WHEN the space shuttle Columbia touched down at the Kennedy Space Center in
Florida last November, none of the seven crew realised how close they had come
to disaster. During its 16-day mission, one door of the shuttle鈥檚 cargo bay was
hit by a projectile that left a crater almost 2 centimetres across and 6
millimetres deep. At the bottom of the crater, among the microscopic debris from
the impact, lay a millimetre lump of fused metals including silver, lead and
tin鈥攖he elements used to make electrical solder. 鈥淲e believe Columbia was
hit by a piece of electronic circuit board,鈥� says Eric Christiansen, chief
analyst at the Hypervelocity Impact Test Facility at the Johnson Space Center in
Houston.
The fragment almost certainly came from a satellite or rocket that had
exploded in orbit. Further examination of the crash site revealed that the
particle had been travelling at 5 kilometres per second. At that speed, it could
have punched a hole through a sheet of aluminium one centimetre thick. Had the
cargo doors not been partially closed, the fragment would have hit vital oxygen
tanks in the shuttle hold causing untold damage.
The incident highlights the growing problem of orbital debris, but the
shuttle is not the only worry. In 1998, NASA plans to launch the International
Space Station which, after its four-year construction, will operate in orbit for
ten years, much longer than a typical two-week shuttle mission. NASA engineers
know that during this time, the station will have to survive numerous collisions
with fragments like the one that hit Columbia last year. They also know that
some impacts will be far worse and have developed a 鈥渂umper鈥� to absorb most of
them. Despite this precaution the odds that one of these collisions will
penetrate the station鈥檚 hull are frighteningly high. At best, this would cause
the station鈥檚 atmosphere to leak into space. At worst, the particle could hit a
crew member causing serious injury or even death.
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If the threat from orbiting debris weren鈥檛 bad enough, in November 1999 the
Earth is due to be bombarded by the heaviest shower of meteors it has seen for
years. Meteors have a much higher velocity than orbiting debris. How the space
station will fare is anybody鈥檚 guess.
But calculating the amount of debris in orbit is no easy task. 鈥淵ou assume a
rate at which debris is created and then you assume a rate at which it re-enters
the atmosphere,鈥� says Gene Stansbery, the physicist who is in charge of orbital
debris models at the Johnson Space Center. Debris creation is governed by
factors such as the number of launches each year, the rate at which the
redundant satellites and rocket stages begin to fragment and the size of
particles they form.
Rockets and satellites break up for a number of reasons. Explosions are a
major cause. In February, the final stage of a Russian Proton rocket exploded on
its journey into space sending at least 200 large metal fragments into orbit,
each with the potential to devastate the shuttle or space station. Another cause
is the erosion of spacecraft surfaces and paintwork caused by highly reactive
atoms of oxygen in the upper atmosphere.
Space dump
Some debris is deliberately dumped in space. For instance, lens caps that
cover sensitive instruments during launch must be jettisoned once the satellite
has settled into an orbit. In 1990, the space shuttle recovered a satellite that
had been in orbit for six years. Analysis on the ground showed that it was
speckled with urine and fecal matter. NASA concluded that it had come from
previous Russian and American space missions.
Eventually most of this debris will re-enter the Earth鈥檚 atmosphere, however.
At low altitudes of around 300 kilometres, where the shuttle and the space
station will orbit, friction with the upper atmosphere acts like a natural
vacuum cleaner, slowing the particles down so they re-enter and burn. But even
this factor in NASA鈥檚 calculations is unreliable because the atmosphere heats up
and expands during periods of high solar activity, increasing the cleaning
effect.
Stansbery calibrates his models using impact analyses from shuttle flights
and other missions, and radar measurements of the number of particles in orbit
made from the ground. NASA鈥檚 Haystack radar system can spot particles as small
as 5 millimetres across passing overhead. By watching a section of the sky and
extrapolating their results, researchers can build up a statistical picture of
what鈥檚 up there. 鈥淭here are around 400 000 pieces of orbital debris that we can
see,鈥� says Stansbery.
Larger objects are tracked by the US Space Command in Colorado Springs, which
has a worldwide network of radars capable of tracking objects larger than 10
centimetres across. Space Command catalogues large fragments so that it can
distinguish between the re-entry of orbital debris and incoming ballistic
missiles. It has more than 8000 objects on its books.
Despite all this data, NASA still gets it wrong. Columbia suffered three
times as many impacts as predicted. Stansbery says the hits generally involved
particles less than 1 millimetre across, such as flecks of paint eroded from
aging rocket stages by atomic oxygen鈥攁 problem that grows as the number of
tanks increases year by year. Although these particles are too small to pierce
the shuttle or the space station, they can damage the heat shields on the
shuttle and its windows, which are expensive to replace.
