Their
90-day warranty expired awhile ago, but NASA's twin Mars rovers Spirit and Opportunity are still trundling along the Martian surface en route to their next
destinations more than five years after landing on the red planet. But just how
long they can keep going is anyone's guess — it could be three days, or it
could be three years.
"We
have no way of knowing what the future holds for the rovers at this
point," said Mars Exploration Rover Mission principal investigator Steve
Squyres of Cornell University. "The mission could easily end tomorrow, but
the miracle could continue."
That
miracle began with the January 2004 landings of the two rovers. Spirit
got to Mars first, touching down at Gusev Crater on Jan. 3. A few weeks later, Opportunity
bounced to a stop in the vast plains of Meridiani Planum on the other side of
the planet.
The road
has been a bumpy one — sometimes literally — with stuck wheels, broken wiring
and Spirit's one bum wheel periodically hampering the mission.
But even
with those hiccups, the past
five years (that's 20 times the planned lifetimes of the rovers) have
yielded many insights into the planet's past and present and taught mission
controllers numerous lessons about running a mission from millions of miles
away.
Rover
lessons
Among the
many remarkable discoveries the rovers have made are the clues that show the
planet has not always been as cold and dry as it is at present — at one time,
it was warm and wet enough to support life.
More than
1,200 sols (or Martian days) into the mission, Spirit analyzed a patch of dirt
and found it was rich in silica, which provided some of the strongest evidence
yet that the Martian surface was once wet.
Opportunity found salty areas in Merdiani
Planum that have garnered
many watery theories to explain how they go there.
These
scientific findings aren't the only useful information to come out of the
mission.
"Spirit
and Opportunity helped invent a whole new discipline — robotic field
science," Squyres said. "They've taught us how to organize a team of
scientists and engineers to operate robotic rovers on a distant planet. We all
had to learn to work together effectively year after year to squeeze the most
possible science from the rovers."
Team members
learned many of the perils of maneuvering a wheeled robot from hundreds of
millions of miles away.
"We
now know how to negotiate sand dunes and piles of rocks," Squyres said,
"and perhaps more importantly, how to avoid them."
That experience
will help with future missions, such as the upcoming Mars Science Laboratory,
currently slated to launch in 2011.
The snags
hit along the sustained mission have left the rovers with some bruises. Spirit,
for example, has been driving backwards since one of its wheels jammed in 2006,
and a broken electrical wire has reduced the movement capabilities of Opportunity's robotic arm.
Spirit also
had a glitch earlier this year that caused it to not report in to mission
controllers as planned, but eventually the rover resumed
normal behavior. Spirit was on its way to its next target, von Braun, a
cap-rock about 800 feet (250 meters away).
Opportunity has fared a little better; it's
been "the lucky vehicle since Day 1," Squyres said. However, the
rover did recently adjust its
course to get around obstacles while working its way toward its next
destination, Endeavor Crater, some 7 miles (12 kilometers) away.
On the
horizon
Endeavor
will be the largest crater that Opportunity has yet investigated; it is about
12 miles (20 km) in diameter and hundreds of meters deep.
"Endeavor
is an intriguing target because the rocks close to it look different from the
ones surrounding the other craters Opportunity has visited," said deputy
principal investigator Ray Arvidson of Washington University in St. Louis. "Part of Endeavor crater's rim is sticking up — Mars' ancient bedrock
exposed — and rocks nearby may be suggestive of acidic lakes on Mars' surface
billions of years ago."
The trip
won't come without its costs though.
"We'll
have to double the odometer reading on a five-year-old vehicle to get
there," Squyres said. "And it will take at least two years to reach
it."
Opportunity can manage about 300 feet (100
meters) of traveling on an average day.
"It'll
be a long march across the plains, but it will be well worth it," Squyres
said. "The deeper the crater, the older the history of Mars we can look
at."
Spirit
won't have quite as long a trip as its target is a bit closer, but the journey
will still take a few months. Spirit's mechanical issues make its trip a little
trickier.
"Spirit
is the more challenging rover to operate," Squyres said. "There's not
as much wind at its location to clean the solar arrays, and that affects the
vehicle's power."
Luckily,
Spirit recently got
a power boost when some Martian winds wiped some of the dust off its solar
panels.
"Also,
Spirit has to travel more challenging terrain," Squyres added. "The
rocks and loose sand at Spirit's location are treacherous. Of course, to top if
all off, Spirit is driving backwards."
But the
trip to von Braun and its subsequent target, a feature dubbed Goddard, will be
worth it.
"Home
Plate, where Spirit spent the winter, is a volcanic structure eroded down so we
can see the layers," Arvidson said. "And we think von Braun and the
neighboring Goddard structure may be made of the same stuff."
The rocks
that Spirit has checked out show evidence of "water-charged explosive
volcanism," Arvidson said. "Such areas could once have supported
life."
Predicting
the end
Even with
the periodic glitches and mechanical issues encountered so far, the rovers have
stood up to the test.
"It's
like a good old car that keeps on running," Arvidson said.
And while
mission controllers are aware of the limitations of the rovers, it's almost
impossible to predict when the rovers might stop for good.
"They're
so far out of warranty it's hard to predict," Arvidson said.
The most
vulnerable parts of the rovers are any parts that move, Arvidson told SPACE.com.
The rovers are also exposed to large swings in temperature throughout the day
that stress their systems.
Though the
rovers will be checking out rocks and other features on the way to their
primary targets, the team will also likely minimize the movement of certain
rover parts needed to investigate the Martian environment, Arvidson said.
"We
want to save those capabilities for the really juicy targets," he said.
Many
mechanical issues can be worked around too, as Spirit is currently doing with
its broken wheel.
"We've
got an enormous amount of functional redundancy built into these
vehicles," Squyres told SPACE.com in an email. "So a lot can
go wrong and still allow us to do good science."
Predicting
how fast different mechanical systems will wear out is difficult though, and of
course that's not the only limiting factor.
"If
nothing mechanical stops us, then sooner or later the capacity of the batteries
will degrade to the point where they no longer can hold enough charge,"
Squyres said. "But we don't know how long that will take either."
If the
rovers get to the point where their roving days are over, they could still
operate for a time as a weather station, something like Viking
1 did when its primary mission was finished, Arvidson said.
"If we
lose mobility completely, then yes, we can poke around at the place where we
stop," Squyres said. "But if the scenery isn't changing, sooner or
later you reach a point of diminishing returns."
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