ROVING MARS --------- John Pazmino NYSkies firstname.lastname@example.org 2005 December 24
Introduction ---------- NYSkies enjoyed two chances to hear Dr Steve Squyres, Cornell University, explain the operation of the Mars rovers Spirit & Opportunity n December 2005. The first occasion was on Monday the 12th at the Hayden Planetarium; the second, on Tuesday the 13th at the Science, Industry, and Business Library. I attended the Library talk, which started at 5:30PM. A couple others at this session also attended the Planetarium talk. Squyres is the project manager for the two rovers on Mars, which now completed one Mars year of work on the planet. The rovers were intended to run for 90 days each, but so far show good health and vitality. NASA will fund the rover mission until they ultimately end their lifes.
General history ------------- Squyres discussed the planning and design of the rovers. Each is a self-operating vehicle about the size and bulk of a golf-cart. To fit into the confines of the launch rocket, the machine is folded up in an intricate way, like origami. It is enclosed by the lander module, itself folded up to protect the rover inside. The lander is surrounded by the descent module to protect it during the descent from orbit to the Mars surface. He detailed the problem of designing the parachute. After many failures in testing, a design that worked was installed on the descent module. It, for both rovers, worked perfectly. A lesser problem came with the airbags. The often in testing burst and lost their cushion. After many trails, the ones on the mission worked well.
Deployment on Mars ---------------- The descent module separated from the lander during the fall to the ground; its pieces were later found and examined by the rovers. The lander opened up like a flower to reveal the rover all curled up on top. The petals when spread out were also the off ramp that the rover used to leave the lander and reach the ground. The rover then unfurled, with precise sequence and timing. The first thing it had to do is fire up under solar power. The rovers have panels, looking like wings, that furnish all the electric and charge an internal battery. Both rovers passed the power up test. After a thoro system check out, the rover rolled off of the lander via one of the petals as a ramp, and started its exploration of Mars. The lander has no further function. The rovers do not have any business with it. In fact, there is no plan to send the rover back to the lander. The rovers will eventually expire several kilometers away.
Solar power --------- The solar panels deliver at Mars about 900 watts. The rover can operate with as little as 300 in limited capability. If the power declined to 200 watts, the rover is dead. One factor in fixing the mission at 90 days was the reasonable prospect of the panels getting covered with dust. This would block sunlight and reduce the power generation. The rover in 1997, Sojourner, died from such obstruction of sunlight from its panels by the perpetual dust swirling around on Mars. By good fortune, the wind at both Spirit and Opportunity sites was enough to repeatedly blow off the dust. Thus, the rovers have regained many times full solar power. Squyres said that future Mars exploration vehicles will use a nuclear power system.
Instruments --------- The rovers have many cameras, including a stereo pair on a mast of about man height. It has an infrared and X-ray spectrometer, a microscope, and a rock-grinder. This last was built and controlled by Honeybee company on Elizabeth St, Manhattan. The grinder has a diamond cutter head, which eventually will get dull. The grinder, called Rock Abrasion Tool or RAT, removes a 9mm diameter circle of rock off of a target to expose the interior. Then the microscope or X-ray spectrometer can be applied to the fresh face. The RAT is the one part of the rover that 'wears out'. The rovers have an articulated arm, which can hold the grinder, X- ray spectrometer, or microscope, on or near a target in front of it. The infrared spectrometer is in the rover's belly and receives input radiation thru the stereo camera. All date are radioed directly from the rover to the Odyssey satellite in Mars orbit and then relayed to Earth.
Lighttime ------- Due to the lighttime between Earth and Mars, it is tricky to run the rovers 'live', as if by a joystick and video monitor. The delay ranged from about 4-1/2 minutes for Mars at opposition to 21 minutes at superior conjunction. To guard against hazardous operation, caused by the delay in signal transfer, the rovers have internal self- decision power. The rover takes pictures with its navigation cameras and hazard avoidance cameras, 'looks' at them by itself, and figures out the need to deal with the terrain at hand. Extensive testing on Earth on terrain similar to Mars demonstrated that the rover can recognize hazardous land ahead of it and stop. It will suggest the response or let JPL send corrective maneuvering commands to it. The rover is surprising agile, able to clime slopes up to 30 degrees and pass over loose sand and gravel It is easy to tip over sideways, as proved by Earth tests.
Wheels and suspension -------------------- The six-wheel configuration is based on previous roving vehicles on the Moon and Mars. Each wheel is articulated to maintain contact with the uneven ground under it. The open mesh 'tire' also comes from prior experience. Each wheel is individually motorized for turning, spinning, reversing, and other movements. The rover can procede when there is momentarily loss of ground contact with one wheel on each side. he running speed of the rover is amazingly slow, a meter or so per minute. The videos showing the rover racing around the planet are speeded up for viewer convenience. So far there are no major breakdowns of the rover wheels or motors. The minor ones that did occur are overcome by work-arounds.
