By Irene Klotz

CAPE CANAVERAL, Florida (Reuters) - A NASA telescope was cleared to launch on Friday on a mission to look for Earth-like planets around other stars and determine whether there are places that could support human-like life beyond our solar system.

Liftoff of the Kepler telescope is scheduled for 10:49 p.m. EST on Friday from Cape Canaveral Air Force Station in Florida.

"This is a historical mission," NASA's space science chief Ed Weiler told reporters on Thursday. "It really attacks some basic human questions that have been asked since that first man or woman looked up in the sky and asked, 'Are we alone?'"

Once in orbit, Kepler will be aimed at a star-rich swath of sky between the constellations Cygnus and Lyra in our own Milky Way galaxy. The telescope has two main tasks on its three-year mission: Stare at the stars and stay still.

Light-collecting devices in the telescope are sensitive enough to detect slight changes in the number of photons emanating from more than 100,000 target stars in the telescope's field of view. Some of changes will be due to planets passing in front of their parent stars and temporarily blocking a bit of light.

Scientists already have found more than 340 planets circling stars beyond our solar system, but none of those worlds are as small as Earth. Kepler is the first instrument designed solely to hunt Earth-sized worlds circling their parent stars at the proper distance for liquid water to exist. Water is believed to be a necessary ingredient for life.

"Kepler is not going to find out about the atmospheres, or whether there is water on these planets," said Gibor Basri, a Kepler scientist with the University of California at Berkeley. "It's really an assay of what the real estate market is out there for rocky planets."

The survey will take about three years, after which scientists expect to be able to announce whether Earth-like planets are common or rare.

"It very possibly could tell us that Earths are very, very common, that we have lots of neighbors out there. Or, it could tell us that Earths are really, really, really rare - perhaps we're the only Earth," Weiler said.

"I think that would be a very bad answer," he added. "I, for one, don't want to live in an empty universe where we're the best there is -- that's a scary thought to many of us."

(Editing by Jane Sutton)