Debris of the Solar System: asteroids Otawara and Siwa
Our tiny corner of the Universe – the Solar System – is home to one star, nine planets, and dozens of planetary satellites. It also contains untold billions of asteroids and comets – the left-over debris from the cosmic construction site that created the planets and their moons. Rosetta's task is to study three of these primitive building blocks at close quarters so that scientists may gain new insights into the events that took place 4.6 billion years ago, during the birth of the Earth and its planetary neighbours.
Two Asteroid Flybys
On the outward leg of its eight-year trek to Comet Wirtanen, Rosetta will make two excursions into the main asteroid belt that lies between the orbits of Mars and Jupiter. On each visit, Rosetta will send back the first detailed pictures of and scientific data on the asteroids 4979 Otawara and 140 Siwa.
These primordial rocks could hardly be more different. 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. Otawara is suspected to be a chunk of once-molten basalt (a type V or SV asteroid), and Siwa seems to be a carbon-rich object, which is blacker than coal (a type C asteroid).
| Asteroid Vital Statistics | 4979 Otawara | 140 Siwa |
|---|---|---|
|
Average distance from Sun (million km) |
324 | 409 |
| Orbital period (years) | 3.19 years | 4.52 years |
| Size (estimated) | 2.6 - 4 km | 110 km |
| Rotation period (estimated) | 2 h 42 | 4 h 38 m |
| Orbital inclination (degrees) | 0.91 | 3.19 |
| Orbital eccentricity | 0.1449 | 0.2157 |
| Asteroid type | V or SV | C |
| Date of discovery | 2 Aug. 1949 | 13 Oct. 1874 |
| Discoverer | K. Reinmuth | J. Palisa |
Rosetta's flyby of Otawara will take place on 12 July 2006, when the asteroid is 283 million km from the Sun. Travelling at a relative velocity of more than 10 km/sec, the spacecraft will pass by Otawara's sunlit side at a distance of about 2200 km. Otawara rotates faster than any asteroid so far visited by spacecraft (about once every 3 hours), which should allow Rosetta to image most of its surface during the flyby.
Rosetta will also obtain spectacular images as it flies to within 3500 km of Siwa on 24 July 2008.The spacecraft will fly past at a relative velocity of 17 km/sec, approaching on the sunlit side and then looking at a crescent phase as it moves away. At this time, Siwa will be about 470 million km from the Earth, so that signals from the spacecraft will take 26 minutes to reach ground stations.
