A mission to search for planets beyond our solar system that could harbour life is set to blast off from Cape Canaveral, Florida, later today at no earlier than 22:48 local time.

The three-year Kepler mission will seek to probe 100,000 stars for Earth-sized objects. Costing $590m, the NASA craft will aim to determine what fraction of stars have an Earth-like planet around them and guide future missions in locating Earth-like twins that can be scrutinized for any signs of life.

If the launch succeeds, the craft will enter into a “trailing orbit” that will fall behind Earth by roughly 18 million km each year. From there it will stare at the same part of the sky in hopes of catching any star that "blinks" as a planet passes in front.

Largest space camera

The Kepler spacecraft has the largest camera to ever be put into space. Its CCD array has more than 94 million pixels that will monitor 105 square degrees of sky (about the size of your hand held at arm's length).

Speaking before the launch, James Fanson, project manager of Kepler at NASA's Jet Propulsion Laboratory (JPL) in California, said that Kepler would be "a major step in our quest to understand if we are alone in the universe." If our type of planet is common, then Kepler might see hundreds of Earth-size transits, but it could also see none at all if terrestrial planets are rare.

Over 330 extrasolar planets are currently known, but most are “gas giants”. These planets have been the easiest to detect with traditional “radial velocity” techniques, which measure the wobble their gravity induces in the star. Kepler will use a different technique, which involves looking for changes in the brightness of a star as a planet crosses in front.

No atmospheric blurring

In the last few years small, dedicated ground-based telescopes have detected more than 50 transiting planets, with radii between 5 and 20 times that of Earth. The big advantages of Kepler over such instruments are that it will not be limited by atmospheric blurring and it will not suffer from daily temperature fluctuations in equipment. It will therefore be able to measure changes of as little as 10 parts per million in the brightness of stars.

Principal investigator William Borucki of NASA Ames Research Center in California compares this to seeing a tiny flea crossing a distant headlight. "What's exciting about Kepler is that it will detect far smaller planets than we are currently able to do from the ground," says Coel Hellier of Keele University in the UK, who is a member of the world's largest ground-based transiting survey called SuperWASP.

Far from easy

Seeing small changes in a star’s brightness caused by a passing planet is far from easy. If someone on another planet were looking at our Sun, they would need to detect a drop of 84 parts per million in brightness to notice our planet. Even if they had this capability, they would have to wait patiently to catch the 13-hour transit that only happens once every year.

Moreover, there is only a 0.5% probability that the geometry is right for seeing an Earth transit — in other words, these outside observers must be viewing the Sun along its orbital plane.

To deal with these low odds, the Kepler mission has selected a large sample: 100,000 stars that are between 150 and 2500 light-years away from Earth in the direction of the Cygnus constellation. If every one of these stars had an Earth-sized planet, Kepler would observe at most 500 of them.

Following act

The planets of greatest interest will be those in the so-called habitable zone, where the planet has temperatures favourable for liquid water — a presumed necessity for life. To tell whether a transiting planet is orbiting in this region, astronomers will observe at least three or four transits from which they will be able to verify the planet’s period.

By including a separate estimate of the host star's mass, they can then calculate the orbital radius using the laws of motion derived by the namesake of the mission, the astronomer Johannes Kepler in 1609.

Kepler will not be the first transit survey from space. The largest current mission is CoRoT, led by the French Space Agency (CNES) with contributions from the European Space Agency (ESA) and other nations.

Smallest exoplanet so far

Launched in 2007, CoRoT has so far detected seven transits and researchers recently announced the confirmation of the smallest extrasolar planet found so far. It is presumed to be rocky, having 1.8 times the radius and 11 times the mass of Earth.

CoRoT’s recent findings bode well for Kepler, which can detect planets over a 10 times larger range of orbital periods than CoRoT. "Kepler has a very good chance of seeing the first Earth-sized planet," says Malcolm Fridlund, ESA's CoRoT project scientist.

But, like all transit detections, Kepler's observations will need to be confirmed by the radial velocity method with ground-based telescopes, such as Keck in Hawaii and the William Herschel Telescope on the Canary Islands that measure the mass of a transiting object.

Transit mimicry

Lots of things can mimic a transit. Some stars like our Sun, for example, can have spots that alter the brightness as they rotate around the surface, while many others have faint companion stars than can appear like a planet when they pass in front.

“The important thing about Kepler is that it will tell us the relative number of small, medium and large mass planets," says Wesley Traub, chief scientist for JPL’s Exoplanet Exploration Program.

Kepler was originally planned to launch yesterday. However, NASA engineers have spent an extra day testing common hardware on the Delta II rocket, which will take the spacecraft into orbit, with that used for the Taurus XL rocket that crashed last month while taking NASA’s Orbiting Carbon Observatory into space.

Kepler will help support potential missions in the coming decade such as NASA's Terrestrial Planet Finder and ESA's Darwin mission that both have their sights on directly imaging an Earth-like planet around a nearby star. "How deep will we have to look? The nearest 100 stars? The nearest 1000 stars? Kepler will help us decide by giving the frequency of Earths in our galaxy," Fanson says.