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For the JWST to be able to make precise infrared observations it is necessary that the telescope and its instruments are cooled so as not to emit their own infrared radiation that might interfere with signals from observed astronomical objects. This is one of the reasons why the JWST orbit will be far away from Earth. Sunlight reflected from the Earth's clouds and oceans creates enough heat to disturb observations.

An L2 orbit provides a benign and stable environment. Credit: ESA

A special point in space, 1.5 million kilometres from Earth, is particularly stable and favourable for spacecraft. It is called L2, or the second Earth-Sun Lagrange point, and this is where will be 'parked'. The distance to this point corresponds to about four times the distance from the Earth to the Moon. The point is located on the opposite side of the Earth with respect to the Sun. (There is another stable point, L1, between the Earth and the Sun from where ESA's SOHO spacecraft observes the Sun.)

The remote location of the JWST at L2 puts very strict limits on its physical specifications. To be able to fly all the way to L2 the JWST can't weigh more than around 6200 kilograms, and it will need powerful radio transmission equipment to 'beam' back data to Earth.

Last Update: 13 October 2009

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