Comparative sizes of asteroids. Photo: Courtesy Observatoire de Paris

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 since 1991 - including distant views of asteroid 2685 Masursky on January 23, 2000 from the Cassini spacecraft - 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.

An artist's concept of the Rosetta comet probe. Photo: DSS

Rosetta's flyby of Otawara will take place on July 11, 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 July 24, 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.