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A scale model of the ExoMars orbiter presented at the Farnborough air show in 2010. Copyright © 2010 Anatoly Zak


The initial assembly of the TGO orbiter at OHB Systems AG in February 2014 shows the propellant tank assembly on the spacecraft. Click to enlarge. Credit: ESA


Trace Gas Orbiter, TGO, core module during integration of its electrical subsystems at Thales Alenia Space in Cannes, France, in November 2014. Click to enlarge. Credit: ESA


Fueling of the TGO orbiter in Baikonur on Feb. 23, 2016. Click to enlarge. Credit: ESA


An artist rendering illustrating the TGO orbiter conducting a braking maneuver to enter orbit around Mars on Oct. 19, 2016. Click to enlarge. Credit: ESA


Testing of the solar panel deployment on the TGO spacecraft on May 29, 2015. Click to enlarge. Credit: ESA


The Atmospheric Chemistry Suite, ACS, is comprised of four sections integrated into a single unit: ACS-NIR (blue), ACS-MIR (green), ACS-TIR (dark red) and a central electronics box (brown). ACS-MIR has a solar occultation aperture; ACS-NIR has a wide-angle aperture for nadir and limb viewing, and ACS-TIR has a one-dimensional scan mechanism that allows it to observe space, a black body, nadir or the Sun. Credit: Space Research Institute, IKI


Methane concentrations on Mars. Click to enlarge. Credit: ESA

Trace Gas Orbiter might help unlock mysteries of Mars

The centerpiece of the ExoMars-2016 project is the nearly 3.4-ton spacecraft designated Trace Gas Orbiter, TGO. After releasing a small probe to the surface of Mars, the TGO should enter orbit around the Red Planet to help answer some of the most intriguing questions about this alien world. The orbiter will scan martian atmosphere for traces of various gasses including methane, which could reveal biological activity on the planet. Designed to operate until at least 2022, the TGO can also relay communications to Earth from its lander and from the follow-on ExoMars-2018 rover designed to search for signs of life on Mars.

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The Trace Gas Orbiter, TGO, spacecraft shown in its fully deployed configuration during the cruise flight between the Earth and Mars, with the Schiaparelli lander still attached (right) Credit: ESA.

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TGO design

The TGO measures approximately 3.5 meters high and two meters across. It is built around a central cylinder serving as a structural backbone designed to evenly distribute loads experienced during launch throughout the rest of the spacecraft. The cylinder extends all the way to the top of the spacecraft, providing a base for the attachment of the 600-kilogram Schiaparelli lander.

Four scientific instruments with a total mass of 135.6 kilograms were mounted on two horizontal panels crossing the orbiter's main structure above and below the navigational star trackers. Such an arrangement aimed to ensure best alignment of the instruments with the trackers, even when severe temperature swings in space cause minute shifts in the spacecraft's structural shape.

The lower service section of the orbiter houses the majority of support systems not directly involved in scientific investigations.

To adjust trajectory on its way to Mars and to easy itself into the planet's orbit, the spacecraft was equipped with a 424-newton main engine. For high-accuracy attitude control, the TGO has 10 primary small thrusters and 10 backup thrusters. All engines burn mixed oxides of nitrogen as oxidizer and monomethylhydrazine as fuel. The spacecraft has one fuel tank and one oxidizer tank, each with a capacity of 1,207 liters. Propellants for the spacecraft were supplied by Gerling Holz in Germany.

A pair of two-section solar panels spanning 17.5 meters when fully unfolded can provide 2,000 watts of power to all onboard systems. Each section of the panel is 3.12 meters long and 1.74 meters wide, making an individual array 7.92 meters long.

For the TGO, solar arrays are the only source of electricity, which can be stored in rechargeable lithium-ion batteries. They will come handy each time the spacecraft flies over the night side of Mars, with the planet eclipsing the Sun. The batteries are able to output around 5,100 watts per hour.

While in transit between the Earth and Mars, the solar arrays will also charge Schiaparelli's rechargeable battery to spare the lander's primary non-rechargeable battery for the operation on the surface of Mars.

The Power Conditioning Unit, PCU, onboard the orbiter is responsible for taking electrical power generated by solar arrays and making it suitable for charging the spacecraft's batteries and powering the payloads. The Power Control and Distribution Unit, PCDU, routes the flow of electricity to all of the spacecraft's subsystems.

All the communications between the spacecraft and mission control will be possible via the 2.2-meter, 65-watt high-gain antenna dish operating in X-band and attached to the service module. In addition, a NASA-provided UHF-transceivers with a single helix antenna will be used to communicate with rovers and landers on the surface of Mars.

The computer brain of the TGO orbiter is known as the Spacecraft Management Unit, SMU.

The TGO spacecraft was assembled at the Cannes facility of the Thales Alenia Space, France, with OHB Systems of Bremen, Germany, serving as the project's co-prime contractor, which built the spacecraft structure, thermal control and propulsion systems.

The TGO/Schiaparelli combination will be launched on a Proton-M rocket with a Briz-M upper stage from Baikonur Cosmodrome in Kazakhstan. Schiaparelli lander will separate on October 16, 2016, three days before reaching Mars.

Around 12 hours after dropping the lander, the TGO will make a trajectory correction to avoid entering the atmosphere of Mars but rather entering orbit around the planet three days later, the TGO spacecraft will maneuver to a final orbit for scientific research. It has a capability to orient itself along all three axis to point its instruments at the surface of the planet.

Although ExoMars is a European-led mission, for the Russian scientists, it is the first chance in years for reviving the nation's planetary exploration program.

