NASA

In a $2.5 billion gamble, a nuclear-powered Mars rover the size of a small car will attempt a pinpoint landing near the base of a 3-mile-high mountain overnight Sunday to search for the building blocks of life in the frozen history of the red planet and evidence of past or present habitability.

In so doing, the Mars Science Laboratory rover, dubbed "Curiosity" in a student naming contest, will climb layer by layer through vast eras of the planet's enigmatic history, possibly shedding light on the transition from a warmer, wetter past to the arid, frigid world of today.

Doug McCuistion, director of Mars exploration at NASA headquarters in Washington, said the mission "could arguably be the most important event in the history of planetary exploration."

"It truly is a major step forward, both in technology and in potential science return and science capability to unlock the mysteries of Mars in places that have never been accessible to humankind in the past."

But getting there will not be easy.

The Mars Science Laboratory spacecraft must first endure entry temperatures of up to 3,800 degrees Fahrenheit, crushing deceleration of up to 15 Gs and the 65,000-pound jerk of a huge parachute inflating at supersonic velocity.

After slowing the spacecraft to a bit less than 200 mph, the parachute will be cut away and a rocket-powered descent stage, carrying the Curiosity rover bolted to its belly, will fall free for a nail-biting one-mile plunge to the surface.

"The Curiosity landing is the hardest NASA robotic mission ever attempted in the history of exploration of Mars, or any of our robotic exploration. This is risky business." <br />--NASA science chief John Grunsfeld

Controlled by the rover's main computer, the descent stage will slow to just 1.7 mph, four of its eight rocket engines will shut down and Curiosity will be lowered on the end of a 25-foot-long tether like a bobber on a fishing line.

With the descent stage maintaining its slow fall, the rover's six wheels are expected to touch down on the floor of Gale Crater around 1:17 a.m EDT (GMT-4). Confirmation will be relayed back to Earth in near real time by NASA's Mars Odyssey satellite.

But because of the distance between Earth and Mars -- about 154 million miles -- it will take 13.8 minutes for confirmation of a successful landing to reach anxious engineers and scientists at NASA's Jet Propulsion Laboratory in Pasadena, Calif. That translates into 1:31 a.m. ET on August 6, Earth-received time.

"MSL holds the potential to look for evidence of habitable environments, if they existed, on Mars in the distant past," said NASA science chief John Grunsfeld, a veteran shuttle spacewalker. "The Curiosity rover has the potential to discover the building blocks of life on Mars, if life ever existed on Mars.

"However, the Curiosity landing is the hardest NASA robotic mission ever attempted in the history of exploration of Mars, or any of our robotic exploration. This is risky business."

Curiosity's novel "sky crane" landing technique has dominated news coverage, in part because it seems so outlandish compared to past missions and because of the perception of increased risk -- a full-up, end-to-end test was not possible in Earth's atmosphere and gravity.

But engineers are confident that the entry, descent and landing system will work as advertised, the first act in the most complex, expensive and scientifically significant robotic Mars mission ever attempted.

NASA

"This rover, the Curiosity rover, is really a rover on steroids," Colleen Hartman, a senior NASA manager, said before launch. "It's an order of magnitude more capable than anything we have ever launched to any planet in the solar system. It will go longer, it will discover more than we can possibly imagine."

Over the course of a planned two-year mission, Curiosity will act as a robotic geologist, using high-definition cameras to photograph its surroundings in exquisite detail, beaming back wide-angle high-resolution panoramas as well as close-up microscopic views through what amounts to a geologist's hand lens.

Equipped with 10 state-of-the-art instruments and a sophisticated robot arm, the rover will drill into rocks and soil, use a rock-vaporizing laser to assess more distant targets and collect rock and soil samples for detailed chemical analysis.

The initial phases of the mission will be focused on the crater floor and an alluvial fan visible from orbit where scientists believe water may have pooled in the distant past.

Onward and upward at Mount Sharp

But the long-range objective is Aeolis Mons, dubbed Mount Sharp by NASA, a huge mound of sedimentary rocks in the center of Gale Crater that rises more than three miles into the thin Martian atmosphere, higher than Mt. Rainier above Seattle.

The instruments aboard Curiosity were not designed to look for signs of life. Rather, the primary goal of the Mars Science Laboratory is to search for carbon compounds and evidence of past or present habitability.

But searching for carbon compounds is only part of Curiosity's mandate. As it works its way up the 15-degree slopes of Aeolis Mons and passes from older to younger layers, the rover is expected to cross over beds marking a geologically sudden transition from a warmer, wetter past to a drier, less hospitable age.

In so doing, hundreds of thousands to tens of millions of years of the planet's evolution will be brought into focus.

"The really cool thing about the Gale stratigraphic succession to me is it's a tour through nearly the entire history of Mars where we can begin to understand these major changes in the environmental history of the planet," project scientist John Grotzinger said in an interview. "And I can't think of another place on Mars where you can go do that."

NASA

To get a sense of the landing site's potential, Grotzinger said the layers making up Aeolis Mons are three times thicker than those in the Grand Canyon, which "takes you ... through 300 million years of Earth history, from the origin of animals to the origin of dinosaurs."

"If you were to have remote sensing data from an orbiter around Earth, looking at Earth and the Grand Canyon 150 years ago, nobody would have ever predicted that that's what you would discover if you went there one day," Grotzinger said. "I don't know what it is that we're going to discover about Mars. But I have to believe it's going to be something really good."

Big ambitions, tough choices

The high-stakes mission comes at a critical time for NASA's planetary exploration program as budget pressures threaten to sharply reduce the scope of the agency's robotic missions.

