Washington - NASA late Friday sent the Kepler satellite into Earth's orbit with instructions to search for Earth-type planets orbiting other stars - a mission which could shed light on the ancient question of life elsewhere in the universe.

The punctual launch from Cape Canaveral in Florida via a Delta-II- rocket was reported on a live blog operated by the Kepler project on the internet.

The Kepler mission, named after the 17th century German astronomer, is targetting about 100,000 stars in the Milky Way galaxy that scientists believe could have planets orbiting in a "habitable" zone.

The orbiting camera will be looking for shadows moving across the stars, which would indicate planetary movement and help measure the length of the orbits.

"If it orbits very fast, it will be a very hot planet," said William Borucki, NASA's principle investigator for the Kepler project, last month. "If the planet has a long orbital period, it will be very cold, we expect it would be forever frozen."

The mission will eliminate both extremes as scientists, like Goldilocks, search for the planets that are "not too hot, not too cold, but just right," Borucki said.

The 591-million-dollar mission is slated to last for three and a half years, but the Kepler camera will be outfitted to last six years, in anticipation that funding would be extended, NASA scientists said.

The mission scientists hope to help resolve many questions: Are we alone in the universe? Are there other planets like Earth?

It "could tell us that we have lots of neighbours or that we are perhaps the only one," said researcher Ed Weiler.

The camera must sort through a large swath of the Milky Way Galaxy containing some 4.5 million stars.

The most advanced cameras ever used in space will focus on some 100,000 to 150,000 stars deemed most likely to have planets orbiting them, scientists said at a pre-launch press briefing.

An Earth-sized planet will be particularly hard to spot because of its size and the fact that it will cross in front of its sun only once a year, scientists said.

Since 1995, more than 340 planets have been found outside our solar system, but these so-called exoplanets have been large, gaseous planets, like Jupiter, which tend to be closer to their stars and easier to spot because of their size. Planets like Earth, which could be capable of housing life, can exist only in a small "habitable zone" within a certain distance from their sun.

The known exoplanets are "nothing at all like our solar system, which we know so well and we have yet to discover a true analog to Earth," said Padi Boyd, a scientist for the programme who called Kepler "essentially a planet sifter for Earths."

Still, Kepler will scan just a small portion of the sky - about as much as would be covered by the hand of a human standing on Earth with an arm outstretched.

And if it finds an Earth-sized planet, scientists would still have a lot of work to do to determine whether conditions there would be hospitable to life because many other conditions, such as the existence of water and temperature, would also have to be right.

The mission will provide "an assessment of the real estate market of other rocky planets, but it won't actually tell us how nice the homes are," said scientist Gibor Basri, noting that will have to wait for other missions.