CAPE CANAVERAL, Fla., Aug. 25— The last in the series of NASA's ''Great Observatories'' streaked into a starry sky early this morning, reaching its intended orbit around the Sun less than an hour later.

Called Sirtf (pronounced SIRT-ef), short for Space Infrared Telescope Facility, the $700 million satellite will scan the cosmic abyss for infant stars and other relatively lukewarm objects hidden by the dusty cores of our galaxy as well as others, some as distant as 10 billion light-years.

''These objects are far away; it takes the light a long time to get to us, so we're seeing them when they were young,'' said Dr. Michael Werner, the spacecraft project scientist at the Jet Propulsion Laboratory in Pasadena, Calif. ''We're looking back in time as well as back in space.''

Sirtf's three scientific instruments will be chilled to minus 457 degrees Fahrenheit (right above absolute zero) by a hefty reserve of liquid helium. The telescope's three-foot-wide mirror will enable scientists to detect celestial bodies like young stars still encircled by a blend of gases, dust and other matter that one day may coalesce into planets. The search will include failed stars, also known as brown dwarfs.

NASA hopes observations will go on for two and a half years and possibly twice as long. The launching was delayed more than a year because of problems with the spacecraft and its Delta 2 rocket.

''Sirtf will be following the Earth around the Sun, kind of like a faithful puppy,'' Dr. Werner said.

Sirtf will also make it possible to study frigid worlds like Pluto and its moon, Charon. They are considered part of the Kuiper belt, a sort of boundary of the solar system populated by more than a million icy bodies.

Without Sirtf, these objects would be just too cold, dim and distant to be observed, said Dr. Dale Cruikshank of NASA. About half of the comets seen in the solar system come from the Kuiper belt, he said.