PASADENA, Calif.--Dutifully executing its complex flight control software, the Mars Science Laboratory silently raced toward its target Sunday, picking up speed as it closed in for a 13,200-mph plunge into the Red Planet's atmosphere and an action-packed seven-minute descent that will require a rocket-powered "sky crane" to lower a one-ton nuclear-powered rover to the surface.
The target is Gale Crater and the goal is a pinpoint landing near the base of a three-mile-high mound of layered rock that represents hundreds of thousands to tens of millions of years of martian history, a frozen record of the planet's changing environment and evolution.
Exploring the crater floor and climbing Mount Sharp over the next two years, the Curiosity rover will look for signs of past or present habitability and search for carbon compounds, the building blocks of life as it is known on Earth.
But first, the rover has to get there and its entry, descent, and landing represent the most challenging robotic descent to the surface of another world ever attempted, a tightly choreographed sequence of autonomously executed events with little margin for error.
"We're about to land a rover that is 10 times heavier than (earlier rovers) with 15 times the payload," Doug McCuistion, director of Mars exploration at NASA Headquarters, told reporters. "Tonight's the Super Bowl of planetary exploration, one yard line, one play left. We score and win, or we don't score and we don't win.
"No matter what happens, I just want the team to know I am really proud and privileged to have worked with these guys and gals. They're amazing. They've done everything humanly possible to make this happen. If we don't do it and we're not successful, we'll pick ourselves up, we'll dust off, we'll do it again. The science is on the surface. We need to keep going back and that's the plan. But I think we're going to stick the landing."
Touchdown was expected at 10:31 p.m. PDT. Adam Steltzner, the leader of the EDL team at the Jet Propulsion Laboratory in Pasadena, Calif., said he was "rationally confident, emotionally terrified."
"This team...has worked really hard for the better part of a decade, and the fruits of that labor get put to the test tonight," he said. "It's a little anxiety provoking. But I will say I slept better last night than I have slept in a couple of years, because she's kind of on her own now. When I look back on the hard work that we've done, I believe the team has done everything that we can to deserve success tonight, although as we all know, we can never guarantee success."
The Mars Science Laboratory spacecraft consists of an interplanetary cruise stage, providing power and communications during the long-flight out from Earth, and the Curiosity rover, cocooned inside a heat shield and aeroshell to protect it from the extreme temperatures of atmospheric entry.
After covering 352 million miles since launch from Cape Canaveral, Fla., last November, the cruise stage is expected to separate from the lander around 10 p.m. But because of the distance between Earth and Mars -- 154 million miles -- it will take radio signals confirming critical events 13.8 minutes to reach the flight control team at JPL. That translates into 10:14 p.m. "Earth-received time."
One minute later, thrusters will fire to stop the entry vehicle's 2-rpm rotation and the spacecraft will re-orient itself heat shield forward and slam into the discernible atmosphere at 10:24 p.m. at an altitude of about 78 miles and a velocity of 13,200 mph. At that point, it will be about 390 miles -- seven minutes -- from touchdown in Gale Crater.
The Mars Science Laboratory is the first spacecraft to attempt a guided entry on another planet. To control its lift, which will allow Curiosity's flight computer to make a pinpoint landing, two 165-pound tungsten weights will be ejected just before entry to change the spacecraft's center of mass. During hypersonic flight, thruster firings will control the orientation of the vehicle's "lift vector" to compensate for actual atmospheric conditions as it precisely controls its path toward Gale Crater.
About one minute and 15 seconds after entry, the spacecraft's heat shield will experience peak temperatures of up to 3,800 degrees Fahrenheit as atmospheric friction provides 90 percent of the spacecraft's deceleration. Ten seconds after peak heating, that deceleration will peak at 10 to 15 times the force of Earth's gravity at sea level.
Plummeting toward Mars, the rover's flight computer will continue to steer the spacecraft, firing thrusters to make subtle changes in the flight path as required by atmospheric density and other variables.
The guided entry phase of flight will come to an end about four minutes after entry. At 10:28 p.m., six 55-pound weights will be ejected to move the center of mass back to the central axis of the spacecraft to help ensure stability when its braking parachute deploys.
