Over 40 years ago the Soviet Union made history by launching its first space satellite. Since then thousands of both manned and unmanned space probes have been launched by this and other countries. Over the years many of these satellites have completed their mission, but have not been retrieved and remain, together with their carrier rockets and other "bits and pieces" (fragments produced by accidental explosions or mechanical and communications failures), in near-earth space and warrant no other name except space junk. Studies associated with this rather unconventional type of "pollution" are discussed by a team of researchers of the RAS Institute of Astronomy: Lydia Rykhlova, Dr. Sc. (Phys. & Math.), Mikhail Smimov, Dr. Sc. (Phys. & Math.) and Anatoly Mikisha, Cand. Sc. (Phys. & Math.).

The problems, which usually escaped our attention at the initial stages of our euphoric achievements in space research came as a bolt from the blue. The year 1961 saw an accident with the US satellite in the TRANSIT series and in 1964 Russian space experts blew up in orbit (by a command from the Mission Control) the COSMOS-50 satellite. This was the beginning of "man-made" technogenic pollution of circumterrestrial orbits. And although its scale continued to grow with time, both experts and the public at large took these alarming reports in their stride.

The problem of space pollution was put on the agenda in earnest in the early 1980s when the conditions in

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Debris in space:

1 - Number of objects, including those not listed in official catalogues:

2 - number of objects listed in catalogues;

3 - fragments of space debris;

4 - space probes;

5 - upper stages of rockets;

6 - other debris.

near-Earth space deteriorated to such as extent that they presented a real hazard not only to manned satellites and orbital stations, but to people in various parts of our planet. On more than one occasion we were spared disasters by sheer good luck. In 1978, for example, the COSMOS-954 satellite fell into the woods of Northern Canada, and a year later pieces of the US SKYLAB hit some desert areas of Australia. During the unsuccessful launch of the US navigational satellite with nuclear power sources on board in 1964 radioactive debris were scattered over the Indian Ocean. Finally, the problems of pollution of near-Earth space were put on the agenda of various international forums, of the annual UN sessions on the peaceful uses of space and its committees on scientific, technical and legal matters. Later on experts singled out as a special branch of astronomy the studies of man-made objects in outer space. This now stands between the traditional studies of meteorites dealing with matter of the Solar system near and within the Earth's atmosphere, and planetary studies of space beyond the celestial confines of the Earth, so to speak.

The first concrete steps for the permanent monitoring of objects in space were made by the defense agencies of the former Soviet Union and the United States within the framework of their anti-missile programs. Both countries began to develop systems of monitoring of near-Earth space equipped with long-range radars and optical gear for the detection, tracking, determination of coordinates and the obtaining of images of such objects, their identification and analysis. Today such systems "keep an eye" on over 10 thousand bits and pieces of various shapes and sizes, beginning from 10 cm and more.

About 8 thousand of them are listed in the official catalogues and the "active" space probes (about 500) make up only an insignificant share of that amount.* The rest are space probes which have exhausted their power resources, final stages of carrier rockets, various pieces of launch equipment and suchlike things. And one has to bear in mind that during the past decade more than 20 thousand objects with a total mass of over 3 thousand tons have been launched into near-Earth and deep space.

The problems of anthropogenic pollution of near-Earth space are high on the agenda of NASA in the United States and of the Institute of Astronomy of the Russian Academy of Sciences in this country.

As has been mentioned before, the monitoring services of near-Earth space maintain catalogues of space junk, covering objects measuring 10-30 cm at the minimum for the lowest orbits (from 200 to 2,000 km) and of about one meter for geostationary orbits (35,800 km). And one can calculate only approximately the number of "pieces" of 1-10 cm (approximately 70,000-150,000), since they cannot be observed with the help of telescopes or radars which means that they do not lend themselves to any classification. And the situation is even worse with smaller pieces (1 cm) of which there must be several millions, to say nothing of gas and dust levels.

To make the dangers involved more clear, suffice it to say that any fragment of more than 1 cm in size hitting a satellite is likely to put it out of action. And should it carry a nuclear reactor, the consequences will be really unpredictable. To reduce the risk of any such occurrences experts have so far have thought of nothing better than what they call building models of debris "populations" not covered by catalogues. The main source of such models are calculations of the likely emergencies in space including the disintegration of space probes and carrier rockets as a result of explosions and high- velocity collisions with space debris. Experts also point to a regularity which consists in the fact that the smaller is the size of the colliding fragments, the more debris they produce.

Studies have born out experts' expectations that the most polluted are the near-Earth orbits where the "space traffic" is the heaviest (altitudes of 850-1,200 km), and the geostationary one which carries weather satellites, probes for the remote

* See: A. Nazarenko, "Cosmos ... in Danger", Science in Russia, No. 3, 1995. - Ed .

