[meteorite-list] Mars Exploration Rovers Update - April 18-30, 2008

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 5 May 2008 12:52:56 -0700 (PDT)
Message-ID: <200805051952.MAA28685_at_zagami.jpl.nasa.gov>

http://marsrovers.jpl.nasa.gov/mission/status.html

SPIRIT UPDATE: "Catch-22": Staying Awake vs. Going to Sleep -
sol 1532-1538, April 24-30, 2008:

Spirit's Tau measurements of atmospheric dust have remained steady, but
solar array input has dropped a bit to 235 watt-hours per sol. Spirit
still has enough energy to squeeze in Moessbauer studies of iron-bearing
minerals at a time of year when the rover's handlers expected Spirit to
be concerned only with survival. At present, the rover's target of
scientific interest is a soil exposure nicknamed after Arthur C. Harmon,
a former Tuskegee airman. Spirit conducted 8 more hours of Moessbauer
integration, for a total of 12 hours. Scientists hope the rover will be
able to collect 36 more hours' worth of data from the same target.
Meanwhile, Spirit continued to acquire panoramic-camera images, using
all 13 color filters, of the "Bonestell panorama," informally named in
honor of famed space artist Chesley Bonestell.

Concerned that cold winter temperatures on Mars might trigger the
survival heaters on the rover electronics module, rover planners took
the extra precaution of disabling those particular heaters on sol 1533
(April 25, 2008) to conserve power. With the heaters turned off, the
rover's handlers must monitor temperatures carefully to make sure the
module doesn't get too cold. Besides the survival heaters, the remaining
means of keeping the module warm enough during the night is to generate
more heat during the day by keeping the rover awake for about one
additional hour. Of late, 39 minutes is the shortest possible awake time
for conducting minimal activities. Another 20-plus minutes of awake time
are needed on days when the rover transmits data to NASA's Odyssey
orbiter during its overhead pass. On other sols, rover planners may
arbitrarily increase the rover's awake time to 50 minutes or longer to
generate enough heat to keep the electronics module alive, even if
science activities do not require Spirit to be awake that long.

In summary, the challenge for Spirit's handlers during each planning
cycle is to recharge the battery enough to do significant science, then
recharge the battery again to transmit data to Odyssey for downlink to
Earth. By keeping the rover awake for shorter periods, they conserve
energy but generate less thermal inertia (heat) for keeping the rover
electronics module alive. The more consecutive sols that go by without
transmitting temperature and power updates to Odyssey and from there to
Earth, the more Spirit's handlers must rely on margin ("wiggle room")
from earlier predictions and keep the rover awake longer to protect the
electronics module. As a result, Spirit is caught in a "catch-22" set of
tradeoffs among power, heat, communications, and science. This delicate
balance will become increasingly more precarious as the rover moves
closer to the winter solstice, with its even colder temperatures and
lower solar array input.

Sol-by-sol summary:

In addition to measuring atmospheric dust levels with the panoramic
camera and receiving direct-from-Earth instructions via the rover's
high-gain antenna, Spirit completed the following activities:

Sol 1532 (April 24, 2008): Spirit spent 8 hours acquiring data from
Arthur C. Harmon with the Moessbauer spectrometer.

Sol 1533: In the morning, Spirit took spot images of the sky with the
panoramic camera for calibration purposes and acquired panoramic-camera
images of the dune field known as "El Dorado." The rover recharged the
battery, disabled the survival heaters on the rover electronics module,
and shortened the "Up_Too_Long" computer sequence to 30 minutes.

Sol 1534: Spirit recharged the battery and relayed data to NASA's
Odyssey orbiter for transmission to Earth.

Sol 1535: Spirit recharged the battery.

Sol 1536: Spirit recharged the battery and relayed data to the Odyssey
orbiter. The rover checked for drift (changes with time) in the
miniature thermal emission spectrometer and surveyed the sky and ground
with the instrument. The rover was awake for a total of 61 minutes.

Sol 1537: In the morning, Spirit acquired full-color images, using all
13 filters of the panoramic camera, of column 12, part 2 of the
Bonestell panorama. The rover recharged the battery, checked for drift
in the miniature thermal emission spectrometer, and surveyed the sky and
ground with the miniature thermal emission spectrometer. Spirit was
awake for a total of 39 minutes.

Sol 1538 (April 30, 2008): Spirit checked for drift in the miniature
thermal emission spectrometer and surveyed the sky and ground with the
instrument. After sending data to Odyssey, the rover used the
alpha-particle X-ray spectrometer to measure argon gas in the Martian
atmosphere. Plans for the next morning called for Spirit to complete
work on column 12, part 3 of the Bonestell panorama.

Odometry:

As of sol 1538 (April 30, 2008), Spirit's total odometry remained at
7,528.0 meters (4.7 miles).

------------------------------------------------------------------------

OPPORTUNITY UPDATE: Opportunity Investigates Arthritic Joint -
sol 1505-1510, April 18-23, 2008:

Opportunity is healthy and all subsystems are performing as expected,
with the exception of the Instrument Deployment Device (the robotic
arm). Power has improved slightly during the last week, primarily as the
result of a better state of charge in the batteries. Given the recent
difficulties with the robotic arm, the rover hasn't been using the
batteries as much as usual. Energy has averaged about 380 watt-hours
(almost enough energy to light four 100-watt bulbs for one hour).

