive the ephemeris. However, the prediction of the events may be unreliable because of the limited precision of the ephemerides. We estimate the precision of our ephemerides as described in the special sections 6 and 7 below. Based on these estimates we attribute a quality index for the resulting event predictions and provide it in Table 1. See the explanation of the meaning of the index in the caption to the table.

More detailed estimates of the accuracy of phenomena prediction can be obtained using the proposed server as described at the end of the Section 7.

Note that we give overestimates predicted circumstances of mutual events for the reasons described in the Section 6 below. Here N is the number of the system in Table 1.

Comparison with the results of Grundy et al. (2018)

Grundy et al. ( 2018) calculated the circumstances and ephemerides of mutual occultations and eclipses in the system of asteroid (617) Patroclus, a Trojan in the orbit of Jupiter, and its satellite Menoetius. We also calculated the ephemerides of the phenomenona in this system for the same dates as in [START_REF] Grundy | The Upcoming Mutual Event Season for the Patroclus -Menoetius Trojan Binary[END_REF] and can now compare results and check our calculations.

The parameters of our solution for the orbit of the satellite Menoetius includes eccentricity, which we find to be 0.0051 ± 0.0068 (1 sigma) based on observations. This is a formal solution with the range including zero value. [START_REF] Grundy | The Upcoming Mutual Event Season for the Patroclus -Menoetius Trojan Binary[END_REF] computed the ephemerides of the phenomena for the case of circular orbit. For our comparison to be correct we had to compute a special solution inferring the parameters of the circular orbit of the satellite from observations and calculating the ephemerides for this case.

We use our tool to calculate the times of beginning and end of the phenomena, as well as their duration, for all the cases given in [START_REF] Grundy | The Upcoming Mutual Event Season for the Patroclus -Menoetius Trojan Binary[END_REF]. We then determine the differences between our results and those of [START_REF] Grundy | The Upcoming Mutual Event Season for the Patroclus -Menoetius Trojan Binary[END_REF] including the durations of the phenomena. Figs. 2 and3 shows the results of our comparison for mutual occultations and mutual eclipses, respectively.

As is evident from the figures, the two sets of calculations compared yield ephemerides that are very close to each other. Small differences can be explained by the fact that in our algorithm ephemeris times are in increments of 7 minutes and do not coincide with the exact times of the beginning and end of the phenomena. The comparison leads us to conclude that both our satellite motion model and the algorithm of detection of mutual occultations and eclipses Asteroids and satellites sorted by number in the Fig. 1. Semi-major axis in astronomical units is indicative of the type of the asteroid. Q is the ephemeris quality established in the following way: if the errors measured along the apparent relative track of satellite exceed the value 0.5 of the semi-major axis of the orbit on the periode 2022-2025, then the ephemeris is considered unreliable and the sign "-" is assigned to the index Q. The sign "!" is assigned to the index Q in opposite case. The values , , are the average radius of the asteroid, radius of the satellite and the uncertainty of the sum of these radii correspondingly adopted in this work (see the Section 8).

N

Some considerations for estimates of ephemeris precision

To simulate mutual phenomena, we made assumption that both bodies are spheres and their visible images are uniform disks. We know, however, that the real bodies are non-spherical. The circumstances of mutual phenomena might change if this fact is taken into account. However, for most of the considered asteroids with satellites, there is no information about the orientation of these elongated bodies at the moments of the ephemeris. In the cases when there are data on asteroid rotation, they are not reliable. Thus, we have no choice but to follow the assumptions made.

Observation errors are random values. This results in that the ephemeris based on these observations are also random. The least squares method gives us the most likely ephemeris. However, because of ephemeris uncertainties, real coordinates may significantly differ from those obtained from the ephemeris. It is inevitable that predictions of mutual phenomena have limited certainty. If, during the observation, the phenomenon did not take place, it dos not mean that the observers wasted their time as such information is also valuable because it can be used to refine the orbit. On the contrary, if we do not observe the phenomenon that has taken place, we will lose valuable information. Thus, we prefer to give overestimated predicted circumstances of mutual events. Due to random observation errors, there is a possibility that real events could not be predicted. Such situations are inevitable. When some mutual phenomena are not predicted but observed this carries additional valuable data that serve to refine the ephemeris.

