COLORADO SPRINGS, Colo. — House and Senate appropriators have pushed back against NASA’s proposed termination of a planned 2011 robotic lunar lander mission, directing the agency to spend $20 million this year to continue work on a follow-on to the 2008 Lunar Reconnaissance Orbiter.

The directive, in the form of a letter from Sen. Barbara Mikulski (D-Md.) and Rep. Alan Mollohan (D-W.Va.), who respectively chair the Senate and House appropriations panels with NASA oversight, responds to a 2007 operating plan the space agency submitted to Capitol Hill in mid March. Besides formalizing NASA’s previously disclosed intentions to cancel the robotic lander , the operating plan details the latest cost estimates for a slew of other missions.

Canceling the lander mission would save NASA $105.8 million this year, according to the operating plan.

But Mollohan and Mikulski are not having it. In an April 10 letter to NASA Administrator Mike Griffin, the lawmakers remind the agency head of past promises to pursue “robust lunar precursor missions” in preparation for human expeditions to the Moon.

“Based upon the President’s plan for lunar exploration and Congress’s support for the [Lunar Precursor and Robotics Program, or LPRP] through subsequent appropriations and authorizations acts, we do not agree with your decision to terminate the LPRP program at this time pending a further examination of program requirements, design, cost and viability,” the letter reads. “Therefore we direct that $20 million be provided to continue planning for a potential LPRP mission during the remainder of [2007].”

NASA wants to cancel the lander in part to help absorb a nearly $700 million shortfall in its exploration systems budget without having to delay near-term work on the Ares 1 rocket and Orion Crew Exploration Vehicle. Those two programs still would be funded at $2.55 billion this year under the operating plan.

NASA also says the robotic lander is not needed to enable a human return to the Moon, a point Griffin made as recently as April 12 during a press briefing at the 23rd National Space Symposium here.

“I do not need a robotic lander to reduce risks for the human landings,” Griffin said. “Everybody who has carefully looked at that has said you don’t need it.”

Griffin said such a mission, while “it would be nice to have,” is not necessary. “Right now, the budget is such that I have to focus on what’s necessary,” he said, adding that the decision to cancel the lander was not taken lightly.

“The lander wasn’t the first thing I removed from the program. It was the last thing,” he said.

No other cancellations are detailed in the operating plan, a copy of which was obtained by Space News.

But Mikulski and Mollohan do take issue with some of the other spending decisions NASA has made in apportioning the $16.2 billion Congress gave the agency for a second year in a row.

Specifically, the lawmakers want NASA to restore half of a $54 million cut it made to education programs, and to provide $5 million for a Solar Probe mission under study at the Johns Hopkins Applied Physics Laboratory in Laurel, Md.

The lawmakers also expressed concern about the long-planned Space Interferometry Mission being reduced to a technology-development effort, but did not call for any restoration of funds. The letter also urged NASA to provide stable funding for its Independent Verification and Validation Facility in Fairmont, W.Va.

The lawmakers are silent on the rest of NASA’s operating plan, which also quantifies the extra funding NASA must spend this year to maintain the launch schedules of major programs including the James Webb Space Telescope and the Mars Science Laboratory.

Keeping James Webb on track to launch in 2013 will cost $478.5 million this year, $9.9 million more than NASA previously anticipated , according to the documents.

Maintaining the Mars Science Laboratory’s 2009 launch date will cost an extra $62.7 million this year, pushing the mission’s total price tag, including launch and several years of operations, to $1.75 billion.

NASA expects to spend $141.7 million completing the Gamma Ray Large Area Telescope this year, $16.6 million more than it was banking on due to problems with the observatory’s avionics and main instrument. Total price tag for the mission, due to launch this November, is now $871.7 million.

The Kepler planet-hunting telescope also is costing more than expected.

Delaying the launch of the planet-hunting mission to November 2008 will require an additional $36.7 million for 2007 and increase the total cost of the project to $649.3 million, according to the operating plan.

Launch vehicle-related concerns that pushed last year’s scheduled launch of the Time History of Events and Macroscale Interactions during Substorms mission by four months — into 2007 — added $3.9 million to the cost of the five-satellite project.

A NASA official said the agency would not comment on specifics of the operating plan since it was still before Congress for review.