A school bus-sized spacecraft carrying the largest telescope ever installed in a planetary probe blasted off early Friday from the Cape Canaveral Air Force Station in Florida, beginning a seven-month journey to the Red Planet.

The Mars Reconnaissance Orbiter, 22 feet in length and weighing more than two tons, will study the Martian surface and atmosphere from orbit, searching for sites where water may have once flowed, identifying potential landing sites for future surface craft and providing a powerful new relay station for transmitting data back to Earth.

It is expected to return more data about Mars than all previous missions put together.

“We’re up, we’re on our way to Mars, and we have a spacecraft that is performing … absolutely perfectly,” said James Graf, project manager for the mission at the Jet Propulsion Laboratory in La Canada Flintridge, Calif.

The Atlas V launch vehicle carrying the Mars Reconnaissance Orbiter, MRO, lifts off the pad Friday at the Cape Canaveral Air Force Station in Cape Canaveral, Fla, on Friday. Circling the planet for at least four years, the orbiter is to provide information on Mars' weather, climate and geology.

Mars was 72 million miles from Earth at launch time, but the craft will travel more than four times that distance to intercept the planet on March 10, 2006. A 30-minute burn of its main engine will slow the craft enough to put it in a highly elliptical orbit that will swing within 100 miles of the surface at its closest point and as far out as 20,000 miles at its highest.

For the rest of the year, the orbiter will perform a delicate maneuver called “aero-braking” in which it will dip into the upper fringes of the atmosphere an estimated 512 times until its speed has slowed enough to bring it into a nearly circular orbit 190 miles above the planet, where it will begin its four-year mission.

The orbiter carries six major science experiments, the largest of which is the High Resolution Imaging Science Experiment or HIRISE, operated out of the University of Arizona. HIRISE is a telescope with a 20-inch mirror, the largest ever sent out of Earth orbit, with a 1,200 megapixel digital camera.

The telescope will be able to resolve objects on the surface the size of a washing machine, said University of Arizona astronomer Alfred McEwen, the principal investigator for the instrument. The high-resolution imaging will allow researchers to map geological features and identify large rocks that would be hazardous to future landers.