Space debris are artificial, non-functional objects orbiting the Earth. These debris are satellites or rockets out of operation, non-operational material released by space operations and fragments from satellites or rockets due to explosions or collisions.

Orbital debris history and population

Nowadays more than 8,000 orbiting objects are known with diameters larger than 10 cm. This population grows at a rate of some 175 objects every year resuting from the 120 annual launchings. One sixth of the orbiting objects are rocket stages, one fifth consists of non-operational payloads, a further 12 % are pieces of hardware released during space operations and, finally, 40 % of these objects are fragments resulting from the over 160 satellites and rocket stages that have been destroyed on orbit, either deliberately (30 % of the cases) or accidentally. Only 6 % of the catalogued objects are operational satellites. Apart from this, there is an estimated 1 to 10 cm size fragment population of more than 100,000 objects and tens of millions of still smaller particles, surpassing by far the natural meteoroid population around our planet.

The majority of space debris is obviously located in the most useful altitude bands, that is to say in LEO (Low Earth Orbit, up to an altitude of about 2,000 km above the Earth surface) and in GEO (Geostationary Earth Orbit, at an altitude of 35,788 ± 300 km). Specifically, the density maxima are located at altitudes of some 850, 1,000, 1,500, 20,000 (semigeosynchronous orbit) and 36,000 km. The maximum density in both altitude bands is comparable (of the order of one object bigger than 10 cm per 100 million cubic km), but the flux is much larger in LEO because of the smaller total volume of the region and because of the higher velocity of its objects. The GTO (Geostationary Transfer Orbits, with perigee at 180-500 km and apogee in GEO), also host a considerable population of space debris.

Some curious facts about the space debris population are:

• The oldest debris still on orbit is the second US satellite, the Vanguard I, launched on 1958, March, the 17th, which worked only for 6 years.
• In 1965, during the first american space walk, the Gemini 4 astronaut Edward White, lost a glove. For a month, the glove stayed on orbit with a speed of 28,000 km / h, becoming the most dangerous garment in history.
• More than 200 objects, most of them rubbish bags, were released by the Mir space station during its first 10 years of operation.
• The biggest amount of space debris created by an only spacecraft destruction was due to the upper stage of a Pegasus rocket launched in 1994. Its explosion in 1996 generated a cloud of some 300,000 fragments bigger than 4 mm and 700 among them were big enough to be catalogued. This explosion alone doubled the Hubble Space Telescope collision risk

Collision risk and probability

On average, the relative velocity between LEO objects is about 10 km/s (a rifle bullet has a speed of some 800 m/s), so an 80 grams object possesses a kinetic energy equivalent to the explosion of 1 kg of TNT, enough to destroy completely a 500 kg satellite on collision. Even an object of 1 cm diameter could considerably damage a satellite if there happened to be a collision.

Given the present population of orbiting objects, the collision probability is not negligible. However, to be sure that an explosion or malfunction has been due to an impact with space debris, one must be able to discard completely the possibility that it has been produced by a failure of the satellite itself, and this is very difficult. Nevertheless, it is reasonably certain that the destruction of Kosmos 1275 in 1981 was due to a collision and it is completely proved that the orbit change of the CERISE French military minisatellite, in July, 1996, was due to the impact of an uncontrolled fragment from an Ariane rocket that had exploded 10 years before and which collided with the CERISE altitude control arm at almost 15 km per second. Apart from this, more than 25 among the 160 explosions or fragmentations that have taken place on orbit have not been explained yet, and some of them could well have been caused by collisions with space debris.

It has been calculated that the probability of two objects with diameters larger than 10 cm crashing in the altitude band 800 to 1000 km is 1/100 per year, so the probability of an impact with these characteristics in the next 10 or 15 years is higher than 50 %. And the probability that some of the satellite destructions that have occurred in this band has been caused by the impact of an object of 1 to 10 cm of diameter is 40-70 %.

Also, the high object flux in LEO can give rise to cascade effects if the debris creation rate is larger than the rate of re-entry into the terrestrial atmosphere. It seems that the critical density for this effect to take place has already been achieved in certain altitude bands (around 900 and 1500 km of altitude) and, if the number of objects continues to increase as usual, this phenomenon will become general in 10 or 15 years, when the LEO object density will be twice the present one, causing an exponential increase in the number of fragments.

