MER-B (Mars Exploration Rover - B), known as Opportunity, is the second of the two rovers of NASA's Mars Exploration Rover Mission. It landed successfully at Meridiani Planum on Mars on January 25, 2004 05:05 Ground UTC (circa 13:15 local time), three weeks after its twin Spirit (MER-A) had landed on the other side of the planet. Its name was chosen through a NASA-sponsored student essay competition. The rover has continued to function effectively over fifteen times longer than NASA planners expected, allowing it to perform extensive geological analysis of Martian rocks and planetary surface features; as of 2008 its mission is ongoing. An archive of updates on its status can be found at the NASA/JPL website
During the next two decades, NASA will conduct several missions to address whether life ever arose on Mars. The search begins with determining whether the Martian environment was ever suitable for life. Life, as we understand it, requires water, so the history of water on Mars is critical to finding out if the martian environment was ever conducive to life. Although the Mars Exploration Rovers do not have the ability to detect life directly, they are offering very important information on the habitability of the environment in the planet's history.
Opportunity (and its twin, Spirit) are six-wheeled, solar-powered robots standing 1.5 m (4.9 ft) high, 2.3 m (7.5 ft) wide and 1.6 m (5.2 ft) long and weighing 180 kg (400 lb). Six wheels on a rocker-bogie system enable mobility over rough terrain. Each wheel has its own motor, the vehicle is steered at front and rear and is designed to operate safely at tilts of up to 30 degrees. Maximum speed is 50 mm/s (2 in/s) although average speed is about a fifth of this.
Solar arrays generate about 140 watts for up to four hours per Martian day (sol) while rechargeable lithium ion batteries store energy for use at night. Opportunity's onboard computer uses a 20 MHz RAD6000 CPU with 128 MB of DRAM, 3 MB of EEPROM, and 256 MB of flash memory. The rover's operating temperature ranges from −40 °C to +40 °C (−40 °F to 104 °F) and radioisotope heaters provide a base level of heating, assisted by electrical heaters when necessary. A gold film and a layer of silica aerogel provide insulation.
Communications depends on an omnidirectional low-gain antenna communicating at a low data rate and a steerable high-gain antenna, both in direct contact with Earth. A low gain antenna is also used to relay data to spacecraft orbiting Mars.
Fixed science instruments include
The rover arm holds the following instruments
The cameras produce 1024-pixel by 1024-pixel images, the data is compressed, stored, and transmitted later.
The primary surface mission for Opportunity was planned to last 90 sols. The mission has received several extensions and as of November 17, 2006 had passed 1,000 sols. From its initial landing, by chance, into an impact crater amidst an otherwise generally flat plain, Opportunity has successfully investigated soil and rock samples and taken panoramic photos of its landing site. Its sampling allowed NASA scientists to make hypotheses concerning the presence of hematite and past presence of water on the surface of Mars.
Following this, it was directed to travel across the surface of Mars to investigate another crater site, Endurance crater, which it investigated from June — December 2004. Subsequently, Opportunity examined the impact site of its own heat shield and discovered an intact meteorite, now known as Heat Shield Rock, on the surface of Mars.
From late April 2005 to early June of that year, Opportunity was perilously lodged in a sand dune, with several wheels buried in the sand. Over a six week period Earth-based physical simulations were performed to decide how best to extract the rover from its position without risking a permanent immobilization of the valuable vehicle. Successful maneuvering a few centimeters at a time eventually freed the rover, which resumed its travels.
Opportunity was directed to proceed in a southerly direction to Erebus crater, a large, shallow, partially buried crater and a stopover on the way south towards "Victoria" crater, between October 2005 and March 2006. It experienced some mechanical problems with its robotic arm.
In late September 2006 Opportunity reached Victoria crater and explored along the rim in a clockwise direction. In June 2007 it returned to Duck Bay, its original arrival point; in September 2007 it entered the crater to begin a detailed study.
In over two earth years of functioning, the rover traversed over 10,000 meters.
Opportunity landed in Meridiani Planum at 354.47°E 1.94°S (planetocentric coordinates), about 24 km downrange (east) of its intended target. Although Meridiani is a flat plain, without the rock fields seen at previous Mars landing sites, Opportunity rolled into an impact crater approximately 20 meters in diameter, with the rim of the crater approximately 10 meters (32 ft) from the rover. NASA Scientists were so excited about landing in a crater that they called the landing a "hole in one", but given that they were not aiming for the crater (or even knew it existed), they were simply engaging in hyperbole. Later, the crater was named Eagle crater and the landing site designated "Challenger Memorial Station. This was the darkest landing site ever visited by a spacecraft on Mars. It would be two weeks before Opportunity was able to get a better look at its surroundings.
