ÐÓ°ÉÔ­´´

Space is now a vast junkyard. Indiscriminate pollution, and in some
instances wilful degradation of the space environment by both the US and
the Soviet Union, have resulted in an estimated 3.5 million pieces of rubbish
in orbit around the Earth.

Debris is a hazard to both sapcecrew and craft. A metal chip 0.5 millimetres
wide and travelling at a relative velocity of 10 kilometres/second could
easily penetrate a space suit and kill an astranaut. A particle a few centimetres
in diameter could destroy part of the Soviet Union’s Mir space station,
or Freedom, the $30 billion station that NASA plans to begin assembling
in space in 1995 (New ÐÓ°ÉÔ­´´, ‘The growing hazard of rubbish in space’,
25 August 1988).

Already, there have been many close encounters. In 1982, part of an
old Soviet rocket missed the shuttle Columbia by only 12 kilometres. In
1983, a paint fleck 0.2 millimetres across chipped Challenger’s windscreen.
Ground crew had to replace the windscreen before the shuttle could be relaunched.

The quantity of space debris is increasing and could leave us Earthbound
within 30 years. In 1988, Reimar Lust, director general of the European
Space Agency (ESA), said: ‘If we fail to take preventative measures, future
generations will inherit an ominous legacy.’ But, until recently, space
debris had a low profile in the glamorous space industry. Most satellite
owners assumed that their spacecraft were launched into limitless space.
Lone voices, such as Donald Kessler at NASA, pointed to the rapidly increasing
amounts of debris. Already, an estimated 3 million kilograms of junk orbit
within 2000 kilometres of Earth.

In Feburary 1989, a report on space debris to the US’s National Security
Council predicted that by 2010, 12 million kilograms of junk would orbit
the Earth at an altitude less than 5500 kilometres (low-Earth orbit or LEO).
NASA’s most recent prediction, of 6 million kilograms by 2010, is less alarmist,
but equally serious. The agency’s figure is lower because there have been
no major explosions in space since 1988, compared with five per year before
then, and fewer launches than predicted have occurred.

Accidental and deliberate explosions create a large percentage – about
40 per cent – of debris in space. Between 1973 and 1981, seven American
Delta rockets exploded in orbit before the manufacturer McDonnell Douglas
found out. When the company discovered what was happening, engineers modified
the rocket so that no fuel was left after the launch, and the explosions
stopped. Between 1964 and 1986, the deliberate destruction of 34 satellites
for security reasons generated 2094 fragments. All but three explosions
were Soviet.

Although the superpowers bear much of the responsibility for space debris,
ESA contributed for the first time in 1986. An Ariane rocket blew up in
the worst single breakup in history. The US Space Command, which tracks
and catalogues objects in space geeater than 10 centimetres, observed 465
objects. ÐÓ°ÉÔ­´´s estimated that the explosion created a further 2300
much smaller particles. After the explosion, debris spread out from the
original orbit and eventually formed a shell around the Earth.

The US Space Command, which comprises 29 sites operating optical and
radar sensors, tracks about 7000 objects. Of these, only 5 per cent are
operating spacecraft. Most tracked objects are in low-Earth orbit, where
they constantly collide with millions of smaller fragments, generating yet
more tiny particles. Another 683 tracked objects are between low-Earth orbit
and an altitude of 36,000 kilometres (geosynchronous orbit). Geosynchronous
orbit, which houses most of the world’s telecommunications satellites, contains
only 453 pieces of tracked debris.

The number of objects per 10 kilometres of altitude is greatest at 800
kilometres, presenting problems which today’s space users have no choice
but to live with. Michael Shara of the Space Telescope Science Institute,
in Baltimore, says there is a 1 per cent chance that a fragement of debris
10 centimetres or larger will destroy the Hubble Space Telescope. Even a
3-centimetre particle could do serious damage. Unfortunately, shielding
against particles larger than 1 centimetre would make satellites too heavy
to launch.

Debris poses additional hazards to Hubble. The fine guidance sensors,
which lock onto stars and help to orientate the telescope, were modified
so that reflected light from debris would not confuse them. Engineers ran
software checks to ensure that light would not damage one of the telescope’s
instruments known as the faint object spectrograph. However, Shara still
expects streaks on pictures that the wide field and planetary camera takes.
In future, Shara would like large observatories to be placed in geosynchronous
orbit, away from the problems of light interference and from the threat
of collision.

Space station at risk

No such option is available for the space station, Freedom. The US is
leading an international effort, incling Europe, Japan and Canada, to build
Freedom. The space station has met many political and technical set backs,
and faces many more unresolved problems, including space debris. Freedom
will be assembled at an altitude of 407 kilometres and will operate at about
463 kilometres, just below the peak density of debris.