Traversing Mars ------------- The original intent was to explore the zone out to 600 meters from the lander. As it turned out, both rovers are now a couple kilometers away from their landers with no plan to come back to them. The traverse is very irregular both to circumnavigate obstacles and hazards and to examine interesting targets. There are many spans of station when the rover sleeps at night or makes a data transmission to Odyssey or to 'think' about the land ahead. Hence the clock speed ends up about three kilometers per Earth year! Opportunity bogged down in a sand trap, raising fears that it would end its mission there. JPL built a simulated sand pit with a clay and sand mixture and planted the Earth engineering model of the rover in it. To better simulate the lesser traction due to lower gravity of Mars, the model was stripped of unneeded parts to make it 1/3 of its full weight. Trials with this sand pit and model resulted in getting Opportunity out of the Mars sand and back on track to continue its explorations.
Lander inspection --------------- The rovers took time to insect the landing site and debris scattered around it. They examined the parachute, airbags and the impact marks from them, and the hear shield. All revealed valuable information about their performance and damage, for factoring into future design of these components. The lander itself was abandoned, there being no interactive function for it after the rover left it. The marks from the airbags allowed study of soil mechanics. The heat shield suffered major searing during the descent and its many layers were split apart. The chute seemed to lie still even under stiff Mars breeze.
Water on Mars ----------- Both rovers found pretty conclusive evidence that their territories were once filled with standing water, like a lake or pond. However, at first, there seemed to be little sign of past water due to the overburden of dust on the ground. Neither rover has tools for digging or drilling under the ground. Therefore, prime destinations were nearby craters, on the chance that the crater was dug out of the ground to reveal underlying structure. It was in these craters that water vestiges were found. The evidence is in many forms. These include, sediment layers, chemicals best formed in water solution, texture of surface soil, the 'blueberries spherules, topographic features like river beds and outwashes. The rovers were not looking specificly for life. They do not, for instance, detect methane or other organic substances. It did find minerals which could be made by biological processes, but exclusively so. These were found in the X-ray spectrograms of various targets in areas once filled or soaked with water. The pictures seem to indicate that the water was in place for some substantial span and then suddenly vanished. There is no certain chronology for Mars, but the water regime could have lasted many millions of years and disappeared in a couple centuries. It is generally believed that the water was gone from the planet for several billion years. The reason for the vagueness of timeline is that we still do not understand the areological processes as we do those on Earth. On the other hand, there is considerable shortterm localized alterations of the land by wind.
Meteorites -------- Among the rocks examined was a real meteorite! Like in Antarctica and deserts of Earth, it would seem plausible to find meteorites on Mars. Several were found, examined, and then left alone. Since these rocks are not native Mars material, there was no real interest to spend much time on them. The air on Mars is only about 1% the density of Earth's, so there would be far fewer 'shooting stars'. Interplanetary particles have a far better chance of surviving the fall thru Mars air and landing as meteorites.
Navigation on Mars ---------------- The rovers know their direction by taking sights on the Sun. There is no global magnetic field on Mars for ordinary marine compasses and inertial guidance may not have survived the landing and rough running. From the local alt-azim of the Sun, as gaged from a color calibration plate, doubling as a simple sundial, the rover can figure out compass directions and headings. In addition, the several orbiting probes take pictures of the rovers. They clearly show the vehicle and wheel tracks, with resolution of something under one meter. These images also help JPL plan upcoming paths to take and targets to study.
Future rovers ----------- It seems sensible to exploit the Mars rover design for future explorations. The rovers lasted far longer than ever hoped, performed amazingly well, collected immense amounts of data. with the development and experimental costs already behind NASA, is it reasonable to field an armada of similar vehicles? It definitely is, but this is not the mindset of NASA. for the next landing on mars, a totally new vehicle is under consideration. It will not be merely a beefed up rover fitted with newer instruments. The two rovers were put together once the go-ahead was given to JPL in 2000. The all-in cost was $800 million dollars, barely more than a single two-week Shuttle flight. The rovers, being built somewhat in a hurry for their launch in mid 2003, used only already proved devices and technology. There is nothing 'new' in them. At every step along the way from launch to landing to deployment to traversing, any stupid little problem could have killed the mission instantly. So far, every thing is running extremely well. Eventually the rovers must expire, even if years from now, with NASA's pledge to continue the mission as long s the rovers are alive.
After the lecture --------------- The talk ran for about an hour and half, partly due to the interest expressed by the audience and diversions to explain some extra feature of the rovers. Squyres stayed for questions and discussion, much of it offered by NYSkiers. He spent time to elaborate his answers and even took into consideration technical suggestions from the audience. ! Outside the classroom Coliseum Books offered Squyres's book 'Roving Mars' for sale. There was no formal book signing. Those who had the book, either purchased on the spot or carried from home, got Squyres to autograph it. We hung around for over a half-hour, until the Library crew started to close the building. It had to repeatedly remind that time's up. It were litterally sweeping the floor around us huddled with Squyres.