Impacts with metallic fragments whose diameters are smaller than 1 millimeter are frequent, as is attested by the over 700 craters with diameters larger than 1.2 mm, observed on the 20 square meters of solar panels in the Hubble Space Telescope. In fact, the probability of an impact with a 0.1 mm object against a 100 squared meter surface on orbit at 400 km of altitude (as is the case for the International Space Station -ISS-) is of 1 every 10 days, although luckily this kind of collisions are seldom fatal.

Some of the experiments aimed to better understand the space debris problem consisted on orbiting spacecrafts deliberately left to stand the debris collisions, before being recovered and studied on the ground. The most famous among these experiments was the Long Duration Exposure Facility (LDEF), which was hitted by tens of thousands of tiny space debris fragments from 1984 to 1990.

It is also possible that two operational satellites collide in the geostationary ring, since, because of the high occupation level of this orbit, it is necessary to locate several active satellites in the same longitude window (that typically measures 0.2 degrees only). For example, given a group of 4 satellites, located in the same longitude window, the probability of two of them approaching at a distance smaller than 50 m is 60 % per year (assuming that they are controlled by different ground-based centres, although usually they will be controlled by the same centre and so the risk will be lower). Globally, the collision probability between two of the 700 objects with diameters larger than 1 m located on the geostationary orbit is 1/500 per year.

A clear example of the problem is that it has been calculated that the new groups of several tens or even hundreds of identical telecommunications satellites (also known as satellite constellations), which will be located on orbits between 1,500 and 2,000 km of altitude, will have, on average, of a problem each year because of space debris impacts. Besides, it is estimated that the collision risk in low orbits because of these satellite constellations will increase by a factor of 10 during the next 50 years.

Shieldings and maneuvers

The need to minimize risks is evident, so the numerous design phases of spacecrafts such as the International Space Station incorporate an exhaustive shielding analysis. Each part of the spacecraft has to be studied separately, depending on if it is more or less vulnerable to collisions, on its importance (obviously the module occupied by the astronauts must be better protected than the solar panels, for example), on the material it has been built with, etc. With this aim laboratory tests as well as computer simulations are done, and shields are built using the most modern materials (and new materials are invented sometimes, which eventually may have applications for everyday use). These shields are used not only to limit collision damages, but also to reduce the amount of debris generated after an impact. The final objective for the International Space Station is to reach an accepted requirement of a risk of penetration smaller than 1 in more than 800 years. Unfortunately, these shields are heavy and complex, and consequently very expensive.

Apart from incorporating shields, many spacecrafts are sometimes forced to maneuver in order to avoid crashing against drifting space debris. Even in this way, on average, one out of eight space shuttle windows must be replaced after each mission because of microimpacts. In this way, after it started to operate, more than 60 windows have had to be replaced, each one costing some 40,000 $. However, not all specacrafts and satellites are able to maneuver. For example, the three cosmonauts on board of the Mir on 1997, September, the 15th, had to shelter in the Soyuz security vehicle while waiting for a USA satellite which was approaching the space station.

Fragments whose sizes are between 1 and 10 cm constitute the biggest problem. Below 1 cm it is necessary (and not unaffordably expensive) to shield the satellites, both because the huge number of this kind of objects on orbit, because their detection is extremely difficult and because an impact with one of them could be dangerous. On the other hand there are relatively few objects bigger than 10 cm and they are comparatively easily detected, so that their orbits can be determined and the satellites can maneuver to avoid colliding. But objects between 1 and 10 cm are too numerous to neglect the probability of collision, too big to shield the spacecrafts against them, too small to be detected in advance and, of course, extremely dangerous in case of impact.

The atmosphere as a natural cleaning method

The status of our atmosphere is not always the same, but changes following the 11-year solar activity cycle. The number of sunspots (areas darker or brighter than average) visible on the solar surface, changes during a cycle, in such a way that during the cycle minima there are almost no spots, whereas during the cycle maxima there are dozens of spots distributed over the solar surface. Between two consecutive maxima or minima there is a 11 year gap.

