WASHINGTON — A fter nixing a proposal to send a series of tiny spacecraft to the Moon starting in mid-2008, NASA recommitted late last month to keeping the Lunar Reconnaissance Orbiter at the head of the line — followed by a still-to-be-defined lunar lander mission run by Alabama’s Marshall Space Flight Center.

The proposal, the brain child of NASA Ames Research Center Director Simon “Pete” Worden, called for inserting a mix of microsatellites and tiny landers into the U.S. space agency’s existing robotic lunar exploration plans in order to make early strides at modest cost.

Worden, a big believer in small satellites, prepared the proposal with the blessing of NASA’s Exploration Systems Mission Directorate, which had assigned Ames in late May responsibility for developing small spacecraft missions to support the agency’s exploration goals. Worden said at the time that much of what NASA wanted to accomplish on the Moon before sending astronauts there around 2020 could be done with a mix of $50 million to $100 million missions.

“We are going to offer NASA some low-cost proposals that will knock their socks off,” Worden said in a May 24 interview.

While some NASA officials viewed Worden’s proposal as a way to enhance the agency’s current lunar robotics program at a modest cost, others saw in it a lower-cost alternative to the bigger, more expensive lander mission taking shape at Marshall Space Flight Center, Huntsville, Ala., sources said.

Worden’s small spacecraft proposal had gathered momentum inside the agency by the time it was presented to senior NASA officials, including the agency’s associate administrator for exploration, Scott Horowitz, at a July 13 meeting at Wallops Flight Center on Virginia’s eastern shore. Sources familiar with the meeting said the proposal was well received and that senior NASA officials emerged from the Wallops gathering in broad agreement on adding small missions to the mix.

The following day, July 14, Ames Research Center issued a

p air of solicitations for propulsion elements that would be needed right away to support a micro-lander mission that could be ready to launch by mid-2008 – a date that would put it ahead of the $550 million Lunar Reconnaissance Orbiter mission, slated to launch in October 2008.

“[NASA Ames] has a requirement for detailed information about components which would be suitable for the lander propulsion system . . . for a 64 [kilogram] total (wet) mass vehicle that will travel on a 5 day trajectory to the moon,” one of the solicitations read. “At minimum, NASA will require one propulsion system to support hover test beginning May 2007 and one propulsion system to support the first flight in June 2008.” The solicitations said NASA intended to acquire the needed components, following a brief study phase, with sole-source awards to industry.

Two weeks later, on July 28, the solicitations were canceled with no explanation given.

Ames Research Center spokeswoman Laura Lewis said in an e-mail that the release of the solicitations had been “premature.”

“We were ahead of the program on this,” she said. “Once we realized it, we pulled the solicitations.”

Lewis said Worden was away on international travel and unavailable for a

n interview.

The manager of NASA’s lunar robotics program, Tony Lavoie, was aware that Ames was preparing the solicitations and would be releasing them soon. Lavoie, on detail to NASA headquarters here from Marshall Space Flight Center , said in an Aug. 9 interview that the agency was seriously considering fitting a microlander into the program as early as mid-2008. If NASA wanted to preserve that option, he said, Ames had to get going with the solicitation.

“The only way to do it with that schedule was to issue a solicitation at that time,” Lavoie said.

NASA pulled the solicitations in late July, Lavoie said, after determining that it would be impractical to push ahead with a 2008 micro lander in light of the program’s existing commitments and overall budget situation.

“We were just looking at the overall schedule pressure, the budget we have, and the threats that were out there,” Lavoie said.

NASA’s 2007 budget request calls for spending $1.9 billion on the lunar robotics program over the next five years, but Lavoie said he is not counting on actually getting that much money to spend.

“The budget is threatened by other activities,” he said. “That budget is not assumed to be given.”

Although Lavoie would not say what “other activities” threaten the lunar robotic budget, other senior NASA officials, said development of the Crew Exploration Vehicle and the continued shortfalls in the space shuttle’s budget are putting serious pressure on the robotics effort. With NASA’s leadership reluctant to take more money out of its science or aeronautics accounts, the lunar robotics program stands out as one of the few available bill payers for human space flight expenses, these officials said.

Against this budget backdrop, Ames’ microlander proposal could be viewed as another new initiative competing for scarce resources or a lower-cost, early alternative to the bigger, more expensive lander mission taking shape at Marshall for a 2011 launch.

Several NASA officials, who asked not to be identified because they were not authorized to speak about internal agency matters, said those issues were still being hashed out inside NASA when Ames released the microlander solicitations, alarming Marshall personnel sensitive to threats to their lander project.

“That really torqued everybody because it showed somebody getting out of the starting gate way too early,” one NASA official said. “It broke down what level of trust there was.”

The resulting stir then reached NASA Administrator Mike Griffin, who brought a month or more of internal wrangling over the lunar robotics program to a close by ruling out a 2008 microlander mission and recommitting the agency to doing the Lunar Reconnaissance Orbiter mission — followed by a mid-sized lander to be built by Marshall.

The solicitations were canceled and NASA’s legislative affairs personnel, eager to avoid a dustup with one of Marshall’s congressional backers, notified the office of U.S. Sen. Richard Shelby (R-Ala.) of the action taken, but did not go into detail about the debate that had just played out inside the agency.

“Our staff was contacted by e-mail toward the end of July regarding the solicitations from Ames,” said Shelby spokeswoman Katie Boyd. “However, no specifics were included and in fact until I received your call and our staff proactively contacted NASA we had no information regarding mid-size versus small or Marshall versus Ames.”

Several days after the Ames microlander solicitations were pulled, Griffin told Space News in a brief interview that microsatellites and tiny landers might still figure into NASA’s lunar exploration plans, but would fall “third in line after the Lunar Reconnaissance Orbiter and what we are calling RLEP 2 . . . the medium-class lander that the Marshall team won.”

Griffin later declined to offer more details on the episode.

“It was an internal discussion and in the end the administrator made the final decision,” Griffin spokesman Dean Acosta said Aug. 3.