Mars Express Overview

European Space Agency

Status

Mars Express orbiter is in successful operation around Mars.

Beagle 2 lander was scheduled to land on Mars at 2.54am GMT, 25th December 2003. Contact has not been made with Beagle 2 and on 6 February 2004, following an assessment of the situation, the Beagle 2 was declared lost.

Objective

To search for subsurface water from orbit, and release a lander to reach the Martian surface. The orbiter is now studying the Martian atmosphere, the planet's structure and its geology.

Mission

Mars Express is Europe's first spacecraft to the Red Planet. It carries seven instruments and a lander. The orbiter instruments are remotely investigating the Martian atmosphere, surface and subsurface. Beagle 2, the lander, was expected to perform on-the-spot measurements and also search for signs of past life.

Mars has always been a source of intrigue and fascination. It is currently the only planet in the Solar System on which there is a strong possibility of finding life - past, or perhaps present. It is a prime candidate for future manned exploration, and even colonisation.

Mars Express, together with its lander, is an important element of the international flotilla of spacecraft destined to explore Mars. The ESA project is also the start of an innovative way of developing building blocks for cheaper assembly of future European space missions. The spacecraft has been built and launched in record time and at a much lower cost than previous, similar missions into outer space.

What's special?

Mars Express comprises a number of essential components - the spacecraft, its instruments, the lander, the ground segment, and the launcher. An experienced team of engineers in ESA and industry and hundreds of international scientists are combining these elements into a space mission.

One of the main objectives is to search for traces of water in the subsurface, through the atmosphere, and all the way up to free space. The lander was designed to perform on-the-spot analyses on the Martian surface. Seven scientific instruments on board the orbiting spacecraft will perform a series of remote sensing experiments designed to shed new light on the Martian atmosphere, the atmospheric structure, and geology.

The Beagle 2 lander, named after the ship in which Charles Darwin set sail to explore uncharted areas of the Earth in 1831, provided an exciting opportunity for Europe to contribute to the search for life on Mars. After coming to rest on the surface, Beagle 2 was expected to perform environmental, exobiological, and geochemical research.

As well as its science objectives, Mars Express will also provide relay communication services between the Earth and the NASA rovers on the surface, so forming a centrepiece of the international effort in Mars exploration.

Scientists hope that the instruments on board Mars Express will detect the presence of water below the surface. This could exist in the form of underground rivers, pools, aquifiers, or permafrost.

Spacecraft

Mars Express is designed to take a payload of seven state-of-the-art scientific instruments and one lander to the Red Planet and allow them to record data for at least one Martian year, or 687 Earth days. The spacecraft will also carry a data relay system for communicating with Earth.

The mission is a test case for new working methods to speed up spacecraft production and minimise mission costs. This novel approach is, among other things, also based on reuse of technology developed for the Rosetta mission to a comet.

Most of the velocity needed for the journey of Mars Express from Earth to Mars is provided by the fourth stage of the Soyuz launcher. This stage is called Fregat and separated from the spacecraft after placing it on a Mars-bound trajectory. The spacecraft uses its on-board means of propulsion mainly for Mars orbit injection and for orbit corrections.

The main engine, an off-the-shelf item, uses a mixture of two propellants which are contained in two tanks each with a capacity of 267 litres. The fuel is fed into the engine using pressurised helium from a 35-litre tank.

Electrical power is provided by the spacecraft's solar panels which are folded against its body during launch and deployed shortly after separation from Fregat. The panels are mounted on a rotating drive mechanism, which tilts them forwards and backwards to catch most sunlight.

When the spacecraft's view of the Sun is obscured during a solar eclipse, an innovative lithium-ion battery, previously charged up by the solar panels, will take over the power supply. Over 1400 eclipses, each lasting up to 90 minutes, are expected during the nominal mission's lifetime. They occur when Mars obscures the spacecraft's view of the Sun. 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 standard household electric fire.

Journey

Mars Express travelled to the Red Planet in seven months arriving in in Mars orbit on 25 December 2003. It set off on its journey from the Baikonur launch pad in Kazakhstan on a Soyuz-Fregat launcher on 2 June 2003. After reaching an altitude of about 200 kilometres, the Fregat upper stage (which carries the spacecraft) fired its own motors to circularise the orbit 200 kilometres above the Earth. Just before completing the first orbit, it fired again to send itself and its cargo into an escape orbit, en-route for Mars.

