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July
20, 2007: Having explored Mars for three-and-a-half
years in what were missions originally designed for three
months, NASA's Mars rovers Spirit and Opportunity are facing
perhaps their biggest challenge.
For
nearly a month, a series of severe Martian summer dust storms
has affected the rover Opportunity and, to a lesser extent,
its twin, Spirit. The dust in the Martian atmosphere over
Opportunity has blocked 99 percent of direct sunlight to the
rover, leaving only the limited diffuse sky light to power
it. Scientists fear the storms might continue for several
days, if not weeks. "We're rooting for our rovers to
survive these storms, but they were never designed for conditions
this intense," says Alan Stern, associate administrator
of NASA's Science Mission Directorate, Washington.
The
animated global map of Mars, below, shows how the opacity
of the Martian atmosphere has increased since late June. Blue
areas denote clear air, red areas are thick with dust:
Above:
Martian atmospheric dust opacities measured by the THEMIS
instrument on the Mars Odyssey satellite. [More]
[Larger movie]
If
the sunlight is further cut back for an extended period, the
rovers will not be able to generate enough power to keep themselves
warm or operate at all, even in a near-dormant state. The
problem is, Mars is a very cold planet, and the rovers depend
on electric heaters to keep some of their vital core electronics
warm.
Before
the dust storms began blocking sunlight last month, Opportunity's
solar panels had been producing about 700 watt hours of electricity
per day, enough to light a 100-watt bulb for seven hours.
When dust in the air reduced the panels' daily output to less
than 400 watt hours, the rover team suspended driving and
most observations, including use of the robotic arm, cameras
and spectrometers to study the site where Opportunity is located.
On
Tuesday, July 17, the output from Opportunity's solar panels
dropped to 148 watt hours, the lowest point for either rover.
On Wednesday, Opportunity's solar-panel output dropped even
lower, to 128 watt hours.
NASA
engineers are taking proactive measures to protect the rovers,
especially Opportunity, which is experiencing the brunt of
the dust storm. The rovers are showing robust survival characteristics.
Spirit, in a location where the storm is currently less severe,
has been instructed to conserve battery power by limiting
its activities.
"We
are taking more aggressive action with both rovers than we
needed before," said John Callas, project manager for
the twin rovers at the Jet Propulsion Laboratory, Pasadena,
Calif.
 Right:
In Aug. 2005, Spririt used its panoramic camera to make this
self-portrait looking down on the rover's solar panels. Dust
in the air means less sunlight and less solar power. [More]
By
Opportunity's 1,236th Martian day, which ended Tuesday, driving
and all science observations had already been suspended. The
rover still used more energy than its solar panels could generate
on that day, drawing down its battery. "The only thing
left to cut were some of the communication sessions,"
Callas said.
To
minimize further the amount of energy Opportunity is using,
mission controllers sent commands on Wednesday, July 18, instructing
the rover to refrain from communicating with Earth on Thursday
and Friday. This is the first time either of the rovers has
been told to skip communications for a day or more in order
to conserve energy. Engineers calculate that skipping communications
sessions should lower daily energy use to less than 130 watt
hours.
A
possible outcome of this storm is that one or both rovers
could be damaged permanently or even disabled. Engineers will
assess the capability of each rover after the storm clears.
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NASA
will provide mission updates as events warrant. The Jet Propulsion
Laboratory manages the Mars Exploration Rover project for
NASA's Science Mission Directorate.
Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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