Mars Rovers Batten Down for Rough Season

Like painted kites, those days and nights
went flyin' by
The world was new, beneath a blue
umbrella sky

Then softer than a piper man,
one day it called to you
And I lost you, I lost you to
the summer wind...

NASA engineers are hoping those words, famously crooned by Frank Sinatra, don't come true this summer for the unflappable Mars rovers, Spirit and Opportunity.

Summer is approaching on Mars, and with it comes the onset of huge wind storms that kick dust around the twin Mars Exploration Rovers and their life-giving solar panels. A storm in July 2007, nearly one Martian year ago, blocked 99 percent of the rovers' sunlight. The storms don't happen every year, but when they do, they start about this time -- on Tuesday, April 21, Mars will be at the closest point to the sun in its 23-month year, and the official start of summer comes a month later.

Scientists at NASA's Jet Propulsion Laboratory and at offices in San Diego and College Station, Texas, are keeping an eye on the warming Martian weather, ready to tell the rovers to rest in advance of a dust storm.

They're still healthy, but like the fittest human, the rovers are not immune from the symptoms of aging. Spirit is having some memory problems, and has had some "senior moments" when its computer rebooted unexpectedly a couple times in the past week. Spirit talked to its controllers on Earth Friday, April 10, through Sunday, April 12, but some of the communications were irregular, according to JPL. The rover apparently reset its computer in the midst of using its dish antenna to communicate with Earth. It has also had some memory gaps in which it's not properly saving data.

John Callas, project manager for Spirit and Opportunity, based at JPL, said Spirit's batteries are charged, the solar arrays are working fine and the rover seems healthy, despite the unusual activity. It is in an automatic mode to take care of itself, and can stay in that mode for some time while engineers figure out what's wrong.

The rovers' software has been upgraded several times in the five years since their mission had been scheduled to end. The latest upgrade was last month, and mission managers are investigating whether that put Spirit on the fritz -- even though Opportunity, halfway across the planet with the same software, is operating just fine.

"We are aware of the reality that we have an aging rover, and there may be age-related effects here," Callas acknowledged.

The rovers have been going strong since January 2004 on a mission that was supposed to last just 90 days. Martian winds are partly to thank for that, because a stiff breeze can help clear dust off the rovers' solar panels.

In February, wind cleared enough dust off Spirit's panels to nearly double its power-gathering capacity. And earlier this month, wind helped increase Opportunity's electrical output by 40 percent. The rovers use about 180 watt-hours per day for basic survival and communications, according to NASA.

But that same wind can cause harm by blocking out the sun. If scientists can anticipate a big storm, they can back off power-eating communications sessions and tell the rovers to stay put. The Mars Reconnaissance Orbiter, which reached the Red Planet in 2006, helps with that.

MRO's Mars Color Imager can see the entire planet every day at a resolution comparable to Earth weather satellites, which allows Bruce Cantor, with the Color Imager team, to provide frequent weather reports. On Nov. 8, Cantor warned the rover team of a dust storm nearing Spirit, and the team was able to lessen Spirit's workload to help save energy, JPL said. Without taking those precautions, Spirit's batteries could have dwindled to dangerously low levels.

As they watch for dust storms, engineers hope the fickle summer wind blows their way and helps clean off Spirit and Opportunity, rather than burying them in more dust.

"We're all hoping we'll get another good cleaning," said Bill Nelson, chief engineer for the rovers at JPL.