It is evident that the results of observations have errors. Each measured coordinate is a random value. Hence, the parameters of the orbit found from observations are random too. The ephemeris obtained from such an orbit are also random. The least-squares method gives us the most likely ephemeris. But there are other possible random realizations of ephemeris. We give variations to the parameters by the Monte Carlo method. Using the parameter covariance matrix, we get the ephemeris variations. Such variations provide us with the error estimates for the ephemeris. For this end, we follow the method described by [START_REF] Emelyanov | Precision of the ephemerides of outer planetary satellites[END_REF].

We do not take into account the solar phase that also leads to some little overestimation of the event prediction.

We give an ephemeris based on the observations used. See the table given in the supplementary material to our article [START_REF] Emelyanov | Orbit determination for sixty two moons of asteroids based on astrometric observations[END_REF]. For 35 of the 62 satellites, we have by 12 or less measured positions for each satellite. This is with seven refined parameters. For most satellites, observations cover only a small part of their orbits. Under these conditions, we cannot count on reliable ephemeris. New observations are necessary which could be used to refine our ephemeris. Our ephemeris server could be appropriate for this purpose.

Estimates of ephemeris precision

The observations from which the asteroid satellite motion models were derived have errors and span short time intervals for some satellites. These circumstances lead to errors in the resulting ephemeris. Estimating the errors of an ephemeris is a difficult task. The least-squares method used to determine the parameters of satellite orbits also yields estimates of parameter errors, which, however, do not allow determining the errors of the ephemeris.

Emelyanov (2010) offers three methods for evaluating ephemeris errors arising due to the errors of the underlying observations. In any case, one needs to know either the accuracy of the observations or the covariance matrix of parameter errors. The required matrix is computed in the process of determining parameters from observations. When applying the proposed methods it is assumed that observation errors are random in nature. We have no choice but to adopt this hypothesis. We also assume that the Keplerian motion model corresponds to the accuracy of observations. Any actual violation of the assumptions made can only increase the errors of the ephemerides.

Given these circumstances, we evaluate the accuracy of the ephemerides by the method proposed in [START_REF] Emelyanov | Precision of the ephemerides of outer planetary satellites[END_REF] using the covariance matrix of parameter errors. The method gives us estimates of the accuracy of satellite coordinates at any given time instants. It is very important how the accuracy estimates are presented. First, the ephemeris errors along the satellite trajectory are more important. Secondly, absolute coordinate errors should be related to the size of the satellite orbit.

The current asteroid satellite ephemeris server provides accuracy estimates in the form of a table that contains errors of the satellite's apparent positions projected along and across the path of apparent relative motion. A special column of the table gives the angular value of the apparent semimajor axis of the satellite's orbit. Finally, the server provides errors along and across the trajectory, expressed in the units of the semi-major axis.

In order to have a clear idea of the accuracy of the ephemeris, the server provides a graph of the errors along the trajectory, expressed in the units of the semi-major axis. A link to this graph appears as soon as any request is made for the ephemeris of a satellite.

The server uses a special sequence of time instants. It takes points on one revolution of the satellite. Then it makes a roll for days, again tracks the satellite's rotation, and so on. This is necessary to track the behavior of the error on one revolution and show the ephemeris errors over a long time interval. The server selects and individually for each satellite. The time interval spans from the time of the first observation to January 1, 2025.

Examples of graphs for assessing the accuracy of ephemerides are shown in Figs 4, 5, 6. As is evident from Figs. 4 and 5, the ephemeris errors for these satellites are much smaller than the sizes of their orbits. These ephemerides can be used at least over the time interval until 2025. For satellite Pichi unem of the asteroid (702) Alauda the ephemeris becomes useless after 2015, as the satellite position errors become as large as the size of its orbit. This is so because the observations cover a very short time interval.

We summarize the uncertainties of the ephemerides for all 62 asteroid in a table, where for each of the 62 satellites, the maximum uncertainties of the predicted satellite positions are given for each of three time intervals: Jan 1, 2022 -Dec 31, 2023, Jan 1, 2023 -Dec 31, 2024, Jan 1, 2024 -Dec 31, 2025. The table is attached to this paper as Supplementary material and is available online. Excerpts from this table are given here in Table 2. Satellites with the best ephemerides are listed at the top of the table, and those with the poorest ephemerides are listed at the bottom. As is evident from the table, the accuracy of the ephemeris depends significantly on the length of the time interval and the number of observations used to determine the orbit.

