President Obama is proposing to cancel NASA's behind-schedule moon program, diverting funds to new commercial space ventures, space technologies research and climate science, the White House announced Monday.

In its 2011 budget request, the Obama administration unveiled an overhaul to NASA's human space program, directing the space agency to embark on a myriad of research programs aimed at a "more capable approach to space exploration."

"The Administration proposes to cancel the Constellation systems program intended to return astronauts to the moon by 2020 and replaces it with a bold new approach that embraces the commercial space industry, forges international partnerships, and develops the game-changing technologies needed to set the stage for a revitalized human space flight program and embark on a 21st century program of space exploration," the White House budget document says.

Budget-makers justified the cancellation by citing the outcome of a blue-ribbon presidential commission that last year reported the Constellation program was "unsustainable" and needed up to $3 billion more each year to get back on track.

The Constellation program includes the Ares rocket family and Orion spacecraft. The first piloted mission of the new system may not occur until 2017, according to independent analysts.

"Using a broad range of criteria, an independent review panel determined that even if fully funded, NASA's program to repeat many of the achievements of the Apollo era, 50 years later, was the least attractive approach to space exploration as compared to potential alternatives," Obama's budget office wrote.

Constellation supporters in Congress are already criticizing the shift in NASA's strategy. Many of the voices are from states with a significant NASA presence.

Sen. Richard Shelby, R-Ala., said the budget begins the "death march" for U.S. human spaceflight.

"The president's proposed NASA budget begins the death march for the future of U.S. human spaceflight," Shelby said in a statement Monday. "The cancellation of the Constellation program and the end of human spaceflight does represent change -- but it is certainly not the change I believe in. Congress cannot and will not sit back and watch the reckless abandonment of sound principles, a proven track record, a steady path to success, and the destruction of our human spaceflight program."

In place of Constellation, NASA should support commercial space transportation programs to ferry astronauts to and from the International Space Station "as soon as possible," the budget said.

NASA would receive $500 million to begin contracts with private industry for crew transportation services, saying they could begin flying sooner at less expensive costs than the Constellation program.

"We cannot continue to coddle the dreams of rocket hobbyists and so-called 'commercial' providers who claim the future of U.S. human spaceflight can be achieved faster and cheaper than Constellation," Shelby said.

Sen. Bill Nelson, D-Fla., said last week the budget is "risking U.S. leadership in space to China and Russia."

"If the $6 billion in extra funding is for a commercial rocket, then the bigger rocket for human exploration will be delayed well into the next decade. That is unacceptable," Nelson said in a statement. "We need a plan that provides America with uninterrupted access to space while also funding exploration to expand the boundaries of our knowledge."

The budget also directs the space agency to extend the International Space Station beyond its planned retirement in 2016, proposing $183 million in 2011 to start the life extension process.

NASA would receive a budget boost in 2011, despite the Obama administration's discretionary spending freeze for the federal government. The request totals about $19 billion for fiscal year 2011, and another $6 billion increase is on tap over the next five years.

President Obama also proposes to expand research and development programs to include future heavy-lift rocket systems designed for exploration at much lower costs that current vehicles.

Other enabling technologies, such as autonomous rendezvous and docking, closed-loop life support systems, in-orbit propellant transfer, and advanced in-space propulsion would also get funding under the spending plan.