The Kepler science spacecraft has successfully lifted off from Florida and begun its multi-year mission to look for planets similar to our own in the galaxy.

Kepler blasted into space atop a Delta II rocket at 10:49 pm EST from Launch Complex 17B at Cape Canaveral Air Force Base.

The US $600 million observatory will use a large electronic camera to survey a swath of the Milky Way for so-called exoplanets transiting through the light coming from stars much like our sun but far, far away.

NASA said today that the survey field contains about 100 000 stars of the appropriate magnitude. Kepler will travel through space in a solar orbit trailing Earth, which will enable it to maintain its focus on targets selected by astronomers, something the Hubble telescope is unable to do.

The objects Kepler "sees" will be detected by a 95-megapixel array of charge-coupled devices to record changes in brightness of 20 parts per million in stars that are thousands of light years away in a portion of the sky visible terrestrially in the constellations Cygnus and Lyra. Then scientists will subject the recorded data to extensive testing to determine the characteristics of the discovered planets.

"We certainly won't find E.T., but we might find E.T.'s home by looking at all of these stars," Bill Boruki, Kepler's principal scientist, said yesterday.

"This is a historical mission, it's not just a science mission," NASA Associate Administrator Ed Weiler said during the prelaunch news conference. "It really attacks some very basic human questions that have been part of our genetic code since that first man or woman looked up in the sky and asked the question: Are we alone?"

For more on Kepler's mission, see NASA Planet Hunter to Search Out Other Earths in the current issue of Spectrum Online.