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OUR most recent odyssey to Mars began in July 1988, with the launch of two spacecraft called Phobos (1 and 2), and ended in March 1989, when ground controllers lost touch with the second craft. The international project involved 14 countries and the European Space Agency. It was the first flight designed especially to study one of the rocky minor bodies of the Solar System. Most of these orbit the Sun as ‘asteroids’, between the orbits of Mars and Jupiter. But Mars has two small moons that are probably asteroids captured by the planet. The space mission was intended to investigate the larger of the two, Phobos – hence its name.

The results from the Phobos mission came in three stages. The first were observations made as the spacecraft travelled between Earth and Mars; the second comprised observations and measurements of Mars; and the third, the observations of the moon Phobos itself.

Unfortunately, the Phobos mission did not carry out its programme in full. Early in September 1988, ground controllers lost contact with Phobos-1 after sending it an incorrect command. This switched off the orientation system of Phobos-1, and its solar panels stopped facing the Sun. With the onboard systems starved of power, the probe could not respond to any commands sent from the Earth.

After the loss of the first probe, the controllers took additional measures to make Phobos-2 more foolproof. They decided to correct its trajectory on the way to Mars only once, instead of twice as originally planned, even though this would increase the height of its orbit over the planet’s surface and produce fewer scientific results.

On 29 January 1989, 200 days after its launch, Phobos-2 went into a highly elongated elliptical orbit above Mars’s equator. Subsequent corrections gradually transformed the orbit into a circle around Mars, 350 kilometres above the orbit of Phobos. The planners chose to make these complex manoeuvres because they had only very scant information about the orbit of Phobos around the planet. All the data had been culled from observations from Earth during the short periods when the faint Martian satellite was visible, and also from the information supplied by three American spacecraft back in 1971 and 1976.

First Phobos-2 observed Mars, its atmosphere and space near the planet when in the elliptical ‘parking orbit’. It carried on observing Mars for a further three days in this orbit. Then, with the probe at a distance of between 860 and 1130 kilometres from Phobos, it began the first television session of the Martian moon.

The spacecraft obtained nine television pictures of Phobos. The controllers needed this information to update their knowledge of the precise positions of Phobos and the space probe, in order to bring them closer together. When the distance between Phobos and the probe had diminished to between 320 and 440 kilometres, the spacecraft took more television images. These pictures were used not only for checking the position of Phobos but also for identifying the shape and details of its terrain.

On 21 March, the probe took up an orbit in which it kept pace with the Martian satellite, swinging between 400 kilometres further out from Mars than Phobos and 200 kilometres closer in. During this period, the probe completed one more television picture-taking session. At the same time, the controllers were preparing to put the probe on the side of Phobos not facing Mars, at a distance of 35 kilometres. Once there, the spacecraft was to have begun an entirely novel phase of its flight, moving along with the moon and making elaborate manoeuvres over its surface. According to the plan, the spacecraft would move in to hover only 50 metres above the Martian satellite. From here it would investigate the surface by bombarding it with lasers and beams of ions. The probe would then have lowered two landing modules – one fixed and the other mobile – onto the surface of Phobos.

But, unexpectedly, on 27 March, radio contact with the probe was lost. Roald Kremnev, the chief designer at the G. N. Babakin Research and Testing Station that built the probe, says that until that point the data from Phobos-2 indicated that all systems were in full working order. We do not have any information yet on the causes of the failure. Mission controllers will reach their final conclusions after processing all the information received during attempts to regain contact with the craft.

Despite the early termination of the mission, the staff of the Institute of Space Research and many foreign colleagues who prepared and participated in the investigations do not see it as a failure. The probe’s instruments have collected new data on the activity of the Sun and interplanetary gas , the surface and magnetic field of Mars, and Phobos.

One unquestionable achievement has been the measurement of ionised gas – plasma – in the vicinity of Mars. Phobos-2 carried instruments to study magnetic fields, electric and magnetic waves, and other phenomena associated with plasma. It measured the magnetic field with two magnetometers, one from Austria and the Soviet Union and the other built by East Germany and the Soviet Union, and the plasma waves with an analyser that was developed by Czechoslovakia and the European Space Agency. Investigation of plasma waves can reveal what happens as the ‘solar wind’ of charged particles from the Sun sweeps past a planet and interacts with its magnetosphere – the region where its magnetism dominates the solar wind.

The first investigations of the Martian magnetosphere were carried out between 1971 and 1974 by the probes Mars-2, Mars-3 and Mars-5. These observations indicated that Mars has a magnetic field, although very weak. They determined the shape and size of the magnetosphere including a long ‘tail’ stretching away from the planet. These studies were limited, however, both by the instruments on board and by the orbits of the craft. The American craft, Mariner-9 and Vikings 1 and 2, had no instruments at all for such studies.

The weak magnetic field of Mars means that the solar wind must reach the planet’s upper atmospheric layers before being stopped by the magnetic field. As a result, the magnetosphere is formed in a region where the solar wind is interacting simultaneously with both the planet’s magnetic field and its atmosphere. In consequence, the Martian magnetosphere must differ substantially both from that of the Earth, where the stronger magnetic field stops the solar wind before it hits the atmosphere, and also from that of Venus. Venus has no magnetism of its own and its magnetosphere is formed by the solar wind disrupting the planet’s ionosphere.

