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Mars Exploration Rovers - Mission Overview
NASA's Mars Exploration Rover Project will deliver two mobile laboratories to the surface of Mars for robotic geological fieldwork, including the examination of rocks and soils that may reveal a history of past water activity. Sequences of launch, cruise and arrival operations are dispatching each rover to a different area of the planet three weeks apart to explore those areas for about three months each. The twin rovers, Spirit and Opportunity, can recognize and maneuver around small obstacles on their way to target rocks selected by scientists from images sent by the rovers. They will conduct unprecedented studies of Mars geology, such as the first microscopic observations of rock samples. They will provide "ground truth" characterization of the landing vicinities that will help to calibrate observations from instruments that view the planet from above on Mars orbiters. NASA selected the sites to be explored, Gusev Crater and Meridiani Planum, from 155 potential locations as the two offering the best combination of safe landing potential and scientific appeal in assessing whether liquid water on Mars has ever made environments conducive to life. While the rovers and the instruments they carry are the centerpieces of the project, each rover mission also depends on the performance of other components: the launch vehicle; a cruise stage; a system for entering Mars' atmosphere, descending through it and landing; a versatile system for deep-space communications; Earth facilities for data processing; and an international team of engineers, scientists and others. Launch The two rover spacecraft were lofted on three-stage Delta II rockets from Florida's Cape Canaveral Air Station. Spirit was launched on June 10, 2003, from Canaveral's Space Launch Complex 17A on a version of the Delta II known as model 7925. Opportunity's launch on July 7, 2003, from Launch Complex 17B used a newer, slightly more powerful version called model 7925H; the H identifies the vehicle as a heavy lifter. Interplanetary Cruise and Approach to Mars Following launch, each spacecraft has spent several months en route to Mars. During this cruise and the approach to Mars, each spacecraft has been attached to a cruise stage that will be jettisoned in the final minutes of the flight. Solar panels on the cruise stage provide electricity for the spacecraft in flight. Flight teams at NASA's Jet Propulsion Laboratory, Pasadena, Calif., have prepared and commanded trajectory correction maneuvers, instrument checkouts and other activities while the twin spacecraft have been speeding toward Mars. The trajectory corrections are carefully calculated firings of thrusters located on the cruise stage to make planned adjustments to the spacecraft's flight path. Each rover spacecraft is scheduled to perform four or five maneuvers, with an optional final maneuver on arrival day if needed to tweak landing targeting. The trajectory correction maneuvers are as follows:
Like NASA's Mars Odyssey orbital mission, the Mars Exploration Rover project is supplementing two traditional tracking schemes with a relatively new triangulation method to improve navigational precision. One of the traditional methods is ranging, which measures the distance to the spacecraft by timing precisely how long it takes for a radio signal to travel to the spacecraft and back. The other is Doppler, which measures the spacecraft's speed relative to Earth by the amount of shift in the pitch of a radio signal from the craft. The newer method, called delta differential one-way range measurement, adds information about the location of the spacecraft in directions perpendicular to the line of sight. Pairs of antennas at Deep Space Network sites on two different continents simultaneously receive signals from the spacecraft, then use the same antennas to observe natural radio waves from a known celestial reference point, such as a quasar. Successful use of this triangulation method is expected to shave several kilometers or miles off the amount of uncertainty in delivering the rovers to their targeted landing sites. The months in which Spirit and Opportunity have been traveling from Earth to Mars have also provided time for testing critical procedures, equipment and software in preparation for arrival. Entry, Descent and Landing The Mars Exploration Rovers will use the same airbag-cushioned landing scheme that successfully delivered Mars Pathfinder to the Red Planet in 1997. About 84 minutes before entering Mars' atmosphere, each rover spacecraft will begin a 14-minute partial rotation to orient its heat shield forward. From that point until the rover deploys its own solar panels after landing, five batteries mounted on the lander will power the spacecraft.