During the solar activity maxima, the radiation flux of our star is slightly stronger than usual, causing an expansion of the Earth atmosphere. This phenomenon is responsible for a stronger atmospheric friction on the space debris, and so for a bigger braking, in such a way that every 11 years a decrease of about 10 % in the number of orbiting fragments is observed. The last time that this phenomenon took place was between 1989 and 1990, when the number of known objects decreased from some 7,300 to about 6,700. During this period of maximum solar activity, an average of 3 catalogued objects were deorbited every day (three times the usual number), cleaning more than 560 tons of metal from the space around our planet in one year. The Skylab and Salyut 7 space stations were among the victims of the 1979-80 and 1989-90 solar activity maxima, respectively.

These objects reentering the atmosphere, either when it coincides with a period of maximum solar activity or not, constitute an evident danger if the objects are too big to completely evaporate. The Skylab example can be used again, since it uncontrolledly reentered the atmosphere in July of 1979, scattering some 20 tons of debris on Australia and the Indic Ocean. Another important case took place in March of 1997, when a Delta rocket tank, weighing 225 kg, crashed only 50 meters away from a farm in Texas. Since 1958, 62 cases of space debris are known to have crashed against the Earth.

Even more worrying are the 1,300 kg of orbiting radioactive material distributed in some 50 satellites. For example, in 1978, the nuclear sovietic satellite Kosmos 954 crashed accidentally in the north of Canada, carrying 30 kg of enriched uranium on board. Since 1988 no more nuclear reactors have been launched, but a considerable quantity of radioactive material is still uncontrolledly orbiting the Earth.

Knowing with enough precision the moment and place of a space debris impact is not yet possible. Although its trajectory is monitored, ten days before its collision the precision is only of 24 hours, and an error of 5 minutes in the reentry time corresponds to an error for the impact site of 2,000 km.

Other solutions

• Minimise the number of debris generated in space missions. It has to be taken into account that during the first 25 years of the space age a lot of nuts, brackets and springs were released when satellites got rid of their propelling rockets. Besides, a lot of missions got rid of the sensors covers or the altitude control devices, or, as said, left the upper rocket stages on orbit. Nowadays, however, the idea is to create the minimum possible amount of space debris.
• Empty out the spare fuel and switch off the electrical systems on board of non-operational spacecrafts in order to avoid eventual explosions.
• Try to remove from their orbit those satellites and rocket stages which are non-functional. The idea is that those on low orbits must be sent towards the Earth, so that they evaporate because of the atmospheric friction, whereas those on high orbits are taken to "graveyard orbits" where they can not disturb future missions. The problem of this measure is that the additional amount of fuel needed to change the orbit is of some 5 to 15 % of the satellite mass and so it is very expensive (the cost to launch one kilogram of material is of some 20,000 $, so the cost of the extra fuel which is necessary to cause a 1 ton satellite reentry would be of some 2,000,000 $).

The OGS role

Regarding this subject, the mise en scene of the 1 m Zeiss telescope in the OGS (Optical Ground Station), located in the Teide Observatory, is very promising. When it will start operating, it will have available a mosaic made of 4 CCDs, each one with 2048x2048 pixels, that will cover a total field of view of 0.7x0.7 degrees. The pointing error is smaller than 10'' and the tracking error is better than 2.5'' per hour. With this telescope we will make firstly a map of the objects located on the geostationary ring and then another one for the objects on geostationary transfer orbits, determining their orbital parameters in both cases.

The OGS technical characteristics and the Teide Observatory sky quality are clearly better than those of the few groups presently working in this field. This fact allows us to hope that the catalogued object population and knowledge about the problem that space debris represent are going to increase spectacularly when the OGS starts providing results. In fact, during the last tests carried out at the OGS in September of 1999, we were able to detect objects with diameters as small as some 25 cm on geostationary orbits. On the other hand, some 70 % of the objects found were still not catalogued. In order to compare, the scarce research groups presently working in space debris surveys with optical telescopes are only able to see those debris bigger than 1 meter of diameter at that distance, and the percentage of uncatalogued GEO objects they find is only 20 to 30 %.

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