After separation from Fregat, the spacecraft's first task was to steady itself by locking onto the Sun using a sun sensor and to unfurl its solar arrays.

Two days and 600 000 kilometres later, ground control sent a message to Mars Express to adjust its trajectory onto what put it on a collision course with Mars. A few minutes burn of the small thrusters produced the desired effect. Mars Express was then hurtling through interplanetary space with an absolute velocity of 116 800 kilometres per hour and a velocity relative to Earth of 10 800 kilometres per hour.

One month before arrival, preparations began for the separation of the Beagle 2 lander. Once more, the small thrusters fired to put Mars Express onto a trajectory that would allow Beagle 2, which had no propulsion of its own, to enter the Martian atmosphere and endure a bumpy ride through the Martian atmosphere down to the correct landing site on the surface.

Beagle 2 was released as late as possible, just six days before Mars Express went into orbit around the Red Planet, to increase the precision of landing sequence. The friction caused by Beagle 2 entering the thin Martian atmosphere would cause it to slow down considerably and then parachutes would have deployed; then approximately one kilometre above the surface, large gas-filled airbags would deploy around the lander to cocoon it as it bounced to rest on the surface.

The rocky ride through the Martian atmosphere to the surface should have taken no longer than ten minutes. Contact with Beagle 2 was attempted by the NASA Mars Odyssey orbiter, several Earth-based telescopes and the Mars Express orbiter itself, however no signal was received from the Beagle 2 lander. The Beagle 2 Management Board met in London on 6 February 2004 and, following an assessment of the situation, declared Beagle 2 lost.

History

Europe's Mars Express is the lowest-cost mission to Mars so far and is seen as a pilot project for new methods of funding and working. The experience gained on Mars Express will provide a good basis to further lower the costs of future ESA missions.

After a 12-month competitive study, proposal and evaluation phase, concluded at the end of 1998, ESA recommended Astrium SAS of Toulouse, France, as prime contractor. The contract for the design and development of this first European spacecraft to visit the planet Mars was signed formally on 30 March 1999.

The relatively low cost of the mission was achieved through new and innovative approaches in the working relationship between ESA, industry, national agencies and the scientific community, and through the reuse of equipment developed for ESA's Rosetta mission. Some of the scientific instruments have a heritage from the Russian Mars 1996 mission.

Partnerships

ESA believes that the scientific community and European industry have gained sufficient experience during past scientific projects for industry to take on more responsibility for the management of interfaces, in particular with the scientific payload.

For this reason, Astrium SAS is taking on tasks that previously would have been done by the project team at ESTEC. These include interacting directly with the Principal Investigators for the scientific payload and with the launch services supplier, Starsem, to ensure that technical interfaces are compatible. As a consequence of this shift in responsibility, the ESA project team is only ten-strong compared with at least 20 for earlier comparable missions.

The Soyuz-Fregat launcher was provided by Starsem, which is jointly owned by Arianespace, Aerospatiale, the Russian Aviation and Space Agency and the Samara Space Centre.

Technical Summary

The Red Planet has always been a source of intrigue and fascination. It is currently the only planet in the Solar System on which there is a strong possibility of finding life - past, or perhaps present. And it is a prime candidate for future manned exploration, and even colonisation.

Mars

The end of 2003 and early 2004 will see a true scientific invasion of Mars as no fewer than six international spacecraft chart a course to the planet within a short time. Europe has waited a long time for the opportunity to mount its own mission to Mars and that dream is now reality. Mars Express, the name of ESA's Mars mission for 2003, marks the opening of a new era for Europe in planetary exploration.

Mars

Mars Express will be an important element of the international flotilla of spacecraft destined to explore Mars in the first decade of the new millennium. The ESA project is also the start of an innovative way of organising the building blocks that form European space missions. The spacecraft was built and launched in record time and at a much lower cost than previous, similar missions into outer space.

Artist impression of the Mars Express orbiter

A scientific water diviner

Mars Express is the first 'flexible' mission of ESA's long-term science exploration programme. The journey to the red planet began on 2 June 2003 with the launch from the Baikonur Cosmodrome on a Soyuz-Fregat rocket. It ended on 25 December 2003 with the successful orbit insertion. Mars Express comprises a number of essential components - the spacecraft and its instruments, the lander, a network of ground and data processing stations, and the launcher itself. These are supported by an experienced team of engineers in ESA and industry and hundreds of international scientists.