The planned sequence of events for entering the atmosphere, descending and landing is essentially the same for each of the two rover missions, though the operation will take several seconds more for Spirit because its landing target is at a slightly lower elevation than Opportunity's. On both spacecraft, 15 minutes before atmospheric entry, the protective aeroshell encasing the lander and rover will separate from the cruise stage, whose role will at that point be finished. Each cruise stage will ultimately impact Mars. Each spacecraft will hit the top of the atmosphere, about 128 kilometers (80 miles) above Mars' surface, at a flight path angle of about 11.5 degrees and a velocity of about 5.4 kilometers per second (12,000 miles per hour). Although Mars has a much thinner atmosphere than Earth does, the friction of traveling through it will heat and slow the spacecraft dramatically. The surface of the heat shield is expected to reach a temperature of 1,447 C (2,637 F). By 4 minutes after atmospheric entry, speed will have decreased to about 430 meters per second (960 miles per hour).
At that point, about 8.5 kilometers (5.3 miles) above the ground, the spacecraft will deploy its parachute. Within 2 minutes, the spacecraft will be bouncing on the surface, but those minutes will be packed with challenging events crucial to the mission's success. Twenty seconds after parachute deployment, the spacecraft will jettison the bottom half of its protective shell, the heat shield, exposing the lander inside. Ten seconds later, the backshell, still attached to the parachute, will begin lowering the lander on a tether-like bridle about 20 meters (66 feet) long. Spooling out the bridle to full length will take 6 seconds. Almost immediately, a radar system on the lander will begin sending pulses toward the ground to measure its altitude. Radar will detect the ground when the craft is about 2.4 kilometers (1.5 miles) above the surface, approximately 35 seconds before landing. The Mars Exploration Rover design has two new tools, absent on Mars Pathfinder, to avoid excessive horizontal speed during ground impact in case of strong winds near the surface. One is a downward-looking camera mounted on the lander. Once the radar has sensed the surface, this camera will take three pictures of the ground about 4 seconds apart and automatically analyze them to estimate the spacecraft's horizontal velocity. The other innovation is a set of three small transverse rockets mounted on the backshell that can be fired in any combination to reduce horizontal velocity or counter-act effects of side-to-side swinging under the parachute and bridle. Eight seconds before touchdown, gas generators will inflate the lander's airbags. Two seconds later, the three main deceleration rockets on the backshell -- and, if needed, one or two of the transverse rockets -- will ignite. After 3 more seconds, when the lan-der should be about 10 to 15 meters (33 to 49 feet) above ground and have zero verti-cal velocity, its bridle will be cut, releasing it from the backshell and parachute. The airbag-protected lander will then be in free fall for a few seconds as it drops toward the ground. The first bounce may take the airbag-protected lander back up to 15 meters (49 feet) or more above the ground. Bouncing and rolling could last several minutes. By com-parison, the airbag-cushioned Mars Pathfinder bounced about 15 times, as high as 15 meters (49 feet), before coming to a rest 2-1/2 minutes later about a kilometer (0.6 mile) from its point of initial impact. Twelve minutes after landing, motors will begin retracting the airbags, a process likely to take about an hour. Then the lander petals will open. No matter which of the four petals is on the bottom when the folded-up lander stops rolling, the petal-opening action will set all four face up, with the rover's base petal in the center. Opening of the petals is expected to take about 20 minutes if the spacecraft has rolled to a stop with its base petal down, about 35 minutes if one of the three side petals is down, or more than an hour if the rolling ended with the lander nose-down. Mars Surface Operations Opening of the four-sided lander will uncover the rover tucked snugly inside. Each rover's first action will be to unfold its solar-array panels. Then, still in a crouch, it will take images of the immediate surroundings with four hazard-identification cameras mounted below the plane of the solar panels. Since the rovers rely on sunlight to generate electrical power, their operations on the surface will run on a schedule timed to the length of the martian day. A martian day, or "sol," lasts 24 hours, 39 minutes and 35 seconds. Each rover will need to spend a week or more completing a series of engineering and scientific tasks before moving off its lander.