Estimating ephemeris uncertainties for predicted mutual phenomena requires special treatment. We call here an apparent position and apparent velocity of the satellite always counting them relative to the main body. The error of the moment of the phenomenon is determined by the error of the relative position of the satellite calculated along the apparent trajectory. The position error across the trajectory determines the depth of the phenomenon, that is, the drop of the light flux from the system. With a certain level of error, the phenomenon may not occur at all. We estimate the uncertainty for mutual phenomena in the following way. First, the uncertainty for the moment of the phenomenon is calculated using the formula = ∕ where is the uncertainty for the relative position along the apparent trajectory, is the apparent velocity of the satellite along the apparent trajectory. Due to an error in the relative position of the satellite at the time of the predicted maximum of the phenomenon, the phenomenon may not occur. We calculate the relative distance of the satellite for each random ephemeris sample. Each time we record whether an event has occurred or not. For all the samples, the probability of whether the event takes place is calculated. This is one more estimate for the uncertainty in predicting the phenomenon. The values and are given as part of the output of the ephemeris. To get this, it is necessary to choose the first year of the mutual event ephemeris 2021, since the uncertainties were calculated in advance beginning from 2021.

Taking into account the shape and size of bodies when calculating the ephemeris of mutual occultations and eclipses

First of all, it should be noted that by now there are very few data on the shapes of asteroids with satellites and about rotation of these bodies. Thus, it is hard to evaluate their influence upon ephemeris. Additional difficulties arise from complex dependence of moments of the phenomena on both the orbital and physical parameters of the bodies.

The primary of an asteroid-satellite system can have non-spherical shape. It can be approximated by the triaxial ellipsoid with semiaxes , , . At the current level of observations, shapes of the satellite cannot be surely determined so we consider it to be spherical.

Suppose that the orientation of the main body in space is known at the time of the ephemeris. Then the primary's shape can be projected on image plane, that is on a plane perpendicular to the line of sight. Projection of the triaxial ellipsoid (its shadow) will always be an ellipse. We denote the semiaxes of this ellipse on the image plane as and .

Let the axis be directed from the center of the primary to the East, with axis pointing to the North. We denote the angle between the axis of the ellipse and the direction from the primary to the satellite as . The configuration is shown in Fig 7.

We introduce the coordinate system , with the origin in the ellipse's center, the axis being directed along the semiaxis . At the beginning or end of the phenomenon, the projection of the satellite will look like a circle touching the ellipse at some point with the coordinates , . Let , be the coordinates of the satellite center. With given , , , , the coordinate of the touch point is determined from the the algebraic equation of the fourth degree

(1 - 2 2 ) √ 2 -2 + = √ 2 -2 . ( 1
)
The value 0 ≤ ≤ can also be calculated by using the dichotomy method to solve the equation

( ) = (1 - 2 2 ) √ 2 -2 + - √ 2 -2 = 0.
with (0) < 0, ( ) > 0.

Table 2 The uncertainties of the ephemeris predictions (Excerpt of the full table available online). Maximum values are selected during each year. The errors are measured along the apparent relative track of the satellite and normalised to the current value of the apparent semi-major axis. Here is the time interval of observations (in years) and is the number of satellite positions used to fit the orbital parameters. The ephemeris provide us with the functions ( ), ( ). For the moments of the beginning or end of the phenomenon we have

Asteroid

( -) 2 + ( -) 2 = 2 , ( 2
)
where is the satellite image radius. At these moments, the distance between the centers of the bodies, , is determined by the relation 2 = 2 + 2 .

In fact, the angle of rotation of the main body is unknown. We could not find in publications the models where the rotation angle could be obtained for any moment of time. The only thing that can be found in publications is the coordinates of the rotation axis of the main body for some asteroids with satellites. The reliability of these data is low.