The mission's main objective is to search for sub-surface water from orbit and deploy a lander onto the Martian surface. Seven scientific instruments onboard the orbiting spacecraft will perform a series of remote sensing experiments designed to shed new light on the Martian atmosphere, the planet's structure, geology and composition.

Beagle 2 lander leaving Mars Express following the Cruise Phase

The lander, called Beagle 2 after the ship in which Charles Darwin set sail to explore unchartered areas of the Earth in 1831, represented an exciting opportunity for Europe to contribute to the search for life on Mars.

While addressing its science objectives, Mars Express will also provide relay communication services between the Earth and various landers deployed on the surface by other nations, thus forming a centre piece of the international effort in Mars exploration.

Searching for the elixir of life

Scientists hope that the instruments onboard Mars Express will detect the presence of water below the surface. This could exist in the form of underground rivers, pools, aquifers or permafrost. Overall, the main goals of the instruments to be carried by the Mars Express orbiter are:

Sharp-eyed, 3D photography to discover more about the surface and geology of Mars.
Looking at the 'invisible' beneath the surface by using radar beams to penetrate below ground. Different materials or structures will send back different radar echoes allowing scientists to produce an accurate 3D survey.
Precise determination of atmospheric circulation and composition to build up an accurate picture of Martian meteorology and climate.
Study of the interaction of the atmosphere with outer space.

Gathering such information on the history and present day circumstances of Mars may also improve our understanding of phenomena that influence our own environment. For example, if we can determine why Martian water disappeared in the past we may learn more about whether a similar fate one day awaits the oceans of Earth. The Mars Express spacecraft and its instruments represent a truly international endeavour - a stereoscopic camera from Germany, a mineralogical mapping device from France and an atmospheric sounder from Italy. The radar instrument, to probe for water at depths of a few kilometres below the surface, has been built jointly between Italy and the Jet Propulsion Laboratory in California. The Beagle 2 landing craft has been designed and built in the UK. As well as the remote observation payload, the orbiter carries a lander communications package to support international Mars lander missions from 2003 to 2007.

Postcards from the surface

If deployment had been successful, the Mars Express Beagle 2 lander would have needed to survive temperatures down to as low as -100 deg C. It carried a variety of scientific experiments powered by solar cells and a rechargeable battery.

The Beagle 2 lander showing its solar panels outspread and its robotic arm
and instruments being deployed (Image courtesy of Beagle 2 project).

Like any self-respecting tourist visiting a new destination for the first time, Beagle 2 would have taken photographs. Panoramic and wide-field cameras were to be used for pictures of the landing site to guide further exploration as the mission progressed.

A microscope would have looked closely at the rocks and soil with a high degree of magnification. Fragments of rocks within reach of Beagle 2's small robotic arm were scheduled to be analysed for the existence of organic matter, water and aqueously-deposited minerals.

The busy lander was also to deploy a mole capable of crawling short distances across the surface at one cm every six seconds (the relative equivalent of six metres an hour) and burrowing beneath large boulders to collect soil samples for a gas analysis system. The primary aim of these experiments was to see if any evidence of past life processes near the landing site remained.

Unmanned Soyuz on launch pad (Image courtesy Starsem)

How Europe gets to Mars

The selection of a Soyuz/Fregat launcher to put Mars Express on its course towards Mars is linked to the flexible approach adopted by ESA. The launcher was procured through Starsem, a Russian/European company. As a relatively low-cost launcher it helped keep the overall cost of the Mars Express mission within a total initial budget of 150 million Euros.

Objectives

In the broad context of planetary science, Mars represents an important transition between the outer volatile-rich, more oxidised regions of the accretion zone of the terrestrial bodies (asteroid belt) and the inner, more refractory and less oxidised regions from which the Earth, Venus and Mercury accreted.

This special position of Mars and its traditional character is also manifested by its size, the degree of internal activity, the age of its surface features, and the density of its atmosphere. These properties are intermediate between those of the large terrestrial planets (Earth, Venus) and the smaller planetary bodies (Mercury, the Moon, the asteroids). Exploration of Mars is crucial for a better understanding of the Earth from the perspective of comparative planetology.

The scientific objectives of the Mars Express mission represent an attempt to fulfill part of the lost scientific goals of the ill-fated Russian Mars-96 mission.

