NASA Administrator Charles Bolden warned House appropriators on Tuesday not to expect an alternative to President Obama’s proposed 2011 budget for the space agency, a spending plan that calls for the controversial cancellation of the Constellation moon program and threatens thousands of job losses.

Bolden’s remarks came during the first of three congressional hearing this week on the $19 billion NASA budget unveiled by the White House in early February.

Key provisions in the budget would replace Constellation’s goal of reaching the moon with human explorers by 2020 with a five-year, $6 billion initiative to develop multiple commercial launch services that are capable to transporting astronauts to and from the International Space Station. Bi-partisan opposition to the cancellation prompted the White House to call for an April 15 space summit in Florida that some lawmakers believe could become a forum for Obama to reverse course.

However, in a polite, but frank exchanges with the top Democrat and Republican on the Appropriations Commerce, Justice and Science Subcommittee Bolden said there is no “Plan B” in the works.

“With no destinations and no timetables, is that a problem?” questioned Alan Mollohan, the West Virginia Democrat who chairs the panel. “You are not really articulating a vision that inspires. Or are you rolling it out piece meal?”

“Unless something is rolled out, then you will have a fairly significant confrontation,” predicted U. S. Rep. Frank Wolf of West Virginia, the panel’s ranking Republican. Wolf is among 15 House Democrats and Republicans who have called on Bolden to huddle with top agency engineers and scientists to re-think Constellation’s demise before the summit. “Is there an effort to develop a compromise, an effort to look at this thing again and compromise? Or is this just the way it is?”

“There is no alternative plan, no alternative budget,” Bolden responded. “If the question is: Am I developing a Plan B. There is no Plan B.”

U. S. Sen. Bill Nelson, a Florida Democrat has been among those most vocal in calling on the White House to set Mars as NASA’s new exploration goal. Nelson has suggested the president might use the summit to call for a near-term initiative to develop a new heavy lift rocket that would enable American explorers to explore the moon, asteroids and eventually Mars as part of a flexible path strategy for deep space exploration.

Bolden assured lawmakers that NASA intends to sponsor a major research and develop effort that will focus on the technologies for a new rocket, advanced life support systems as well as faster in-space propulsion capabilities that could pull all of those destinations within range. However, it is premature to set production dates for an exploration rocket, let alone a timeline for the first Mars landing, he said.

“We are in competition with China and Russia, and this would put us at a disadvantage,” said U. S. Rep C. A. “Dutch” Ruppersberger, a Maryland Democrat who voiced a concern shared by several of the appropriators.

However, Bolden defended the president’s spending plan and the decision to cancel Constellation by summarizing the findings and recommendations of the Augustine Committee. The White House chartered study committee concluded last year that the six-year-old moon initiative outlined by President Bush was so under funded it could not achieve a lunar landing until well after 2030.

New investments in technology coupled with a push to turn the transportation of astronauts to Earth orbit over to less costly commercial carriers will save money, while leading to advances that will get Americans back to the moon and other deep space destinations faster than would Constellation, Bolden predicted.

“I’m convinced that with the technologies we can develop, we will get there much sooner,” the 63-year-old administrator testified. “We could not go to Mars probably in my life time, to be quite honest. I am convinced that given the opportunity to expend some funds on research and development we can actually do that.”

At one point, Bolden told the lawmakers they need not be concerned for America’s leadership even if China reached the moon ahead of an U. S. lunar return.

As the managing partner in the 15-nation International Space Station, the United States is the undisputed world leader in human space exploration; even if China were to set down on the lunar surface first, most will know America has already been to the moon and has its sights set on new more challenging destinations, he told the panel.

“Sometimes you lead not by being out front, where you are very visible, but you lead by being very influential and being able to get people to do the things you need to do, ” Bolden said. “We do that every day on the International Space Station.”

Later, Bolden told the panel he’d had a change of heart.

“China, it does make a difference to me who is first,” said the retired Marine Corps general and former NASA astronaut. “It’s important that we be first all the time.”