Graphic (collage) showing relative sizes of possible target asteroids and other known asteroids

Debris of the Solar System: Asteroids

Our tiny corner of the Universe - the Solar System - is home to one star, eight planets and dozens of planetary satellites. It also contains countless asteroids and comets – the debris of the cosmic construction site that created the planets and their moons.

The majority of Solar System asteroids populate the main asteroid belt, lying between the orbit of Mars and Jupiter. However, asteroids can also be found wandering in other areas of the Solar System, sometimes in the vicinity of planets.

Rosetta’s task is to study one comet and two asteroids at close quarters so that scientists may gain new insights into the events that took place 4600 million years ago, during the birth of the Earth and its planetary neighbours.

Rosetta’s asteroid fly-bys

On the outward leg of its ten-year trek to Comet 67P/Churyumov-Gerasimenko, Rosetta will perform two incursions into the main asteroid belt and perform observations of two asteroids:

  • (2867) Steins on 5 September 2008, from a distance of 800 km
  • (21) Lutetia on 10 July 2010, from a distance of 3000 km

Rosetta will obtain a wealth of science data as it flies by these rocks composed of primitive material.

Asteroid types

Asteroids are very different in shape and size (ranging from a few kilometres to more than 100 kilometres), as well as in their composition. Rosetta will provide information on the mass and density of the asteroids that it approaches, thus telling us more about their bulk composition. It will also measure their subsurface temperature and look for gases and dust around them.

Based on their albedo, or refelectivity (a white, perfectly reflecting surface has an albedo of 1 whereas a black, perfectly absorbing surface has an albedo of 0), spectral characteristics and similarities to known types of meteorites, asteroids are classified into at least 14 different classes, whereby the following three major types occur:

C-type (carbonaceous)

  • More than 75% of known asteroids
  • Very dark, with an albedo of 0.03-0.09
  • Similar to carbonaceous chondrite meteorites
  • Composition is thought to be similar to the Sun, depleted in Hydrogen, helium, and other volatiles
  • Inhabit the main belt's outer regions

S-type (silicaceous)

  • About 17% of known asteroids
  • Relatively bright with an albedo of 0.10-0.22
  • Composition is metallic iron mixed with iron- and magnesium-silicates.
  • Dominate the inner asteroid belt.

M-type (metallic)

  • Includes most of the rest of the known asteroids
  • Relatively bright, albedo of 0.10-0.18
  • Pure nickel-iron composition
  • M-type asteroids inhabit the main belt's middle region

In addition to these major types there are several less common types of asteroids.

Steins is an E-type asteroid, which is defined by a high albedo and a featureless spectrum. They resemble the characteristics of the Aubrite meteorites and are believed to have suffered at least partial melting. Steins therefore may be a fragment of a larger asteroid that fragmented a long time ago.

Mission Asteroid Date of observation Distance and speed Type size
Galileo
(951) Gaspra 29 October 1991 1600 km at 8 km/s S-type 18.2 x 10.5 x 8.9 km
(243) Ida 28 August 1993 2391 km at 12.38 km/s S-type 56 x 24 x 21 km
Dactyl (Ida's satellite) 1.6 x 1.4 x 1.2 km
NEAR Shoemaker
(253) Mathilde 27 June 1997 1212 km at 9.93 km/s C-type 60 km diameter
(433) Eros 23 December 1999* S-type S-type 33 x 13 x 13 km
Deep Space 1
(9986) Braille 29 July 1999 13 500 km, 15.5 km/s V-type 2.1 x 1 x 1 km
Stardust
(5535) Annefrank 2 Nov 2002 3100 km at 7.4 km/s S-type 6.6 x 5.0 x 3.4 km
Hayabusa
(25143) Itokawa 12 November 2005** S-type 540 x 180 m
Rosetta
(2867) Steins 5 September 2008 about 800 km at 8.62 km/s E-type 4.6 km diameter
(21) Lutetia 10 July 2010 3000 km at 15 km/s C-type 95 km diameter

* arrival 23 December 1999, touch down 12 February 2001
**12 November 2005 (touch down for sampling)

Last update: 18 November 2009

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