For the first time, astronomers have found an object on a certain
collision course with Earth. Fortunately, it is so small it is not
expected to cause any damage, burning up in the atmosphere somewhere
above northern Sudan in the wee hours of Tuesday morning. It may,
however, produce a brilliant 'shooting star'.
--------------------

The space rock, dubbed 2008 TC3, was first spotted on Monday in a survey by the Mt Lemmon Observatory near Tucson, Arizona.
Its
brightness suggests it is no more than about 5 metres across – so small
it will likely be destroyed in the atmosphere, says Andrea Milani
Comparetti of the University of Pisa in Italy.
Rocks
of such size are thought to hit the atmosphere every few months, says
Steve Chesley, an astronomer at NASA's Jet Propulsion Laboratory.
"The event is not unusual – what is unique is that it's been predicted beforehand," Milani told New Scientist.
"This is the first time we see something arriving, compute that it's
going to impact, and announce it is going to impact before it happens."
The
rock is due to hit the atmosphere above northern Sudan on Tuesday at
0246 GMT. It will be travelling from west to east, and may be visible
from a few hundred kilometres away.

Long trail

The
meteor it produces is likely to be spectacular. The rock will release
about 1 kiloton of energy in the atmosphere – the equivalent of a
low-energy nuclear bomb, says Milani. But it's not clear whether it
will do so all at once or over a longer period, perhaps lasting a
minute or so.
It
will hit the atmosphere at an angle of 20°, so "it will make a long
trail in the atmosphere", says Milani. "But we cannot honestly predict
how long it will be. [The rock] might end up quite far – above the Red
Sea or Saudi Arabia – or it might explode and disappear sooner."
If it disintegrates all at once, it would produce a bright flash of light and a loud sonic boom, says Chesley.
This
space rock is so small it is unlikely to cause any damage. "The only
concern is that [the explosions] might be interpreted as something
else, that is man-made explosions. Thus in this case, the earlier the
public worldwide is aware that this is a natural phenomenon, which
involves no risk, the better," Milani's team wrote on a popular
astronomy listserv.

Impact probability

The
object's discovery is a reminder that larger and potentially more
dangerous rocks might also be on a collision course with Earth.
Milani and Chesley are members of the only two groups in the world that calculate the probability that a given space rock
will hit the Earth. They both say that they are delighted at how
quickly this meteorite was determined to be on a collision course with
Earth – since it was only discovered at about 0630 GMT on Monday.
"For
us, [we feel] satisfaction because our computation worked and because
this kind of accident – which is without any risk that anybody [would
be] hurt – will make people more aware of the fact that something has to be done about asteroids in case a bigger one arrives," Milani told New Scientist.
"The
fact that we're able to make this prediction proves the system's
working," says Chesley. "These sized objects are not the ones we're
most concerned about – there are tens of thousands of much larger
objects that could cause real damage on the ground that are still yet
to be found."
Despite
the advanced warning, there is probably too little time to mount a
mission to observe the atmospheric impact from an aeroplane, as sometimes happens during known meteor showers,
says Milani. "But now that this is out in the public, anybody who has a
telescope is going to be pointing it in that direction," Chesley says.
Comets and Asteroids – Learn more about the threat to human civilisation in our special report.