Tau, a measure of direct sunlight (and thus of dust in the atmosphere)
has been fairly steady at 0.62, meaning that about half the sunlight
streaming through the atmosphere of Mars reaches the ground. The rest is
either absorbed or scattered. Like direct sunlight, scattered light
generates power. Absorbed sunlight does not.

Opportunity's dust factor has been fairly steady at 0.7, meaning that
about 70 percent of the sunlight hitting the solar arrays penetrates the
dust layer to make electricity.

In mid-may (May 12, 2008), Mars will reach aphelion, its farthest point
from the Sun. At that time it will be 249 million kilometers (155
million miles) from the Sun, about 1.6 times farther from the Sun than
Earth ever gets.

On June 25, 2008, Opportunity will enter the winter solstice. This is
the date when the Sun is lowest in the sky that marks the official start
of the six-Earth-month Martian winter.

Ever since sol 654 (Nov. 25, 2005), Opportunity has experienced
occasional motor stalls in the shoulder joint of the robotic arm known
as Joint 1. Joint 1 is the shoulder azimuth joint, the one that swings
the arm out from the rover, and left or right in front of the rover. The
motor has worked long past its expected lifetime.

Apparently random in occurrence, the stalls have been accompanied by
step increases in electrical resistance, which is consistent with a
broken winding within the motor. (The rover's motors have bifilar coils,
consisting of two parallel windings. If one wire breaks, a second coiled
wire provides some torque to turn the motor. Torque is a force that
causes rotation about an axis. With one wire broken, there is less
torque during part of each rotation.)

Each motor has magnetic detents -- permanent magnets that pull the rotor
into a fixed position to prevent it from rotating when necessary. The
detents can also prevent the motor from turning when engineers want it
to turn. At that point, it takes a bit more torque to start the motor
turning. If a broken coil happens to align with a detent, the remaining,
unbroken coil has a hard time starting to rotate. This can result in a
stall.

Opportunity's handlers have been living with this for nearly 900 Martian
days and until now, they have overcome every stall simply by trying the
motion again.

To minimize the chance of getting "stuck" in an unfavorable position
where Joint 1 is permanently stalled, the rover's handlers keep the
robotic arm deployed (that is, unfolded and suspended in front of the
rover) except when driving. This approach is known as the
"Stow/Go/Unstow" strategy. When rover drivers want to drive, they stow
the arm (that is, fold it up with the elbow in horizontal position and
the fist against the chest). After the drive, they promptly unstow the
arm (move the elbow down and the turret, or fist, up).

On Sol 1502 (April 15, 2008), during a routine post-drive unstow,
Opportunity experienced a Joint 1 stall that was quantitatively
different from prior stalls. Tests since then have continued to result
in stalls with significantly higher electrical resistance five to 10
times greater than previously measured values. Motor currents have been
very low, consistent with higher electrical resistance.

Engineers are conducting diagnostic tests of Joint 1 and other
components. So far, every attempt to move Joint 1 has failed, producing
at most a single motor revolution. The joint seems to stall in the same
spot each time. The rover's handlers are pursuing a slow and steady
approach of fully understanding the problem and possibly devising
strategies for living with or working around the issue. They already
know that even if the joint is permanently stalled, Opportunity can
still do some science observations with instruments on the robotic arm.

Sol-by-sol summary:

In addition to receiving morning instructions directly from Earth via
the rover's high-gain antenna, relaying data back to Earth via the UHF
antenna on the Mars Odyssey orbiter, and measuring atmospheric dust with
the panoramic camera, Opportunity completed the following activities:

Sol 1505 (April 18, 2008): Opportunity ran diagnostic tests of the
robotic arm.

Sol 1506: Opportunity acquired six time-lapse movie frames in search of
morning clouds with the navigation camera and surveyed the horizon with
the panoramic camera. Later, the rover acquired a 3-by-1 panel of images
with the navigation camera.

Sol 1507: Opportunity took morning spot images of the sky for
calibration purposes with the panoramic camera. The rover ran more
diagnostic tests of the robotic arm.

Sol 1508: Opportunity began work on a 360-degree, two-tier panorama of
lossless-compression (highly detailed and precise) images with the
navigation camera.

Sol 1509: Opportunity spent most of the day characterizing dust in the
atmosphere.

Sol 1510 (April 23, 2008): Opportunity acquired full-color images, using
all 13 filters of the panoramic camera, of one of the two trenches made
by the rover's wheels, dubbed "Williams." At 11:05 a.m. local Mars time,
Opportunity acquired the left-hand view of a mosaic of panoramic-camera
images. The rover acquired part 2 of the lossless-compression,
360-degree panorama with the navigation camera and surveyed the sky at
high Sun with the panoramic camera. The following morning, Opportunity
was to acquire full-color images, using all 13 filters of the panoramic
camera, of the other wheel trench, dubbed "Harland."

Odometry:

As of sol 1510 (April 10, 2008), Opportunity's total odometry was
11,689.53 meters (7.26 miles).
Received on Mon 05 May 2008 03:52:56 PM PDT