The spacecraft

Spacecraft facts
Spacecraft bus dimensions 1.5 x 1.8 x 1.4 m
Spacecraft mass 1270 kg (including 93 kg of payload and 570 kg fuel)
Thrust of main engine 400 N
Attitude thrusters Two sets of four, each delivering 10 Newtons each
Solar arrays Two triple-junction Ga As;
5.7 square metres; generating 800 Watts
near Earth and 1100 Watts at Venus
Power storage Three lithium-ion batteries
Antennas Two high-gain dishes, HGA1 = 1.3 m diameter,
HGA2 = 0.3 m in diameter, 2 low-gain antennas
Venus Express ready for shipping to BaikonurAccess the image

The Venus Express spacecraft is a virtual twin of Mars Express. The spacecraft body itself, termed a ‘bus’, is a honeycomb aluminium box about one and a half metres wide. With its solar arrays extended, it measures about eight metres across.

As for Mars Express, the scientific instruments are mounted in the bus. Only minor changes from Mars Express were required to accommodate the revised instruments payload, which is concentrated on three sides of the spacecraft.

However, the environment of Venus is very different from that of Mars. This required some design changes that make Venus Express more suited to operating around Venus.

Thermal control
Flying to an inner Solar System planet like Venus, at half the distance to the Sun as compared to Mars, means that the effect of solar illumination and ionising radiation on the spacecraft is much higher. In fact, the heating of the spacecraft is four times greater at Venus than at Mars.

To keep Venus Express within temperatures that are safe for the spacecraft, the radiators on the spacecraft surface have been increased in area and efficiency. The spacecraft coating, called ‘multi-layer insulation’ or MLI, is composed of 23 layers, packaged differently from Mars Express.

Moreover, for Venus Express, the MLI is gold instead of black, which provides more capability to reflect radiation away. In general, Mars Express was designed to keep warm while Venus Express is modified to stay cool.

Artist's impression of Venus Express orbiting VenusAccess the image

At Venus, the Sun appears twice as powerful as on Earth, so solar radiation to power the spacecraft is plentiful. Venus Express’s solar arrays could be made smaller (almost half size) than those of Mars Express.

The two symmetric solar arrays, counting about six square metres, are based on a ‘triple junction’ gallium arsenide (GaAs) technology, different from the silicon-based technology used for Mars Express.

The solar cells, each composed by four layers of gallium arsenide, are more tolerant to high temperatures (up to around 120 °C), so they are more suitable for hot environments. Moreover, they are able of exploiting a wider range of solar radiation. The solar cells are separated by aluminium strips that help to reject heat. All together, these aluminium strips cover half of the overall solar array surface.

When the spacecraft is in shadow (eclipse) or when its power demand exceeds the capacity of the solar arrays, electrical power is supplied by three lithium-ion batteries that are charged by the solar-generated power.

The gravity of Venus, almost the same as Earth’s, is about eight times higher than that of Mars. This, plus the fact that the gravitational pull of the Sun is stronger at Venus, means that Venus Express needs more energy to brake and be captured into orbit around Venus.

This energy is provided by part of the 570 kilograms of propellant on board (about 20% more than for Mars Express). The propellant mass is almost half of the overall spacecraft weight!

Major spacecraft manoeuvres, like the injection into orbit around Venus, are performed by firing the main engine located at the bottom floor of the spacecraft, while minor manoeuvres are made using four pairs of thrusters located at the four bottom corners of the spacecraft.

The thrusters are used for small trajectory corrections, spacecraft attitude changes, and to correct the altitude of the Venus Express orbit’s pericentre about every 50 days. In fact, due to the gravitational pull of the Sun while the spacecraft is farther away from the planet, the pericentre naturally drifts upwards at a rate of about 1.5 kilometres per day.

For a spacecraft in orbit around Venus, it is not always possible to point a single antenna dish at Earth while always keeping the cold face of the spacecraft, hosting delicate instruments, away from the Sun.

To overcome this pointing constraint, Venus Express has two high-gain antennas, mounted on different spacecraft faces. The main high-gain antenna, used for most of the communication with Earth, is a dish measuring 1.3 metres diameter.

The second and smaller high-gain antenna (30 centimetres diameter) is used to communicate with Earth when the spacecraft is in the part of the orbit closest to our planet (less than 0.78 Astronomical Units* away).

Two low-gain antennas are also mounted on board, to communicate with Earth during launch, in the first days of the cruise and, should it occur, during severe ‘safe modes’.

*One Astronomical Unit, defined as the mean distance between the Sun and Earth, is about 150 million kilometres

Navigation and attitude control
Venus Express is a three-axis stabilised spacecraft. Through three different systems on board, the spacecraft can acquire data about its position in space, attitude (orientation) and change of velocity.

These on-board systems consist of two star trackers, two Sun sensors and a set composed by three laser gyroscopes and three accelerometers. They provide data necessary to re-orient the spacecraft and the solar arrays when necessary.

The actual spacecraft re-orientation manoeuvre or trajectory correction is performed by means of so-called ‘reaction wheels’ or by firing the thrusters.

Data storage
The Venus Express on-board computer is responsible for supervising and managing the overall spacecraft functioning, for handling all data acquired by instruments and sensors and for sending commands through the whole spacecraft.

Part of the computer is the Solid State Mass Memory, which has a capacity of 12 gigabits. All scientific data collected by the instruments are stored here until they can be downloaded to Earth during the appropriate orbital phase.

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