Global high-resolution photogeology (including topography, morphology, paleoclimatology, etc...) at 10 m resolution
Super-resolution photogeology of selected areas of the planet (2 m/pixel)
Global high spatial resolution mineralogical mapping of the Martian surface at kilometer scale down to several 100 m resolution
Global atmospheric circulation characterisation, and high-resolution mapping of the atmospheric composition
Subsurface structure characterisation at kilometer scale down to the permafrost
Surface-atmosphere interaction; interaction of the atmosphere with the interplanetary medium
Structure of the interior, atmosphere and environment via radio science measurements
Surface geochemistry and exobiology

The Spacecraft

The Spacecraft Bus

Exploded Diagram of Mars Express

Mars Express spacecraft is designed to take a payload of seven state-of-the-art scientific instruments and one lander to the red planet and allow them to record data for at least one Martian year, or 687 Earth days. The spacecraft will also carry a data relay system for communicating with Earth.

The mission is a test case for new working methods to speed up spacecraft production and minimise mission costs. These new methods have two major impacts on spacecraft design. Weight is being kept to an absolute minimum: 116 kg is allowed for the seven instruments and 60 kg for the lander. And off-the-shelf technology, or technology developed for the Rosetta mission to a comet, is being used wherever possible.

The instruments will sit inside the spacecraft bus which is a honeycomb aluminium box just 1.5 m long by 1.8 m wide by 1.4 m high. The lander, Beagle 2, is attached to the outside of the bus. Payload, lander, spacecraft and on-board fuel will weigh a maximum of 1223 kg at launch.

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 0.15o. 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 deg C) by radiating excess heat into space. 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 deg C).

The plan is to keep the inside of the spacecraft at 10-20 deg 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 deg C in the shade and up to 150 deg 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 deg C).

Orbiter Instruments in Brief

ASPERA Energetic Neutral Atoms Analyser

The Earth and Mars, like the other planets, swim deep inside a plasma of charged particles (ions and electrons) racing outward from the Sun called the solar wind. ASPERA will study how the solar wind interacts with the Martian atmosphere and thus throw light on the mechanisms by which water vapour and other gases could have escaped from Mars in the past. The instrument will use a technique known as energetic neutral atom imaging to visualise the charged and neutral gas environments around Mars.

Dr. Rickard Lundin, Swedish Institute of Space Science, Kiruna, Sweden

HRSC High/Super Resolution Stereo Colour Imager

The HRSC is a stereoscopic camera that will photograph the Martian surface to reveal detail as small as 2 m. The images will be used to produce a geological map showing the location of different minerals and rock types. The HRSC will make use of a modified second flight model of the High Resolution Stereo Camera originally developed for the Mars 96 mission.

Prof. Gerhard Neukum, Freie Universitat, Berlin, Germany

MaRS Radio Science Experiment

MaRS will use radio waves to study both the surface and atmosphere. It will measure local variations in gravity over the surface of Mars and will provide pressure and temperature profiles of the atmosphere.

Dr. Martin Patzold, University of Cologne, Germany

MARSIS Subsurface Sounding Radar/Altimeter

The primary objective of MARSIS is to map the distribution of water and ice in the upper portions of the Martian crust. Using techniques similar to oil prospecting on Earth, the instrument will analyse reflections of radio waves in the upper 2-3 km of Martian crust to reveal the subsurface structure. It will be able to distinguish between dry, frozen and wet soil.

Prof. Giovanni Picardi, Universita di Roma 'La Sapienza', Rome, Italy

OMEGA IR Mineralogical Mapping Spectrometer

Omega will determine the mineral content of the Martian surface and the molecular composition of the atmosphere by analysing sunlight reflected from the surface and diffused through the atmosphere. It will also perform similar analyses on heat radiation emitted from the surface. Information from Omega will contribute to our understanding of the structure of the Martian landscape and the role played by water over timescales ranging from seasons to billions of years. Like HRSC, OMEGA was originally developed for the Mars 96 mission.

Dr. Jean-Pierre Bibring, Institut d'Astrophysique Spatiale, Orsay, France

PFS Planetary Fourier Spectrometer

The Martian atmosphere consists mainly of carbon dioxide and nitrogen with a very small proportion of water vapour and ozone. PFS will measure the global atmospheric distribution of water vapour and other minor constituents with greater accuracy than previous missions.

