A picture of Darwin free-flyers
Copyright Alcatel

Permission to reproduce this picture may be sought from the makers, Alcatel Space Industries, in Cannes, France. The person to contact is Dr Glenn Lund ( glenn.lund@space.alcatel.fr ).

  A concept for the `Free-Flyer' version.

This figure shows an artists impression by the Alcatel company of the Darwin spacecraft, operating some 1.5 million kilometres from the Earth, in the direction away from the Sun. Alcatel started in December 1997 a contract with ESA to perform a study of this concept. This picture is copyright Alcatel.

The infrared light from the target planetary system comes down from the top right of the picture into the six individual telescopes, each of which has a collecting mirror of 1.5 metres diameter.

In this version, called the `free-flyer', the six individual telescopes are mounted are separate spacecraft. The central spacecraft holds the hub with the optics which combines the light from the individual telescopes. The individual spacecraft are moved around by very precise small rocket-type engines on each of them.

In the hub, the light is combined in a technique called nulling interferometry. In this the light from the bright star is cancelled out, leaving only the faint light from the planets around the star.

In interferometry, one gets some of the information one would get from a telescope as big as the separation between the individual telescopes. For Darwin, this would be between 50 and 500 metres. To get enough of this information to build up a good picture, one has to move the individual telescopes around to different relative positions and repeat the `exposures'.

The optical distances between the telescopes and the central hub have to be controled to about a billionth of a meter. The actual distances between the spacecraft are measured with lasers to within a few millionths of a metre. There is a seventh, small, spacecraft, shown below the telescopes and the hub. This is used both in the distance measuring, and for the communications with Earth.

As well as taking a picture, the interferometer can take spectra of the objects it is looking at.

Alan J Penny - Rutherford Appleton Laboratory (RAL)

Last modified - 1999 July 27th