The shape, that is the semiaxes of the approximating ellipsoid, are known only for some of the objects in consideration. For the objects listed in the Table 1, the semiaxes , , are known only for the first six ones. Therefore, we have to consider the angle in Fig 7 as a random value varying in the range 0 ≤ < 2 , considering the direction from the primary to the satellite to be constant.

We have calculated the average value of and the corresponding rms for those six asteroids with satellites for which the semiaxes , , are known. Since the orientation of the bodies is unknown, to overestimate the values for ( ), we took to be equal to the maximum of the semiaxes , , , while is equal to the minimum of them. With the given value for , the value for was calculated by iterations using the relations (1) and ( 2), assuming in the zero approximation = √ 2 + 2 + 2 and taking into account the relations = cos( ), = sin( ). Then, the averaged value and its uncertainty were calculated. In fact, we assumed that the main body is a sphere with the radius = -2 determined with the uncertainty = . The initial data for asteroids with satellites were taken from [START_REF] Johnston | Asteroids with Satellites[END_REF]. This database gives also the values for the satellite radia, , and the corresponding uncertainties, . For the systems for which the sizes of the primary , , are given in [START_REF] Johnston | Asteroids with Satellites[END_REF], we have defined and in the way described above. For other systems, and were taken directly from [START_REF] Johnston | Asteroids with Satellites[END_REF]. Then we used the sum = + as a random variable with uncertainty = √ 2 + 2 . The values , , adopted in our work are given in the Table 1. In most cases when mutual occultations and eclipses in the systems of asteroids with satellites are observed, in the beginning and in the end of the event the components either approach or move away from one another. Errors in the moments of phenomena arise due to the errors of orbital parameters, as well as because of the uncertainty of the mutual distance . That is why the ephemeris server generates the value which is calculated using the relation = ∕

, where is the relative velocity of the satellite along the apparent relative trajectory. It is the uncertainty which arises because of uncertainty in the sizes and shapes of the components. The value for is given separately in the output of the server.

Note that, when calculating the probability of whether the event takes place or not (see above), the mutual distance between the bodies was taken as a set of 300 samples of a random variable with uncertainty . Thus, this probability takes into account not only errors of the model arising from the errors of observations but also the errors in the sizes and shapes of the objects.

As result, if the year 2021 is chosen as the initial date of the ephemeris of mutual phenomena, in the line with the moment of apparent approach of the satellite to the asteroid, near the "max" code, the ephemeris server will output three values: , , .

Conclusions

We developed an asteroid satellite ephemeris calculation tool -an ephemeris server, which we propose here for the use by the community. Ephemerides are calculated with motion models based on all available observations. The satellite orbit parameters were found earlier and published in [START_REF] Emelyanov | Orbit determination for sixty two moons of asteroids based on astrometric observations[END_REF].

The server includes motion models of 62 asteroid satellites. When requesting an ephemeris the user can choose from different ways of setting the ephemeris time instants and various types of satellite coordinates. The ephemerides of the satellites include the right ascension and declination of the asteroid. The server also allows the user to determine the conditions of visibility of an asteroid at any observatory from IAU list.

Ephemerides are needed for new observations and inevitable deviations of the results of new observations from the computed ephemeris provide valuable information for further refinements of satellite motion models.

Of particular value are photometric observations of mutual occultations and eclipses in asteroid systems with satellites. These phenomena occur rarely and not in all systems. The ephemeris server gives the beginning and end time instants for each such event. In this study we calculate the time periods of the phenomena and show the corresponding graphs for the time interval from 2021 to 2025. The graphs allow the user select an object and plan observations. We found that mutual occultations and eclipses are to occur for 21 of the 62 satellites considered.

To test the method for determining the ephemeris of asteroid satellites, we compared our computed times of mutual occultations and eclipses for the system of asteroid (617) Patroclus and its satellite Menoetius with the results obtained by [START_REF] Grundy | The Upcoming Mutual Event Season for the Patroclus -Menoetius Trojan Binary[END_REF]. We conclude from the comparison that our satellite motion model and algorithm of detection of mutual occultations and eclipses are correct and yield results that are very close to those of [START_REF] Grundy | The Upcoming Mutual Event Season for the Patroclus -Menoetius Trojan Binary[END_REF].

