(CNN) -- The much-celebrated Mars rover Curiosity is headed for Mount Sharp, where it will help scientists explore the question of life on Mars as it climbs up and up.

Meanwhile, however, NASA's budget for planetary exploration is slated to go down, down, down.

Scientists are basking in the success of Curiosity's stunning landing on August 6, proving that a complicated system involving a parachute and a sky crane can safely deliver a 2,000-pound vehicle to Mars. The $2.6 billion Curiosity will spend years roaming the planet, snapping photos and gathering scientific data.

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Given the budget constraints facing the space agency, however, there are limits on what the rover, and NASA, will be able to do on the surface of the Red Planet. Although astronauts brought back thousands of moon rocks during the Apollo Mission, there's never been a sample of Martian material returned to Earth. Such a mission is considered a priority, so scientists can do more detailed chemical analyses.

But it may not happen anytime soon.

"We're optimistic, given the success of our program, but we're anxious, too," said Richard Zurek, chief scientist for the Mars Program Office at NASA's Jet Propulsion Laboratory in Pasadena, California. "Like all of us, we're anxious about our country's ability to be able to support and do these kinds of things."

NASA's budget for Mars exploration is slated to take a huge hit in 2013, dropping from $587 million to $361 million. It will then further decline to $228 million in 2014 and $189 million in 2015, rising slightly in 2016 before sloping upward to $503 million in 2017.

Researchers landed Curiosity on Gale Crater, which is 96 miles across and may have once hosted a lake. Mount Sharp, in the middle of the crater, is composed of hundreds of rock layers accumulated over time. The rover will climb a small portion of the 3-mile-high mountain, testing different layers to search for organic molecules that could indicate the presence of life on the barren planet.

"We've demonstrated that we've got a landing system that worked. And it worked well," Zurek said, referring to Curiosity's dramatic touchdown. "The question now is: What will we use this for? Or will we have to step back from that because those kinds of missions -- of putting something that takes a metric ton down to the surface -- they're just too expensive for our future?"

The early days

Curiosity is the latest in a long trajectory of missions that have allowed humanity to study Mars more extensively than any other planet apart from our own. A series of visits from Earth-made spacecrafts has taught us that Mars used to be a warmer, wetter place, perhaps with liquid water and even life.

Decades of research have led scientists to understand Martian history and pinpoint places on the planet where liquid water may have once flowed -- targets for future investigation, if money allows.

But getting to Mars didn't happen on the first try.

The USSR initiated a number of failed attempts to go to Mars in the early 1960s, including Sputnik 24, a lander that never left Earth's orbit. The United States also started out with bad luck: a failed flyby attempt by Mariner 3 in 1964.

Earthlings' first successful landing on Mars happened in May 1971 with the Soviet Union's Mars 3 lander. It failed after sending 20 seconds of video data to the orbiter, however.

NASA claimed a big success the same year: Mariner 9 marked the first time a U.S. spacecraft had orbited a planet other than our own. This orbiter discovered river and channel-like features on Mars, and took the first high-resolution images of Mars' moons Phobos and Deimos.

Hopes were high for NASA's Viking mission, launched in 1975, which included two landers equipped to search for tiny organisms. The orbiters mapped the surface of the planet, while the landers monitored the weather and sent back color panoramic views of Mars.

"It was extraordinary engineering achievement. A huge amount of science came out," said Scott Hubbard, former head of NASA's Mars program and author of the book "Exploring Mars: Chronicles from a Decade of Discovery."

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But the expectation of evidence of life -- namely, that the lander's arm would be able to put material into its chemistry kit and see organic molecules -- fell through. These molecules wouldn't prove that life existed, but they would be a signal that life was once possible there. There just wasn't conclusive evidence that Mars had these molecules.

Then came almost two decades of inactivity. NASA didn't send any other spacecraft to Mars until 1993 with the failed Mars Observer, followed by the successful orbiter Mars Global Surveyor in 1996.

Why the break? The space-shuttle program was one reason, said Zurek. NASA's priority at the time was manned vehicles, and budgets were tight. The Challenger disaster of 1986 also set NASA back.