To smarten up its predictions NASA must find better ways of modelling how the
space environment is likely to change over the next few decades, stresses
Nicholas Johnson, principal scientist for Kaman Sciences Corporation in Colorado
Springs. 鈥淲e鈥檝e been in this game long enough that we shouldn鈥檛 be doing
back-of-the-envelope kind of calculations,鈥� he says. 鈥淏ut that, in essence, is
still what we鈥檙e doing.鈥�
Such is the concern over the threat from debris that in February the White
House produced a report outlining ways to combat the problem. Key options
included better design for launch vehicles and spacecraft. Boosters, for
example, can shed key parts such as nose cones and payload ejection devices
before reaching orbital velocity. And any that do reach orbit would deliberately
use up any left-over fuel and discharge any pressurised containers which can
otherwise cause explosions.
The report says that even simple measures could help. Lens caps could be
tethered to their instruments and old spacecraft deliberately manoeuvred back
into the atmosphere or sent into higher 鈥済raveyard鈥� orbits out of harm鈥檚 way.
鈥淎s a result of these efforts, the growth rate of orbital debris will decline,
although the overall debris population will still increase,鈥� notes the White
House study.
Researchers are also working on ways of flying the space shuttle to minimise
damage. For example, once in orbit the shuttle flies backwards, engines first.
This is because the engines are no longer needed once the shuttle is in space
and so can be used to absorb the impact of any debris approaching the shuttle
head on. This minimises any damage to the heat shields at the front of the craft
which are vital later in the mission during re-entry.
Similar measures can be adopted with the space station. For instance, it can
be oriented to present the smallest possible target to oncoming debris.
Nevertheless, the space station鈥檚 bumpers will have to cope with many hits. NASA
has plumped for a design based on an idea proposed by Fred Whipple of the
Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, to protect
spacecraft from meteor showers. Whipple鈥檚 idea is a 鈥渕eteor bumper鈥濃�攁
sheet of metal placed a few centimetres from the ship鈥檚 hull that acts as a
shield. When a meteoroid strikes, the shield absorbs the shock of the impact and
the resulting fragments and vapour dissipate benignly between the bumper and
vehicle鈥檚 skin.
Whipple鈥檚 idea has since been refined. The ideal material for the bumper must
be flexible so that it can be wrapped around the station and light so that it
can be lifted into orbit. But one of the most important factors is its
density, says Christiansen. When something strikes the bumper, the impact sends
a shockwave through the fragment causing it to explode. This shockwave is
highest when the density of the bumper and the fragment are the same. 鈥淭he ideal
density is around 2.8 times higher than water, about the same as aluminium,鈥� he
says.
Indeed, the outer layer of the station鈥檚 shield will be aluminium sheeting
about 2 millimetres thick. But aluminium tends to break up during an impact
creating more fragments that could damage the layer below. So NASA engineers
have developed a woven blanket of carbon fibres coated with ceramics that will
sit in the gap between the outer shield and the hull of the space station.
Although ceramics are brittle in their pure form, their density matches
aluminium. 鈥淭he Kevlar blanket provides strength and prevents the fragmentation
that occurs with aluminium,鈥� says Christiansen.
At the Johnson Space Center鈥檚 Hypervelocity Impact Test Facility,
Christiansen and a senior engineer, Jeanne Lee Crews, are testing the shield
using light gas guns that fire projectiles into it at 7.5 kilometres per second.
To date, their Whipple shield has performed superbly, says Christiansen.
Projectiles tend to fragment and spread out as they pass though each layer. 鈥淏y
the time they reach the inner pressure vessel, their energy has dissipated,鈥� he
explains. The greater the space between the layers the more protection the
shield will give the ship. Any fragments that penetrate the aluminium layer will
then spread over a wider area before hitting the carbon fibre/ceramic
blanket.
The most vulnerable parts of the station, living quarters and those facing in
the direction of travel, for example, will be given shielding with a space up to
30 centimetres between the outer aluminium shield and the hull. On the other
hand, some less vulnerable areas will have no shielding at all.
Direct hit
But even the most heavily protected areas can only withstand so much.
Christiansen says the gas gun tests show that the shield can withstand a direct
hit from an aluminium sphere 1.3 centimetres across. 鈥淚t will protect against
larger particles if the impact is oblique,鈥� he adds. If these hit head on, they
could cause the atmosphere inside the station to leak into space, hit vital
machinery and possibly even injure or kill an astronaut inside, admits
Christiansen.
Meteors will be travelling even faster than orbiting debris at around 20
kilometres per second. In November 1999, the Earth will pass through a cloud of
dust will produce a shower of over 10 000 meteors鈥攖he heaviest in 33
years. 鈥淲e are anticipating a very intense storm at that time,鈥� says Walter
Marker, a senior researcher at the Johnson Space Center. If necessary, NASA can
add more shielding and angle delicate solar panels to present a small surface
area to the incoming storm and then cross their fingers.
So how likely is the station to be holed? Christiansen has calculated this
probability based on the relatively crude orbital debris models that are
available today and by calculating the vulnerable surface area of the fully
constructed space station in 2002. 鈥淭he chances of penetration are one per cent
per year,鈥� he says. That is frighteningly high: the figures imply that during
its 10-year lifetime, the station has a one in ten chance of being holed by
debris. But Crews is resigned to the threat: 鈥淭here is no way to shield against
别惫别谤测迟丑颈苍驳鈥�.