[meteorite-list] Spirit Team Facing Critical Decision As Martian Winter Approaches

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Apr 5 16:23:25 2006
Message-ID: <200604051815.k35IFp121450_at_zagami.jpl.nasa.gov>

http://www.spacedaily.com/reports/Spirit_Team_Facing_Critical_Decision_As_Martian_Winter_Approaches.html

Spirit Team Facing Critical Decision As Martian Winter Approaches
by Phil Berardelli
SpaceDaily
April 5, 2006

Washington DC (SPX) - Mars rover mission controllers announced Tuesday
what had been suspecting for several days: Spirit has permanently lost
its right-front wheel. The disclosure came exactly two years and three
months after the rover landed in a formation called Gusev Crater, near
the Martian equator - long after its planned 90-day mission had expired.

Because of the mechanical failure, controllers at NASA's Jet Propulsion
Laboratory said they are having a difficult time getting the rover to
move toward a north-facing slope to survive the cold and dark Martian
winter, which is still more than a month away.

Spirit must position its solar panels northward in winter to collect
enough solar energy to operate while the Sun lays low along the red
planet's horizon.

"Essentially, it's a race against time," controllers said in a
statement. Even though the period of minimum sunshine in the Martian
winter is more than 100 days away, Spirit already receives only enough
power to drive for about an hour a Martian day, or sol, on flat ground.

"The climate is changing rapidly, and we have to put our primary
emphasis on keeping the rover safe," said Steve Squyres, the principal
investigator for both rovers. "Assuring survival has to take priority
over science until we've got the vehicle on safer ground."

The problem is the route Spirit must take to reach that safer ground
seems more and more difficult, because of the softness of the soil. That
dragging front wheel has made progress painfully slow, and could
paralyze the rover, perhaps permanently.

So the team has devised what might turn out to be a last-ditch strategy:
They have reversed Spirit's course, driving it back downhill for a while
before moving laterally to a different slope that should be easier to
climb and will provide the maximum available sunlight for the rover's
solar panels.

So far, the results are mixed. Spirit drove about 10 meters (33 feet) in
one day after the wheel stopped working, but then lost traction on
following days because of wheel slippage in the loose soil. A further
problem has been caused by the rover's suspension system, which keeps
all six wheels in contact with the ground.

"Engineers designed the rovers so well that Spirit's weight remains
evenly distributed on all six wheels," said Al Herrera, a computer
sequencing specialist for Columbus Technologies and Services Inc. who
oversees uplink commands to the rover. "That keeps the rovers balanced
so they don't tip or roll backward."

It also prevents relief against the dragging right-front wheel
disrupting Spirit's progress, however, because it continues to maintain
equal weight on it.

Spirit's wheel difficulty emerged March 13, on sol 779, when the motor
suddenly began drawing more electrical current than normal, then stopped
drawing current altogether while the rover was turning to adjust the
orientation of its communications antenna.

Engineers determined that the fault occurred in a drive actuator, one of
10 motors that operate the six wheels. Following tests in the duplicate
rover facility at JPL, engineers determined the motor could no longer be
used.

Spirit's problem is long overdue. The rover and its twin, Opportunity,
have lasted more than two years, or nearly nine times longer than their
mission design. Spirit's wheel motors have rotated more than 13 million
times.

Still, controllers are trying to maintain their optimism. "It is too
early to tell how serious this is," said John Callas, the rover's
project manager. "The nature of the terrain is a dominant factor."
Received on Wed 05 Apr 2006 02:15:51 PM PDT