During its complicated loops around Mars, Phobos-2 was able to investigate the Martian magnetosphere, and its tail, in detail. The spacecraft found that the Martian magnetosphere, like that of the Earth, has distinct structures, such as a magnetopause (the boundary of the magnetosphere), a plasma layer in the tail, and a shock wave in the flow of the solar wind in front of the magnetosphere. It was also able to discern some other, finer structural details of the Martian magnetosphere.

The magnetosphere is largely filled with a relatively cold plasma that comes from the atmosphere of Mars. But Phobos-2 also found islands of hotter plasma, which scientists think come from the solar wind. This suggests that Mars’s weak magnetic field is closely interwoven with the interplanetary magnetic field, creating a natural ‘magnetic’ channel for solar plasma to penetrate into the magnetosphere. The close relationship makes it difficult for us to distinguish how much of the measured magnetism comes from the interplanetary material and how much is due to the planet. As a result, we cannot tell with any certainty the strength or direction of Mars’s own magnetic field.

The interplay between the planet’s magnetism and the solar wind has important consequences for the Martian atmosphere, helping its gas to leak away into space. The lines of magnetic force from Mars connect with those in the solar wind, forming a channel by which ions from the atmosphere can escape. Phobos-2 observed this process in action. Its plasma instruments measured separately the flow of plasma in the solar wind (in the main, hydrogen ions) and of the plasma coming from the planet’s atmosphere (mainly ions of carbon dioxide, and of molecular and atomic oxygen). The measurements from the atmosphere revealed the rate at which planetary ions are escasping into space. This flow is between 2 X 1025 and 5 X 1025 ions per second. In other words the atmosphere of Mars is losing between 1 and 2 kilograms of its substance every second. This may not seem to be a lot, but when we take into account the thinness of the Martian atmosphere (the pressure on the planet’s surface is 1/170th of that on the Earth), such losses may have a substantial effect on its evolution.

The estimated loss of gas from the Martian atmosphere is practically equal to losses along the magnetic tail of the Earth’s magnetosphere. For the Earth, such a loss is negligibly small; it would take 10 billion years – twice the age of the Solar System – for the Earth’s atmosphere to disappear. But in the case of Mars, this rate of loss means that the planet would lose its atmosphere in a time much shorter than the age of the Solar System. If the planet’s gases are continuously replenished by the evaporation of water from ice frozen in the soil, the present rate of loss is equivalent to the disappearance from the planet’s surface of a layer of water 1 to 2 metres deep during the history of Mars. So the weakness of its magnetic field may have been responsible for considerable erosion of its atmosphere and possibly to the loss of much of the water with which it formed.

Also unexpected was the discovery of beams of accelerated ions in the magnetosphere, similar to beams of electrons and ions that form the Earth’s aurorae when they hit the atmosphere. Unlike the Earth, where these particles form radiation belts or Van Allen belts, around the equator, Mars has no permanent radiation belts. And despite the similarity of the ion beams, the spacecraft did not detect aurorae at Mars.

Among the most interesting results on Mars itself are the infrared images of the planet’s surface, taken by the Soviet-made Thermoscan instrument. In these images we see the planet in the heat radiation that it produces rather than reflected sunlight. So the brightness in these images indicates the temperature of the surface. The heart of the Thermoscan is a highly sensitive infrared detector cooled by liquid nitrogen. Never before has an instrument like this been put in a long-distance spacecraft. Nor has a visible image of a planet been built up from its thermal emission, with the sole exception of the Earth – its ‘thermal portraits’ are transmitted regularly from weather satellites.

From a circular orbit 6000 kilometres in radius, the Thermoscan surveyed a considerable region around the equator of Mars in a sweep approximately 1500 kilometres wide and with a resolution of some 2 kilometres. Its thermal images are remarkable for their sharpness and high contrast – superior to the best television pictures of Mars. The differences in temperature that are revealed indicate the physical characteristics of the surface, especially the degree to which the soil is fragmented. Thus, the thermal images simultaneously offer information on the large-scale features of the surface and on its microstructure. The Thermoscan also detected radiation of shorter wavelengths reflected by the planet’s surface. A comparison of the brightness at these visible wavelengths with those at the infrared wavelengths will be very important in interpreting the data.

Another Soviet-built instrument registered Mars’s emission not just in two, but in 16 parts of the spectrum. Six of them were in the thermal infrared band, and 10 at shorter wavelengths in the near ultraviolet and visible parts of the spectrum. This instrument could not construct images. But thanks to the large number of wavelengths that it observed, it enjoyed a number of advantages. For example, it made measurements at infrared wavelengths absorbed by carbon dioxide, which allowed it to determine the temperature of the Martian stratosphere. Its observations at shorter wavelengths should help us to understand the nature of aerosol particles in the atmosphere of the planet.

