A Day in the Life of Spirit
The details of both rovers on Mars today is about the mileage on their odometers.
Mobility is such an important scientific asset that daily mission reports have come to read like road maps. Turn left, move forward 10 meters, check one’s directional bearings. While a fascinated public may clamor for more pictures or startling conclusions about the red planet’s history, the rovers march on their long treks. One way to examine this progress is to outline what a few days provide as far as learning how to navigate a robotic explorer several hundred million miles away.
A day on the rover, if one were actually riding the 300-pound, golf-cart sized vehicles, might be mildly frustrating for the passenger. First the rovers don’t move too fast. Like the tortoise, a day’s journey is better measured from its start-to-finish, while ignoring the pace itself. When a mission report shows that a hundred meter day was a banner trip, one must bear in mind that the rovers travel at maximum speeds using only a mildly perceptible crawl. The mobile laboratory was designed for a top speed of 5 centimeters per second on a flat surface; however, on average, it will only move at 1 centimeter per second for a maximum of 4 hours each day.
|Two years ago, this was the state of the rovers. Disassembled in piles on tables, the rover had not yet taken shape. Credit: NASA/JPL|
The rovers are basically on their own as far as finding a good set of roadsigns. To get a single command from Earth to Mars, the one-way lightspeed travels for 10 minutes so the rovers largely navigate autonomously. Depending on the hazards detected, the rovers can crawl cautiously or go full throttle across flat plains. Its routine is not to follow a straight line ‘as the crow flies’, particularly at the Spirit site which is much rockier than Opportunity’s. A given trajectory chosen by the rover’s navigation can easily include 25% backward or sidewise motion compared to forward driving on target.
Consider what happened over the last five sols.
Spirit took it easy the morning of sol 112, which ended at 8:30 a. m. PDT on April 27 , and didn’t begin operations until 11:45 a.m. Mars Local Solar time, to conserve energy for an afternoon drive. Before taking off, Spirit gathered some soil and atmospheric observations with the mini thermal emission spectrometer and panoramic camera.
|One year ago, this was the state of the rovers. Assembled and folded up, the rover systems were still being checked months before launch. Credit: NASA/JPL|
Then the drive began. Spirit’s updated autonomous navigation software proved its worth again this sol. During a long auto-navigation segment, the rover encountered a hazard and was able to back up and find a way around it. Spirit continued to drive backwards towards its intended goal point, using the rear hazard avoidance cameras to navigate the way. When the allotted drive time was up, Spirit turned back around and made one last short drive to its resting place for the night. Spirit’s odometer records backwards and forwards driving and logged another 88.6 meters (290.7 feet) for the sol 112 drive. The actual distance covered was about 60 meters (197 feet).
On Sol 113, which ended at 9:09 a.m. PDT on April 28, Spirit woke up earlier than normal, 9:00 a.m. Mars Local Solar time, to do morning atmospheric science. One objective of the early sky scan was to image morning clouds with the panoramic camera. Spirit then began an intense study of a soil spot called "MayFly." During her examination of the area, Spirit took panoramic camera and mini thermal emission spectrometer images in parallel, conducted a two-hour Mössbauer integration and finished off with a look through the microscopic imager. The rover then stowed the instrument arm to prepare for digging a trench.
|Today, this is the state of the rovers. Shown here approaching Endurance Crater, the Opportunity rover faces considerably fewer road obstacles than Spirit on its way to the Columbia Hills. Credit: NASA/JPL|
Rover planners intended for Spirit to use its wheels to dig a trench at the MayFly spot, but hazard avoidance camera images of the area showed a potato-size rock that could have potentially fallen into the wheel hollow in the process. Rather than take that risk, controllers decided to back the rover up 10 centimeters (3.9 inches) to a clearer spot. After the final positioning, Spirit used its wheels to dig a 6-centimeter (2.4-inch) trench. Spirit finished the sol with hazard avoidance camera images of the trench, which was used to plan Mössbauer, alpha particle X-ray spectrometer and microscopic imager work on sol 114.
On sol 114, which ended at 9:49 a.m. PDT on April 29, 2004 Spirit continued to investigate the trenched area with the Mössbauer spectrometer, alpha particle X-ray spectrometer and the microscopic imager.
On Sol 114, which ended at 9:49 a.m. April 29 PDT, Spirit performed a lot of science activities in the trench called "Big Hole" using the microscopic imager, Mössbauer spectrometer and alpha particle X-ray spectrometer. Opportunity also studied the rover tracks and the crater rim.
Sol 116 started with a repeat of the microscopic imaging of a target in the trench due to minor communication glitches on sol 115. Spirit then stowed the arm, backed away from Big Hole trench, and took panoramic camera images of the trench before it continued on its trek toward the Columbia Hills. The drive on sol 116, which ended at 11:08 a.m. May 1 PDT, established a new drive record of 90.8 meters (298 feet) for Spirit!
On sol 117, which ended at 11:47 a.m. May 2, Spirit drove 37 meters (121 feet) to a small ridge, where the vehicle experienced a pitch up of 12.2 degrees. Engineers believe that the change in tilt caused the vehicle to recompute its "goodness map," which helps the rover autonomously drive over the martian terrain, and the rover declared that it was not safe to continue its drive. One good thing that came out of this is that the end-of-drive tilt positioned the solar arrays to maximize afternoon solar exposure, and the rover’s battery state of charge is in good health.
The science team refers to these chores as ‘touch and go’ days–the rover’s main task is surveying. After nearly 120 martian days, there of course is not a typical day yet. But the last week illustrates that even when science rides in the ‘backseat’ to engineering goals, the mobile laboratory offers a full menu of activities. The journey is more than just about getting there.
Related Web Pages
Mars Exploration Rovers
Travler’s Guide to Mars
Mars Rover: The Owner’s Manual
Pancam- Surveying the Martian Scene
Alpha Proton X-ray Spectrometer
Mars Rover: The Owner’s Manual
Reverse Robotic Origami