Debris merely jolting the cabin wall will cause shrapnel-like pieces
to fly out across the cabin at high velocity. Larger projectiles, causing
a small pinhole in the cabin wall, could produce shrapnel and a sonic boom,
generating heat with the possibility of fire. Debris 1 to 5 centimetres
across could cause a 10-centimetre hole, so cabin pressure would drop very
rapidly. Particles roughly 5 to 10 centimetres across could penetrate one
side of the space station and emerge out of the other side, creating a huge
hole.

To counter these dangers, shielding will protect Freedom from particles
smaller than 1 centimetres. Particles between 1 and 10 centimetres in diameter
will be detected by equipment mounted on the space station, perhaps by infrared
sensors picking up the heat the debris emits. Although an infrared detector
would show the existence of an object, it would not show its velocity and
distance from the space station. One solution would be to place a radar
by the side of the infrared sensor. Alternatively NASA could install two
infrared sensors that would give distance and velocity by traingulation.
Despite working on the detectors, NASA does not know yet what to do about
the debris 1 to 10 centimetres across that infrared sensors will detect.
One possibility is that an electrongun or laser could destroy the debirs
or, with warning, the astronauts could move to a more heavily shielded area
of the space station.

Pieces of debris greater than 10 centimetres in diameter will be detected
from the ground by the US Space Command. Theoretically, Freedom could then
manoeuvre out of the way, but to do so would be expensive and difficult.

Kessler, from NASA, claims that the risk of being hit in the cabin by
particles greater than 10 centimetres is less per year than dying on an
American freeway. Astronauts working outside Freedom are in more danger,
because tiny particles can penetrate space suits. ‘Lives are at risk, but
100 per cent safety isn’t possible, and NASA’s goal is to achieve an acceptable
risk,’ says Kessler.

Experts have calculated various figures for the probability of debris
damaging or destroying Freedom. ‘A number has been defined, but what does
it mean to crew or station? There is one chance in 2000 per year that debris
will penetrate a critical element of the space station. But there are about
20 critical elements, and we don’t know how critical a critical element
is. So loss of crew or station must be an educated guess, and we err on
the conservative side. NASA wants Freedom to last more than 30 years and
would therefore like a large safety margin,’ says Kessler.

Future predictions should be more accurate. The deep space radar at
Goldstone, California has periodically tracked particles as small as 2 millimetres.
And NASA has asked a Space Command facility, called Haystack, to count and
measure objects between 1 and 10 centimetres. In future, sophisticated radars
built as part of the Strategic Defense Initiative could track particles
down to 0.5 millimetres.

Some speculations about the future of space junk would make calculations
by any of the spacefaring nations of safety factors in space redundant.
One hypothesis is that so much debris will accumulate that random collisions
will set off an avalanche of secondary collisions in a runaway cascade known
as the Kessler Syndrome. The resulting belt of small debris would make space
flight impossible for several centuries.

NASA has postulated that such a critical mass will be reached before
the middle of the next century. Dietrich Rex and Peter Eichler, from the
Technical University of Braunschweig, West Germany, believe the critical
mass could be reached much sooner. They say that the critical mass for a
chain reaction is only 2 or 3 times the present population of 70,000 fragments
of 1 centimetre across or larger, and that spaceflight could be impossible
within 20 to 50 years (New ÐÓ°ÉÔ­´´, Technology, 21 October 1989). Kessler
agrees: ‘We are so close to runaway that within 30 years we could become
Earthbound. First, orbits below 1000 kilometres would become unsafe so everyone
would fly at higher altitudes. But atmospheric drag doesn’t act there, so
debris would soon accumulate rather than fall to Earth. We can’t go higher
and put men into the radiation belts, and it becomes more expensive to launch
into even higher altitudes, so we could become Earthbound out of economics.’

Even if all space activities stopped now, Rex’s computer models show
more than half the objects currently in orbit will still be there in 50
years. Atmospheric drag will cause particles in fairly low altitudes to
re-enter the Earth’s atmosphere and burn up, reducing the peak at 800 kilometres
by half within 5 years. But the second peak, at 1500 kilometres, will remain
unchanged. After 100 years, the 800 kilometre peak will be eliminated, but
the 1500 kilometre peak will be reduced by only 15 per cent.

In an attempt to reduce the growth of junk in low-Earth orbit, NASA
decided in 1981 that the number of rocket explosions in low-Earth orbit
must be limited, and that engineers should design spacecraft from which
nothing is ejected an altitude greater than 300 kilometres. Japan and ESA
followed suit.

