Mars Beckons for UK Scientists
After a lull lasting 20 years, the exploration of Mars is once again beginning in earnest with the launch of three spacecraft towards the Red Planet within a month. First to go on 5 November is the American Mars Global Surveyor. It will shortly be followed by the Russian-led Mars 96 and the American Mars Pathfinder missions. UK scientists are contributing to this new wave of exploration by participating in the ambitious Mars 96 project.
Mars 96 is scheduled for lift-off from Baikonur cosmodrome in Kazakhstan on 16 November. Included in the spacecraft payload are 23 scientific instruments on an orbiter, two penetrators and two small stations.
One of the instruments installed on the orbiter is the Fonema (Fast Omni-directional Non-scanning Energy Mass Analyser), one of a package of experiments designed to study the way in which the charged particles of the solar wind interact with Mars. Fonema is located on the corner of one of the solar panels which provide power for the spacecraft. It was designed and developed at the Mullard Space Science Laboratory at Holmbury in Surrey under the leadership of Professor Alan Johnstone and Dr. Adrian James.
The instrument is intended to study the 'hot' ions (20 eV to 8 keV) - protons, alpha particles, and positively charged oxygen, carbon and nitrogen atoms - which occur in near-Martian space. This will be done by using a new type of mass spectrometer. Ions entering the instrument will be reflected by a central electrostatic mirror to one of 36 detectors which surround it. The mass and energy of each ion can be measured from the cloud of electrons generated when it strikes a detector and from the degree by which the ion is deflected as it travels through the analyser.
The direction from which each ion arrives can also be approximately determined from which of the analysers it strikes and the position of the hit. This gives a three-dimensional distribution across almost the entire sky. The novel feature of the equipment, which has won international acclaim, is its ability to complete each all-sky measurement in a mere 125 milliseconds, about 20 times faster than previously achieved by any similar instrument.
The experiment will provide evidence about whether Mars possesses a magnetic field and about the nature of the plasma processes taking place in near-Martian space. It should also measure the rate at which oxygen molecules are escaping from the atmosphere during the breakup of water. This should enable scientists to determine what happened to the water which once flowed on Mars and how long ago the planet began to dry out.
Another experiment package which will be studying the interactions between Mars and the solar wind is the ELISMA (Electromagnetic Investigation of Mars) complex on the orbiter. ELISMA is the main dedicated plasma wave experiment on the mission and comprises a series of sensors mounted on booms projecting from one of the solar arrays. The various instruments have been built by an international consortium including nine institutes from six European countries. Among the participants are the Universities of Sussex and Sheffield which have provided the Digital Wave Processor (DWP) component of the wave experiment.
The DWP has been developed jointly by Dr. Paul Gough at Sussex and Dr. Hugo Alleyne at Sheffield. The Sussex contributions include a fault-tolerant processing unit and data compression software, while Sheffield was responsible for the overall design of the hardware and the integration of the DWP with the other ELISMA instruments.
The DWP is particularly important since the other experiments in the wave complex rely on it to control and access their data. In return, the UK DWP institutes have full access rights to all ELISMA acquired data.
ELISMA will be switched on several times during the cruise phase to Mars. An initial early turn-on will allow calibration with known phenomena whilst still within the Earth's magnetosphere. Approximately one month later the Earth's distant magnetotail will be encountered. When the spacecraft arrives at Mars in September 1997, results will be gathered about the nature of the interaction of the planet with the solar wind.
This interaction is likely to be intermediate between that of the magnetised planets such as the Earth and Jupiter and that of Venus which has no magnetic field worth mentioning. Earlier missions have shown the existence of a bow shock and a pressure boundary that probably changes in character dynamically with time between an ionopause and a magnetopause. There is also some evidence of trapped particles, or radiation belts, at Mars.
ELISMA will study the role of magnetic and electric field fluctuations in the near-Martian environment. It will allow the team to map the electromagnetic fields and study waves in the ionosphere/magnetosphere of the planet. Scientists will also learn whether Mars has magnetic substorms and auroras (Northern Lights), and whether plasma waves are also generated by summer dust storms and lightning in the lower atmosphere.
While the Mars 96 orbiter surveys the planet and its atmosphere from above, a number of experiments will be carried out on the surface. They include the launch of two large dart-shaped penetrators from the mother craft, the first time that such devices have been used on a planetary mission. The penetrators will be partially buried in the surface after striking it at a velocity of about 80 m/sec. Tiltmeters for the penetrators, together with all of the data processing and control electronics for the lower part of each penetrator, have been provided by the University of Kent.
The tiltmeter is on the forebody of the penetrator which separates from the rear section after impact. "It will measure the final resting angle relative to the vertical," said project manager Howard Shaw. "It is used for calibration of other instruments, notably the seismometer and the impact accelerometer, both Russian. Although it may be buried 6 metres underground, the Russians are confident that the two parts (of the penetrators) will come to rest in a straight line relative to each other, so the tiltmeter angle will also be relevant for the magnetometer, the meteorological studies such as wind direction, and for measuring the angle of the horizon as seen by the camera on the top part."
The Kent electronics will control the other experiments within the penetrator. These will make measurements of soil temperature and chemical composition as well as measure 'Marsquakes'. The results will then be passed on to Earth via the radio transmitter links.
The mass of the spacecraft at launch is expected to be about 6.6 tonnes, including a science payload of 645 kg and over 3 tonnes of fuel for trajectory correction and spacecraft stabilisation. Apart from detailed studies of the planet by the orbiter, the mission plan includes the release of four landers - two small stations and two penetrators.
The small stations will be released three to five days before the main craft arrives in orbit around the planet. They will parachute onto the surface and send back pictures of the local environment together with data on the changing weather, and the nature of the surface material. Among their payload will be two CD-Roms entitled Visions of Earth. Intended as 'time capsules' to be found by future Mars explorers, they include more than 70 novels, stories and articlesdocumenting our fascination with the Red Planet over the centuries.
The orbiter is scheduled to arrive at Mars on 12 September 1997. It will slowly be manoeuvred into a highly elliptical near-polar orbit. Once the two penetrators are dropped onto the planet, the orbiter will spend the next year mapping the planet at high resolution and studying its composition. Some instruments will also study the Martian atmosphere and ionosphere.
The penetrators will embed themselves in the surface and then carry out similar studies to the small stations. All four surface sites will radio back their data to the Mars 96 orbiter or the Mars Global Surveyor as they pass overhead.
WWW LinksFurther information on the Fonema experiment is available on the World Wide Web at... http://mssla3.mssl.ucl.ac.uk/www_plasma/homepage.html
Details of the Sussex participation are available at... http://www.susx.ac.uk/engg/research/space/
General information on the Mars 96 mission is found at... http://www.iki.rssi.ru/mars96/mars96hp.html
or on the NASA site at... http://www.jpl.nasa.gov/files/misc/m96pkt.pdf
Dr Jacqueline Mitton, RAS Public Relations Officer
Peter Bond, Space Science Advisor