Mars Express is a space exploration mission by the European Space Agency (ESA) exploring the planet Mars and its moons since 2003, and the first planetary mission attempted by ESA. Mars Express consisted of two parts, the Mars Express Orbiter and Beagle 2, a lander designed to perform exobiology and geochemistry research. Although the lander failed to fully deploy after it landed on the Martian surface, the orbiter has been successfully performing scientific measurements since early 2004, namely, high-resolution imaging and mineralogical mapping of the surface, radar sounding of the subsurface structure down to the permafrost, precise determination of the atmospheric circulation and composition, and study of the interaction of the atmosphere with the interplanetary medium. Due to the valuable science return and the highly flexible mission profile, Mars Express has been granted several mission extensions. The latest was approved on March 7, 2023, consisting of a confirmed operating period until December 31, 2026, and a further provisional extension to December 31, 2028. Arriving at Mars in 2003, 22 years, 8 months and 2 days ago (and counting), it is the second longest surviving, continually active spacecraft in orbit around a planet other than Earth, behind only NASA's still active 2001 Mars Odyssey. It has acted as a central satellite communications node in Mars orbit, conducting radio occultation experiments with the joint ESA and Roscosmos Trace Gas Orbiter, communicating with China's Zhurong rover, and is planned to communicate with Japan's Martian Moons eXploration Phobos sample return.
Name "Express" in the name originally referred to the speed and efficiency with which the spacecraft was designed and built. However, "Express" also describes the spacecraft's relatively short interplanetary voyage, a result of being launched when the orbits of Earth and Mars brought them closer than they had been in about 60,000 years.
Background The Mars Express mission is dedicated to the study of the interior, subsurface, surface, atmosphere, and environment of the planet Mars. The spacecraft carried seven scientific instruments, a small lander, a lander relay, and a Visual Monitoring Camera, all designed to contribute to solving the mystery of Mars's missing water. Some of the instruments on the orbiter, including the camera systems and some spectrometers, reuse designs from the failed launch of the Russian Mars 96 mission in 1996 (European countries had provided much of the instrumentation and financing for that unsuccessful mission). The scientific objectives of the Mars Express represent an attempt to fulfill in part the lost scientific goals of this failed Russian mission, complemented by exobiology research with Beagle-2. The design of Mars Express is based on ESA's Rosetta mission, on which a considerable sum was spent on development. The same design was also used for ESA's Venus Express mission in order to increase reliability and reduce development cost and time. The total initial Mars Express budget excluding the lander was €150 million. The prime contractor for the construction of Mars Express orbiter was EADS Astrium Satellites.
Orbiter and subsystems
Structure The Mars Express orbiter is a cube-shaped spacecraft with two solar panel wings extending from opposite sides. The launch mass of 1223 kg includes a main bus with 113 kg of payload, the 60 kg lander, and 457 kg of propellant. The main body is 1.5 m × 1.8 m × 1.4 m in size, with an aluminium honeycomb structure covered by an aluminium skin. The solar panels measure about 12 m tip-to-tip. Two 20 m long wire dipole antennas extend from opposite side faces perpendicular to the solar panels as part of the radar sounder.
Propulsion The Soyuz/Fregat launcher provided most of the thrust Mars Express needed to reach Mars. The final stage of the Soyuz, Fregat was jettisoned once the probe was safely on a course for Mars. The spacecraft's on-board means of propulsion was used to slow the probe for Mars orbit insertion and subsequently for orbit corrections. The body is built around the main propulsion system, which consists of a bipropellant 400 N main engine. The two 267-liter propellant tanks have a total capacity of 595 kg. Approximately 370 kg are needed for the nominal mission. Pressurized helium from a 35-liter tank is used to force fuel into the engine. Trajectory corrections will be made using a set of eight 10 N thrusters, one attached to each corner of the spacecraft bus. The spacecraft configuration is optimized for a Soyuz/Fregat, and was fully compatible with a Delta II launch vehicle.
Power Spacecraft power is provided by the solar panels which contain 11.42 square meters of silicon cells. The originally planned power was to be 660 W at 1.5 AU but a faulty connection has reduced the amount of power available by 30%, to about 460 W. This loss of power does not significantly affect the science return of the mission. Power is stored in three lithium-ion batteries with a total capacity of 64.8 Ah for use during eclipses. The power is fully regulated at 28 V, and the Terma power module (also used in Rosetta) is redundant. During routine phase, the spacecraft's power consumption is in the range of 450–550 W.
Attitude control - avionics Attitude control (3-axis stabilization) is achieved using two 3-axis inertial measurement units, a set of two star cameras and two Sun sensors, gyroscopes, accelerometers, and four 12 N·m·s reaction wheels. Pointing accuracy is 0.04 degree with respect to the inertial reference frame and 0.8 degree with respect to the Mars orbital frame. Three on-board systems help Mars Express maintain a very precise pointing accuracy, which is essential to allow the spacecraft to use some of the science instruments.
Communications The communications subsystem is composed of three antennas: A 1.6 m diameter parabolic dish high-gain antenna and two omnidirectional antennas. The first one provide links (telecommands uplink and telemetry downlink) in both X-band (8.4 GHz) and S-band (2.1 GHz) and is used during nominal science phase around Mars. The low gain antennas are used during launch and early operations to Mars and for eventual contingencies once in orbit. Two Mars lander relay UHF antennas are mounted on the top face for communication with the Beagle 2 or other landers, using a Melacom transceiver.
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