NASA, however, has no policy to protect geosynchronous orbit. The agency
believes that their is no immediate problem. Relative velocities are low,
so collisions have a lower impact, and, because smaller quantities of junk
exist in geosynchronous orbit than in low-Earth orbit, collisions are less
likely. However, bunching satellites increases the probability of collision,
and, in 1980, three American military satellites in geosynchronous oribt
had to take evasive action.

In 1988, EAS was the first agency to publish a policy for debris covering
both geosynchronous and low-Earth orbit. EAS’s main concern is that growth
in geosynchronous orbit is faster than in low-Earth orbit, and that debris
may remain in the higher orbit for millions of years. ESA wants satellite
owners to boost their defunct satellites out of geosynchronous orbit. The
International Telecommunication Union, the regulatory body for geosynchronous
orbit, is currently debating the idea, but the US is resisting. William
Djinis, a spokesman for NASA, says that boosting is not the answer because
particles created by collisions between boosted satellites and meteorites
would rain down on geosynchronous orbit. Boosting also carries a 2 per cent
risk of explosion, which would multiply the debris in orbit. So, NASA is
awaiting the results of studies before deciding to raise or lower the orbit
of dying satellites from geosynchronous orbit.

The keenest supporters of boosting are the equatorial nations, which
fear that the geostationary orbit will be clogged up with dead satellites
by the time they have developed technology to launch and operate their own
satellites. Geosynchronous orbits pass directly over the equator, and the
equatorial nations, claimed sovereignty over the orbit in 1976.

In the meantime, ESA and NASA have commissioned the University of Kent
to compile a database of all sizes of debris in all orbits, which should
enable the agencies to work out launch trajectories that avoid collisions.
However, neither superpower provides information about their military satellites,
according to Heiner Klinkrad of ESA’s technical directorate. The US gave
no data about KH11, a military satellite which broke up in March this year,
even though it could have landed on Europe. A Soviet news agency revealed
KH11 had split into four.

The ESA would also like improved early warning about the uncontrolled
re-entry of large objects. When NASA’s Skylab decayed over southwest Australia
and the Indian Ocean in 1979, it left a trail 1000 kilometres long. Nobody
was killed or injured, although about 500 large fragments hit the Earth,
some at supersonic speed.

A disturbing aspect of the debris problem is the presence of nuclear
power sources in the maximum-density region of 800 to 1000 kilometres. The
majority of the nuclear power sources are on Soviet spy satellites which
are boosted to parking orbits higher than 800 kilometres at the end of their
mission.

Both superpowers are highly secretive about their nuclear-powered satellites,
but a paper to the United Nations in February this year revealed that 56
Soviet and American radioactive satellites, are orbiting between 800 and
1100 kilometres. Nicholas Johnson, an expert on Soviet space at Teledyne
Brown Engineering, Colorado, feared, as recently as 1987, that the probability
of a radioactive satellite colliding with apiece of space debris was ‘a
virtual certainty’. Now he believes that the situation has improved. First,
because increased solar activity has heated the atmosphere so that it has
expanded, increasing the drag on satellites which would otherwise be above
the atmosphere. The increased drag slows down orbiting debris at lower altitudes
so it re-enters, thus cleaning out low-Earth orbit. Secondly, fewer spacecraft
are injected into low-Earth orbit than in the past. Says Johnson: ‘We have
a situation now, not a problem.’

If a collision happened, Rex and Wolfgang Pfeffer showed that 97 per
cent of the radioactive fragments would enter the atmosphere within 25 years
and that uranium particles would accumulate on the surface of any space
station orbiting below within 8 to 25 years. As a result of these studies
and an increased willingness by the Soviets to reach consensus, the space
powers broke a 10-year deadlock in April this year, and established international
guidelines on the safe operation of nuclear power sources. According to
these guidelines, if an accident happens, an individual on Earth must not
be exposed to more than 1 millisieverts per person per year. If an accident
to a reactor on Earth exposed people to these levels, authorities would
not be obliged to take action.

The guidelines also restrict the use of nuclear power sources to missions
which cannot be operated by non nuclear sources. Both the legal and scientific
subcommittees of the Committee for the Peaceful Use of Outer Space have
passed the guidelines, which have now reached the main agenda of the organisation.
A complete set of principles governing nuclear safety in space will, if
passed by the General Assembly, become international law by 1991 or 1992.

Further agreement between spacefaring nations is essential to reduce
debris. Johnson suggests that the UN should play a role in detecting debris,
and ESA would like the UN involved in regulating debris. However, Dan Jacobs,
a NASA spokesman, rejects this idea as premature. Jacobs says: ‘The space
players themselves need to understand the issues first.’

Barbara Wood-Kaczmar is a freelance journalist.