Scientists were intrigued by the abundance of rock outcrops dispersed throughout the crater, as well as the crater's soil, which appeared to be a mixture of coarse gray grains and fine reddish grains. This sweeping look at the unusual rock outcropping near Opportunity was captured by the rover's panoramic camera. Scientists believe the seemingly layered rocks are either volcanic ash deposits or sediments laid down by wind or water. It was given the name Opportunity Ledge.
Geologists said that the layers—some no thicker than a finger—indicate the rocks likely originated either from sediments carried by water or wind, or from falling volcanic ash. "We should be able to distinguish between those two hypotheses", said Dr. Andrew Knoll of Harvard University, Cambridge, a member of the science team for Opportunity and its twin, Spirit. If the rocks are sedimentary, water is a more likely source than wind, he said.
These layered rocks measure only 10 centimeters (4 inches) tall and are thought to be either volcanic ash deposits or sediments carried by water or wind. The layers are very thin measuring just a few millimeters thick in some cases.
A picture received on February 10 (taken on Sol 16) showed that the thin layers in the bedrock converge and diverge at low angles, suggesting that some "moving current" such as volcanic flow, wind, or water formed these rocks. The discovery of these layers was significant for scientists who had planned this mission to test the "water hypothesis" rigorously.
On Sol 30, Opportunity used its Rock Abrasion Tool (RAT) for the first time to investigate the rocks around El Capitan. The image on the right-hand side shows a close up view taken after the drilling and cleaning process was complete. Due to chance, two spherules were also cut partially, and seem to show scratches and other marks made by the diamond-crusted grind tool. The black areas are artifacts of the imaging process, when parts of the picture are missing.
During a press conference on March 2 mission scientists discussed their conclusions about the bedrock, and the evidence for the presence of liquid water during their formation. They presented the following reasoning to explain the small, elongated voids in the rock visible on the surface and after grinding into it (see last two images below).
These voids are consistent with features known to geologists as "vugs". These are formed when crystals form inside a rock matrix and are later removed through erosive processes, leaving behind voids. Some of the features in this picture are "disk-like", which is consistent with certain types of crystals, notably sulfate minerals.
Additionally, mission members presented first data from the Mossbauer spectrometer taken at the bedrock site. The iron spectrum obtained from the rock El Capitan shows strong evidence for the mineral jarosite. This mineral contains hydroxyl radicals, which indicates the presence of water when the minerals were formed. Mini-TES data from the same rock showed that it consists of a considerable amount of sulfates.
The rover alternately pushed soil forward and backward out of the trench with its right front wheel while other wheels held the rover in place. The rover turned slightly between bouts of digging to widen the hole. "We took a patient, gentle approach to digging", Biesiadecki said. The process lasted 22 minutes.
The resulting trench—the first dug by either Mars Exploration Rover—is about 50 centimeters (20 inches) long and 10 centimeters (4 inches) deep. "It came out deeper than I expected", said Dr. Rob Sullivan of Cornell University, Ithaca, N.Y., a science-team member who worked closely with engineers to plan the digging.
Two features that caught scientists' attention were the clotty texture of soil in the upper wall of the trench and the brightness of soil on the trench floor, Sullivan said.
By inspecting the sides and floor of a hole it dug, Opportunity is finding some things it did not see beforehand, including round pebbles that are shiny and soil so fine-grained that the rover's microscope cannot make out individual particles.
"What's underneath is different than what's at the immediate surface", said Dr. Albert Yen, rover science team member at NASA's Jet Propulsion Laboratory, Pasadena, Calif.
Initial scientific results from the Opportunity rover were published in the journal Science
On April 30, 2004 Opportunity reached Endurance crater, which was known to have many layers of rocks. In May the rover circumnavigated the crater, and made observations with Mini-TES and the panoramic camera. The rock "Lion Stone" was investigated and found to be similar in composition to the layers found in Eagle crater.
On June 4, 2004 mission members announced their intention to drive Opportunity into Endurance, even if it should turn out to be impossible to get back out, targeting the various rock layers that were identified in the pictures from the crater rim. "This is a crucial and careful decision for the Mars Exploration Rovers' extended mission", said Dr. Edward Weiler, NASA's associate administrator for space science. Dr Squyres, principal investigator from Cornell University said: "Answering the question of what came before the evaporites is the most significant scientific issue we can address with Opportunity at this time."
A first drive into the crater was executed June 8 and Opportunity backed out again the same day. It was found that the angle of the surface was well inside the safety margin (about 18 degrees), and the full excursion towards the rock layer of interest was started. During Sols 134 (June 12), 135, and 137 the rover drove deeper and deeper into the crater. Although some wheel slip was observed, driving was discovered to be possible even at slope angles up to 30 degrees.
