Scientists
at NASA are keeping close tabs on two clouds of debris from Tuesday collision between
U.S. and Russian satellites to determine how much of a risk they pose to the
agency's Earth-watching spacecraft and, possibly, the Hubble Space Telescope.
The rare
collision between a U.S. Iridium 33 communications satellite and the
defunct Russian military communications satellite Cosmos 2251 is unprecedented,
marking the first time two intact satellites orbiting Earth have accidentally
crashed into and obliterated one another, NASA officials said. Their smash-up created
two large
clouds of space debris that are currently being tracked by the U.S. Space
Surveillance Network.
The debris
poses a greater risk to science satellites than to the
International Space Station, which is currently home to two American
astronauts and a Russian cosmonaut, since the collision occurred 490 miles (790
km) above Siberia. The space station flies in an orbit about 220 miles (354 km)
above Earth.
"This is
like over 400 kilometers above the station, so we do believe that some of the
debris is going down through station altitude. But it's a very, very small
minority of the debris clouds," said Nicholas Johnson, chief scientist of NASA's
Orbital Debris Program Office at the Johnson Space Center in Houston. "For
robotic spacecraft at higher altitudes, the answer's a little bit different. So
one by one we'll be looking at those."
Earth
observations satellites, such as NASA's Aqua and Aura spacecraft in orbit 438 miles
(705 km) above Earth, are particularly vulnerable - though the risk of an impact is
still low - and there's another satellite in a 497-mile (800-km) orbit just
above the impact level, Johnson told SPACE.com late Wednesday. The
Hubble Space Telescope orbits the Earth at about 372 miles (600 km), Johnson
said.
"That's a
little bit farther away, but it's a lot bigger too. All that matters," Johnson
said of Hubble. "It's about how close you are to the debris cloud and how big
you are."
It will be
weeks before the U.S. Space Surveillance Network pins down an accurate count of
the number of individual debris pieces created in the event, but unofficial
estimates put the damage somewhere in the 500 count. The U.S. Space
Surveillance Network is currently tracking more than 18,000 separate man-made
objects and debris at any given time, officials with the U.S. Strategic Command
said Wednesday.
"This is
the first time we've had two intact spacecraft collide, so it is a big deal,"
Johnson said. "But you know, it's not unexpected."
Johnson
said that some satellites fly within a few hundred meters of each other every
day. Each year, there are about six instances in which old satellites and
satellite parts break apart in what scientists call "fragmentation
events." Satellite components or spent rocket stages have accidentally collided
three times before in the last 20 years.
In June
1997, an unmanned Russian Progress cargo ship slammed into Russia's Mir Space
Station, damaging a solar array and radiator, and punching a hole in the ship's
hull that depressurized one of its modules. Unlike Tuesday's collision, that
Progress spacecraft was deliberately heading for Mir, where it was expected to
dock in a rendezvous system test.
"This was
going to happen," Johnson said of Tuesday's accidental collision. "There was no doubt that it was going to happen."
Johnson
said the chances of a satellite being damaged by the debris from Tuesday's collision
are admittedly low, but as the collision itself proved, such things can happen.
He does not expect to see reports of many secondary impacts from the event.
He pointed
out that when China intentionally destroyed one of its aging weather satellites
during a 2007
anti-satellite test, the impact created a cloud of more 2,500 pieces of
debris.
"We don't know
if any of them have hit any other satellite, alive or dead," Johnson said of
the Chinese test debris. "So the odds are still very small, but they're bigger
today than they were two days ago."
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