WASHINGTON — NASA has developed a draft Mars exploration plan that entails sending a single robotic mission to the red planet every couple of years, a downsizing of previous ambitions that must still be approved by top agency officials.

But while the plans for scouring Mars with orbiters and landers have been scaled back significantly since the agency made exploration of the Moon a major focus, NASA still intends to spend in excess of $600 million a year exploring the fourth planet from the Sun.

NASA’s draft Mars exploration agenda for the next decade — drawn up over the past year with the help of outside scientists — was presented to an ad hoc committee of the Space Studies Board during a March 29 meeting at the National Research Council here. The committee has been asked to comment on the plan by June. NASA’s Science Mission Directorate is supposed to weigh in around the same time in advance of final agency-level approval, which is expected in late summer or early autumn.

NASA currently has four science spacecraft in orbit around Mars, including the European-led Mars Express, and a pair of identical Mars Exploration Rovers that have lasted much longer than expected. The Mars Reconnaissance Orbiter, the newest spacecraft on the scene, arrived in mid-March and is expected to begin its two-year science mission in October. A competitively selected stationary lander called Phoenix is slated to launch in August 2007, followed in late 2009 by the Mars Science Laboratory, which includes a brawny rover powered by a long-lasting nuclear battery.

Since the 1996 Mars Pathfinder mission, NASA has generally sent two spacecraft to Mars roughly every two years, taking advantage of favorable planetary alignment that shortens the trip. NASA now plans to launch only one mission per two-year window.

NASA’s plans for the next decade of Mars exploration have been largely up in the air since the agency canceled a planned 2009 Mars Telecommunications Orbiter last summer, scrapped plans to do at least two technology-driven missions meant to help set the stage for eventual human exploration, and postponed indefinitely a long called for mission to collect samples from the planet’s surface and return them to Earth for study.

Although Mars sample return and so-called human precursor missions still do not enter into the mix, NASA can now say what it expects to be doing on Mars through 2016 — assuming the proposed plan meets with the approval of the senior agency officials who still have to review it.

Under NASA’s draft plan, the first mission of the next decade would be a competitively selected Scout mission. NASA is soliciting proposals for the 2011 launch opportunity this spring and intends to select three finalists in October for further study, according to Doug McCuistion, director of NASA’s Mars Exploration Program.

Next up would be a Mars science and telecommunications orbiter that would launch in 2013 to continue detailed reconnaissance of the planet and serve as a data relay for other spacecraft exploring the planet.

In 2016, NASA would send a lander mission. McCuistion said the two options under consideration are a so-called Astrobiology Field Laboratory that would look for evidence of past life or one or two mid-sized rovers similar to Spirit and Opportunity, the scrappy rovers that have been exploring opposite sides of the planet since January 2004.

Beyond 2016, McCuistion said, NASA’s plans are not as crisp, but ideas currently on the table include sending to Mars a mix of Scout missions, sample collection expeditions and a network of small landers that would in concert .

In presenting NASA’s draft Mars exploration agenda to the Space Studies Board committee, McCuistion did not sugar coat the challenges he and his colleagues faced in drawing up what he termed a viable plan for the next decade.

NASA’s 2007 budget request includes $3.1 billion less for science through 2010 than the agency’s previous five-year plan. Hardest hit of NASA’s science divisions was Solar Systems Exploration, of which Mars exploration is a part. It lost $2.9 billion in the new five-year plan, most of which came out of the Mars program.

After a $50 million increase in 2007 to cover peak spending on the Mars Science Laboratory, NASA’s Mars budget drops to about $630 million and then remains essentially flat into the next decade. Under NASA’s prior five-year plan, the Mars Exploration Program stood to head into the next decade with annual budgets about twice as big as what it now expects to get.

The absence of real growth in NASA’s Mars exploration budget for at least the remainder of the decade forced McCuistion and his colleagues to make tough choices about what he dubbed NASA’s core Mars program.

Because NASA is reducing its planned technology spending by about 50 percent, McCuistion said NASA will not, for example, be developing pinpoint landing technology meant to enable the Mars Science Laboratory to touch down within 100 meters to a kilometer of its chosen landing spot instead of the 20 kilometer range possible today.

McCuistion said he also cut the operations budgets of the existing Mars missions by 10 to 20 percent. “I’ve taken money out of all of them to be able to pay the bills,” he said. “Some are squeezed. No doubt about it.” He said NASA will conduct a senior review in late spring to look at which Mars missions NASA can afford to continue operating and which have completed their objectives and should be turned off.

McCuistion said he also had to make fairly deep cuts to the program’s management account. The amount of budget NASA’s Mars projects hold in reserve to address unforeseen problems has been reduced by about half. “We are at the lowest level of program reserves the program has ever had,” he said. “It’s a dangerous place to be.”

Telling the Space Studies Board committee that he did not like making the cuts he had to make, he said they were necessary, later adding that $600 million is still a “big pot of money” for a single program.

“Are we a little fragile? Yeah, we are a little fragile. But we still have a program that’s viable for the next decade,” he said.

Brian Berger is editor in chief of SpaceNews.com and the SpaceNews magazine. He joined SpaceNews.com in 1998, spending his first decade with the publication covering NASA. His reporting on the 2003 Space Shuttle Columbia accident was...