WASHINGTON -- NASA is making a cosmic U-turn on the road to Mars.
For the past two decades, the U.S. space agency has been practically obsessed with Mars. It has hardly missed an opportunity about every two years to fling robotic spacecraft at the red planet.
This summer, the most high-tech rover ever, Curiosity, will land near the Martian equator in search of the chemical building blocks of life.
The more scientists study Mars, the closer they get to answering whether microbial life once existed there, a clue to the ultimate question: Are we alone?
Presidents have long talked about sending astronauts to Mars.
Two years ago, President Barack Obama stood in Kennedy Space Center and said it was more of a priority than going to the moon and wanted astronauts there by the mid-2030s, but robotic Mars missions slated for 2016 and 2018 were cut from the president's new budget proposal, even though NASA has spent $64 million on early designs with the European Space Agency for the two missions.
The most ambitious Mars flight yet and one the National Academy of Sciences endorsed as the No. 1 solar system priority -- a plan to grab Martian rocks and soil and bring them back to Earth -- is on indefinite hold.
"We're really at a crossroads," NASA planetary sciences chief Jim Green said. NASA will skip the 2016 launch opportunity and if officials are lucky, they hope still to salvage something relatively cheap for 2018, when Mars passes closest to Earth.
But it won't be the large rock-collecting mission that scientists had been counting on.
What a new mission for 2018 would be is still not clear even to NASA senior officials.
To scientists, the message from the White House seems simple: Bye-bye, Mars.
On Monday, upset Mars researchers are meeting with NASA officials to figure out how to reboot the program beyond the 2013 mission.
If Obama's budget sails through as outlined, "in essence, it is the end of the Mars program," said Phil Christensen, a Mars researcher at Arizona State University.
It's like "we've just flown Apollo 10 and now we're going to cancel the Apollo program when we're one step from landing," he said.
It's not that NASA officials don't think Mars is worth exploring further; it's just that they don't think they can afford it anymore.
Obama has proposed cutting 10 other federal agency budgets this year including Defense, Homeland Security and Education.
NASA's 0.3 percent budget cut was among the smallest.
In fact, the $28.3 billion cuts to the Defense Department dwarf NASA's entire $17.7 spending plan for 2013.
"We're trying to identify a way to (explore Mars) in these very difficult fiscal times," NASA Administrator Charles Bolden said last week at the Jet Propulsion Laboratory in Pasadena, Calif., the epicenter of Mars research.
Researchers are partly to blame because they promise to do a mission cheaply and when they get approval, costs soar, said Alan Stern, a former NASA sciences chief.
He called it "committing suicide in slow motion" and said it's been happening in the Mars program since 2006.
An even more over-budget space telescope forced more cuts to NASA science.
The Curiosity mission costs $2.5 billion -- almost $1 billion over budget.
Many scientists believe the life question can only be answered by examining Martian samples back on Earth and that astronauts should not set foot on Mars before that happens.
Stern said: "We are probably back to being 15 to 20 years from a Mars sample return."
If NASA ignores Mars for a decade, it runs the risk of a brain drain, said Ed Weiler, who resigned last year as NASA's sciences chief because of budget battles over Mars.
"Landing on Mars is a uniquely American talent and there aren't too many things that are uniquely American," Weiler said.
In 19 tries, Russia has had little to no success when it comes to Mars. The European Space Agency currently has a spacecraft circling the planet but its lander crashed.
NASA has had six Martian failures during its 20 tries.
The Europeans are talking to the Russians and Chinese to replace the U.S. in the upcoming missions.
Earthlings have been captivated by Mars since the 1900s when amateur astronomer Percival Lowell saw what looked like canals.
The life question was tackled by the twin Viking spacecraft, which landed in 1976. Their rudimentary experiments failed to turn up signs of life and NASA lost interest.
The space agency launched an ambitious probe in 1992 but lost contact with it right before it was to slip into orbit.
After that failure, NASA came up with a blueprint for Mars: Each mission followed up on discoveries found in the previous flight, and all focused on water, a key element for life.
"It's become a more interesting planet every time we go back there," said Wesley Huntress, who spearheaded the new Mars program and went on to run NASA's sciences division.
That's why the Planetary Society and others are lobbying to save the Mars program, starting with meetings this week.
Bill Nye, the former science television personality who heads the society, said exploring Mars is central to humanity's future: "We do it to learn more about ourselves, to learn more about our relationship with the cosmos, to learn more about our place in space."
By SETH BORENSTEIN and ALICIA CHANG, AP Science Writers
Alicia Chang reported from Pasadena, Calif.
Follow her at http://twitter.com/SciWriAlicia.
Follow Seth Borenstein at http://twitter.com/borenbears.
The Associated Press