ESA Science & Technology - Engineering

Telecoms

The circular dish attached to one face of the spacecraft bus is a 1.6 m 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 at the beginning of its journey, communication is via a low gain antenna which is a 40 cm aerial protruding from the spacecraft bus.

For up to six hours during the spacecraft's 7.5 hour Martian orbit, the high gain antenna will point towards Earth for communications between the spacecraft and three ground stations. During the remaining 1.5 hours, the spacecraft will point towards the Martian surface so that the on-board instruments can make observations. Each time the spacecraft passes over Beagle 2 on the Martian surface, the lander will automatically relay data collected by its instruments to a special UHF antenna on the spacecraft.

The Beagle data, together with that collected by the instruments on the orbiter, will be sent back to Earth during the communications phase at a rate of up to 230 kbit/s. The European Space Operations Control Centre (ESOC) in Darmstadt will communicate with the spacecraft via the ESA ground station in Perth, Australia. The spacecraft will send housekeeping data on instrument temperatures, voltages and spacecraft orientation, for example, and the ground station will send back software commands to control the spacecraft and its instruments over the following few days.

Signals to Earth will be in the X-band (7.1 GHZ) and those from Earth will be the S band (2.1 GHZ).

As scientific data cannot be transmitted back to Earth as soon as it is collected, it will be stored on the spacecraft's computer which has 12 Gbits of solid state mass memory. The computer will control all aspects of the spacecraft's functioning including switching instruments on and off, assessing the spacecraft's orientation in space and sending commands to change it. The control and data management software is being developed for the Rosetta mission.

Attitude control

To communicate with a 34 m satellite dish on Earth up to 400 million km away and conduct sensitive scientific experiments, Mars Express must maintain a pointing accuracy of 0.15^o. So it is essential that the spacecraft knows not just where it is but in which direction it is pointing. There are three on-board systems to help:

• Like navigators before the advent of radar, two star trackers, one attached to two opposite sides of the spacecraft bus, assess the direction in which the spacecraft is pointing by automatically identifying patterns of stars seen through small telescopes.
• Three innovative laser gyros, one for each axis of spacecraft rotation, offer a frame of reference against which spacecraft rotation can be measured. The gyros are under development for Rosetta.
• Two coarse sun sensors, also under development for Rosetta, allow the spacecraft to orient itself with respect to the Sun. This is how the spacecraft first determines its orientation after separating from the launcher upper stage. The sun sensors can also be used to right the spacecraft if at any time it accidentally goes into an uncontrolled spin.

Small corrections to the spacecraft's orientation can be achieved by altering the rotation of spinning (off-the-shelf) reaction wheels attached to the underside of the bus. Such changes are necessary, for example, to correct jitter which could disturb observations when the thrusters are fired. The reaction wheels are also used to rotate the spacecraft slowly as it moves round its orbit so that the instruments or antenna are kept pointing in the right direction.

Electrical Power

Most of the power needed to propel Mars Express from Earth to Mars is provided by the four stage Soyuz-Fregat launcher which will separate from the spacecraft after placing it on a Mars-bound trajectory. The spacecraft uses its on-board means of propulsion solely for orbit corrections.

The main engine, an off-the-shelf item attached to the underside of the spacecraft bus, is capable of delivering a force of 400 N. It uses a mixture of two propellants which are contained in two tanks each with 267 litre capacity. Fuel is fed into the engine using pressurised helium from a 35 litre tank.

"The main engine is pretty powerful," says Rudi Schmidt, Mars Express project manager at ESA's technical centre, ESTEC in Noordwijk, the Netherlands. "It can propel the spacecraft a long way. It's used to decelerate the spacecraft to go into orbit around Mars. By the time Mars Express gets to its final orbit, most of the propellant is used up."

Corrections to the spacecraft's trajectory en route for Mars will be achieved by firing two or more of the eight 10 N attitude thrusters which are attached to each corner of the spacecraft bus and are fuelled by the same bi-propellant mixture as the main engine. The attitude thrusters are being developed for the Cluster mission which puts similar demands and constraints on spacecraft design. "The attitude thrusters are also back-up," says Schmidt. "They could do the job of the main engine if they had to, although we would not be able to reach the same final orbit."

Electrical power is provided by the spacecraft's solar panels which are folded against its body during launch and deploy shortly after the launcher housing has been jettisoned.The panels are mounted on a drive mechanism, also under development for the Rosetta mission, which tilts them forwards and backwards to catch most sunlight. The panels themselves are off-the-shelf technology. Their surface area, 11 m², is larger than those used on near-Earth orbiting satellites to compensate for the drop in sunlight intensity at Mars.

When the spacecraft's view of the Sun is obscured during a solar eclipse, an innovative lithium-ion battery (67.5 Ah), previously charged up by the solar panels, will take over the power supply. 1400 eclipses, lasting up to 90 minutes, are expected during the nominal mission's lifetime. They occur when the spacecraft is in polar orbit around Mars and the red planet obscures its view of the Sun. When Mars is at its maximum distance from the Sun (aphelion), the solar panels will be capable of delivering 650 Watts which is more than enough to meet the mission's maximum requirement of 500 Watts, just half that of a single bar 1 kW electric fire!

Thermal Control

As well as getting to Mars, the spacecraft has to provide a benign environment for the instruments and on-board equipment. That means keeping some parts of the spacecraft warm and other parts cold. 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 (HSC) 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 plan is to keep the inside of the spacecraft at 10-20°C by encapsulating the whole thing in thermal blankets and to cool those instruments that need it. The thermal blankets will be made from gold-plated aluminium-tin alloy. "We will design the thermal isolation so that the spacecraft doesn't get warm when the Sun or Mars shines on it, nor cold when it's on its interplanetary cruise. This is a challenging problem for the mission engineers" says Schmidt.

Material not covered by insulation may face temperatures of -100°C in the shade and up to 150°C in sunlight. Such temperature variations can cause material to shrink and expand unacceptably. Major external equipment on Mars Express, such as the solar array and high gain antenna, would require a large amount of power to keep them at room temperature - so they are made from composite materials which can withstand wide temperature variations without significant deformation.

The instruments that need to be kept cold will be attached to radiators that face deep space. Instrument and radiator will be thermally insulated from the rest of the spacecraft. Cooling will be through loss of heat to space which is very cold (about -270°C).

Table of Facts

Table 1: Mass and power budget

Table 2: General facts and figures

Table 3: Power supply

Table 4: Temperature requirements and thermal specification