The Obama administration's fiscal 2013 budget request calls for $17.7 billion for NASA, but it cuts $300 million from planetary science, most of it from the Mars program.

As a result, NASA has backed out of a 2008 agreement with the European Space Agency to share the costs of two ambitious Mars missions known as ExoMars, which called for launch of an orbiter in 2016 and two rovers in 2018.

Along with searching for signs of past or present life on Mars, the missions also would have tested technologies needed for a long-sought sample return mission.

"Tough choices had to be made," NASA Administrator Charlie Bolden said when the budget was unveiled earlier this year. "This means we will not be moving forward with the planned 2016 and 2018 ExoMars mission. ... Instead, we'll develop an integrated strategy to ensure the next steps in Mars exploration will support science as well as human exploration goals."

"I'm really focused on the question, not if life evolved [on Mars], but if it did evolve, where would it be preserved? And where are the places we need to go to find the best potential records of things that could be clues that would lead us on future missions toward the discovery of biosignatures?" <br />--Project scientist John Grotzinger

The Curiosity rover is the only so-called "flagship" mission currently in the Mars pipeline, and it takes years to plan, design and build new spacecraft. Aerospace engineer Robert Zubrin, president of the Mars Society and author of "The Case for Mars," said in an interview that he believes the fate of NASA's Mars program rests firmly on Curiosity's shoulders.

If Curiosity fails, he said, "not only do you lose this mission, but I think we lose the rest of the decade. On the other hand, if this succeeds, it will be a brilliant mission, it will be the best Mars mission ever flown and I think we have a real chance of not only reversing the missions that were cut but moving on towards sample return."

Launch originally was planned for 2009, but in 2008, the flight was delayed two years to verify the integrity of the myriad actuators used in the rover's mobility system and robot arm, a delay that added $400 million to the project's price tag.

Curiosity's journey finally got under way on Nov. 26, 2011, when a United Launch Alliance Atlas 5 rocket boosted the craft into space. The spacecraft has performed in near flawless fashion during the long cruise to Mars and now the stage is set for entry, descent and landing Aug. 6.

Acting as a robotic geologist, Curiosity is well suited for its trailblazing mission, dwarfing the hugely successful Spirit and Opportunity rovers both in size and scientific capability. The instruments carried by each of the earlier rovers weighed about 11 pounds. The 10 aboard Curiosity weigh 165 pounds.

Not counting its robot arm, Curiosity is 10 feet long, nine feet wide and seven feet high measured to the top of its main camera mast. Its mobility system is similar in design to that used by Spirit and Opportunity, but its six 20-inch-wide wheels are twice the size of the earlier models. Each wheel has its own drive motor and the four corner wheels are independently steerable.

NASA's earlier rovers were solar-powered, forcing them to shut down at night and to hibernate in winter months to conserve power and heat. MSL is powered by a radioisotope thermoelectric generator, using the heat produced by the decay of radioactive plutonium dioxide to generate electricity. Excess heat is used to keep electronics and other sensitive systems from getting too cold.

Curiosity is equipped with redundant computers, using one at a time and keeping the other as a backup. The computers feature radiation-resistant PowerPC 750 processors operating at 200MHz with 2GB of flash memory storage, about eight times more than Spirit and Opportunity.

Independent of the weather and the sun, Curiosity is designed to operate for at least one martian year -- two Earth years -- and to rove at least 12 miles. But engineers expect it to continue operating well beyond its design specification, both in time and distance.

NASA

"It could last a long time if we haven't made a mistake," said Project Manager Pete Theisinger. "If Mars doesn't get us, it could last a long time."

The heart of the spacecraft is the most sophisticated instrument package ever sent to Mars.

The Sample Analysis at Mars, or SAM, instruments will be used to analyze soil and rock fragments delivered by the lander's robot arm. It includes a gas chromatograph, a mass spectrometer and a laser spectrometer to look for carbon compounds and measure isotope ratios, which will shed light on the history and distribution of water and the evolution of the martian atmosphere.

"You've got to have water for life as we know it," Grotzinger told CNET in an earlier interview. "The second thing is you need a source of energy. ... And then the important thing is, you need the fundamental building block, which is carbon."

Whether or not life existed on Mars "verges more on philosophy, really," he said. "We don't know how life originated on Earth. I'm really focused on the question, not if life evolved, but if it did evolve, where would it be preserved? And where are the places we need to go to find the best potential records of things that could be clues that would lead us on future missions toward the discovery of biosignatures?"

Another instrument, called CheMin, uses X-ray diffraction to identify the minerals in collected rocks and soils. The Mars Hands Lens Imager, mounted on the robot arm, will take close-up photos of selected samples while the Alpha Particle X-ray Spectrometer, also on the arm, measures the abundances of various elements.

A camera mounted on a mast atop the rover will take high-resolution stereo pictures as well as high-definition video. Another mast-mounted instrument known as ChemCam will use a laser to vaporize the surface layers of nearby rocks, a spectrometer to measure the types of materials present in the debris and a camera to photograph the site.

A Radiation Assessment Detector will will characterize the radiation environment at the surface, a key factor in planning for eventual crewed missions, while a suite of Spanish instruments called the Rover Environmental Monitoring Station monitors the martian weather.

An instrument provided by Russia, the Dynamic Albedo of Neutrons experiment, will look for signs of water or ice below the surface.

"The team is very excited. They've come to the end of a long journey," said Theisinger. "I think the team feels that they've done everything they can do to make this successful.

"That being said, success is not assured. We could have any (number of) problems that could end up in an end of mission. But I think the team is very positive, morale is good. ... I think the team is really up for this."