Opportunity spent roughly 180 sols inside the crater, before backing out of it again in late December 2004. Scientific results of the sedimentary geology of the crater were published in the journal Earth and Planetary Science Letters
After exiting Endurance crater, in January 2005 Opportunity went to examine its own discarded heat shield. While in the vicinity of the heat shield, it happened to come upon an object which was immediately suspected and soon confirmed to be a meteorite. The meteorite was promptly named Heat Shield Rock, and is the first meteorite identified on another planet (although the Bench Crater and Hadley Rille were found on the Moon)
After about 25 Sols of observations Opportunity headed south for a crater named Argo, nearly 300 m from the heat shield.
Opportunity reached Vostok Crater on Sol 399, finding it mostly filled with sand and lined by outcrops. It was then ordered south into what has been called "etched terrain", to search for more bedrock.
By Sol 415, Opportunity stopped by some soil ripples to investigate the differences between soil in the trough of a ripple and its crest. Various soil targets included "Mobarak" in the trough, named in honor of Persian New Year, and "Norooz" and "Mayberooz" on the crest. By Sol 421, the rover left the ripple for "Viking" crater.
The rover's condition was simulated on Earth prior to any attempt to move, out of concern that the rover might become permanently immobilized. After various simulations intended to mimic the properties and behavior of Martian sand were completed, the rover executed its first wheel movements on May 13, 2005 (Sol 463), intentionally advancing only a few centimeters, after which mission members evaluated the results.
During Sol 465 and 466 more drive commands were executed, and with each trial the rover moved another couple of centimeters. At the end of each movement, panoramic images were acquired to investigate the atmosphere and the surrounding dune field. The sand dune escape maneuver was successfully completed on June 4, 2005 (Sol 484), and all six wheels of Opportunity were on firmer ground. After studying "Purgatory" from sol 498 to sol 510, Opportunity proceeded southwards towards "Erebus crater".
New programming to measure the percentage of slip in the wheels was successful in preventing the rover from getting stuck. Another "Purgatory"-like incident was averted on sol 603, when onboard slip check software stopped a drive after slip reached 44.5%. It proceeded over many ripples and 'half-pipes' taking photographs after each sol's journey.
On sol 628 (November 3, 2005) Opportunity woke up in the midst of a mild dust storm that lasted three days. The rover was able to drive in self protective auto-mode during the storm but could not take any post drive images. Less than three weeks later, another cleaning event cleared the dust off of the solar array so as to produce around 720 watt-hours (80% of max). On sol 649 (December 1, 2005), it was discovered the motor used to stow the robotic arm for travel was stalling. This problem took nearly two weeks to fix. Since then, the arm is only stowed for travel and is extended at night to save the arm from getting stuck.
Opportunity's "shoulder" joint has had troubles since Sol 2 (Jan. 25, 2004), the rover's second day on Mars. Engineers discovered that the heater on the shoulder azimuth joint, which controls side-to-side motion of the robotic arm, was stuck in the "on" position. Closer investigation revealed that the on-off switch had probably failed during assembly, test, and launch operations on Earth. Fortunately for Opportunity, the rover was equipped with a built-in safety mechanism called a "T-stat box" (thermostatic switch) that provided protection against overheating. When the shoulder azimuth joint, also known as Joint 1, got too hot, the T-stat switch automatically opened and temporarily disabled the heater. When the joint got cold again, the T-stat closed. As a result, the heater stayed on all night but not all day.
The safety mechanism worked until Opportunity approached the first winter on Mars. As the Sun began to retreat lower in the sky and solar power levels dropped, it became clear that Opportunity would not be able to keep the batteries charged with a heater draining power all night long. On Sol 122 (May 28, 2004), rover operators began using a procedure known as "deep sleep," during which Opportunity disconnected the batteries at night. Deep sleep prevented the stuck heater (and everything else on the rover except the clock and the battery heaters) from drawing power. When the Sun came up the next morning and sunlight began hitting the solar arrays, the batteries automatically reconnected, the robotic arm became operational, the shoulder joint warmed up, and the thermostatic switch opened, disabling the heater. As a result, the shoulder joint was extremely hot during the day and extremely cold at night. Such huge temperature swings, which tend to make electric motors wear out faster, were taking place every sol.
This strategy worked for Opportunity until Sol 654 (Nov. 25, 2005), when the Joint-1 azimuth motor stalled because of increased electrical resistance. Rover operators responded by delivering higher-than-normal current to the motor. This approach also worked, though Joint 1 continued to stall periodically. Typically, the rover's handlers simply tried again the next sol and the joint worked. They determined that the Joint-1 motor stalls were most likely due to damage caused by the extreme temperature cycles the joint experienced during deep sleep. As a precaution, they started keeping the robotic arm out in front of the rover overnight, rather than stowing it underneath the rover deck, where it would be virtually unusable in the event of a Joint-1 motor failure. They stowed the arm only while driving and unstowed it immediately at the end of each drive.
