The Phoenix Lander lowers itself onto Mars using a set of powerful thrusters. No airbags for this tricky touch down on the red planet. Image
Credit: JPL/Corby Waste
DENVER, Colorado Getting the Phoenix Mars Lander down and dirty on the red planet is an engineering saga stretching out over a decade. Its mission "raises from the ashes" a spacecraft and instruments from two prior tries to reach the red planet: the Mars Polar Lander that failed to phone home in 1999 and a 2001 lander that was mothballed and shelved by NASA.
The builders of those two earlier spacecraft here at Lockheed Martin Space Systems have taken lessons learned to send Phoenix on its way.
Fingers are crossed for sure, but confidence is high that, indeed, the flight of the Phoenix will open up a new chapter in Mars exploration.
Shimmy and shake, dig deeper
In putting together NASA's next Mars lander, special attention was placed on the spacecraft's gang of a dozen pulse-firing descent thrusters that use rapidly opening and closing valves. The vibration they create was found to shimmy and shake throughout the spacecraft structure. A similar system was utilized on the failed Mars Polar Lander.
"That was a huge risk item to get retired very early," said Ed Sedivy, the company's Phoenix program manager. He led the Lockheed Martin engineering team in designing, constructing, and testing the Phoenix spacecraft.
Other problems that were spotlighted in a failure investigation board looking into the loss of the Mars Polar Lander as well as those identified by company experts were tackled by testing, testing and more testing.
"If you look back at the Mars Polar Lander experience, that mission did not have every opportunity to be successful," Sedivy told SPACE.com.
Having another chance to work out those problems was key, said Timothy Priser, the Phoenix entry, decent and landing phase lead at Lockheed Martin Space Systems. "We were given an opportunity to do it again. And engineers when they are given that opportunity are going to dig deeper and they are going to make it better," he added.
Not on the radar screen
An early problem Phoenix designers also wrestled with was the prospect that the lander's radar might lock onto the spacecraft's own discarded heat shield and not on the rapidly approaching Martian landscape.
That issue was not on the radar screen, quite literally, for the Mars Polar Lander mission nor the Mars Surveyor 2001 program, Priser said. Radar behavior improvements through the use of an algorithm in the computer brains of the Phoenix lander have solved this concern, he added.
Additionally, in the landing profile for Phoenix, there is a 70 second delay between heat shield jettison and the time the space probe starts to use its radar to gauge distance to ground, speed of descent and horizontal velocity.
"It turned out to be a pretty simple fix ... and the delay in the timeline is what will mitigate it," Priser said.
Big wild card
One of the unknowns that the 772 pound (350 kilogram) Phoenix will encounter happens as the craft sets its three legs onto the arctic terrain. The ground texture shows polygonal cracking a fractured surface.
"The reconnaissance of the landing sites has been like nothing we've ever been the beneficiary of in the past," Sedivy said. He points to the super-powerful camera gear aboard NASA's Mars Reconnaissance Orbiter (MRO), adding: "It is visual imagery with resolution far better than we've had in the past."
Still, what truly awaits Phoenix in terms of arctic surface geology and cold temperatures is a puzzle.
"It's the thing that's the big wild card for us. We're going to a place that nobody has ever put an asset before," Sedivy explained. "It could be thermally more hostile than we were expecting ... or it could be more benign. We just don't know."
With descent thrusters firing, Phoenix will be speeding at about 5.4 miles per hour (2.4 meters per second) in vertical velocity just before touchdown. But the horizontal velocity could be up to 1.4 meters per second, advised Tim Gasparrini, deputy program manager for the Phoenix entry, descent and landing at Lockheed Martin Space Systems.
What will truly greet the lander in terms of slopes, polygons, and rocks is not known. "It's somewhat of a complicated analysis to do in terms of touchdown," Gasparrini said. "But we've got a reasonably high probability of landing success even given all factors."
Phoenix fan dance
Nevertheless, Sedivy said he's nervous when trying to second-guess the depth of those polygonal fractures and what may await Phoenix in terms of any crevasses.
"If they are narrow, the lander won't care. If they are not so narrow, the lander might care," Sedivy noted.
Phoenix is about exploration, Gasparrini said, "and this is one of the risks of that exploration." All the MRO imagery and data gathered about the landing site has been phenomenal, he said, "but it's not the same as being there ... and Phoenix is going to be there."
Once the Phoenix footpads detect contact with the ground, the lander's thrusters will shut down. The blast effect from those thrusters from soil liquefaction and gas-soil bursting is expected to toss light soil and dust up into the air.
Indeed, before the lander's solar arrays are unfurled, there's a 20 minute wait, a period of time thought long enough for churned up dust and dirt to settle.
Then the Phoenix fan dance begins: two nearly circular decagons of energy-generating solar cells extend from opposite sides of the lander, unfolded in Chinese fan-like fashion.
"It's really important to get the solar arrays open, to get ourselves power-positive," Sedivy said. "You don't begin to breathe again until you see your solar arrays deployed."
That crucial opening day deployment of solar arrays comes down to a wax job.
Electrical power is applied to a paraffin actuator that, when that wax melts, allows the arrays to spread outward. That process should take some two to five minutes, with the variability due to temperature in the landing area and how long it takes for the wax to melt, Sedivy explained.
In the event that one array fails to fold out, Gasparrini responded: "We can run a mission. It'll be a little different and may not get everything quite done that you want done. But you would be able to run a mission ... one that is tailored to meet your power constraints."
Sedivy added: "On paper, we've got a pretty respectable mission with one solar array."
But the bad news comes if both solar arrays are a no-show. Phoenix will be alive on batteries for about 31 hours. That trickling down of energy and a ticking countdown clock would mean taking risks. A fast-paced, priority-setting scenario would have to be scripted in order to maximize scientific output from the dying lander.
"It's not the sort of activity you'd like to see after spending the last four years designing the vehicle," Sedivy said. "So being diligent engineers, there are contingency procedures in place. To be ill-prepared for surprises is not in anybody's best interest," he pointed out.
The first photos from Phoenix are meant to provide engineering data. That is, a footpad picture will convey technical information about the surface upon which the lander is resting. Imagery is also to be taken of pie-shaped sections of the solar arrays. That should reveal if they are under full-tension and latched in place or a problem has occurred during deployment.
The cost of the Phoenix mission breaks out into a U.S. investment of $420 million, including development, science instruments, launch and operations; plus the Canadian Space Agency's investment of $37 million for the weather station carried by the Mars lander.
In coming to full-stop on Mars, the spacecraft will have traveled about 422 million miles (679 million km).
As landing day for Phoenix approaches, the designers, builders and operators of the spacecraft feel that they've treated concerns flagged by the Mars Polar Lander loss and learned from the shelved Mars Surveyor 2001 mission.
Confidence-building on the Phoenix project via extensive testing was done. A laundry list of issues has been addressed that might crop up to hinder the lander from safely reaching the red planet and operating successfully at the Martian polar north.
Yet, fingers are still crossed. Getting to Mars is a balance of risk, hard work and a good dose of luck.
"I think we've done everything that we could have done," Sedivy concluded.
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