For almost half a century, the United States has dominated the exploration of Mars, from the first grainy black-and-white pictures of the craggy surface to the more recent discovery of ice.
Now, budget woes are pushing NASA toward a joint exploration venture with Europe.
By 2016, the U.S. may unite with the European Space Agency for future Mars trips — a move that would mark a significant shift for NASA.
Details of such a union could come by the end of this month.
In May, NASA's space sciences chief Ed Weiler said he believed a partnership was the best avenue to pursue shared science goals "if we can lose a little bit of our ego and nationalism."
A NASA presentation to the Mars science community in March indicated that the two space agencies would likely take turns being the leader.
As Marcello Coradini of the European agency has put it: "In terms of willingness, we all agree that we have to work together. The discussion is not on the 'if,' it's on the 'how' we work together."
The impetus for the unprecedented discussions comes down to money. After delaying the launch of its powerful Mars Science Laboratory to 2011, NASA had to slash its technology spending and scale back its future Mars vision to pay for the $2.3 billion next-generation, nuclear-powered rover.
The Europeans, too, have money problems. They lack the cash to send up ExoMars, a new drill-toting rover scheduled to launch in 2016.
NASA is trying to figure out how to help Europe land on Mars while sending up its own less capable orbiter during the same launch window.
"That's a difficult partnership because we had an existing mission and they had an existing mission and to merge two existing missions is challenging," said Doug McCuistion, who heads NASA's Mars exploration program. "Frankly, we have backed off quite a bit on our mission requirements. They've backed off somewhat."
Still unresolved is who will pay for the rocket that will blast both out of the Earth's atmosphere and what joint projects to pursue beyond 2016.
While an international collaboration makes financial sense, it is also fraught with risks. The European Space Agency has never successfully landed a spacecraft on Mars, though it has an orbiter circling the planet.
Coradini, ESA coordinator for solar system missions, envisions trade-offs in the partnership.
"We reverse the shared responsibilities. As with any good family, one day it's the husband doing the dishes and another day it's the wife," he said. "If it's always the husband or always the wife, then we're bound for a divorce."
Talk of a possible marriage has unnerved some space advocates who worry a union will decrease competition and cause the U.S. to lose its edge, holding it hostage to foreign politics.
"NASA should be showing off its stuff and not saying, 'We can't do it unless we have the cooperation,'" said Robert Zubrin, a former Lockheed Martin Corp. engineer who now heads the Mars Society advocacy group.
The trans-Atlantic dialogue comes at a critical point. NASA is polishing a new public relations message to replace its decade-old focus on finding water.
At the same time, Mars is facing competition from other solar system bodies emerging as promising places to search for signs of life.
While international pairings are nothing new, they tend to be more common on deep space missions.
NASA united with the European and Italian space agencies to launch the Cassini-Huygens mission to Saturn and its moon Titan.
Earlier this year, NASA and the European agency announced plans to be partners on a 2020 mission to Jupiter's ice-covered moon Europa.
With Mars, however, NASA has kept a do-it-yourself attitude while letting other countries add instruments to NASA spacecraft for their own data-gathering.
That NASA and the Europeans are considering pooling resources reflects a budget reality: It has become too expensive for one nation to pay to go to Mars alone, especially with a long-term goal of returning Martian rocks and soil to Earth estimated to cost at least $5 billion.
The growing pains afflicting the U.S. Mars program are partly due to it being a victim of its own success.
Past missions have raised more questions about whether the planet once had an environment that could support microbial life. To get answers requires more expensive study.
The early American Mars missions were quick flybys. In 1965, Mariner 4 became the world's first spacecraft to return 21 close-up images. The failures of an orbiter and lander in 1999 led to a revamp of the program.
The last decade arguably has been the golden age of Mars exploration. The plucky twin rovers Spirit and Opportunity are still active after five years. Before it froze to death last year, the Phoenix lander gathered up ice and tested water.
"Now the question is, where do we go from here?" said Scott Hubbard, a former NASA Mars czar who teaches at Stanford University.
Arizona State University Mars scientist Phil Christensen believes a partnership is inevitable given the cost, but he would rather see the space agencies collaborate after 2016.
"We're very close to the point where we can't keep on doing it alone," he said.