ASTRONOMERS may have discovered a new class of asteroid. The bad news is that
because these orbiting rocks are extremely hard to spot, they could pose a risk
to Earth鈥攁nd there may be lots of them.
Most asteroids orbit the Sun further out than the Earth, and some cross the
Earth鈥檚 orbit. Even at their brightest, asteroids are dim and hard to see.
Between the Earth and the Sun they are hardest of all to spot. They can only be
seen near the horizon at dawn or in twilight, and reflect little sunlight
towards the Earth.
Now astronomers David Tholen and Rob Whiteley at the University of Hawaii
claim to have seen the first asteroid with an orbit that is entirely within the
Earth鈥檚. They first saw it in February, using a telescope on Mauna Kea. When
they found it again, the next day, they realised that its orbit does not carry
it far out in the Solar System. 鈥淚t鈥檚 overwhelmingly likely that it is entirely
interior to the Earth鈥檚 orbit,鈥� says Whiteley.
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The asteroid is thought to be about 40 metres in diameter. Equipment problems
stopped the astronomers making any more observations, and they have not found
the asteroid again. 鈥淭his object is now technically lost,鈥� says Whiteley.
Gareth Williams of the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Massachusetts, points out that calculations of the asteroid鈥檚 orbit
are not very precise. But he agrees that it might be part of a new group of
Earth-approaching asteroids that haven鈥檛 been accounted for. 鈥淚t鈥檚 likely that
there鈥檚 a large population of these objects,鈥� he says. 鈥淭hey鈥檙e difficult to
discover, hard to follow up, and people haven鈥檛 been looking for them.鈥�
