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Mars Exploration Rover Mission
I INTRODUCTION
Mars Exploration Rover Mission, robotic exploration of Mars carried out by the National Aeronautics and Space Administration (NASA) of the United States beginning in 2003. The primary goal of the mission was to land two highly mobile vehicles known as rovers at separate landing sites on Mars so that they could search for evidence that liquid water once existed on the planet's surface. Equipped with scientific instruments, the rovers landed at sites chosen because they were likely to provide information on whether liquid water once existed on Mars. Water is essential to life, and scientists believe that if water once existed on Mars, then life might have evolved there in the distant past. The mission was regarded as a major scientific and engineering success, resulting in important new discoveries about the so-called Red Planet, including the finding that large areas of Mars once had liquid water.
The rovers were designed, built, and tested from 2000 to 2003 by a large team led by the California Institute of Technology's Jet Propulsion Laboratory (JPL) in Pasadena, California. JPL is managed by the California Institute of Technology for NASA. The rover team included scientists and engineers from NASA, professors at U.S. and European universities, and private contractors. More than 1,000 people around the world were directly involved in the rovers' design, assembly, and software development work. Once the rovers landed on Mars, a worldwide team of several hundred people became involved in the rovers' day-to-day operations.
Assembly, test, launch, and a year of operations of each rover cost about $425 million, or about the same amount of money as it cost to make the movies Titanic (1997) and Pearl Harbor (2001). This amount was also equivalent to what it costs to launch a single space-shuttle mission. The rovers are part of a new class of less expensive NASA spacecraft, with costs similar to other recent Mars orbiters and smaller missions such as the Lunar Prospector. These missions cost hundreds of millions of dollars compared with the Viking spacecraft of the 1970s, or the more recent Hubble Space Telescope or Cassini Saturn orbiter missions, which each cost several billion dollars.
The first rover, Spirit, was launched on a Delta rocket from Cape Canaveral, Florida, on June 10, 2003. Spirit was targeted to land inside Gusev Crater, a 160-km (100-mi) wide crater in the southern hemisphere of Mars. Images and other data from orbiting spacecraft such as Viking, Mars Global Surveyor (MGS), and Mars Odyssey showed an enormous meandering channel, possibly a dry riverbed, which ends in Gusev Crater. This finding suggested that Gusev may once have been a water-filled lake. One of Spirit's primary goals was to search for evidence of whether Gusev ever contained liquid water, and if so, for how long.
The second rover, Opportunity, also part of a larger spacecraft, was launched on a similar Delta rocket from Cape Canaveral, Florida, on July 7, 2003. Opportunity was targeted to a landing site halfway around the planet from Gusev, in a smooth region just south of the equator called Meridiani Planum. Meridiani is among the flattest places on Mars and thus was judged to be a safer rover landing site than Gusev. The decision to land in Meridiani was mostly based on scientific observations from the MGS orbiter, which had detected evidence for deposits of the coarse-grained mineral hematite (Fe2O3) in this part of the planet. Coarse-grained hematite often forms on Earth in the presence of substantial amounts of liquid water. Opportunity's primary goals were to confirm the presence of hematite in Meridiani, to determine if it was formed by liquid water, and if so, to provide clues about how much water was there and how long it lasted.
After a seven-month interplanetary cruise to Mars traveling at a speed of about 19,000 km/h (12,000 mph), the spacecraft that carried each rover went through a six-minute thrill ride to decelerate and land. Each spacecraft used a heat shield to protect it from the heat of atmospheric entry, a parachute to slow the craft, retro-rockets to bring the craft to a near-hovered stop just above the surface, and finally a set of inflated airbags to cushion the blow of impact and allow the spacecraft to bounce around to a gentle stop. The Spirit spacecraft landed successfully on January 4, 2004, and the Opportunity spacecraft landed successfully on January 24, 2004. Once operators on Earth determined that it was safe to begin the mission, each spacecraft unfolded and deployed its various instruments, and the rovers rolled out on a ramp onto the surface of the planet. Spirit rolled onto the surface on January 15, and Opportunity was deployed onto the surface on January 31.
The landing procedure was similar to that of the successful 1997 Mars Pathfinder mission. However, the two identical rovers differ significantly from the earlier Sojourner rover on the Pathfinder mission. The Mars Exploration rovers are larger and heavier. Whereas Sojourner was 65 cm (2 ft) long with a mass of 10 kg (22 lb on Earth; 8 lb on Mars), each of the Mars Exploration rovers is 1.6 m (5.2 ft) long and has a mass of 174 kg (384 lb on Earth; 144 lb on Mars). The six-wheeled Mars Exploration rovers also have a suspension system that enables them to ride over rocks bigger than 26 cm (10 in) and to tilt up to about 30 degrees without turning over.
II THE ROVERS' INSTRUMENTS AND NAVIGATION
Each rover is outfitted with a suite of sophisticated scientific instruments that enable scientists to study in detail the geology and composition of materials at each landing site. Two of the instruments are mounted on the rover mast, about 1.5 m (5 ft) above the surface. One of these instruments is a high-resolution digital camera system called Pancam. The Pancam takes three-dimensional images of an area to reveal its topography and to calculate the distances to various targets of interest. It also takes color images in ultraviolet, visible, and near-infrared wavelengths for studies of Mars's rocks, soils, and atmosphere. The other instrument is an infrared spectrometer called Mini-TES. A spectrometer is a scientific instrument that disperses the light emitted or reflected by a substance into a spectrum (individual colors). Because every substance has its own unique spectrum, scientists can use a spectrometer to identify the substance and study its chemical properties. Mini-TES was used to determine the mineral composition of rocks and soils around the rovers and to study the way the atmosphere heats up and cools down as a result of daily weather patterns or occasional dust storms.
Still more instruments are mounted on a flexible robotic arm used to study rocks and soils up close. These instruments include a camera called the Microscopic Imager with a magnifying power on the scale of a handheld magnifying lens and a grinding device called the Rock Abrasion Tool (RAT) used to clean off dusty rock surfaces. The RAT can also grind a few millimeters to a centimeter (up to about half an inch) into rocks to expose their interiors for detailed study. Two different types of spectrometers are also attached to the robotic arm—an Alpha Particle X-Ray spectrometer for determining the elemental composition of rocks and soils (such as the amount of iron, silicon, aluminum, sulfur, and other chemical elements) and a M?ssbauer spectrometer for identifying iron-bearing minerals.
The rovers are also equipped with high-tech engineering systems for power, communications, temperature control, and onboard data processing. The rovers use solar panels to generate electricity during the daytime. This electricity is then used to drive and operate the instruments. Some of that power is also used to recharge a set of internal batteries that keep the rovers alive during the extremely cold Martian night when temperatures reach about -100C (-150F). These batteries also enable a limited number of nighttime scientific observations.
The rovers can communicate with Earth in three different ways. Two of the ways involve using direct-to-Earth high-gain or low-gain antenna radio systems to send data and receive instructions in "real time," or almost simultaneously. In this case, “real time” means about 10 minutes for data to be sent to Earth and another 10 minutes for instructions to be sent back to the rovers. A high-gain antenna has a narrow radiowave beam that can be directed at a specific target on Earth, while the low-gain antenna has a wider beam for a more reliable signal. The high-gain antenna can send more data to Earth than the low-gain antenna, but an even better way of transmitting data involves using an ultrahigh frequency (UHF) radio system. The UHF systems on the rovers send data to MGS or Mars Odyssey as they pass over the landing sites. Those satellites then relay the data to Earth. Because the UHF relay system can transmit much more data using significantly less power than the direct-to-Earth systems, the UHF system has been used to transmit more than 80 percent of the data from both rovers during the course of the mission.
The rovers are able to do much of their own driving. They had to be given significant onboard data processing and decision-making capabilities to do this driving. "Real time," or simultaneous, commanding or driving by JPL operators is not possible because it can take 20 minutes or more for radio signals to travel round trip from Earth to Mars. For driving, front- and rear-mounted Hazard Avoidance cameras and mast-mounted navigation cameras provide image data for onboard feature-detection and navigation software. For example, the cameras provide the images needed for the navigation software to instruct the rovers' computers on how to approach rock targets, dig shallow trenches with its wheels, or traverse 100 m (328 ft) or more per sol, or Martian day. (A Martian day is about 40 minutes longer than an Earth day.)
III A TYPICAL DAY, OR SOL
Each rover's sol begins when the Sun rises high enough in the sky to allow the solar panels to provide adequate power to operate the computer and the instruments. This usually occurs between about 9 to 10 AM local solar time on Mars. Shortly after waking up, the rover receives its daily set of instructions via a high-gain antenna communications from Earth. The rovers typically operate until 3 to 5 PM, when the solar power starts to diminish. During the afternoon, the rovers usually establish a communications link with the MGS or Mars Odyssey orbiters as they pass overhead. These orbiting satellites relay that sol's scientific and engineering data back to Earth. The rovers then shut down for most of the night (using battery power to keep the systems warm), usually waking up again only for a short time during the predawn hours to relay more data back to Earth through the orbiters.
Back on Earth, scientists and engineers analyze data communications from the rovers to ensure that the instruments and systems are operating correctly and to decide where to drive or what new scientific measurements to make the following sol. During the rover night, new commands and sequences are tested, developed, and converted by the team into the stream of radio signals that are sent to the rover after it wakes up the next sol. With two rovers working half a planet apart, running both has been a round-the-clock process for the rover operations team at JPL and other institutions around the world.
IV SPIRIT'S LANDING SITE
Spirit's landing site within Gusev Crater, at latitude 14.6 south and longitude 175.5 east, is a gently rolling plain with about 5 to 10 percent of the surface area covered by rocks. Distant features visible on the horizon include the rim of a crater created by a meteorite impact. The crater is 210 m (690 ft) in diameter and is called Bonneville. It is located about 300 m (980 ft) to the northeast of Gusev Crater. Also visible are a small set of hills, about 100 to 150 m (330 to 500 ft) high, called the Columbia Hills, which are about 3 to 4 km (about 2 to 3 mi) to the east. Isolated hills and mesas are visible about 10 to 30 km (6 to 9 mi) to the south and southwest, and even the rim of Gusev itself is faintly visible nearly 80 km (50 mi) from the landing site.
Spirit spent its first 11 sols on Mars obtaining a large set of panoramic images of the site and preparing to drive off its lander platform. Shortly after driving off and analyzing its first rock, however, the rover suffered a serious software error and the mission was nearly lost. Ground controllers were able to diagnose and repair the problem and develop procedures to avoid its recurrence; by sol 28 the rover was back in perfect health and the mission was resumed.
V SCIENTIFIC FINDINGS FROM SPIRIT
Initial analysis of the chemical elements and minerals found in materials in Gusev revealed that most of the rocks and soils around the lander were covered by a layer of the bright, iron-rich dust that gets distributed throughout the planet by the famous Martian dust storms. The RAT was used to dig through the dust and expose "fresh" rock surfaces for analysis. Those studies showed that the rocks are very similar to a class of volcanic rocks on the Earth called basalts. The scientific team on Earth then decided to drive the rover to Bonneville Crater in the hopes of discovering different kinds of rocks that may have been brought to the surface by the meteorite impact. While the terrain got much rockier by the time the rover reached the crater around sol 67, the rocks themselves continued to be mostly volcanic in nature, with only an occasional hint of water-formed minerals or coatings being discovered on some rocks.
The lack of strong evidence from Spirit for water-formed rocks, minerals, or sediments within Gusev was somewhat puzzling to scientists. This was because photographic images taken by various Mars orbiters suggested that the crater once had liquid water flowing into it and perhaps even ponding for significant periods of time. Some scientists concluded that there must have been significant volcanic activity in and around Gusev Crater after the water was there, generating a layer of volcanic materials that buried any evidence for the region's presumed watery past. In an attempt to test this hypothesis, Spirit was directed on a long trip to the Columbia Hills, where it was hoped that climbing above the volcanic deposits and onto a region that looked geologically different would reveal evidence of different geologic processes.
Spirit finally reached the base of the Columbia Hills around sol 180 (early July 2004). By this time the rover had lasted twice as long as scientists originally anticipated. On the 110th sol, after a nearly 2.5 km (1.6 mi) journey from Bonneville, the rover traveled across soil marked by dark wind streaks. It also went through small, bright, circular depressions called hollows and around several modest-sized impact craters, stopping occasionally to obtain panoramic views and to analyze the composition of the rocks and soils. As Spirit began climbing the hills in late 2004, more evidence for the action of liquid water was found in the form of unusually weathered rocks and the presence of hematite and water-formed iron oxide goethite (FeO[OH]).
VI THE OPPORTUNITY LANDING SITE
Opportunity landed in Meridiani Planum, near latitude 1.9 south and longitude 354.5 east. It landed within a tiny impact crater called Eagle that is 20 m (66 ft) in diameter and 2 m (7 ft) deep. The landing site and its surroundings are among the darkest places on Mars, within one of the classical dark markings that have been studied from telescopes on Earth for centuries. In a stroke of incredible luck, the rover landed right next to an outcrop of bright rocks projecting from the soil that looked unlike anything anyone had ever seen on Mars before.
