The mood among mission controllers was subdued. Scientists at the National Aeronautics and Space Administration's Jet Propulsion Laboratory (JPL), in the foothills of southern California's San Gabriel Mountains, were worried about the landing of their spacecraft on Mars.

With good reason.

Three months earlier, in September 1999, the Mars Climate Orbiter had arrived at the red planet, only to burn up in the atmosphere instead of taking up its intended orbit. Now, on Dec. 3, NASA officials were nervous about the fate of the Mars Polar Lander as it approached the planet, even though they had taken all the precautions they could to prevent a similar mishap.

"The tension was palpable," recalls Scott Hubbard, then a deputy director at NASA's Ames Research Center Laboratory, who was at the event. "Everyone was on edge."

Their anxiety turned out to be prophetic. When the $110 million lander finally touched down on the planet, it did so with all the subtlety of Wile E. Coyote's anvil falling off the cliff. Investigators later posited that the most likely cause was a bogus "I've landed" signal sent to the computerized flight-control system when, in fact, the lander was still 131 feet up. It had deployed its legs as planned. But that triggered an errant signal that caused the landing rockets to shut down prematurely, leaving the craft to crash into Mars at about 50 miles per hour. Controllers were never able to make contact with the lander or the two probes it ferried.

The debacle marked a low point in NASA's decades-old quest to explore Mars. The loss of the lander was the third failure in six years. In the 13 years since, however, the effort has undergone a remarkable turnaround. The agency has overseen the most comprehensive, systematic – and successful – effort to investigate another planet since the dawn of the Space Age. A string of six triumphant orbiters, landers, and rovers has helped unlock mysteries about a planet that has captivated humans since the Babylonians.

Now, as the most sophisticated rover humans have ever sent into the cosmos inches its way along the planet's ruddy surface, mankind may be reaching a hinge moment in the study of Mars. Curiosity's slow journey across Gale Crater marks a shift from tracing the history of water on Mars to focusing on efforts that could help answer a question humans have been asking since Dutch astronomer Christiaan Huygens drew up the first practical sketches of the planet in the mid-1600s: Did Mars ever host life?

Curiosity's landing alone was euphoric for NASA: Its soft touchdown in August, at the end of a "sky crane" tether, heralded a new era of precision landings that was watched by millions around the world, creating a triumphant moment for an often-beleaguered space agency.

Centimeter by centimeter, Curiosity is hunting for evidence that Gale, a desiccated ding in the planet's surface with a central peak towering 18,000 feet above the crater floor, may have harbored conditions that permitted primitive life to exist early in Mars' history. But that's just one spot on the planet – akin to trying to figure out the biological history of Earth by landing solely in rural Chile (which scientists often use as a stand-in for Mars).

Over the next eight years other missions will help fill in elements of the planet's profile. In December, NASA will send aloft MAVEN (Mars Atmosphere and Volatile Evolution Mission), an orbiter intended to tease out the story of how Mars' atmosphere has changed over its history. The launch follows a Mars-orbiter mission India is planning for November. In 2016, the United States will send a lander, InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport), to probe the Martian interior. NASA will also play a supporting role in ExoMars, a joint project between the European Space Agency and Russia's Roscosmos that aims to launch spacecraft to orbit and land on the red planet in 2016 and 2018.

Finally, in 2020, NASA will attempt to put another Curiosity-size rover on Mars. It is widely expected to begin a campaign to achieve one of the holy grails of space science: eventually return a rock or soil sample from Mars. It's an objective that just a few months ago had been axed from NASA's budget.

"It's huge," says Dr. Hubbard of the recent announcement of the 2020 mission.

The technology honed and science gleaned from these missions, past and future, will also add to the knowledge humans will need if they are ever to achieve one long-sought dream: to walk on Mars and perhaps eventually colonize it. While both political and practical problems make a manned mission to Mars seem remote for now, the idea remains a source of fascination and planning in scientific circles and space agencies around the world.

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Mars is certainly a tough destination to reach. The grimness in the mission control room in 1999 is a reminder of that. Human error, technical glitches, and the challenge of orchestrating a rendezvous between two objects after traveling more than 300 million miles, at speeds of up to 50,000 m.p.h., make landing a craft on Mars or orbiting it very difficult. This is to say nothing of dust storms, wayward winds, and frigid temperatures once a spacecraft lands.

The result is that, in the 52 years since humans have been trying to send spacecraft to Mars, only 36 percent of the missions have achieved their primary objectives. That includes 40 launches by a variety of entities – Russia, the US, Europe, and Japan.

Russia, starting back when it was the USSR, was the first to try to send a probe to Mars, in 1960, but it remains 1 for 19 in attempts that were fully successful. The Russians are the only ones to have successfully landed probes on Venus, so their poor track record for Mars remains something of an enigma.

The US has fared better with its missions to Mars. It enjoys a 73 percent success rate over the course of 19 launches, with craft typically far outlasting their initial "warranty." NASA's rover Opportunity, for instance, arrived at Meridiani Planum, just south of the Martian equator, Jan. 25, 2004, and it's still exploring its patch of the red planet.

Its primary mission was slated for just over 90 days. Opportunity's twin, Spirit, was supposed to be another 90-day wonder, but it soldiered on for six years before falling silent.

The last decade's worth of missions to Mars have been framed in three words: "follow the water" – the search for past or present evidence of a fluid deemed essential for organic life. But the effort also has contributed to a more sophisticated grasp of the birth and evolution, not only of Mars, but of all the planets in the solar system over its 4.6 billion-year history.

Initially, the notion was that the terrestrial or inner planets "all started out with the same brownie mix," says Darby Dyar, a planetary geologist at Mount Holyoke College in South Hadley, Mass., and a member of Curiosity's science team.

"It went into the oven, and it came out differently as a function of how far [planets] were from the sun. The last 10 years has challenged that assumption and shown us that there are specific and fundamental differences among the terrestrial planets, even though [they are] really close together."

The geological processes that shape these planets "are fundamentally the same," she says. "But the way they play out in detail, which we can see with the exploration program we've had over the last 10 years, is very different."

It turns out, for instance, that Mars' crust has much higher levels of sulfur than Earth's crust. While the distinction might seem esoteric, it suggests a far different environment early in the planet's history than Earth's – one that would have a direct influence on any life that might have emerged then, Dr. Dyar says.

Efforts to follow the water have revealed a remarkable planet whose prospects for serving as a habitat – even if it ultimately didn't host life of any sort – seem to increase with each new mission.

From high above the planet, two orbiters – Mars Global Surveyor and Mars Odyssey (the first mission in a revamped NASA Mars exploration program following the 1999 debacles) – returned images of fans and flows of sediment from the earliest period of Martian history. The teams interpreting the images, published in 2003, concluded that the features represented evidence of long-lived flows of water moving across the surface.

At the same time, images from Mars Odyssey showed channels cut into the sides of craters and along other slopes, suggesting brief catastrophic releases of water in the geologically recent past.

A year later, after Opportunity landed, it found tiny spheres of minerals sprinkled across the surface and embedded in a layer of sediment that was part of a rocky outcropping the rover was examining. Researchers dubbed the spheres "blueberries," later identified as made of the mineral hematite, which form in water-saturated soil deposits. Last September, the team said it had discovered similar spheres in a formation at Endeavour Crater, but with different compositions. In addition, the rover has detected veins of gypsum and clays in the rocks, further suggesting water in the area early on.

Curiosity has added to the growing evidence. In its brief sojourn, the rover has already helped scientists identify rounded rocks, as well as rocks bound up in cemented clumps of soil, that point to the presence of an ancient riverbed etched on the floor of Gale Crater billions of years ago.

Comparing timelines for the geological evolutions of Earth and Mars, as well as for the emergence of life on Earth, "it becomes abundantly clear that, yes, there probably was persistent standing water on Mars long enough" for life to have evolved in the same fashion it did on Earth, Dyar says.

Now, Curiosity, with its ability to zap, drill, and analyze rock and soil, while rolling across the surface of Mars (and leaving JPL's initials in Morse code in its tracks), is taking the next step: hunting for potential ancient habitats. Yet even if it comes up dry, the mission holds the promise of writing a new chapter in humanity's Book of Mars, and perhaps even Book of Earth.

"I can't imagine being disappointed scientifically, even if we don't find carbon" or features that strongly indicate that the area was not only habitable, but in fact did support life, said John Grotzinger, the mission's lead scientist, during a prelanding briefing.

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Curiosity's path up Gale Crater's Mt. Sharp is essentially a stroll through geological time and environmental history on Mars – one that represents a strong contrast to that on Earth, where life did evolve.

"Even in the case where life was never present on Mars, I still see [the mission] as an extraordinary opportunity to get a bearing on our own existence on Earth," Dr. Grotzinger said.

Whatever Curiosity uncovers about the past, it still leaves open the possibility that habitats may exist on the red planet today. Indeed, the prospect of extant life seems less far-fetched than it did in the years following the Viking results (see timeline). One of the remarkable discoveries from Odyssey is the presence of vast amounts of frozen slurry below the Martian surface, which is either ice layers tens of feet thick or a mix of ice and soil. The deposits extend from the poles deep into temperate latitudes.

The presence of the ice was confirmed in 2008 by the Phoenix Mars Lander, whose sampling scoop discovered it just beneath the surface at its landing site, informally known as Green Valley, in the north polar region. To the researchers' surprise, the lander also detected snow falling during its six-month mission.

Radar on the Mars Reconnaissance Orbiter uncovered evidence that the lower-latitude deposits may represent the remains of glaciers retreating as the planet undergoes a warming trend – part of long-term swings in the planet's average warmth tied to its tilt and orbit. Shaded by rock and soil on the surface, some of these formations are up to a half-mile thick and extend for miles. Flash melting of remnant glaciers when erosion exposes some of their ice may be responsible for relatively young-looking gullies orbiters have spotted on crater walls.