Dr. Vittorio Formisano, Istituto di Fisica dello Spazio Interplanetario, Rome, Italy

SPICAM UV and IR Atmospheric Spectrometer

SPICAM will measure the composition of the Martian atmosphere over smaller volumes than the PFS instrument. It will measure ozone using a technique similar to that used on the Mariner 9 spacecraft which first discovered ozone on Mars. SPICAM will also use the technique of stellar occultation, to measure the vertical profiles of carbon dioxide, temperature, ozone, aerosols and clouds.

Dr. Jean-Loup Bertaux, Service d'Aeronomie, Verrieres-le-Buisson, France

Instrument Design

ASPERA-3: Analyser of Space Plasmas and Energetic Atoms

The ASPERA-3 (Analyser of Space Plasmas and Energetic Atoms) instrument is made up of two components:

the Main Unit, comprising the mechanical scanner, digital processing unit (DPU), Neutral Particle Imager (NPI), Neutral Particle Detector (NPD) and Electron Spectrometer (ELS)
the Ion Mass Analyser (IMA), mounted separately

ASPERA-3 Main Unit with the ELS protective cover removed. The NPI
particle entrance and the two NPD entry ports are protected by red
covers. Image courtesy Swedish Institute of Space Physics © 2003.

Mechanical Scanner

The mechanical scanner sweeps the three sensors mounted on it through 180 degrees to give the ASPERA-3 instrument 4p steradian (unit sphere) coverage when the spacecraft is 3-axis stabilised. The scanner is equipped with two stepper motors, which turn a worm screw. The screw drives a worm wheel, which is attached to the moving part of the scanner. The scanner payload can be turned to any arbitrary angle or perform continuous scanning. The operational rotation rates are 1.5, 3.0 and 6.0 degrees per second. The system offers an angular positioning accuracy of 0.2 degrees.

Digital Processing Unit

The Digital Processing Unit's main task is to control the sensors and the mechanical scanner. The DPU processes, compresses and stores the sensor data and forwards it (together with housekeeping data) to the spacecraft telemetry system. It also receives and implements commands sent to the ASPERA-3 instrument by the spacecraft telecommand system.

The primary design drivers for the Digital Processing Unit (DPU) are optimum use of the allocated telemetry rate and correct handling of telecommands. The ASPERA-3 instrument makes extensive use of sophisticated lossless data compression to enhance the scientific data yield. The principal compression method used is based on the Rice algorithm, an adaptive compression technique that remains efficient over a wide range of input data entropy conditions. This is achieved by employing multiple encoders, each of which is optimised to compress data in a particular entropy range. The structure of the algorithm also permits a simple interface to data packetisation schemes, such as those used for space data communications, without the need to carry auxiliary information across packet boundaries.

Neutral Particle Imager

In the Neutral Particle Imager, incoming particles pass between two 150 mm diameter discs, which are separated by 3 mm and have a 5 kV potential between them. Charged particles are deflected by the electric field and captured, but neutral particles pass between the discs. The space between the discs is divided into 32 sectors by plastic spokes, forming 32 azimuth collimators with an aperture of 9 degrees by 18 degrees each. Neutrals that pass through the deflector system hit a 32-sided conical target at a grazing angle of incidence (20 degrees). The interaction between the neutral particles and the target results in production of secondary electrons and ions, and / or reflection of the primary neutrals. The particles leaving the target are detected by a Micro Channel Plate (MCP) stack with 32 anodes. The signal from the MCP gives the direction of the primary incoming neutral particle. The MCP is operated in such a way as to detect sputtered ions of the target material, ions resulting from stripping of the primary neutrals and neutrals reflected from the target surface. In order to improve the angular resolution and collimate the particles leaving the interaction surface, 32 separating walls are attached to the target, forming a star-like structure. This configuration allows the particles to experience multiple reflections and reach the MCP. The target is specially coated to prevent incoming ultraviolet photons that strike it from producing erroneous results.

The Neutral Particle Imager covers 4p steradians in one 180-degree sweep by the mechanical scanner and produces an image of the ENA distribution in the form of an azimuth x elevation matrix. The direction vector of 32 elements is read out once every 62.5 ms.

