The spacecraft

Spacecraft facts
Spacecraft bus dimensions 1.5 x 1.8 x 1.4 m
Thrust of main spacecraft engine 400 N
Attitude thrusters 8 at 10 N each
Propellant tanks volume 2x270 = 540 litres
Pointing performance 0.15°
Solar array area 11.42 m²
Lithium batteries 3 at 22.5 Amp hours each
Mars Express spacecraft

Propulsion
Most of the energy needed to propel Mars Express from Earth to Mars was provided by the four-stage Soyuz/Fregat launcher. The Fregat upper stage separated from the spacecraft after placing it on a Mars-bound trajectory.

The spacecraft used its on-board means of propulsion solely for orbit corrections and to slow the spacecraft down for Mars orbit insertion.

The main engine on the underside of the spacecraft body (or 'bus') is capable of delivering a force of 400 Newtons, which will reduce the speed of the spacecraft by 2880 kilometres per hour in 30 minutes (400 Newtons is equivalent to the force needed on Earth to suspend a 40 kilogram weight above the ground).

Eight attitude thrusters attached to each corner of the spacecraft bus can deliver 10 Newtons each.

Electrical power
This is provided by the spacecraft's solar panels which were deployed shortly after launch. When Mars is at its maximum distance from the Sun (aphelion), the solar panels are still be capable of delivering 650 Watts which is more than enough to meet the mission's maximum requirement of 500 Watts, equivalent to just five ordinary 100 Watt light bulbs!

When the spacecraft's view of the Sun is obscured by Mars during a solar eclipse, a lithium-ion battery (67.5 Amp hours), previously charged up by the solar panels, takes over the power supply.

Mars Express spacecraft
Mars Express mounted on the Fregat upper stage

Communications
The circular dish attached to one face of the spacecraft bus is a 1.6-metre diameter high-gain antenna for receiving and transmitting radio signals when the spacecraft is a long way from Earth.

When it is close to Earth, communication is via a 40 centimetre-long low-gain antenna, which protrudes from the spacecraft bus.

Data storage
As scientific data cannot be transmitted back to Earth as soon as it is collected, they will be stored on the spacecraft computer until transmission is possible. The computer has 12 Gbits of solid-state mass memory.

Control
The on-board computers control all aspects of the spacecraft functioning including switching instruments on and off, assessing the spacecraft orientation in space and issuing commands to change it.

Navigation
Three on-board systems help Mars Express maintain a very precise pointing accuracy, which is essential to allow the spacecraft to communicate with a 34-metre dish on Earth up to 400 million kilometres away. They are two star trackers; six laser gyros; two coarse Sun sensors.

Thermal control
The spacecraft must provide a benign environment for the instruments and on-board equipment. Two instruments, PFS and OMEGA, have infrared detectors that need to be kept at very low temperatures (about -180°C). The sensors on the camera (HRSC) also need to be kept cool. But the rest of the instruments and on-board equipment function best at room temperatures (10-20°C).

The spacecraft is encapsulated in thermal blankets made from gold-plated aluminium-tin alloy, to keep the interior at 10-20°C. The instruments that need to be kept cold are thermally insulated from the warm interior of the spacecraft and attached to radiators that lose heat to space, which is very cold (about -270°C).

Last update: 10 October 2005

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