Spirit Is Ailing But Still Plodding
Scientists Unsure How Rover's Stint On Mars Will End
By Joel Achenbach
Washington Post Staff Writer
Sunday, April 19, 2009 6:49 PM
The old rover was supposed to work for only 90 days, enough time to
crawl two-thirds of a mile across the Martian desert. More than five
years later, Spirit has put five miles on its odometer and is still
rolling along -- but getting mighty cranky.
The rover, one of two NASA vehicles operating on Mars, has a broken
right wheel. It has dust on its solar panels. It's operating at about
30 percent of normal power. Various sensors and software programs
have gone screwy.
Then, on April 9, Spirit refused to wake up. The rover is designed to
sleep at night, when there is no sunlight hitting the solar panels.
But Spirit snoozed right through its wake-up call. It happened three
times in succession. Finally a backup timer got Spirit up and moving
again after a 27-hour slumber.
John Callas, project manager for the Mars rovers at the Jet
Propulsion Laboratory in Pasadena, Calif., said he doesn't have an
explanation for what ailed Spirit. Nor can he explain why the rover
repeatedly rebooted itself when engineers at JPL tried to send it
commands. The engineers found a fix -- they relayed commands via a
spacecraft that orbits Mars -- but the incidents suggest that Spirit
is getting erratic. Or maybe just old.
By any measure, Spirit and its sister rover, Opportunity, which is a
good bit healthier, have been triumphs of the civilian space program.
Spirit may yet operate for several more years, or it may be on its
last leg. In any event, it is providing a tutorial on how even the
most exquisitely designed machines eventually die.
"I don't think anyone can tell you how these rovers will eventually
end on Mars," Callas said. "Will they gradually degrade until their
mechanical functionality goes or will they have a catastrophic end,
where something inside the rover breaks?"
When the rovers were designed, the presumption was that dust would
blanket the solar panels within a few months of being on Mars and
that the rovers would grind to a halt. Martian winds blew to the
rescue, cleaning the panels periodically and letting the rovers
extend their missions for many years.
Spirit is now driving around a plateau called Home Plate in a valley
known as the Inner Basin. Opportunity, on the other side of Mars, has
left Victoria Crater and is rambling toward a large crater named Endeavour.
Day-to-day life on Mars can be rocky. The temperature can swing 150
degrees Fahrenheit between night and day because of the thin
atmosphere. That heating and cooling cycle puts stress on metal.
"Metal parts and glass parts expand and contract as the temperature
changes," explains John Casani, who has worked on robotic space
missions for decades at JPL. "If you take a piece of metal and keep
bending it back and forth, pretty soon it's going to break."
Casani said spacecraft -- the ones that stick to space and don't try
to land on a planet or some such feat -- exist in a much more stable
environment. Thus JPL is still getting data from the two Voyager
craft launched in 1977. According to JPL, the oldest functioning
spacecraft is Voyager 2 (launched slightly earlier, strangely enough,
than Voyager 1), which is zipping toward interstellar space far
beyond Neptune's orbit.
There's not much to see out there, and, in any event, no operational
camera to see it with, as the spacecraft's batteries, powered by
radioisotope decay, slowly become enfeebled. But a few instruments
still function, and scientists have rebuffed efforts to shut down the
Spacecraft can also be sent to their deaths. Casani was the project
manager on Galileo, a probe to the Jupiter system. When Galileo ran
low on propellant, engineers knew that eventually it would lose
attitude control and start tumbling. That raised the fear that
Galileo would crash into one of Jupiter's moons, which potentially
harbor life, and might be contaminated by stowaway Earth microbes. So
Casani and his colleagues used the last bit of propellant to send
Galileo into Jupiter, where it burned up in the thick atmosphere.
When things go wrong, scientists and engineers often have a
workaround. Sometimes they just get lucky, which is what happened
when Spirit's right-front wheel broke three years ago.
The other five wheels on the rover were functional and were capable
of dragging the broken wheel across the surface. The inoperative
wheel, locked, gouged a trench as it went along. By examining that
trench, Spirit was able to detect a certain kind of silica that
offered evidence of ancient hot springs on Mars.
"When life hands you lemons, you make lemonade," Callas said.
So that's not really a broken wheel on Spirit -- it's a scientific instrument.
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