ExoMars Trace Gas Orbiter
ExoMars Trace Gas Orbiter with Europe’s entry, descent and landing demonstration vehicle
|Operator||ESA and Russian Federal Space Agency|
|Major contractors||Thales Alenia Space|
|Launch date||January 2016|
|Launch vehicle||Proton rocket|
|Mission duration||1 Mars year orbital science mission
Telecom asset until 2022
|Orbital insertion date||October 2016|
|Mass||3,130 kg (6,900 lb)|
|Power||20m2 photovoltaic array (2000 W)|
|Apoapsis||400 km near-circular altitude|
The ExoMars Trace Gas Orbiter (TGO) is a collaborative project between the European Space Agency (ESA) and the Russian Federal Space Agency (Roscosmos) to send a robotic orbiter-carrier to Mars in 2016 as part of the European-led ExoMars mission.
The TGO would deliver the ExoMars EDM Schiaparelli lander and then proceed to map the sources of methane on Mars and other gases, and in doing so, help select the landing site for the ExoMars rover to be launched on 2018.
Investigations with space and Earth-based observatories, have demonstrated the presence of small amounts of methane on the atmosphere of Mars that has been shown to vary with location and time. This may indicate the presence of life on Mars, but may also be produced by a geochemical process, volcanic or hydrothermal activity.
The challenge to discern the source of methane in the atmosphere of Mars, prompted the independent planning of two orbiters that would carry instruments in order to determine if its formation is of biological or geological origin, as well as its decomposition products such as formaldehyde and methanol.
Attempted collaboration with NASA
NASA's Mars Science Orbiter (MSO) was originally envisioned in 2008 as an all NASA endeavor aiming for a late 2013 launch window. NASA and ESA officials agreed to pool resources and technical expertise and collaborate to launch only one orbiter. The agreement, called Mars Joint Exploration Initiative, was signed on July 2009 and proposed to utilize an Atlas rocket launcher instead of a Soyuz rocket, which significantly altered the technical and financial setting of the European ExoMars mission. Since the ExoMars rover was originally planned to be carried by the TGO, a prospective agreement would require that the rover lose enough weight to fit aboard the Atlas launch vehicle with NASA's orbiter. Instead of reducing the rover's mass, it was nearly doubled when the mission was combined with other projects to a multi-spacecraft programme divided over two Atlas V-launches: the ExoMars Trace Gas Orbiter (TGO) was merged into the project, carrying a static meteorological lander slated for launch in 2016. The European orbiter would carry several instruments originally meant for NASA's MSO, so NASA scaled down the objectives and focused on atmosphere trace gases detection instruments for their incorporation in ESA's ExoMars Trace Gas Orbiter.
Under the FY2013 Budget President Obama released on February 13, 2012, NASA terminated its participation in ExoMars due to budgetary cuts in order to pay for the cost overruns of the James Webb Space Telescope. With NASA's funding for this project completely cancelled, most of ExoMars' plans had to be restructured.
Collaboration with Russia
On March 15, 2012, the ESA's ruling council announced it will press ahead with its ExoMars program in partnership with the Russian space agency (Roscosmos), which plans to contribute two heavy-lift Proton launch vehicles and an additional entry, descent and landing system to the 2018 mission.
Under the collaboration proposal with Roscosmos, the ExoMars mission will be split into two parts: the orbiter/lander mission in 2016 that would include the TGO and a static lander build by ESA; this would be followed by the ExoMars rover mission in 2018 —also to be launched with a Russian Proton rocket.
As of April 2015, the 600 kg descent module Schiaparelli is being integrated to the orbiter for some mechanical and thermal-vacuum tests. An ESA official stated that the mission is likely to be enhanced by the contribution of Russian science instruments that will fly on the TGO.
The proposed specifications are:
- Central tube that is 1.194 metres (3.92 ft) in diameter
- 20m2 solar arrays entirely covered with cells and capable of rotating one degree, generating about 2000 W of power at Mars
- 2 modules of lithium-ion batteries with approximately 5100 watt hour total capacity to provide power during eclipses over the prime mission
- 2.2 metres (7.2 ft) X band high gain antenna with a 2 axes-pointing mechanism and 65 W RF Travelling Wave Tube Amplifier to communicate with Earth
- Electra UHF-Band transceivers with a single helical antenna to communicate with surface rovers and landers
- Thermal control
- Spacecraft yaw axis control to ensure the three faces containing the science payload remain cold
- 3,130 kg (6,900 lb)
- Up to 135.6 kg (299 lb) of scientific instruments
The TGO would separate from the ExoMars stationary lander and provide it with telecommunication relay for 8 sols after landing. Then the TGO would aerobrake for seven months into a more circular orbit for science observations and would provide communications relay for the ExoMars rover to be launched in 2018, and would continue serving as a relay satellite for future landed missions until 2022.
