NASA's proposal leaves Europe without a key partner for a proposed two-spacecraft mission to collect and return soil samples from Mars. The so-called Exobiology on Mars, or ExoMars, probes are targeted to launch in 2016 and 2018.

The United States wants to recast its Mars exploration program to include both science and human exploration goals, NASA Administrator Charlie Bolden said at a news conference to unveil the agency's proposed budget for the fiscal year beginning October 1. The agency's current budget is $17.8 billion.

"Despite a constrained fiscal environment, this budget continues to aggressively implement the space exploration program agreed to by the president and a bipartisan majority in Congress," Bolden said. "The time for debate about our future is over. We have a solid plan."

"The Holy Grail of Mars exploration is sample return," NASA's associate administrator, John Grunsfeld, told reporters on a conference call. "Those (ExoMars) missions on our current budget trajectory were not executable."

Instead, the agency wants to combine elements of its human space program with technology demonstrations and space science for an alternative Mars mission, such as an advanced communications satellite, that could launch in 2018 or 2020. Earth and Mars favorably align for launches every 18 months.

Overall, the Obama administration wants to spend $4.9 billion on space science, including $1.2 billion for planetary missions, $628 million on the successor to the Hubble Space Telescope - the delayed and over-budget James Webb Space Telescope - and $1.8 billion on Earth science projects.

FOCUS ON HUMAN SPACE PROGRAMS

Most of NASA's proposed budget remains focused on human space programs, with $3 billion to support International Space Station research, maintenance and staffing, and $3.9 billion to design a capsule and rocket that can fly astronauts to the moon, asteroids, Mars and other destinations beyond the station's 240-mile (385-km) orbit.

The budget includes a final $70.6 million to close out the space shuttle program and transfer the three retired orbiters to museums. The aging shuttles were retired last year.

It also includes $830 million to spur development of privately owned space taxis to ferry astronauts to the space station, a $100 billion laboratory owned by the United States, Russia, Europe, Japan and Canada.

The new spending plan includes $99 million that could be spent to speed up space taxi development or to buy more seats aboard Russian Soyuz capsules, which are currently the only vehicles flying crew to the space station. NASA pays Russia about $60 million per person for rides.

The budget also includes $699 million for space technology research and development; $552 million for aeronautics; $619 million for construction, including development of a multi-use launch pad at the Kennedy Space Center; $100 million for education; and $2.8 billion for support of the agency's 10 centers.

For the fiscal 2012 year that ends on September 30, NASA intends to spend $3 billion on overhead, including administrative travel and information technology.

The plan drew immediate fire from the non-profit Planetary Society, a space science advocacy group.

"The priorities reflected in this budget would take us down the wrong path," Planetary Society chief Bill Nye said in a statement.

"Science is the part of NASA that's actually conducting interesting and scientifically important missions. The country needs more of these robotic space exploration missions, not less," Nye said.

(Editing by Jane Sutton)

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