Last Monday, Valentine’s Day, NASA’s NEAR satellite achieved a notable
‘first’ when it entered orbit around a near-Earth asteroid named 433
Eros. While this romantic rendezvous should provide a host of new
information about the potato-shaped little world, Eros is just one
out of many millions of asteroids roaming the Solar System.

Further insights into the nature of these ancient rocks will be gained
later this decade when ESA’s Rosetta spacecraft encounters two
contrasting asteroids which inhabit the main asteroid belt between
Mars and Jupiter.

Apart from the nine planets, our Solar System contains a multitude
of minor planets, officially known as asteroids. Once known as the
‘vermin of the skies,’ scientists now recognise that these battered
chunks of rock contain important clues to the events that took place
during the formation of the Solar System. However, despite a handful
of asteroid flybys by spacecraft during the 1990’s, many mysteries
still remain.

One difficulty is that the asteroids are not all made of the same
material. Some are stony, some are rich in iron and others seem to
contain large amounts of carbon. Most of the asteroids so far visited
by spacecraft have been of the stony type (including Eros). Apart
from Mathilde, a coal-black object which is not much denser than
water, little is known about the other asteroid types.

Rosetta will be able to clear up some of the mystery during its eight
year odyssey to Comet Wirtanen. As the spacecraft swings around the
inner Solar System, it will make two trips across the main asteroid
belt, enabling it to examine two very different asteroids, 140 Siwa
and 4979 Otawara.

The asteroids to be visited by Rosetta are two very contrasting
objects. Siwa will be the largest asteroid ever encountered by a
spacecraft, while (apart from a tiny asteroid moon called Dactyl),
Otawara will be the smallest.

Rosetta’s flyby of Otawara will take place on 11 July, 2006, when
the asteroid is 1.89 AU (1.89 times the Earth’s distance) from the
Sun. Travelling at a relative velocity of 10.63 km/sec, the spacecraft
will pass by Otawara’s sunlit side at a distance of about 1595 km.

Apart from its orbit, very little was known about Otawara until
recent ground-based studies were undertaken in order to learn
more about its physical and chemical make-up. Observations were
obtained during an international campaign using telescopes in
France, Chile and the USA.

Otawara is likely to be a stony object rich in the minerals pyroxene
and/or olivine. However, it may be a member of the asteroid family
named after its largest member, 4 Vesta. Assuming that the asteroid
is quite dark, its diameter is probably only 2.6 – 4 km. Its density is
uncertain, but is probably 2 – 2.5 times greater than water (i.e. twice
the density of Mathilde).

Studies of changes in its reflected light — its light curve — indicate
that Otawara rotates quite quickly, once every 2.7 hours. This is faster
than any asteroid so far visited by spacecraft. Such rapid spin will be
an advantage during Rosetta’s flyby, enabling its instruments to image
and measure the asteroid’s characteristics at high resolution during
one complete rotation.

With a diameter of around 110 km, Siwa is much larger than any
asteroid so far examined by spacecraft. Spectral studies indicate
that it is a very black, primitive, carbon-rich object which has
probably been less altered by collisions than its smaller cousins.
Figures for its rotation period vary between 18.5 hours and 22 hours.

Rosetta will obtain spectacular images and high resolution data as it
flies to within 3,000 km of Siwa on 24 July 2008. The spacecraft will
fly past at a velocity of 17.04 km/sec, approaching the sunlit side and
looking at a crescent phase as it moves away. At this time, Siwa will
be at 2.75 AU from the Sun and 3.11 AU from the Earth, so signals
from the spacecraft will take 26 minutes to reach ground stations.

Following these short-lived encounters, Rosetta will travel beyond the
asteroid belt before closing in on another of the Solar System’s smaller
inhabitants — Comet Wirtanen. It will then complete an historic double
by entering orbit around the comet’s nucleus and deploying a lander on
its icy surface.


Comparative sizes of asteroids (courtesy Observatoire de Paris)