PASADENA, Calif. NASA will announce on Monday whether it intends to launch in 2003 a satellite to orbit Mars or a rover to roam the planets surface.

NASA had originally planned to launch two missions, both an orbiter and a lander, to Mars every 26 months.

But the back-to-back losses of a such a pair of Martian probes last year has forced the American space agency to scale back its immediate exploratory plans for the Red Planet.

Already, NASA has scrapped plans for a Mars lander for the 2001 launch opportunity, opting to send only a satellite to the planet during that time. The 2003 mission opportunity marks the second chapter in the agencys retrenched Martian exploration campaign.

The decision on which roughly $400 million mission to pursue, comes at the end of a fast and furious two-month study of the likelihood for success, launch-readiness, scientific value, cost and public appeal of either option.

Two teams fielded proposals: Lockheed Martin Astronautics, with the Mars 2003 Surveyor Orbiter, and NASAs own Jet Propulsion Laboratory (JPL), with the Mars 2003 Geology Rover.

(A third, dark-horse proposal, now being reviewed independently by NASA, seeks to send the scrapped 2001 Mars lander to the Red Planet in 2003 as well. Proponents argue the mission could cost as little as $100 million more than what has already been spent on the mothballed spacecraft and its science instruments.)

NASA has remained tight-lipped about the decision process, leaving many members of both teams in the dark as of late Thursday.

Should NASA pick the orbiter, the agency could have as many as three satellites in orbit around Mars come 2003, assuming Mars Global Surveyor remains healthy and a 2001 orbiter arrives without a hitch after its launch next April.

(Two international satellites, Japans Nozomi and the European Mars Express, may well join the NASA orbiters as well, making for an intensive and international cluster of spacecraft in orbit around Mars by early 2004.)

The proposed 2003 orbiter, larger than either Global Surveyor or the 2001 satellite, would carry five instruments, including a camera capable of viewing objects as small as 2 feet (60 centimeters) across. It would last in orbit anywhere from six to 10 years.

(A novel British proposal, unlikely to go forward for 2003, calls for the orbiter to carry a carbon copy of the Beagle 2 Martian lander, the original of which will ride piggyback to the planet aboard the Mars Express orbiter.)

Were NASA instead opt for the rover, it could mark the agencys return to the planets surface for the first time since 1997s Pathfinder. Indeed, like Pathfinder, a cocoon of airbags would swaddle the large rover during its bounce-down on Mars.

Unlike Pathfinder and its plucky rover Sojourner, the 2003 version would be all rover and no lander. Or better, the lander portion of the spacecraft would be dead on arrival, leaving the larger rover to scurry up to 0.6 mile (1 kilometer) across Mars, acting as an independent field geologist.

The announcement will be made during a 2 p.m. Eastern Daylight Time (18:00 GMT) press conference at NASA Headquarters in Washington, D.C.

Scheduled to speak are: NASA Associate Administrator Edward Weiler; Scott Hubbard, NASA Mars program director; Jim Garvin, NASA Mars program scientist and NASA JPL Mars Program Office Manager, Firouz Naderi.