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Diagram of space debris distribution in near-Earth space: at altitude of 850-1,200 km and within 38,500 km.

sounding of the Earth and most of the nuclear-powered space gear. The latter can survive in these orbits for hundreds of years and a disaster can occur in a collision with no fragment of less than 0.1 cm in diameter flying at a bullet velocity of 10 km/s.

The geostationary orbit is "populated" with stationary probes-mainly communications satellites of which there are now more than 800. The annual increment ranges from 20 to 30 and the old ones which go out of action become space junk. And one should also bear in mind that man-made objects reaching this region are in danger of collisions with micrometeorites and bolides. This problem is one of the central ones for our scientists working within the framework of the Space Debris Program.

Studies of near-Earth space in the region of manned flights (altitude of about 400 km) have proved that even despite its maximum pollution with space junk this junk happens to have a relatively short life span. Its orbits diminish with time until it enters the upper atmosphere and bums up. But the geostationary orbit is something quite different, and it was believed for a long time that it is not subject to the mechanism of "self- purification". But a series of studies, conducted at the RAS Institute of Astronomy and focusing on the long-term evolution of high-orbit space objects under the effect of light pressure, have now altered that view. It has been proved that even in that region, although at far greater time intervals, the orbit is "automatically cleared" of the used probes.

Both - natural heavenly bodies and fragments of man-made probes - call for the setting up of a special "identification service". The thing is that the satellites which have remained in space for 20 years and more and which have since been written off as junk (of TRANSIT series, NOAA, COSMOS and the upper stages of the VOSTOKs) produce different numbers of fragments (6 to 50 on the average).

Finally, as space probes become more and more complicated-whole "factories" placed into orbit-they require more powerful carrier rockets which add to the volumes of space pollution. For example, the blasts of the second stages of seven DELTA rockets (because of some residual fuel and overheating) added more than 1,300 fragments observed and catalogued. Studies over the past few years by specialists of the RAS Institute of Astronomy and NASA have proved that more than 40 percent of the "scrap iron" in low circumterrestrial orbits are fragments produced by the explosions of the second stages of rockets and satellites.

And the problem is far more complicated as concerns geostationary probes. At the end of their service life, when their orbits are not corrected any more (since that moment they are called "passive") they begin to obey the laws of celestial mechanics - leave their (geostationary) orbit or even re-enter it at some moment of time. And bearing in mind that satellite launches at this altitude have been conducted on a "mass scale" since 1963, any spontaneous reentries of such abandoned probes can have catastrophic consequences. This underlines the importance of keeping a close check on all objects and their movements in the geostationary orbit. This can be done with the help of large wide-angle photo cameras which can "screen" within one night the whole visible area from one observation station. The biggest camera of this kind, installed at the Zvenigorod Observatory (near Moscow) of the RAS Institute of Astronomy, can cover an area of about 100 ⁰ of longitude and register all geostationary objects more than 1 m in size.

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The authors of the article are of the opinion that collisions and explosions of satellites in the geostationary orbit occur as often as at lower altitudes, but the search for and investigation of their fragments pose a special problem. The first step towards its solution, strange as it may seem, is recording the fact of an explosion. Its visual registration being rather improbable (rare case), experts more often rely on indirect "clues", such as changing orbit parameters of a satellite or its drift velocity.

Different types of equipment have been suggested for tracing bits of space junk in geostationary orbit. For example, the Russian EXPRESS-2 satellite launched in September 1996, carried a Geostationary Impact Detector device developed by experts of the European Space Agency for tracing small fragments floating in space and for studies of minor components of interplanetary dust. At the present time specialists of the RAS Institute of Space Studies are developing a cooled telescope for similar observations which is to be installed on board a geostationary satellite. This instrument can detect asteroids approaching the Earth and also meteorites, bolides and mini- comets (according to data from the POLAR satellite these can be registered in the atmosphere at the rate of tens per minute).

The evolution of the "junk" surrounding our planet cannot be accurately predicted because of the steadily growing number of "customers" using near-Earth space (including commercial launches), the advent of new technologies of launching minor probes and even "constellations" of communications satellites (of the IRIDIUM type), and, finally, because of the unpredictability of any future blasts and collisions of these pieces of equipment in orbit.

On the strength of their analysis the authors of the article believe that the vast amount of fragments of different nature and origin floating in near-Earth space makes it practically impossible to keep an eye on all of them on a constant basis. But still and all, there are ways and means which can help experts to approach as close as possible the solution of this problem.

First of all, this includes improving the methods of modelling of fragments of space litter on the basis of special experiments and matching model parameters with the available data. Another area are the studies of the common regularities of the processes of migration of matter in the solar system, the identification and catalogization of our likely "guests" from space. Finally, there must be constant observations of artificial objects in space, their photometric control and identification.

A. M. Mikisha, L. V. Rykhlova, M. A. Smirnov, "Pollution of Space", Vestnik RAN, Vol. 71, No. 1, 2001

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