Zurek believes we may be in a phase like that today, with not enough money to go around for all the programs NASA wants to develop. Also, the priority is once again in a new project: The Space Launch System, which will be the largest and most powerful rocket ever built.

Exploring Mars' surface

A game-changer was the discovery of a meteorite that appears to have come from Mars, found in Allan Hills, Antarctica, in 1984. Some scientists said they thought they saw evidence of tiny fossils that looked like dried-out life from Mars embedded in the meteorite.

No one knew where the meteorite came from, but it ignited a huge amount of interest in returning to Mars, Hubbard said. It also stressed the need to return samples from Mars to Earth, so they could be fully analyzed by multiple scientists.

"Without the Mars rock, we might not have Curiosity today," said Hubbard, whose job as the "first Mars czar" was to plan the next decade of exploration.

The first wheeled vehicle to land on Mars was during NASA's 1997 Mars Pathfinder mission, which consisted of a stationary lander and a surface rover called Sojourner. The rover was a mere 23 pounds and about the size of a large microwave oven. The mission demonstrated airbag technology in landing the rover without damage.

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Japan got in on the Mars action in 1998 with the Nozomi orbiter, but the spacecraft failed to communicate.

At NASA, Hubbard's goals included understanding Mars as a system, looking for potentially habitable environments, examining whether the planet was habitable in the past and preparing to return samples from Mars to Earth.

"The sequence of landers was strategically planned to have ever-increasing capability to go forward and do more," he said.

NASA's 2001 Mars Odyssey mission helped relay communications to Earth from the missions that would follow.

Europe entered the Mars race in 2003 with the Mars Express Orbiter and the Beagle 2 lander. Mars Express is still operating, but the lander never communicated from the surface.

Before Curiosity, NASA's big rover project was the Mars Exploration Rover Mission: two golf-cart-sized rovers called Spirit and Opportunity. They arrived on Mars in January 2004. Each rover was only supposed to operate for 90 days, but both far exceeded that time frame.

Spirit lasted for several years before it lost one of its wheels. As it hobbled along, it broke through the crust of the surface and got stuck in an area called Troy.

During its escape, another of Spirit's wheels failed. But while stuck, Spirit had a breakthrough: it found soil rich in sulfates, which are a component of steam. This suggests there may have once been conditions on Mars able to support life.

Signs of water?

As long as Curiosity has healthy mobility, it shouldn't find itself stuck in this kind of situation, said John Callas, project manager of the mission. The newest rover also uses brushless motors, which don't have the wear mechanism of the motors in Spirit and Opportunity.

Spirit stopped communicating in 2010. Opportunity, meanwhile, still chugs along and sent back its 100,000th picture in July. The rover also found a vein of the mineral gypsum, which indicates water may have once flowed on Mars.

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The Mars Reconnaissance Orbiter, launched in 2005, conducted analyses that helped scientists determine where Curiosity should land. The orbiter also found in 2008 a new kind of mineral, called opaline silica, which suggested that liquid water had been on Mars as recently as 2 billion years ago. The theory is that volcanic activity or meteorite impact on Mars created materials that liquid water would have altered, and the result is opaline silica.

MRO camera imagery also suggested that briny water may flow near Mars' equator in its southern hemisphere. These surface features appear to fade in the winter and reutrn in the spring. The possible liquid flows are extremely small and narrow.

Further clues to water on Mars came from the Phoenix Mars Lander, which arrived on Mars in May 2008. The lander studied the thin Martian atmosphere, consisting of 95% carbon dioxide. Isotopes of carbon and oxygen can tell a lot about whether water existed on the planet.

Data from Phoenix suggested that the Martian surface has interacted with liquid water, and even in modern times. The data also indicated that the planet had volcanic activity as recently as several million years ago. The chemical signature seen in the carbon dioxide points toward liquid water that was primarily at temperatures near freezing.

Also, the lander's robotic arm camera discovered possible evidence of ice at its landing site.

