Part of ESA's Horizons 2000 programme, IRSI/Darwin is a project for a space based infrared interferometer. IRSI stands for Infra-Red Space Interferometer, but the project is commonly referred as just Darwin (and will be for the remainder of the writeup). Darwin is due to be launched in 2014 or later, using an Ariane 5 rocket.

Mission goals

Darwin's goal is to find extrasolar planets capable of supporting life as we know it. Its primary target will be stars similar to our sun and within 50 light years or so from earth. Although many exoplanets have been discovered in recent years using existing equipment, they have not been discovered directly, instead astronomers have inferred their presence from their gravitational influence. In order to accomplish its goal, Darwin needs to be able to observe planets directly.

A candle next to a lighthouse

A planet is not an easy thing to observe, mainly because it's a very small, dim thing next to another very bright thing. In visible light the contrast is roughly of the order of 1 billion to 1. When observing in the mid-infrared this contrast drops to 1 million to 1, which is one of the reasons that Darwin is designed to observe that portion of the spectrum. The other, main, reason is of course because we expect to see something. While Darwin is supposed to find life, it won't be able to see the actual living organisms (for one thing there might not be much to see). Instead it will look for things that we believe to be signs of life or conditions favourable to life, for example water or oxygen. Presence of such substances in the atmosphere would result in the absorption of certain wavelengths of infrared light, and this is what Darwin will be looking for: absorption lines in the spectra of planets.

Why space?

Darwin will be space based because the earth's atmosphere is a terrible handicap when it comes to observing the skies. Not only do turbulences and dust blur your picture but the atmosphere is actually opaque to the wavelengths the Darwin must observe. "Pollution" from local heat sources is also a big concern when observing in the infrared. Space on the other hand is free of these troubles (although zodiacal dust remains a concern). Darwin is designed to operate at around 30K, with the actual detector cooled down to just 8K.

Darwin will be positioned at the L2 Lagrange point (that's around 1.5 million km beyond earth). The L2 Lagrange point is starting to be quite a popular place (the James Webb Space Telescope will also be placed there) because of its advantages:

  • It's a stable point: little or no fuel is required to stay there.
  • The earth blocks some of the sun's radiation, simplifying cooling operations.
  • Sufficient distance from Earth ensures that heat radiated from Earth is not a concern.
  • Staying put is far simpler than orbiting the Earth: the constant passing in and out of the Earth's shadow would cause the telescopes to warm up and cool down, distorting their view.
The downside of course is that unlike an object in low Earth orbit, repairs to Darwin will be impossible.

A cluster of satellites

All this does not allow you to escape fact the that you still need a very big telescope, with a main mirror of at least 15m in diameter. It is currently not feasible to deploy such a large mirror in space. So how does Darwin do it? The answer is interferometry. Without going into the details this technique allows you to combine several small telescopes to act like a single bigger one. A specific variant of interferometry known as nulling interferometry allows one to combine the light from several small telescopes in a way that eliminates the light coming from the star, thus enabling one to observe objects normally hidden by the star's glare. Darwin will be capable of using both these methods.

Darwin calls for a total of 8 satellites. Of these, 6 are the actual telescopes, one is the "hub" used to build up the picture and the last is a master satellite that serves as a coordinator and as a communications relay with earth. Darwin's 6 telescopes will be placed at the vertices of a regular hexagon, with the 6 sided hub sitting at the centre of the polygon. The communications relay is "behind" the 7 other satellites. While initially it was thought that the hub and communications relay parts could be combined, the constraints of keeping the antenna pointed very precisely at earth and keeping the hub in very precise alignment with the 6 telescopes turned out to be incompatible. The current configuration also allows the master satellite to monitor the position of the 7 other satellites.

Each of the telescopes will be of the Cassegrain type (like Hubble) and will be around 2.8 metres long with a diameter of 1.5 metres. To allow the telescopes to reposition themselves, they will be equipped with small thrusters. Ion drives are probable candidates because of their very low fuel consumption: 5 kg of fuel would be enough for the mission's planned duration of 5 years. The satellites would be equipped with large solar panels for all their electrical needs, which would double as sunshields (in order to keep the detector equipment nice and cool). While obviously bigger telescopes would yield better results, the main constraint is that all 8 satellites need to fit into the nose cone of a single Ariane 5 rocket. Although it is true that this mission would use a modified nose cone, there are still hard limits on its diameter.

What's the big deal with positionning?

It's a constraint inherent to interferometry. Interferometry requires that the telescopes and the hub be placed very precisely, in the case of Darwin the precision needed is of the order of 20 nm. As you can imagine positioning 7 satellites with that kind of precision is not going to be easy. The plan is to use a technology related to GPS: radio signals sent at regular intervals will allow the position of the satellites to be monitored very precisely. If a satellite is detected as moving out of alignment it will be commanded to use its thrusters to reposition itself. The technology that makes this hi-precision placement possible will be tested by ESA's SMART-2 mission (although this mission will not perform any kind of interferometry experiments).

Darwin is an ambitious project that is currently still in its development phase. Originally conceived in 1993, the mission's scope has been extended to providing images, which should be ten times more detailed than what the James Webb Space Telescope will provide. Many of its goals and plans are similar to those of NASA's Terrestrial Planet Finder and it is possible that NASA and ESA may collaborate. If Darwin is built it will be a giant step towards answering the question "Are we alone?".

Sources: http://www.esa.int/export/esaSC/120382_index_0_m.html
http://www.esa.int/export/esaSC/SEMZ0E1A6BD_index_0.html
http://www.star.ucl.ac.uk/~rhdt/diploma/lecture_10/

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