The mission would require detection, characterization of spatial and temporal variation, and localization of sources for a broad suite of atmospheric trace gases:
Nature of the methane source requires measurements of a suite of trace gases in order to characterize potential biochemical and geochemical processes at work. The orbiter would require very high sensitivity to (at least) the following molecules and their isotopomers: water (H2O), hydroperoxyl (HO2), nitrogen dioxide (NO2), nitrous oxide (N2O), methane (CH4), acetylene (C2H2), ethylene (C2H4), ethane (C2H6), formaldehyde (H2CO), hydrogen cyanide (HCN), hydrogen sulfide (H2S), carbonyl sulfide (OCS), sulfur dioxide (SO2), hydrogen chloride (HCl), carbon monoxide (CO) and ozone (O3). Detection sensitivities are at levels of 100 parts per trillion, improved to 10 parts per trillion or better by averaging spectra which could be taken at several spectra per second 
- Spatial and temporal variability: Latitude-longitude coverage multiple times in a Mars year to determine regional sources and seasonal variations (reported to be large, but still controversial with present understanding of Mars gas-phase photochemistry.)
- Correlation of concentration observations with environmental parameters of temperature, dust and ice aerosols (potential sites for heterogeneous chemistry.)
- Mapping of multiple tracers (e.g., aerosols, water vapor, CO, CH4) with different photochemical lifetimes and correlations helps constrain model simulations and points to source/sink regions.
- To achieve the spatial resolution required to localize sources might require tracing molecules at the ~1 part per billion concentration.
Like Mars Reconnaissance Orbiter, the Trace Gas Orbiter is a hybrid science-telecom orbiter. Development of the spacecraft's science instruments is well under way. Its maximum science payload mass is projected to be about 115 kg and consist of:
- NOMAD has two infrared and one ultraviolet spectrometer channels.
- ACS has three infrared channels 
- NOMAD and ACS will provide the most extensive spectral coverage of Martian atmospheric processes so far. Twice per orbit, at local sunrise and sunset, they will be able to observe the Sun as it shines through the atmosphere. Detection of atmospheric trace species at parts-per-billion (ppb) level will be possible.
- CaSSIS is a high-resolution (4.5 m/pixel), colour stereo camera for building accurate digital elevation models of the Martian surface. It will also be an important tool for characterizing candidate landing site locations for future missions.
- FREND is a neutron detector that can provide information on the presence of hydrogen, in the form of water or hydrated minerals, in the top metre layer of the Martian surface.
Due to the challenges of entry, descent, and landing, Mars landers are highly constrained in mass, volume, and power. For landed missions, this places severe constraints on antenna size and transmission power, which in turn greatly reduce direct-to-Earth communication capability in comparison to orbital spacecraft. As an example, the capability downlinks on Spirit and Opportunity have only 1/600th the capability of the Mars Reconnaissance Orbiter downlink. Relay communication addresses this problem by allowing Mars surface spacecraft to communicate using higher data rates over short-range links to nearby Mars orbiters, while the orbiter takes on the task of communicating over the long-distance link back to Earth. This relay strategy offers a variety of key benefits to Mars landers: increased data return volume, reduced energy requirements, reduced communications system mass, increased communications opportunities, robust critical event communications and in situ navigation aide. NASA will provide an Electra telecommunications relay and navigation instrument to assure communications between probes and rovers on the surface of Mars and controllers on Earth. The TGO would provide the EDM lander and ExoMars rover with telecommunication relay and would continue serving as a relay satellite for future landed missions until 2022.
- Curiosity rover
- Mars 2020 rover
- Mars Exploration Joint Initiative
- Mars Express orbiter
- Mars Global Surveyor
- Mars Orbiter Mission (Mangalyaan)
- MAVEN orbiter
- Water on Mars
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