A time-lapse image of the dust storm's progress from the Opportunity rover, from June 14, 2007 through July 19, 2007. The images are approximately true color composites, generated using data from the rover's panoramic camera's various filters. The numbers across the top of the image report a measurement of atmospheric opacity, called "tau." The lower the number, the clearer the sky.
Mars' globe-engulfing dust storm has died down during the past several weeks, but the two robotic rovers on its surface now face the fallout of dust from the thin atmosphere.
Conditions were so bad in early August that just before the launch of the Mars-bound Phoenix spacecraft, rover scientist Mark Lemmon feared the demise of the Opportunity rover.
"There was one sol [Martian day] when there was real uncertainty we'd hear from Opportunity," said Lemmon, a planetary scientist at Texas A&M University. He added that the plucky robotic explorer almost entered a power-saving mode that would have been dangerous "uncharted territory" for the rover team.
Still, Lemmon thinks the Mars rovers will persevere through the dusty conditions.
"Mars could throw worse storms at us, but for this season I think we have seen the worst," he told SPACE.com in an e-mail. "We got a good demonstration that Mars could kill them."
Thin layers of reddish powder have been accumulating on Spirit and Opportunity's solar panels due to fallout, blocking light at levels comparable to conditions during the worst periods of the storm, Lemmon said.
Scientists use the measure "tau" to describe the light blockage, where zero is perfectly clear and light blockage diminishes as the figure increases. Tau grew to greater than 5 during the peak of the dust storm.
"Unless tau goes to 5 for several sols in a row, it looks like she's out of the woods," Lemmon said of the Opportunity rover, which has born the brunt of the dusty assault. Tau is currently around 3.3, he added.
Stifled light can threaten the Mars rovers because their electronics need to stay above -40 degrees Fahrenheit (-40 degrees Celsius)?below that and metal circuits can shrink, snap and claim the rovers' lives.
On the martian surface, temperatures are a bone-chilling average of about -81 degrees Fahrenheit (-63 degrees Celsius).
To combat the cold, the rovers rely on small internal radioactive heaters to stay warm and regularly go into "sleep" mode to conserve energy. Lemmon reported that both robots now gather above 240 watt-hours per sol, enough to power a 120-watt light bulb for two hours. He said at least 160 watt-hours per sol is required to keep the electronics alive and well.
"Spirit's panels are still cleaner than before the storm, Opportunity's panels are dustier," Lemmon said. He added that mission managers are thinking of ways to minimize accumulating dust, such as tilting the solar panels, but most options seem more risky than simply toughing it out.
"Once the dust settles, we can pray for wind," he said. "Not too much, just enough to clean the panels."
Lemmon said the rovers' journey has been an amazing one, especially because they were expected to survive for only three months after landing. They have explored the surface of Mars now for nearly three years and seven months.
"I know we're all as stunned as anyone when we look back on how much the rovers have been able to do for us," Lemmon said.
As of sol 1,282, or August 13, 2007, Spirit surpassed the nuclear-power Viking Lander 2's record and is now the second-oldest operational robot on Mars.
"A solar-powered vehicle is now in second place ahead of one of the nukes, despite these dust storms," Lemmon noted, referring to the nuclear-powered Viking landers. Spirit and Opportunity technically are solar-powered, although they do have the radioactive heaters.
While it may take another grueling 959 sols (which will occur in April 2010 on Earth) for Spirit to surpass the first Viking lander's operational record, Lemmon said the rover team isn't planning on closing shop anytime soon.
"[Programmers] apparently developed a 2010 patch for a small rover flight software glitch," Lemmon said. "Just in case."
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