Back to Mars: After '99 failure, NASA sets sights on lander touchdown

After a nine-month, 422-million-mile trip from Earth that began last August, NASA's Phoenix Mars Lander will catch up to the red planet within hours to begin its three-month science mission.

Its mission is series of soil analysis projects that involve digging lightly into the Mars surface to study the history of water, ice and life potential in the planet's soil. But before those soil analysis projects can even get under way, the Phoenix has to successfully land on the planet's surface, which, as NASA knows by experience, isn't a sure thing.

More than eight years ago, in December 1999, NASA's Mars Polar Lander project came to a disastrous end when the craft's descent engines shut down early as it prepared to land on the Martian surface. The lander went of control from a high altitude, crashed and was destroyed.

Since that mission, changes were made in the design of the Phoenix, the next-generation lander that is expected to touch down on Mars at 7:38 p.m. US EDT Sunday. Once the Phoenix lands, NASA won't know if the mission was successful until 15 minutes and 20 seconds later. That's how long it will take for radio signals to get back to Earth with confirmation of a safe landing. The radio signals move at the speed of light and will travel approximately 171 million miles to reach Earth.

The system improvements on the lander stem from the results of a NASA review board, which investigated the failure of the Mars Polar Lander (MPL) mission, said David Spencer, the Phoenix deputy project manager at NASA's Jet Propulsion Laboratory in Pasadena, California.

One key change centered on the lander's 16 descent rockets, or thrusters, that are used to gently lower the spacecraft to the planet's surface in the northern hemisphere of Mars, Spencer said.

The most probable cause of failure in the MPL mission, according to the NASA review board, was a physical "jolt" that occurred as the landing gear unfolded and the lander began its descent. The jolt tripped a touchdown sensor mounted on one of the lander's three landing gear footpads, as the legs were deployed about 700 meters above the surface.

When software controlling the descent engine thrusting came online, it ... saw that the touchdown sensor said we had already touched down, and so it erroneously ... shut off the descent engines" too early.

"The sensors weren't supposed to be activated at that point, "but because of a testing issue, that problem was not uncovered" until it was too late, Spencer said. "Fast-forward to Phoenix -- we've redone all of that testing."

Another problem that was uncovered has also been fixed. It was discovered that a set of electrical connectors between the lander and its protective shell, called the cruise module, could bind up because of the cold of space and fail to separate when the lander is set to detach and descend to the surface. The module, essentially a container that holds the lander, carries a propulsion system, guidance system and the communications hardware that gets the vehicle from Earth to Mars. NASA added small heaters to the connectors so that the disconnection works reliably, Spencer said.

Because of the time delay because of the millions of miles between the planets, "there's no way you can joy-stick the spacecraft" to control its second-by-second approach and landing on the Mars surface. The entire approach and descent has to be coordinated ahead of time, leaving no room for instant midcourse corrections, Spencer said.

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Mission personnel scoured massive amounts of data on the Mars surface and topography to be sure that the landing area is safe for the spacecraft. "Everything needs to be pre-programmed and autonomous onboard the lander during the seven minutes of entry, descent and landing," Spencer said. The lander also has to slow from about 13,000 mph, as it reaches the Martian atmosphere, to about 5 mph when it touches down, in a span of seven minutes.

Much of the critical data has come from a new high-resolution camera, HiRISE (High Resolution Imaging Science Experiment), that NASA launched in 2005 on board its Mars Reconnaissance Orbiter, a specially equipped science satellite designed to orbit the planet and study the history of water on Mars.

The orbiter, which has been circling Mars since arriving there in 2006, has a powerful and diverse set of science instruments. HiRISE was developed by researchers in the department of planetary sciences at the University of Arizona, which is a key partner in the day-to-day Phoenix mission as part of NASA's low-cost interplanetary exploration effort, called the Scout Program. Also involved in the mission is Lockheed Martin Space Systems.

The new camera, or "imager," is able to provide scientists with their most-detailed photographs so far of the Martian surface in preparation for the Phoenix landing.

"HiRISE can actually see the rocks on the surface of Mars, and so what we've done is selected a landing site that is as hazard-free as possible, from a rock standpoint as well as a slope standpoint," Spencer said. "HiRISE can see rocks that are on the order of a meter to a meter and half in diameter. Those rocks are a threat to us and we can also extrapolate ... the sizes to the smaller-sized rocks as well" to avoid them.

Using HiRISE, scientists were able "to pick out a 'sweet spot' within our landing latitude range," he said. Once on the Martian soil, a robotic arm on the lander will dig below the surface and access what scientists expect will be ice within about six centimeters of the surface, based on the photos provided by HiRISE. The soil samples will then be scooped up and delivered into the lander's analytic instruments that can perform chemical analysis and provide compositions of the soil and elements that may be ingrained in the ice.

As the Phoenix travels to Mars, NASA has made several scheduled midcourse trajectory corrections to get the lander to just the right spot on Sunday, but otherwise, the mission has been going very smoothly, Spencer said.

"We've had a very safe and quiet cruise from Earth to Mars," he said. "So far the spacecraft has performed very reliably. The navigation has been excellent to date. We are right on target for our desired landing at 68 degrees north latitude."

Now scientists can only wait as the landing time approaches to see if all goes according to plan.

"What I can say is that we have done everything that we can do to make sure that the system is as robust as possible," Spencer said. "There are no guarantees in this business. It's a very challenging and difficult environment to operate in, but this program expends a great deal of our resources to ensure the reliability of the system, and so we are confident that we will have a successful outcome. But there are no guarantees."