Exploring Mars on wheels
THE MISSION "MARS EXPLORATION ROVERS": EXPLORING MARS ON WHEELS
By virtue of its size, chemical composition and climate, Mars is the known planet more like our home planet, Earth. Thus, since the Space Age began in 1957 it has been the target most explored by interplanetary missions. No less than 17 Soviet missions, 17 other American missions, and even Europe and Japan have tried to discover the secrets of Mars. But it is a difficult art: about 40% of these spacecraft have been lost.
Still, those which have succeeded have rewritten entire encyclopedias on our neighbor. The first successful mission, the flyby probe Mariner 4, of the United States of America, transmitted in 1964 the first 22 photographs from close range, where they appear a multitude of huge craters like those of the Moon. It was bad news, since it means that Mars has almost no erosion, that is, it has been a desert without rain for a very long time. But the mood improved in 1971 when Mariner 9 became the first artificial satellite to orbit another planet. Among its 7000 photographs appear what look like dry riverbeds, many forming deltas, which means that yes there was rain and liquid water on the martian surface, although in some distant past.
In 1976 came the Viking spacecraft (two orbiters and two landers) which gave us the first complete picture of Mars, including photos and studies from its very surface. And also came the first analysis in search of living organisms: by the results, Mars is more inhospitable than the most inhospitable place on Earth, Antarctica.
After a break of two decades, in 1996 NASA decided to hold a series of simple missions in a systematic way, each time Earth and Mars get close together, which by their orbital motions occurs every 26 months. Each mission includes relatively small spacecraft designed to answer specific questions.
In the long term, NASA hopes that this series of missions will help answer the following: Is there, or was, life on Mars? How is the climate of Mars? How is the geology of Mars? How to explore Mars with human beings?
The first of these simple missions was Mars Pathfinder, with the recalled rover Sojourner, which traveled a hundred meters over martian ground. For the fourth season in 2003, it was decided to extend this concept and create rovers that can carry instruments for km over alien land.
SELECTING THE BAGGAGE
So, NASA called for bids for the 820-million dollars contract for the "Mars Exploration Rovers". The best proposal was by a team led by Cornell University, whose objectives include mineralogical analysis and maps, studying geological processes, comparing these down-on-the-ground data with those obtained in turn by the orbiters up high, for calibration purposes; research of ferrous minerals that would betray the former presence of water, petrographic analysis, and climate studies.
Cornell scientists prepared high-quality electronic cameras with different color filters, for analyzing chemical compositions via the light. A couple of these cameras, mounted at a certain height on a pole, can make three-dimensional images. Also, a periscope directs light to the base, where there is another camera, infrared, prepared by Arizona State University. Comparing the infrared colors with the infrared colors of terrestrial rocks it is possible to know whether certain minerals that exist here also exist on Mars. The U.S. Geological Survey provided a low-power microscope. The Max Planck Institute for Chemistry, Germany, provided an X-ray and alpha particles spectrometer, to detect and identify different chemical elements in the rocks. Johannes Gutenberg University, Mainz, Germany was responsible for a spectrometer that by a so-called Mössbauer effect could detect ferrous minerals and infer the moisture level at ancient times. To analyze the interior of rocks, the company Honeybee Robotics manufactured a special polisher drill. The University of Copenhagen, Denmark, supplied calibrated magnets of different strengths, to "filter" selectively the various types of ferrous particles in the dust blown by the winds. A robotic arm allows bringing some of the instruments close to the rocks.
The electronic components are stored in a box thermally insulated from the cold martian environment. In that box there are also batteries, radio equipment, accelerometers and gyroscopes to measure the movements and vehicle control computers. Six motorized wheels were attached to this box, and then the mast with the cameras and weather sensors. Solar panels provide electricity to make things work. Different antennae permit receiving commands from the Earth and returning the results of the observations back.
The robotic vehicle so assembled is 1,6 m long, 1,5 m height and has mass of 174 kg.
The California Institute of Technology prepared two identical vehicles. A 9-year-old Russian immigrant girl won an essay contest and named them "Spirit" and "Opportunity".
For the trip to Mars, the wheels and solar panels are folded along the body, and each rover was placed in a "flower" made of aircraft materials, that when closed looks like a three-sided pyramid. To save weight, the use of a complicated descent engine were avoided and instead on each side of the pyramid airbags were placed to cushion the fall. A parachute was also provided, though it would be of little help because the martian air has the density of the Earth's at 30 km altitude; it would be aided by solid-fuel cartridges. This "flower" in turn was placed inside a ceramic capsule to protect it from the intense heat when it hits the atmosphere of Mars falling from outer space. A radar system and associated accelerometers, gyroscopes and computers would control the activation and opening of the different components for the automatic landing.
Each ceramic capsule, sterilized and sealed, received a bulky ring with solar photocells, maneuvering engines and navigation electronics to do the cruise from Earth to the next planet.
Each of the spacecraft was placed on the tip of a three-stage Boeing Delta II rocket, of nearly 40-meters high and over 100 tons. "Spirit" was launched toward Mars on 10 June 2003 and "Opportunity" followed on 7 July 2003.
STROLLING ON MARS
The location chosen for landing "Spirit" was the plain Meridiani Planum, where it arrived on 4 January 2004. "Opportunity" was directed to the big Gusev Crater, where it fell on 25 January 2004. Both places are near the equator, although on opposite sides of the planet, and are believed to have been places where water once existed.
As communication with Earth, through giant antennas of 70 m in Australia, Spain and California, have a delay of 20 minutes by the going out and coming in of the signal between the two planets, the vehicles have sophisticated automated driving systems. Different front-and-rear stereo cameras take pictures that are analyzed by the electronic brain in search of the best way forward, without the intervention of ground controllers. Robots move with cautious little steps of 1 meter each time, but are capable of traveling 100 meters per day.
The spacecraft had a performance guarantee of 90 days, but ended up lasting many years. "Spirit" covered almost 8 km until 25 May 2011, and "Opportunity" more than 45 km with a last communication on 10 June 2018, for a lifespan of almost 15 years , a real feat in this rough terrain many millions km away from the nearest mechanic.
These vehicles have dodged stones, fought dust, climbed mountains and even descended to the bottom of small craters. They have sent more than 340 thousand pictures of a bleak desert landscape. And most important: with their sophisticated instruments they have detected that, in the distant past of planet Mars, there was indeed water on its surface.
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Based on a lecture given at the USP, on 12 June 2004. Originally published in ABC Color, on 25 March 2007. Photo: The "postcard" or minipanorama was taken by NASA's rover Spirit on the martian day, or sol, 582 (23 August 2005), just as the rover was completing its intrepid climb to Husband Hill. The summit appears to be a windswept plain with scattered rocks, little sand dunes and small bedrock outcrops. The view here that takes our breath away is heading North, looking down to the alluvial formations and bedrock outcrops of the "Tennessee Valley", a region that Spirit could not visit during its climb to the top of the hill.
The postcard, near-real colors, covers about 90 degrees and consists of images obtained by the rover's panoramic camera during 18 individual shots in different directions. Every time it pointed a target, the rover used three of its panoramic filters (600, 530 and 480 nanometers). Photo credit: NASA / JPL-Caltech / Cornell.