Evolution of the Trace Gas Orbiter specifications, according to ESA:

Spacecraft bus dimensions
3.5 by 2 by 2 meters
Solar panels span
17.5 meters
Liftoff mass of the spacecraft composite
4,400 kilograms
4,332 kilograms
Orbiter module mass
1,365 kilograms
Oxidizer mass (mixed oxides of nitrogen, MON)
~1,500 kilograms
Fuel mass (monomethylhydrazine, MMH)
~1,000 kilograms
Payload mass
125 kilograms
135.6* kilograms
Schiaparelli lander mass
600 kilograms
600.0 kilograms
Main engine thrust
424 newtons
Power supply capability from solar panels
2,000 watts
Power battery capacity
~5,100 watts per hour
High-gain communications antenna diameter
2.2 meters
2.2 meters
Launch date
2016 March
Mission end

*112 kilograms, according to other sources


Known ExoMars-2016 project participants:

Company Responsibility
Thales Alenia Space Italy, France System integrator
OHB Systems, Germany Spacecraft structure, thermal control and propulsion systems
GKNPTs Khrunichev, Russia Proton-M/Briz-M launch vehicle
Deimos, Spain Analysis of entry and descent on Mars for the Schiaparelli lander
RUAG, Switzerland Spacecraft Management Unit, SMU; Separation system
Aerosekur, Italy HEPA filters
Honeywell, US Avionics
Selex-Galileo, Italy Star trackers, photovoltaic assemblies for solar arrays; Power Conditioning Unit, PCU; Power Control and Distribution Unit, PCDU
Rockwell Collins, Germany Reaction wheels
MDA, Canada High-gain antenna
Kongsberg, Norway Driving mechanism for solar array
ABSL, UK Batteries
Patria, Finland Solar array panels
TNO, Netherlands Sun sensor


Science payload: Probing methane mystery

The TGO orbiter will carry four scientific instruments, two of which were built in Europe and another pair was supplied by Moscow-based Space Research Institute, IKI.

The primary goal of the instruments is to map the distribution of the methane gas on Mars, which was first detected by Europe's Mars Express orbiter in 2004. NASA's Curiosity rover later confirmed the findings. The exact origin of the chemical is unknown, while both biological and non-biological sources of methane on Mars have been hypothesized. However on Earth, methane is produced almost entirely through biological activity with only tiny addition from volcanic and hydrothermal events. In any case, due to a relatively short life span of methane in geological terms, its detection on Mars hints the existence of a very recent source. Hopefully, instruments on the TGO will help to resolve the mystery of the Martian methane.

The TGO will be able to sniff very small concentrations of gases, making up less than one percent of the atmospheric mixture. They include methane, water vapour and nitrogen dioxide.

The instruments will focus on hydrocarbons and sulphur, some of which could be signatures of active biological or geological processes, at present or in the past.

According to ESA, TGO's instruments will be able to detect trace gases with an improved accuracy of three orders of magnitude compared to previous measurements.


Science instruments on the TGO orbiter of the ExoMars-2016 mission. Credit: ESA

TGO instruments overview:

Principal investigator/developer
Atmospheric Chemistry Suite, ACS
Three infrared spectrometers to investigate the chemistry, aerosols, and structure of the atmosphere.
Oleg Korablev, Space
Research Institute, Moscow, Russia
Color and Stereo Surface Imaging System, CaSSIS
High-resolution camera for obtaining color and stereo images.
Nicolas Thomas, University of Bern, Switzerland.
Fine Resolution Epithermal Neutron Detector, FREND
Neutron detector to map surface hydrogen.
Igor Mitrofanov, Space Research Institute, IKI, Moscow, Russia
Nadir and Occultation for Mars Discovery, NOMAD
Three spectrometers, two infrared and one ultraviolet, to perform high-sensitivity
orbital identification of atmospheric components, including methane
Ann Carine Vandaele, Belgian
Institute for Space Aeronomy, Brussels, Belgium

Color and Stereo Surface Imaging System, CaSSIS

The high-resolution camera capturing details up to five meters per pixel will obtain color and stereo images of the martian surface covering a wide swath. It will provide the geological and dynamic context for sources of trace gases detected by NOMAD and ACS.

Fine Resolution Epithermal Neutron Detector, FREND

The instrument will map distribution of hydrogen down to a meter deep, making it possible to reveal deposits of water-ice near the surface. FREND’s mapping of shallow subsurface water-ice promised to be up to 10 times better than previous measurements.

Nadir and Occultation for Mars Discovery, NOMAD

NOMAD combines three spectrometers, two infrared and one ultraviolet, to perform high-sensitivity orbital identification of atmospheric components, including methane and many others. It will capture data either via solar occultation or by looking straight down at the surface (at nadir) for a reflected light.

Atmospheric Chemistry Suite, ACS

ACS is a suite of three infrared spectrometers to investigate the chemistry, aerosols and structure of the atmosphere. ACS will complement NOMAD by extending the coverage at infrared wavelengths.

Components of the ACS suite (as of 2012):

Instrument Purpose
Fourier spectrometer, ACS-TIR Temperature, aerosol measurements
Near-infrared Echelle spectrometer, ACS-NIR Measurement of vertical profile of CO, water, oxygen, night glow registration
Mid-infrared Echelle spectrometer, ACS-MIR Measurement of methane, aerosols


Structure of scientific information processing in the ExoMars-2016 project. Credit: ESA

Communications function

In addition to its prime science mission, the TGO also carries a sophisticated radio relay system provided by NASA. The Electra Proximity Payload will act as a communications relay and navigation aid.


Next chapter: Schiaparelli EDM lander


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Page author: Anatoly Zak; Last update: October 15, 2016

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