Seconds later, at an altitude of about seven miles and a velocity of some 900 miles per hour, the huge chute will unfurl and inflate to a diameter of 70 feet, delivering a 65,000-pound jolt to the still-supersonic spacecraft. The heat shield will be jettisoned about 24 seconds later, at an altitude of about five miles and a descent rate of 280 mph, exposing the rover's undercarriage to view.
A sophisticated radar altimeter then will begin measuring altitude and velocity, feeding those data to the rover's flight computer while a high-definition camera begins recording video of the remaining few minutes of the descent.
Six minutes after entry, now one mile up and falling toward the surface at roughly 180 mph, the rover and its rocket pack will be cut away from the parachute and backshell, falling like a rock through the thin martian atmosphere.
An instant later, eight hydrazine-burning rocket engines, two at each corner of the descent stage, will ignite to stabilize and quickly slow the craft's vertical velocity to less than 2 mph. About 16 seconds before touchdown, at an altitude of just under 70 feet, Curiosity will be lowered on the end of a 25-foot-long bridle made up of three cables. As the support and data cables unreel, the rover's six motorized wheels will snap into position for touchdown.
Finally, seven minutes after the entry began and descending at a gentle 1.7 mph, Curiosity's wheels will touch the surface of Mars. Radio confirmation of landing is expected at 10:31 a.m., about 3 p.m. local time on Mars, but atmospheric conditions and other factors could result in a touchdown 40 seconds or so to either side of that target.
Whatever the actual timing, Curiosity's flight computer, sensing "weight on wheels," will send commands to fire small explosive devices that will sever the cables connecting the rover to the still-firing propulsion system. Its work complete, the descent stage will fly away to a crash landing at least 500 feet away and possibly twice that far.
Engineers on Earth, meanwhile, will be awaiting confirmation of a successful touchdown, relying on NASA's Mars Odyssey spacecraft to relay UHF telemetry from Curiosity directly back to Earth in near realtime. Odyssey experienced an orientation control problem in June, but engineers were confident the aging spacecraft would re-position itself as required to receive entry, descent and landing telemetry from Curiosity.
"We have three different signals we would use to confirm touchdown and we need all three of those things to look right before we say so," said Steltzner. "One of those is a message from the spacecraft that says 'I touched down, and this is the velocity I touched down at and where I think I am.'
"The rover has an inertial measurement unit, a gyro and an accelerometer set, and we look at that stream to say the rover's not moving at all, that signal says 'I think I'm on the ground and I'm not moving.' And the third is, we wait a safe period of time and confirm we're getting continuous UHF (radio) transmission. And frankly, that's there to make sure the descent stage hasn't fallen back down on top of the rover. When all three of those signals are positive, we declare touchdown confirmation."
The first photograph from the surface, a low-resolution thumbnail image from a hazard-avoidance camera on the rear of the lander, will be ready for transmission within four minutes or so of landing. But Odyssey may be over the horizon by that point. If so, the image will be relayed back to Earth one orbit later, around 12:30 a.m.
If Odyssey's orientation cannot be precisely controlled for any reason, engineers could have to wait up to eight hours or more for landing confirmation. Curiosity will beam some data directly back to JPL during its descent, but Earth will drop below the martian horizon after parachute deploy and direct communications will be interrupted.
NASA's Mars Reconnaissance Orbiter will be directly overhead during the descent and will record Curiosity's UHF telemetry throughout the landing sequence. But unlike Odyssey, MRO was not designed for "bent-pipe" direct relay to Earth and it would take about eight hours for engineers at JPL to receive and decode the recorded data.
Regardless of how long it takes for landing confirmation to arrive, Curiosity represents "one of the greatest feats in planetary exploration ever, not to mention just this decade," said McCuistion. "I think it shows the leadership that the United States has had in the exploration of Mars, and our success rate has been pretty darn good recently."
But the stakes are high and failure is clearly an option.
"If it fails, I think we need to know how it failed," McCuistion said. "Does it mean the end of the program? No, it means we've had a setback, it means we'll step back a notch, we will learn what happened. ... So a failure is a setback, it's not a disaster. We can learn from it and we'll move forward."