Saturday, April 14, 2007

ESA's Darwin

Search For Extrasolar Planets And Extraterrestrial Life Improved With Darwin's Frictionless Optics

ESA's Darwin mission aims to discover extrasolar planets and examine their atmospheres for the presence of certain life-related chemicals such as oxygen and carbon dioxide.

The major technical challenge lies in distinguishing, or resolving, the light from an extrasolar planet from the hugely overwhelming radiation emitted by the planet's nearby star.
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The multi-satellite Darwin mission will use optical interferometry in which at least three separate orbiting telescopes jointly operate as an equivalent single telescope with a much larger effective aperture, thus achieving the required resolution. With this method, multiple smaller telescopes having actual apertures of, for example, 3 metres, can combine to provide an effective aperture of several tens to hundreds of metres, depending on the distance between the individual telescopes.

Creating delicate phase delays
Darwin will use nulling interferometry, a specific interferometric technique used to shield the overwhelming star emissions by precisely delaying the radiation coming from some of the telescopes by a small amount. This, in combination with achromatic - or colour independent - phase shifters, will cancel out the bright star radiation while allowing the much fainter extrasolar planet light to be detected.

A component known as an Optical Delay Line (ODL) is at the core of such interferometric observations. An ODL is a sophisticated opto-mechanical device that can introduce well-defined variations, or delays, in the optical path of a light beam and includes a moving mirror positioned with extremely good accuracy.

Precise movement using magnetic levitation
To demonstrate the critical technology required by Darwin, ESA's Technology Research Programme has sponsored the design and testing of an ODL that uses magnetic levitation for precise, frictionless mirror movement.

Sub-nanometre resolution to be incorporated in future flight mechanism
The ODL has also been thoroughly tested in Darwin's demanding cryogenic environment, at 40 Kelvin - or about -233 Celsius.

Darwin's ODLs are uniquely engineered to operate at cryogenic temperatures to avoid self-interference from the satellites' own thermal radiation. This is mandatory as Darwin will conduct observations at mid-infrared wavelengths, where the planet-to-starlight brightness ratio is relaxed compared to that in visible wavelengths, and where life-related marker chemicals such as water, ozone and carbon dioxide can be detected.

The ODLs will be used in Darwin for co-phasing the light collected by the separate telescopes within a central hub spacecraft, which is responsible for the correct recombination of the light beams and hence achieving the high-performance resolution of a single very large telescope.

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