This strategy worked for Opportunity until Sol 1502 (April 15, 2008), when the motor stalled at the beginning of an unstowing operation at the end of a drive, when the arm was still tucked underneath the rover. The motor continued to stall on all subsequent attempts, sol after sol. Engineers performed tests at various times of day to measure electrical resistance. They found that the resistance was lowest (essentially normal) when the joint was at its warmest -- in the morning, following deep sleep, after the heater had been on for several hours, and just before the T-stat opened. They decided to try to unstow the arm one more time under these conditions.
At 08:30 Mars time on Sol 1531 (May 14, 2008), they allowed Opportunity to direct as much current as possible to the warm, joint-1 azimuth motor in order to get the robotic arm into a usable position, in front of the rover. It worked.
Because Opportunity will likely never again stow the robotic arm, engineers are working on a strategy for driving the rover safely with the arm deployed in front.
Opportunity observed numerous outcroppings around Erebus crater. It also collaborated with ESA's Mars Express by using the miniature thermal emission spectrometer and panoramic camera, and took images of a transit across the sun by Phobos. On sol 760 (March 22, 2006), Opportunity began the journey to its next destination, "Victoria" crater.
Victoria crater is a massive impact crater approximately 7 kilometers from the original landing site. Victoria's diameter is six times larger than Endurance crater. Scientists believed that rock outcrops along the walls of Victoria would yield more information about the geologic history of Mars, if the rover survived long enough to investigate them.
On Sol 951 (September 26, 2006) Opportunity reached the rim of Victoria Crater and transmitted the first substantial views of Victoria, including the dune field at the bottom of the crater. The Mars Reconnaissance Orbiter recently photographed Opportunity at the rim of the crater.
A series of cleaning events beginning on Sol 1151 (2007-04-20) have allowed Opportunity's solar energy production to rise to above 800 watt-hours per Sol. By Sol 1164 (2007-05-04) the solar array current was peaking above 4.0 amps, values not seen since Sol 18 (2004-02-10). However, with the recent advent of extensive dust storms on Mars (in-line with Mars' six Earth-year global dust storm cycle), power levels have dropped to 280 watt-hours.
Towards the end of June, 2007, a series of dust storms began clouding the Martian atmosphere with dust. The storms intensified and by July 20, both Opportunity and Spirit were facing the real possibility of system failure due to lack of power. NASA released a statement to the press which said (in part) "We're rooting for our rovers to survive these storms, but they were never designed for conditions this intense" . The key problem caused by the dust storm was a dramatic reduction in solar power. There was so much dust in the atmosphere that it blocked 99 percent of direct sunlight to the rover. The Spirit rover, on the other side of the planet was getting slightly more sunlight than Opportunity.
Normally the solar arrays are able to generate about 700 watt-hours of energy per day. During the storms, the power generated was greatly reduced. If the rovers get less than 150 watt-hours per day they have to start draining their batteries. If the batteries run dry, key electrical elements are likely to fail due to the intense cold. On July 18, 2007, the rover's solar-panel only generated 128 watt-hours, the lowest level ever. NASA responded by commanding Opportunity to only communicate with Earth once every three days, the first time this has happened since the start of the mission.
The dust storms continued through July and at the end of the month, NASA announced that the rovers, even under their very-low-power mode were barely getting enough energy to survive. If the temperature of the Opportunity's electronics module continued to drop, according to the announcement, "there is a real risk that Opportunity will trip a low-power fault. When a low-power fault is tripped, the rover's systems take the batteries off-line, putting the rover to sleep and then checking each sol to see if there is sufficient available energy to wake up and perform daily fault communications. If there is not sufficient energy, Opportunity will stay asleep. Depending on the weather conditions, Opportunity could stay asleep for days, weeks or even months, all the while trying to charge its batteries with whatever available sunlight there might be. It was quite possible that the rover would never wake up from a low-power fault.
By August 7 2007 the storms appeared to be weakening, and although power levels were still low they were sufficient for Opportunity to begin taking and returning images. By August 21 dust levels were still improving, the batteries were fully charged and Opportunity was able to make its first drive since the dust storms began.
The rover exited the crater on August 29, 2008 after experiencing a current spike similar to the one that preceded the malfunction of twin Spirit's right front wheel. It will now investigate sets of "dark cobbles" on the Meridiani plains on its journey to Endeavour crater.
Endeavour is a 22 km wide crater to the south-east of Victoria. Rover drivers estimate the 12 km journey could take about two years at current speed. Scientists expect to see a much deeper stack of rock layers at the crater than those examined by Opportunity in Victoria.