Phoenix Mars Lander Prepared to Weather Dust Storms

Scientists handling the next Mars mission say the only way a significant dust storm like the one raging on Mars in recent days could seriously harm their spacecraft is if one were to suddenly whip up during the critical descent phase.

The dust storm currently engulfing the southern hemisphere of Mars and threatening two robotic surface rovers, Spirit and Opportunity, comes as NASA's Phoenix lander is gearing up for an Aug. 3 launch.

Like Spirit and Opportunity, Phoenix will have no choice but to weather out a dust storm if one occurs during its time on the red planet. But based on past missions, this shouldn't be a problem, said Christopher McKay, a planetary scientist at NASA's Ames Research Center and a member of the Phoenix team.

"The Viking landers sat through one of the largest dust storms we've seen and you could barely tell," McKay told SPACE.com. "It's not like a dust storm on Earth where your tent gets buried in dust or you can't see six or seven inches in front of you. Mars' atmosphere is very thin. In some cases the winds are high, but globally the winds are not."

Aging rovers

If Spirit and Opportunity do incur damage from the current dust storm, it will be because they are already on the margin of their ability to function anyway in terms of power, McKay said. Originally designed for only a three-month mission, the plucky rovers have continued operating for an astonishing three years. Aged as they are and despite concern from rover mission officials, McKay is optimistic the rovers will survive this latest ordeal.

The rovers are "way past their warranty date," McKay said. "It could be that under the low power condition of the dust storm, they may have some breakdowns that can’t be repaired but I actually would be surprised, especially since they both just survived the winter. This should be a piece of cake for them."

Like the rovers, Phoenix will power its electronics using sunlight harvested by solar panels. Peter Smith, Phoenix principal investigator at the University of Arizona, estimates the lander could last 2 or 3 days without sunlight. However, Smith does not expect this to ever be an issue because Martian dust storms typically just scatter sunlight rather than blocking it out completely. "It’s not that you get zero solar energy," Smith said. "It would take a heck of a dust storm to get zero."

Furthermore, Phoenix is set to land on Mars' northern arctic region, where Smith said dust storms, even global ones, don't typically last very long. Also, when the spacecraft reaches Mars next May, it will be southern summer on the planet--a time when dust storms are rare for the north.

Risky business

The Phoenix scientists say the only time a dust storm could seriously threaten the mission is if one were to kick up as Phoenix is trying to land. When the lander reaches Mars, it will punch through the Martian atmosphere while encased in a protective shell fitted with a heat shield. After decelerating to 1.7 times the speed of sound, a parachute will deploy, the heat shield will be discarded, and the lander's legs will extend. The lander will glide through the Martian atmosphere until it is about half a mile (1 km) above the surface, at which point it will cut the threads attaching it to the parachute, and fire up retro-thrusters that bring it down for a soft landing.

If a dust storm were to occur during this critical phase of the mission, it could mean bad news.

"Landing is a risky business, and it always will be," Smith said. "That six-and-a-half minutes of descent is going to be the scariest part of the mission.”

Unlike the Viking landers, which could enter into orbit around Mars until conditions were deemed fit for landing, Phoenix will have no option but to barrel directly into the Martian atmosphere once it reaches its crimson target. "It's simply not designed to go into orbit," Smith said in a telephone interview. "We just don’t have the thrusters on the outside of the spacecraft to slow it down like that."

In the unlikely event that a dust storm occurs while Phoenix is landing, it is still possible that the spacecraft can make it to the surface safely. "We've done some calculations that show we can come down through a dust storm," Smith said. "The dust itself doesn't hurt the spacecraft. What it does is it heats the atmosphere and changes its density. If you knew [a dust storm] was coming and you could calculate the changes to the atmosphere, you could probably survive it very nicely."

The Phoenix mission will be the first in NASA's Scout series, a class of small, low-cost Martian spacecrafts. The lander will touch down on Mar's water-ice-rich northern polar region, where it will use its robotic arm to drill into the the arctic terrain in search of clues about the planet's geological, and perhaps biological, history.

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