In addition the flotilla will be able to carry out high-resolution imaging using aperture synthesis, to provide pictures of celestial objects with unprecedented detail.
Darwin observes in the infrared since life on Earth leaves its mark at these wavelengths. On Earth, biological activity produces gases that mingle with our atmosphere. For example, plants give out oxygen and animals expel carbon dioxide and methane.
This will show the drop in light caused by specific gases being in the atmosphere, allowing them to be identified. If they are the same as those produced by life on Earth, rather than by non-biological processes, Darwin will have found evidence for life on another world.
On Earth, the atmosphere blocks the mid-infrared wavelengths of light that Darwin is designed to observe. The light that Darwin is looking for is absorbed by our atmosphere. At room temperature, the telescopes would themselves emit infrared radiation, swamping their own observations. It would be like using a normal telescope to perform optical astronomy with a wall of floodlights pointing into it.
In space, it is very cold. The telescope is designed to work at just 40K (–233°C) while the actual detector has to be reduced in temperature further to just 8K (–265°C). This stops the telescope radiating its own infrared signal and allows it to search for the faint light of distant planets.
Darwin consists of four (or possibly five) separate spacecraft. Three of the spacecraft will carry 3-4 metre 'space telescopes', or more accurately light collectors, based on the Herschel design. These will redirect light to the central hub spacecraft.
For Darwin's objective to find and investigate Earth-like planets, it will use a technique called 'nulling interferometry'. The light reaching some of the telescopes will be very slightly delayed before it is combined. This will cause light from the central star to be 'cancelled out' in the resultant data.
Light from planets, however, is already delayed between one telescope and the other since the planet is to one side of where the telescopes are pointed. By delaying the light a second time the light from the planet will combine constructively and the planet can thus be seen. If it were not for this 'nulling', the starlight would overwhelm the planet's feeble glow.
In 'imaging' mode, Darwin will work like a single large telescope, with a diameter of up to several 100 metres, providing images of many types of celestial object in detail.
For Darwin to work, the telescopes and the hub must stay in formation with millimetre precision. ESA is confident of achieving this aim using a variation of the highly successful Global Positioning System (GPS) that provides so much of the satellite-based navigation on Earth.
But this is not enough, when the light collected by the telescopes is recombined it has to be done at very high precision. A deviation of more than just 100 thousandths of a millimetre will ruin the observation.
Although this sounds an impossible feat of accuracy, ESA has already together with European industry made pre-developments of the necessary metrology and optical equipment that would allow such precision.
The spacecraft will probably be equipped with tiny ion engines that need just five kilograms of fuel to last the entire five-year mission. The ion engines expel small particles at very high velocity such that the spacecraft moves slightly in the opposite direction.
Darwin's launch date is to be defined in the context of ESA's Cosmic Vision scientific programme. For the launch, ESA will use two launches with Soyuz-Fregat rockets, probably from ESA's Spaceport at Kourou in French Guiana.
These gases, and other substances, such as water, leave their fingerprints by absorbing certain colours (wavelengths) of infrared light. Darwin will split the light from an extrasolar planet into its constituent colours, using an instrument called a spectrometer.
Instead of an orbit around the Earth, Darwin will be placed far away, beyond the Moon. At a distance of 1.5 million kilometres from Earth, in the opposite direction from the Sun, Darwin will operate from a special location known as Lagrangian Point L2.