The ephemerides obtained from the server have unavoidable errors. The main source of ephemeris errors is the inaccuracy of observational data. The accuracy of the ephemeris degrades over time. The shorter the observational arc or time interval, the larger the degradation rate. As a result, the ephemeris provided by the server should be considered as the most probable satellite positions found based on available observations. Estimating the accuracy of an asteroid satellite ephemeris is a particularly difficult problem. We solved this problem using the method described in [START_REF] Emelyanov | Precision of the ephemerides of outer planetary satellites[END_REF]. Thus, the asteroid satellite ephemeris server provides not only the ephemerides but also the uncertainties of the predicted satellite positions.

Our analysis of the precision of the ephemerides for the entire ensemble of 62 asteroid satellites leads us to the following conclusions.

In the time interval until 2025 the ephemerides for 13 satellites are unreliable. They need to be searched for again because we do not know on which side of the asteroid the satellite will be in the near future. For 28 satellites, the errors of the ephemerides until 2025 amount to 0.1-0.5 of the semimajor axis of the orbit. Only 21 satellites have reliable ephemerides with errors not exceeding 0.1 of the the apparent semi-major axis of the orbit. For nine of them the ephemerides are fairly accurate with errors no greater than 0.02 of the apparent semi-major axis of the orbit.

For 9 satellites from the list in the Table 1, the server shows that the probability of detecting a mutual phenomenon does not exceed 0.2. However, you can count on the predicted moment of the phenomenon as the most likely.

Obviously, the low accuracy of the ephemerides of some satellites is due to the fact that their orbits are determined from a small number of observations spanning a short time interval [START_REF] Emelyanov | Orbit determination for sixty two moons of asteroids based on astrometric observations[END_REF] as is evident from Table 2.

Although the accuracy of the ephemerides of asteroid satellites is currently not high, we plan to maintain the functionality of the ephemeris server and improve the accuracy as new observations appear.

This tool is open for more precise orbital parameters and new satellites to be discovered in the future. We hope to work on this initial release and to consider adding updates of astrometry and additional systems.

The proposed tool can be used with more accurate orbital parameters and applied to new satellites that will be discovered in the future. We hope to work on this initial release and to consider further upgrading it by using refined astrometry and incorporating additional systems.

We also plan to publish a detailed description of the method for assessing the accuracy of ephemerides, which we used for asteroid satellites.

Figure 1 :

 1 Figure1: Periods of the mutial occultations and eclipses in the systems of asreroids with satellites. Here N is the number of the system in Table1.

Figure 2 :

 2 Figure 2: Comparison of the ephemerides of the mutual occultations in the system of asteroid (617) Patroclus with the results of Grundy et al. (2018).

Figure 3 :

 3 Figure 3: Comparison of the ephemerides of the mutual eclipses in the system of asteroid (617) Patroclus with the results of Grundy et al. (2018).

Figure 4 :

 4 Figure 4: Uncertainty -standard deviation ( ) -of the ephemeris of asteroid (87) Sylvia's satellite Remus, calculated along the apparent relative trajectory of the satellite and expressed in the units of the apparent semi-major axis of the orbit. The bold line on the axis shows the observation time interval.

Figure 5 :

 5 Figure 5: Uncertainty -standard deviation ( ) -of the ephemeris of asteroid 2000 CF105 satellite, calculated along the apparent relative trajectory of the satellite and expressed in the units of the apparent semi-major axis of the orbit. The bold line on the axis shows the observation time interval, which spans approximately 9 years.

Figure 6 :

 6 Figure 6: Uncertainty -standard deviation ( ) -of the ephemeris of asteroid (702) Alauda's satellite Pichi unem, calculated along the apparent relative trajectory of the satellite and expressed in the units of the apparent semi-major axis of the orbit. The bold line on the axis shows the observation time interval, which spans approximately 50 days.

Figure 7 :

 7 Figure 7: Apparent configuration of asteroid and satellite at the moment of the beginning of mutual occultation.

Table 1

 1

N. V. Emelyanov, A. E. Drozdov: Preprint submitted to Elsevier

Acknowledgements

This work was supported by the Russian Foundation for Basic Research, project no. 19-52-15015-CNRS-a and by the program Projets de recherche conjoints France (CNRS) -Russia (RFBR) for mutual scientific missions.