NASA's planet-hunting Kepler telescope is poised for a late-night launch Friday night to begin seeking out Earth-like planets circling distant stars.

The $600 million Kepler spacecraft is slated to blast off from Florida's Cape Canaveral Air Force Station Friday at 10:49 p.m. EST (0349 March 7 GMT) on a mission that could profoundly change how humans perceive their role in the universe.

"It very possibly could tell us that Earths are very, very common, that we have lots of neighbors out there," said Ed Weiler, NASA's associate administrator for science missions. "Or it could tell us that Earths are really, really rare, and we're all alone out there."

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Named after Johannes Kepler, the 17th century German scientist who pioneered the laws of planetary motion, the Kepler the spacecraft is NASA's first mission dedicated seeking out planets like Earth orbiting stars at just the right distance to allow liquid water — a vital ingredient for life on our own world — to exist on the surface.

"Kepler is essentially a planet-sifter for Earths," said Patricia Boyd, NASA's Kepler program scientist, adding that the mission is expected to take a census of Earth-like planets to see how common they are in our Milky Way galaxy. "The answer to that question could fundamentally shift our picture of our place in the universe."

Astronomers have discovered nearly 340 extrasolar planets since 1995, but most of them are gas giants, like Jupiter, or larger.

"What we're really interested in are rocky planets like that of the Earth," said William Borucki, Kepler's principal investigator at NASA's Ames Research Center in Moffett Field, Calif.

The forecast for tonight's planned launch appears pristine, with a less than five percent chance of foul weather thwarting Kepler's liftoff, mission managers said.

The mission has two launch opportunities; a three-minute window at its first launch time and another window that opens at 11:13 p.m. EST (0414 March 7 GMT).

NASA delayed Kepler's launch by one day last week to allow extra rocket checks on the spacecraft's Delta 2 booster to ensure it was fit to fly.

The precaution, stemmed from the Feb. 24 failure of a different rocket carrying a NASA Earth-watching satellite, found Kepler's booster in fine shape for tonight's planned liftoff, mission managers said.

Strange New Earths

After launch, Kepler is designed to turn its unblinking camera eye at a patch of sky between 600 and 3,000 light-years from Earth in the direction of the constellations Cygnus and Lyra. The target zone covers an area similar to what a human hand could cover when held at arm's length.

Kepler will stare at the region for at least 3 1/2 years, measuring the light from 100,000 stars every half hour with a 95 million-pixel camera to watch for the slight dip in a star's brightness that signals a planet moving across it as seen from Earth. It's the equivalent of trying to spot a flea crawl across a car headlight from miles away, NASA has said.

"We certainly won't find E.T.," Borucki said. "But we will find E.T.'s home by looking at all of these stars."

But spotting planets the size of Earth is hard work. Kepler will seek out planets that circle their parent stars in just the right orbit, a so-called habitable or "Goldilocks" zone that is neither too hot nor cold for liquid water to exist.

For example, last month European scientists using the COROT space telescope announced the discovery of COROT-Exo-7b, a small exoplanet with a mass that weighs in at just twice the size of the Earth.

But while the planet's status as the smallest exoplanet has caused some debate, researchers are sure the alien world orbits very close to its parent star, making the trip once every 20 hours.

Surface temperatures on COROT-Exo-7b are estimated at 1,832 to 2,732 degrees Fahrenheit (1,000 to 1,500 degrees Celsius).

"If that planet has an ocean, it flows with molten lead," said Borucki, adding that a planet circling a star from too far out faces a different problem. "Too far out and they're too cold. They're probably frozen solid."

So Kepler will be hunting for planets that move across their stars, or transit, about once every Earth year. Prime candidates will be ones the space telescope spots three times during its initial mission, mission researchers said.

Kepler will scout for its Earth-like quarry from an orbit that trails behind the Earth and circles the sun once every 371 days. While the spacecraft is designed to last 3 1/2 years, it carries enough fuel for up to six years of planet-hunting just in case its mission is extended.

"We're very proud of the vehicle we have built," said Jim Fanson, Kepler's project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "This is a crowning achievement for NASA and a monumental step for our search for other Earths around other stars."

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