A one-ton European Space Agency satellite, which for four years has being mapping the Earth’s gravity, has run out of fuel and will reenter the atmosphere in two weeks. While its descent is constantly monitored, the impact location is still unknown.
The ESA’s Gravity field and steady-state Ocean Circulation
Explorer (GOCE), which was launched from Russia in March 2009,
managed to spend 2 1/2 more years on its mission than was
initially expected. During these extra years of work, it was
taken to the lowest altitude orbit of any research satellite to
capture the gravity data of unequalled accuracy.
On Monday, GOCE’s supply of xenon fuel was finally depleted and
the ESA declared the mission over.
The work of the flight control team did not end with that,
though, as the 1,100 kilogram satellite will still be orbiting
for about two weeks, before its system stops working and it
plunges back into the atmosphere from an altitude of 227
kilometers (139 miles).
The last part is particularly tricky, as even with all the latest
technologies available, the space agency is unable to predict
where parts of GOCE might fall.
While most of the satellite is expected to burn up during
reentry, Reuters reports that as many as 50 fragments with a
total mass of up to 275 kilograms have been predicted to reach
the surface and splash into the ocean – or hit land.
“When and where these parts might land cannot yet be
predicted, but the affected area will be narrowed down closer to
the time of reentry,” the ESA said in a statement on its
website.
Such uncertainty is caused by constant changes in the upper
atmosphere, which are strongly influenced by solar activity.
An international campaign involving the Inter-Agency Space Debris
Coordination Committee is monitoring the situation continuously,
the ESA said, promising to “keep the relevant authorities
permanently updated.”
Space agencies usually consider the risk of someone being hit by
a piece of space junk as extremely low, although this assumption
is based on the fact that only a tiny fraction of the planet’s
surface is densely populated.
According to NASA’s data, based on 50 years of observations, each
day an average of one tracked piece of orbital debris falls back
to Earth. The agency states that so far “no serious injury or
significant property damage caused by reentering debris has been
confirmed.”
However, when the 13-ton Russian probe Phobos-Grunt unexpectedly
failed to boost its engines in near-Earth orbit and started
slowly descending for its final downfall in January 2012, it sent
chills down the spines of many space-watchers. Luckily, most of
the probe’s debris eventually landed in the Pacific Ocean.
Phobos-Grunt’s failure was not the first uncontrolled fall of a
huge manmade object from space. In September 2011, NASA’s 6-ton
Upper Atmosphere Research Satellite plunged back to Earth with
“a small potential risk to the public.” The largest of the
parts that might have reached the surface was estimated to weigh
160 kilograms.
NASA’s satellite was later confirmed to have fallen in the
Pacific several hundred kilometers away from American Samoa. Just
a month later it was followed by Germany’s 2.4-ton X-ray ROSAT
telescope, which was feared to survive in chunks as heavy as 400
kilograms. None of the chunks were reported to have reached the
surface following reentry, however.