Diamond in space

Researchers use the OSIRIS camera aboard the Rosetta space probe to reveal the shape of the Steins planetoid

January 26, 2010

When viewed through a telescope, the Steins planetoid is an inconspicuous spot of light. Viewed in more detail, it shows itself to be a kind of debris heap with a diamond-like shape and large craters on its surface. A team headed by Horst Uwe Keller from the Max Planck Institute for Solar System Research in Katlenburg-Lindau has taken a closer look. The scientists used the OSIRIS camera system aboard the European space probe Rosetta for this task. (Science, January 8, 2010)

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A diamond-like flying heap of debris: Images of the Steins asteroid, taken by the OSIRIS camera system aboard the Rosetta space probe
A diamond-like flying heap of debris: Images of the Steins asteroid, taken by the OSIRIS camera system aboard the Rosetta space probe

The images reveal that Steins is not a large chunk of rock, but more an accumulation of many small individual fragments, similar to a heap of debris. This would also explain its conical, diamond-like shape, which was apparently formed through YORP. This is the abbreviation for the Yarkovsky-O'Keefe-Radzievskii-Paddack effect and describes the effect of solar radiation on a small celestial body such as a planetoid. The sun heats up its surface; if the heat is re-emitted as radiation, a small angular momentum results. The rotational speed can thus slow down or increase; the spatial position of the axis of rotation can change - and ultimately the shape of the planetoid as well.

"The detailed analysis of our images indicates that the YORP effect can also play a crucial role for asteroids in the main belt. Something that wasn't clear until now," says Horst Uwe Keller from the Max Planck Institute for Solar System Research, who headed the development and construction of the OSIRIS camera.

The European space probe Rosetta flew past the Steins asteroid on September 5, 2008 at a distance of a mere 800 kilometres. The speed at this fleeting rendezvous was 30,000 kilometres per hour. Steins is an object measuring some 5.3 kilometres in the asteroid belt between the orbits of Mars and Jupiter, where more than 400,000 of these celestial bodies are to be found. It is one of the rare E-type asteroids whose surfaces consist of the mineral enstatite and which has a high reflective power (albedo), meaning they are relatively bright.

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Rosetta was launched aboard an Ariane 5 rocket in 2004 and will reach its ultimate target, the 67P/Churyumov-Gerasimenko comet, first in 2014. The plan is for the space probe to change course into an orbit around the comet and drop off a lander called Philae on the comet's nucleus for the first time in the history of space exploration. Rosetta's tortuous trajectory passes through the asteroid belt, among other places.

Teaming up together on the project are scientists from 14 European countries and researchers from Canada, USA and Australia, including the three Max Planck Institutes for Solar System Research (Katlenburg-Lindau), Chemistry (Mainz) and Extraterrestrial Physics (Garching).

On board Rosetta and Philae are eleven and nine scientific instruments respectively, which will investigate the target comet. The Max Planck Institute for Solar System Research has provided five instruments on both Rosetta and Philae as a member of international teams. The overall control of the OSIRIS camera system, the COSIMA dust detector and the COSAC gas chromatograph lies with the Lindau Institute.

HOR / NK

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