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Curiosity rover on track for daring descent to Mars
Posted: August 4, 2012

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The Mars Science Laboratory rover, still attached to its drum-shaped interplanetary cruise stage, closed in on the red planet Saturday, steadily accelerating under the increasing tug of the planet's gravity as it streaked toward a precisely targeted plunge into the martian atmosphere overnight Sunday for a high-stakes descent to the surface.

The Curiosity rover, on the move in Gale Crater. Credit: NASA/JPL-Caltech
"The spacecraft and ground systems are all healthy and performing as expected," said MSL mission manager Arthur Amador. "The spacecraft is now in the EDL (entry, descent and landing) approach configuration, in our final approach orientation, pointing our medium gain antenna within a degree of the Earth. We've got a strong telecom signal, receiving data at 2,000 bits per second.

"The power subsystem is healthy, our rover batteries are charged to 100 percent. The thermal and propulsion systems are nominal with stable temperatures and pressures and the DSN (Deep Space Network) continues to perform well, tracking the spacecraft continuously and conducting two differential ranging passes per day."

As of Saturday morning, the Mars Science Laboratory was just 2.8 million miles from Mars after logging nearly 350 million miles since launch from Cape Canaveral last November. Flight controllers decided Friday the spacecraft's path toward Mars was accurate enough to forego a pre-planned trajectory correction maneuver.

"We're now right on target to fly through the eye of a needle, that is, our target at the top of the Mars atmosphere," said Amador. "The target is a box that's 3 kilometers (1.9 miles) by 12 kilometers (7.5 miles) in dimension. And we're flying right through it."

A high-precision atmospheric entry is just the first step in a complex, high-speed series of events designed to get the nuclear-powered Mars Science Laboratory rover Curiosity safely to its landing ellipse on the floor of Gale Crater, within easy roving distance of a 3-mile-high mound of layered rock that represents a record of the red planet's enigmatic history.

Equipped with a robot arm, a drill, sample scoop, state-of-the-art instruments and a suite of cameras, Curiosity is the most sophisticated robotic lander ever sent to another planet. Over the course of a planned two-year mission, the rover will search for carbon compounds, one of the key building blocks of life as it is known on Earth, and assess whether habitable environments ever existed, or still exist, on the red planet.

"This is a very complicated vehicle, it's way more complicated than (previous Mars rovers) or other vehicles we've flown in the past, and so it's going to take us a while to first check it out and then get into the science ... that everybody wants to do," said Richard Cook, MSL deputy project manager.

"We're going to spend almost the entire month of August really checking out the vehicle, getting the first images. We'll obviously be getting science data during that but we'll also be doing engineering checkouts of the instruments, of the sampling system, changing flight software, doing other things. Hopefully by early September we'll be at the point where we can do our first drive and have the vehicle begin to move around a little bit."

But first, it has to get there.

Tipping the scales at one ton, Curiosity is the largest rover ever sent to Mars and it will hit the upper atmosphere at a blistering 13,200 mph. Protected by a massive insulating heat shield, the spacecraft's computer will fire rocket thrusters to adjust its lift during the hypersonic phase of the descent, dipping or climbing as required and flying through broad S-turns to bleed off speed while keeping the craft on course for a pinpoint landing.

After slowing to around 1,000 mph, a huge supersonic parachute will deploy, the heat shield will be jettisoned and a sophisticated radar altimeter will begin sounding the surface. After slowing to less than 200 mph, the parachute will be jettisoned and Curiosity, bolted to the belly of a rocket-powered descent stage, will fall free for the final drop to the surface.

Unlike past landers, Curiosity's jet pack does not have legs. Instead, it will act like a flying crane, lowering the rover directly to the surface on the end of a 25-foot-long bridle as the "sky crane" slowly descends. When the flight computer senses "weight on wheels," the bridle will be cut and Curiosity will be ready for initial tests and checkout.

Touchdown is expected at 1:17 a.m. EDT (GMT-4) Monday, but it will take radio signals confirming the event 13.8 minutes to cross the 154-million-mile gulf between Earth and Mars. That translates to 1:31 a.m. "Earth-received time."

Engineers at the Jet Propulsion Laboratory in Pasadena, Calif., will be relying on NASA's Mars Odyssey orbiter to relay entry, descent and landing telemetry back to Earth and to confirm a successful touchdown. If Odyssey has problems, or if any issues crop up with Curiosity's transmitter, it could take several hours for that long-awaited confirmation to arrive.

Given the complexity of the entry, descent and landing sequence -- and the mission's $2.5 billion price tag -- NASA managers and engineers are understandably anxious.

"Like all missions, MSL started with something that was pretty scary and pretty risky and that was called launch," said Doug McCuistion, director of Mars exploration at NASA Headquarters. "Unlike most missions, we haven't finished the scary and risky stuff yet. MSL still has to put Curiosity on the surface after it goes through the atmosphere, our 'seven minutes of terror,' which you'll continue to hear about.

"So can we do this? Yeah, I think we can do this. I'm confident. The team's done an amazing job, we have the A-plus team on this. They've done everything possible to ensure success, but that risk still exists, it's going to be tough. If it's not successful, we're going to learn. … We'll pick ourselves up, we'll dust ourselves off, we'll look at this and we'll do it again. This will not be the end."

The spacecraft's computer began executing EDL programming Monday and "our trajectory inbound to Mars has been right down the pipe, so we canceled last night's opportunity to perform our fifth trajectory correction maneuver," said Amador.

"During the hours that we have left here before the landing, the flight team will remain vigilant, monitoring and assessing the health of the spacecraft and tracking its trajectory and preparing any necessary changes to guidance and entry parameters. We have several opportunities to make final parameter updates, one today and two additional opportunities tomorrow if we need them.

"We have one more significant activity to perform with the spacecraft late tonight and that's to command the final enable and activation of the contingency software on our backup computer," Amador said. "The team's confident and thrilled to be finally arriving at Mars. We're reminding ourselves to breathe every so often."

John Glenn Mission Patch

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The historic first orbital flight by an American is marked by this commemorative patch for John Glenn and Friendship 7.