Another instrument, the French mapping infrared spectrometer, measured the planet’s spectrum at 128 wavelengths in the near infrared. A series of absorption bands characteristic of various minerals is located in this range. The data should allow researchers to map rocks made up of different minerals over the planet’s surface. In particular, they will show the content of water bound in the structure of minerals. ÐÓ°ÉÔ­´´s are also going to make maps of pressures and height of the terrain, using information from absorption lines of carbon dioxide, and to evaluate how much water vapour the atmosphere holds.

Even preliminary analysis of these data shows considerable variations in the mineralogical make-up of the surface. The amount of water bound up in some of the rocks suggests that they are sedimentary, indicating that they formed in bodies of water that no longer exist on Mars. If so, this would provide an important clue to understanding the planet’s evolution.

Phobos-2 also studied the vertical structure of the planet’s atmosphere, using two linked instruments, one made in France, the other in the USSR. This new method involves measuring the spectrum of solar radiation that passes through the Martian atmosphere when the Sun is seen near the edge of the planet. At this point the sunlight crosses the largest possible amount of atmospheric gas and dust. Successive spectra correspond to different heights above the planet’s surface, so an analysis can reveal the distribution of various components in the atmosphere with altitude. The instruments observed at wavelengths corresponding to spectral lines of carbon dioxide, ozone and water vapour, and so for the first time we now have a vertical section of the relative amounts of different atmospheric gases on Mars. Preliminary analysis of the data has shown that at altitudes of 20 to 60 kilometres above Mars, the content of water vapour in the atmosphere is on average close to one ten-thousandth of that of the main component, carbon dioxide. The content of ozone varies considerably with altitude.

One of the major achievements of the mission was the series of television pictures of Phobos. The television system comprised three cameras, a spectrometer, a control system and a video recording system. The light detector in each of the cameras and the spectrometer consisted of a single crystal of silicon, the surface of which contained hundreds of thousands of sensors. These transformed incident light into electric signals in proportion to its brightness.

Many Soviet and foreign scientists helped to develop this system. Soviet specialists were in overall charge and they also devised and built the optics and the light-detecting silicon chips. East German scientists developed a video memory to store more than 1000 pictures for subsequent transmission to Earth, while Bulgarians developed the electronics and microprocessor units, and undertook perhaps the most difficult part of the job – piecing together the system from its separate elements. At various stages, scientists and experts from France, the US and Finland also gave a helping hand.

The controllers initially used the television pictures to help them to navigate. The pictures increased the accuracy of predicting the position of the Martian moon approximately tenfold. As a result, the controllers could correct the probe’s orbit and bring it within 200 kilometres of Phobos. The motion of Phobos and its gravitational effect on the spacecraft will provide information on the mass, density and internal structure of the moon.

The spacecraft obtained some 40 images of Phobos, from distances that ranged between 200 and 400 kilometres. They cover more than 80 per cent of its surface. The pictures taken at the minimum distance show details as small as 40 metres across. The photographs from Phobos-2 complement those from Mariner-9 and the Viking orbiters, both in terms of the surface covered and in spectral zones. Phobos-2, for example, produced more detailed photographs of an area west of the largest crater on Phobos, Stickney. The new photographs will do much to determine the shape of Phobos and improve our maps. For the first time Phobos was also photographed in the near infrared. The television pictures show how the brightness of the surface depends on the angle at which light strikes it, so providing information on the size and shape of the microscopic particles that make up its surface.

A combination of instruments on Phobos-2 provided, for the first time, a spectrum that stretched all the way from the ultraviolet to the infrared, at wavelengths ranging from 0.32 to 3.2 micrometres. This shows that Phobos is very dark, reflecting only 4 per cent of the light falling on it, and that the reflectivity is almost the same at all wavelengths. These properties are similar to a type of meteorite – the carbonaceous chondrites – that occasionally falls to Earth. But Phobos has less water than a typical carbonaceous chondrite, and the surface varies in composition from place to place. The findings show that the temperature of the surface of Phobos is about 300 K (27 Degree C).

ÐÓ°ÉÔ­´´s around the world still have a great deal of data from Phobos-2 to analyse. As well as its successes in observing Mars and Phobos, the spacecraft has been important as the proving ground for future Soviet missions. ‘The Phobos spacecraft is a new modification of interplanetary probes and has been developed for more sophisticated tasks of space investigation up till the year 2000,’ says Vyacheslav Kovtunenko, the general designer at the G. N. Babakin Centre. Previous Soviet probes to the planets, including the Venera craft that went to Venus, had their instruments encased in a cylindrical vessel. The instruments on Phobos were arranged in blocks on the outside of the craft. This more versatile arrangement allowed the craft to carry an additional 540 kilograms of scientific payload.

‘It is, therefore, an entirely new type of probe,’ Kovtunenko continues. ‘We believe it has lived up to expectations. The only malfunction, apart from an operator’s mistake on Phobos-1, was due to the more complex problem of integrating command devices used for scientific and control purposes. Today we are fully aware of the kind of changes that need to be effected in the space probe to make it dependable in future launches.’

Yuri Zaitsev is head of the Institute of Space Research at the USSR Academy of Sciences.