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MAVEN and beyond

The next Mars mission is the Mars Atmosphere and Volatile Evolution Mission, or MAVEN, which is scheduled to launch late next year. The instruments for it are already built, Zurek said.

Evidence gained through previous missions suggests that early in its history -- perhaps as much as 4 billion years ago -- Mars was a wetter place. Where did that water go?

It could be that water is frozen into the planet's surface, as observations from Phoenix suggested. But the water could have also been lost to space. Mars doesn't have a global magnetic field like the Earth does, so charged particles emitted from the sun -- the solar wind -- affect Mars in a much bigger way. The current theory is that the solar wind "stole" most of Mars' atmosphere, so radiation from the sun quickly boils any remaining liquid water.

MAVEN will measure the interaction of solar wind with current atmosphere, Zurek said. Using models of the sun's history, scientists may be able to estimate how much water the planet could have lost.

"MAVEN is the next mission. We don't want it to be the last mission in Mars exploration for a similar period, like in the '80s where there were no Mars missions, and everything was sort of on hold," Zurek said.

Meanwhile, Europe plans to launch the ExoMars Orbiter and a ground-based probe in 2016. The probe, called the Entry, Descent and Landing Demonstrator Module, isn't expected to last long, and is meant to be a demonstration of landing technologies. It will perform limited scientific experiments during its brief life.

European space officials and NASA were going to work together on a trace gas orbiter, but NASA pulled out because of budgetary reasons. Instead, the European Space Agency is working with Russia's Roscosmos. The plan is to launch the ExoMars rover in 2018, carrying European and Russian instruments.

Future questions

Exploration of the moon has been more extensive than Mars in many respects. In addition to fly-bys and orbiters, six Apollo missions sent humans to the moon and brought them back between 1963 and 1972.

The moon is closer to Earth than Mars, but also has a less diverse terrain. The interior of the moon is still mysterious, which is why NASA sent twin lunar orbiters called Gravity Recovery And Interior Laboratory, or GRAIL, to investigate. But consider that while sending an SUV-sized rover to Mars is a celebrated achievement, astronauts drove a "moon buggy" 40 years ago.

We haven't figured out how to send people to Mars yet. The planet is cold and dry, with dust storms that can circle the entire planet and an atmosphere that would be toxic to humans.

"Mars is the most Earth-like planet we have found, for certain," says James Wray, assistant professor at Georgia Institute of Technology and member of the Curiosity science team. "That's not saying as much as you would like it to be saying."

NASA still has a lot of questions about Mars. Has the planet hosted life? Does it host life now, in a place we have yet to discover? Has the climate on Mars changed in a way we can understand? And could Mars be a near-term destination for astronauts?

To send a mission to Mars that would bring back samples, there are a few fundamental engineering questions that need to be answered, says Hubbard: Can scientists pinpoint where they want to go and have a spacecraft land there? Can we collect a soil sample? Can we re-enter the Earth's atmosphere?

Science has proven the answers to the first three questions are "yes." But a fourth question -- can we safely load the Mars sample onto a spacecraft that's returning to Earth? -- is trickier. A rocket would have to be carried to Mars, sit there for a year, blast off and then rendezvous with another spacecraft.

Such an endeavor would require millions of dollars of research and development work. But NASA's projected budget for Mars exploration has "now gotten to the point that it's so low that you could not contemplate doing what I just described," Hubbard said.

The next step after MAVEN remains unclear. There are three other sites that had been considered for Curiosity that could be worth exploring with a surface-based spacecraft. Budget permitting, Hubbard also would eventually like to land another rover on Mars, because a stationary device with a six-foot arm isn't nearly as good as exploring the varied terrains of the planet.

"Mars is far more diverse than anybody thought," he said. "Roving is the way to go. Even though it's more difficult, that's what you need."

NASA officials are hopeful the current dire budgetary situation won't last too long.

Congress has signaled that money will be put back into planetary sciences, said Hubbard, who believes scientists will continue to be vocal about the value of Mars exploration.

"I think this [space science] community is going to make some noise," he said.

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