Neutral Particle Detector

The Neutral Particle Detector consists of two identical pinhole cameras each with a 90-degree Field of View (FoV) in the instrument azimuth plane and arranged to cover a FoV of 180 degrees. Particles approaching the pinholes pass between a pair of quadrant deflector plates separated by 4.5 mm and with and 8 kV potential between them. Charged particles with energies up to 70 keV are deflected, while neutrals proceed into the camera. The deflector plates also function as a collimator in the instrument elevation direction.

The collimated Energetic Neutral Atom (ENA) beam emerging from the 4.5 x 4.5 mm pinhole hits a target at a grazing angle (20 degrees) and causes secondary electron emission. The secondary electrons are detected by one of two Micro Channel Plate (MCP) electron multiplier assemblies. The MCP output provides a start signal to the electronics that measures the time of flight of the ENAs over a fixed distance. The incoming ENAs are reflected from the target nearly specularly and travel to a second target. Again, secondary electrons are produced and detected by three more MCPs, which pass a stop signal to the time of flight electronics. The time of flight between the two targets gives the velocity of the incoming particle. Which of the three 'stop' MCPs detects the incoming particle determines its (instrument relative) azimuth direction.

Since secondary electron yield depends on both incident particle mass and velocity, the mass can be determined, given that the velocity is known, by analysing the height distribution of the pulses from the MCPs.

The effects of ultraviolet radiation are suppressed by coating the targets appropriately and checking for coincidence between the start and stop signals used for the time of flight calculations.

As the mechanical scanner moves the NPD through 180 degrees, a 2p steradian (half sphere) coverage of the incident particle field is obtained.

Electron Spectrometer

The Electron Spectrometer determines the energy spectrum of incoming electrons in each of sixteen 22.5-degree sectors.

The Electron Spectrometer is based around a spherical section electrostatic analyser of 'top hat' design. The electrostatic analyser consists of two concentric hemispherical electrodes, the outer of which has a central hole, through which electrons are admitted, covered by the 'top hat' and collimator. Electrons arriving from any azimuth angle and within the elevation field of view of the collimator pass under the 'top hat' and are deflected through the central hole in the outer hemisphere by a positive potential on the inner hemisphere. The electrostatic field between the hemispheres will deflect electrons having an energy in a particular range such that they travel between the electrodes. Electrons with energies outside the selected range will be captured.

These energy band filtered electrons exit the annular gap between the hemispheres and hit a Micro Channel Plate (MCP) electron multiplier. Beyond the MCP, the electrons strike one of sixteen anodes, each defining a 22.5 degree sector of incident azimuth angle.

By varying the electrostatic potential between the hemispheres of the electrostatic analyser, the energy of the electrons selected by the filter can be changed. The voltage applied to the inner hemisphere is swept once every four seconds and the number of anode hits per sample interval is recorded to give an energy spectrum for the incoming electrons in each sector. As the ELS sensor is moved through 180 degrees by the mechanical scanner, a complete 4p steradian (whole sphere) angular distribution of electrons is measured.

Ion Mass Analyser

The Ion Mass Analyser determines the mass spectrum of incoming ions in a selectable energy range. The mass range and resolution of the spectrum are also selectable.

ASPERA-3 Ion Mass Analyser with red protective cover removed to expose the
particle entrance. Image courtesy Swedish Institute of Space Physics © 2003.

Ions arriving at the IMA pass through an outer, grounded grid and enter the deflection system. The deflection system comprises two curved, charged plates that deflect ions arriving in the instrument elevation range from 45 degrees above to 45 degrees below instrument azimuth plane and from any azimuth angle into the entrance of the electrostatic analyser.

The electrostatic analyser consists of two concentric hemispheres with a variable electric field between them. Ions that lie within the energy pass band of the analyser travel between the hemispheres, exit the annular space separating them, and travel on towards the magnetic mass analyser. The electrostatic potential between the hemispheres determines the energy range of the ions that pass through the analyser.

In the magnetic mass analyser, the ions pass through a static, cylindrical magnetic field, which deflects light ions towards the centre of the cylinder more than heavy ones. An electrostatic potential can be applied between the electrostatic analyser and the magnet assembly to accelerate the ions. Varying this potential allows selection of the mass range to be analysed and the mass resolution.

As the ions leave the magnetic mass analyser they hit a Micro Channel Plate (MCP). The electrons exiting the MCP are detected by an imaging anode system. A system of 32 concentric rings measures the radial impact position, which corresponds to ion mass and 16 sector anodes measure azimuthal impact position, which corresponds to ion azimuth angle.