"I didn't see this one coming," Dyar says of the evidence of the hidden glaciers and their implication for changes in Mars' climate.

As for the potential biological significance of the discoveries, she points to a recently published analysis of a microbial community in Antarctica. These bacteria live in tiny brine channels that snake through a sheet of ice at least 90 feet thick that caps Lake Vida in East Antarctica. Temperatures in the microbes' briny home hover around 8 degrees F.

The team, led by Alison Murray, a microbial ecologist with the Desert Research Institute in Reno, Nev., notes that a half-mile-thick layer of permafrost beneath the surrounding landscape isolates the frozen lake from any influx of ground water from the surrounding valley. The lightless depths at which the bacteria live isolate them from any seasonal meltwater that might pond on the surface of the ice.

The team estimates that the briny ecosystem has been isolated for millenniums. The microbes appear to be thriving on residual organic and inorganic matter trapped in the system after it was isolated and frozen, as well as on hydrogen produced by chemical reactions as brine interacts with layers of sediment in the ice.

Could there be similar microbial ecosystems on Mars?

Scientists speculate that microbes represent one possible source for wisps of methane detected in Mars' atmosphere. But the detection remains controversial, and methane can come from geological as well as biological processes.

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As early as 1948, Wernher von Braun, a German rocket scientist who later would head NASA's Marshall Space Flight Center in Huntsville, Ala., was already formulating serious plans for sending humans to Mars. His seminal book, "The Mars Project," envisioned a fleet of 10 spacecraft, assembled at a space station orbiting the Earth, traveling to the red planet with 70 crew members.

The space travelers would be ferried from ships orbiting Mars down to the surface in winged spacecraft, fitted with skis, that would land on one of the planet's frozen poles. They would spend 443 days on Mars before returning to Earth.

Von Braun's scheme put hard physics and rocket-fuel numbers to a vision typically the province of science fiction – a vision that many scientists and space agencies aspire to today. It's one more reason the search for water and other scientific data being sent back from the red planet is so important.

The extensive presence of ice near the surface of Mars, for instance, might represent a resource for human explorers some day for activities ranging from growing crops inside special habitat modules to making rocket fuel by separating and liquefying water's oxygen and hydrogen.

Curiosity is gathering information, too, that might one day be useful to architects of a human mission. Even as the rover scours Gale Crater for signs that it might once have boasted an environment capable of supporting life, it is measuring radiation levels at the surface. In fact, the instrument responsible for the measurements, known as the Radiation Assessment Detector (RAD), has been monitoring radiation levels since the craft left Earth.

"One of our objectives is to make precursor measurements to help in the planning for future human exploration," notes Don Hassler, a physicist at the Southwest Research Institute in Boulder, Colo., and the lead scientist on the instrument, which was paid for by NASA's human-exploration program.

The exposure of space travelers to radiation is one of the major impediments to a mission to Mars. On Earth, humans are shielded from cell-damaging rays by the atmosphere. But astronauts on a prolonged planetary mission would be exposed to the combined effects of cosmic rays, which stream in from all directions from sources outside the solar system, and particles emitted from the sun, which come from steady solar winds and intense solar storms.

RAD gathered data on the intensity level of both sources during its seven-month journey to Mars – including bursts from five solar storms. "That was fortuitous from our point of view because it gave us an opportunity to characterize the effect of those," Dr. Hassler says.

The research could prove crucial in designing shielding for a spacecraft or for planning the right timing for a future Mars mission – when to go during the sun's 11-year sunspot cycle that would be least dangerous to astronauts.

While spacecraft could be lined to prevent radiation from reaching crew members, some rays are always going to get through. Thus another solution scientists are looking at to limit radiation exposure is reducing the time it takes to get to Mars. "That's the one where maybe some breakthrough is waiting to be made in propulsion that gets you there faster," says Cary Zeitlin, a RAD project scientist.

Assuming scientists opt for using NASA's announced 2020 rover to begin a program dedicated to returning soil and rock samples from Mars, that, too, would provide information to inform future human exploration. Late last month, NASA formally asked scientists to propose experiments the rover would carry out. At its most fundamental level, a sample-return program represents a scaled-down dry run for the sequence of missions needed to send humans.

"It's pretty simple," says John Grunsfeld, NASA's associate administrator for the science mission directorate. "If you can send a mission to Mars, acquire a sample, and return that sample back to Earth, that's a good model for sending a human to Mars and returning that human to Earth."

A sample-return program also could help avoid a debate that nearly squelched the Apollo moon program. The concern centered on viruses astronauts might bring back – something people worry even more about with travelers to Mars.

"A lot of planetary protection people were dead-set against it [sending an astronaut to the moon] and could have blocked the lunar exploration program," notes Mount Holyoke's Dyar, who earned her PhD studying lunar samples. "We were very close on that one."

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A sample return program for Mars would allow the most precise analysis possible, from multiple labs, to hunt for signs of organic molecules and better understand the environment to which astronauts would be exposed.

"To me, the holy grail is always gonna be: Return samples," she says.