Opportunity rolled onto the surface 7 sols after landing. It immediately began exploring the sandy materials adjacent to its landing platform. Spectroscopic data from the MGS orbiter had suggested that hematite would be found at the landing site, and scientists were quickly able to confirm the presence of large quantities of the mineral there. The discovery provided an important verification of the ability of orbital observations to predict certain characteristics of potential landing sites on Mars.
Among its first surprises the rover discovered that the ground in this area is covered by a large number of nearly spherical, millimeter- to centimeter-sized particles informally dubbed "blueberries" because they appear bluer than the surrounding soils in Pancam images. By sol 13, Opportunity was driven over to the outcrop to begin what would be more than 50 sols of detailed studies and discoveries of this material.
VII OPPORTUNITY'S SCIENTIFIC FINDINGS
The Eagle Crater outcrop is one of the most fascinating places ever explored on Mars. It provided scientists with direct and unambiguous evidence for the past presence and action of liquid water on the planet's surface. Among the main pieces of evidence is the observation that the rocks in the outcrop show abundant, fine-scale layering, suggesting the periodic deposition and erosion of sediments. On Earth layers of sedimentary rock like this are usually formed by the action of water. In some places, the layers exhibit what geologists call cross-bedding, indicating complex interactions between waves and sediments in a shallow-water marine environment.
Another important piece of evidence was the finding that the outcrop contains an enormous amount of sulfur, chlorine, and other elements characteristic of minerals formed when salty water evaporates. And finally, the “blueberries” were discovered to be hematite-rich and eroding out of the outcrop materials. Scientists concluded that these round grains are what geologists call concretions. Concretions are deposits that are formed in porous rocks saturated with water containing dissolved iron and other elements. These and other findings imply that the Eagle Crater outcrop is a sedimentary deposit of rocks formed after water that once covered this region evaporated. This process of water formation and evaporation may have occurred many times.
Scientists know that the Meridiani region, including the Eagle Crater outcrop, is an ancient area dating back to the first few billion years of Martian history. The initial Opportunity findings, however, did not reveal directly how much water once existed there, exactly when it was there, or for how long. Because water is essential to life, these are critical questions for assessing whether Mars could have been habitable for living things in the distant past.
In an attempt to explore those questions, the rover left Eagle Crater on sol 57 to explore the vast flat and dark plains of Meridiani and to attempt to reach Endurance Crater. This crater, with a 200-m (660-ft) diameter, was 25 m (82 ft) deep and 700 m (2,300 ft) from Eagle Crater. From orbital and early Pancam images, Endurance Crater appeared to contain similar bright outcrop deposits like those found at Eagle Crater.
Opportunity reached Endurance Crater on sol 95 (late April 2004) and discovered a fascinating variety of layered outcrop materials along the crater's inner walls. Studies of the upper layers revealed the same kinds of water-formed materials as at Eagle Crater, confirming that the water was regional in extent, rather than isolated to just one area. As the rover carefully inched down into the crater to study the deeper layers during a period of more than 90 sols, more of the same pieces of evidence for the watery past of this region were discovered. Although scientists have not yet completed their analysis of data from Endurance Crater, they believe liquid water may have been even more extensive in the early history of Mars than they previously thought, based on the amazing discoveries at Eagle Crater.
Opportunity continued exploring deeper into Endurance Crater for more than six months until the slopes got too steep to go further. Then, around sol 315 (mid-December 2004), the rover climbed back out and proceeded south across the plains to the site near where the rover first landed back on sol 1. From there, the rover will continue on to the next interesting target identified by the scientific team.
The Mars Exploration Rovers Spirit and Opportunity have fulfilled their primary mission objectives and have made important new discoveries about Mars. As of early 2005 both rovers continued to operate well and to obtain new data. No one knows how long these amazing robotic field geologists will continue to survive and thrive in the harsh Martian environment, but as long as they are capable of making exciting new discoveries, NASA is planning to continue operating them.
Contributed By:
Jim Bell
Mars Rover Instruments
The Mars rovers, Spirit and Opportunity, carry a variety of instruments that help the rovers navigate, take panoramic images, and send data from their scientific instruments to Earth. Depicted in this artist's rendition, for example, are the Pancam (a pair of panoramic cameras), the solar array panels that help power the rovers, and the high-gain and low-gain antennas, which send data to Earth and receive instructions from scientists on Earth.
NASA