Pre-programmed actions before the first martian sunset -- about four hours after landing for each rover -- include taking stereo wide-angle pictures with the hazard-identification cameras on the front and back of the rover, raising the camera mast and begin-ning to take navigation camera images in each direction around the rover. If time per-mits, the higher resolution panoramic camera may also take images that first day on the surface. After the first-day images are transmitted to Earth, possibly on the follow-ing day, they will help engineers begin to plan the safest route for the rover's later departure from the lander. The rover will also use the panoramic camera to locate the Sun in the sky, allowing it to calculate its orientation and point its high-gain antenna toward Earth. Each rover goes through several stages in rising from its crouching position to stand at its full height while still on the lander base petal. First it unfolds its front wheel assembly. Then a lift mechanism raises the rover so its suspension rockers can drop and latch into deployed position. The rover is lowered back down for a check that the sus-pension system supports its weight. Rear wheels extend. Middle wheels descend. Connecting cables are cut. The flight team sends a "go" command for each step only after thorough checks of the preceding step, so standup requires a number of days. Once the rover is at its full height atop the lander platform, it will take a 360-degree high-resolution, stereo, color panorama with its panoramic camera and a matching 360-degree panorama with its miniature thermal emission spectrometer before moving off the lander. Scientists will rely heavily on those images to decide which rocks and soils the rover should go examine. Unlike Mars Pathfinder, when each Mars Exploration Rover rolls off its lander, the lander's role in the mission will have ended. A new chapter in Mars exploration will begin. In the next few sols after roll-off, the rover will finish checking and calibrating its science instruments and move to whichever nearby rock or patch of soil the science team has selected as the first target by analyzing the panoramic and infrared images taken earlier. The rover will examine each target up close, then begin moving on the follow-ing sol toward its next target. Its maximum travel in one day will likely be about 20 meters (approximately 65 feet), possibly less if the landing region is rough or more if the region is quite smooth. The rover will cover less than the maximum on most travel days as it maneuvers itself to avoid hazards on the way. To coordinate their work with the rovers, flight team engineers and scientists operating the rovers from NASA's Jet Propulsion Laboratory in Pasadena, Calif., will be living on a martian schedule, too. The nearly 40-minute difference from Earth's day length means that, by about two weeks after the rovers land on Mars, team members' wake-up times and meal times will have shifted by about 9 hours. After the second rover reaches Mars, its team will be working on a different martian schedule that the first rover's team because the two chosen landing sites are about halfway around Mars from each other. When it's noon at Meridiani, it's midnight at Gusev. Each rover will typically transmit each sol's accumulation of data in the martian afternoon. The flight team will analyze that data, refine plans for the next sol's rover activity, and send updated commands to the rover the next martian morning. Each rover has a prime-mission goal of operating for at least 90 martian sols (92 Earth days) after landing, though environmental conditions such as dust storms could cut the mission shorter. Mars' distance from the Sun varies much more than Earth's does, and Mars will have passed the closest point to the Sun in its 23-month elliptical orbit about 5 months before the rovers arrive. The distance between Mars and the Sun will therefore increase by about 7 percent between mid-January and mid-April 2004, resulting in two principal consequences for how long the rovers can keep working. The rovers land at the end of summer in Mars' southern hemisphere, and with the onset of autumn the decreasing intensity of solar radiation reaching their solar panels will lessen the amount of electrical power produced. Also, colder nights will increase the need for electrically powered heating to keep the batteries warm enough to work. On top of those factors, a less predictable but possibly most important element limiting the rovers' lifetime will be the accumulation of dust on their solar panels. Communications Like all of NASA's interplanetary missions, the Mars Exploration Rover project will rely on the agency's Deep Space Network to track and communicate with both spacecraft. During the critical minutes of arrival at Mars, Spirit and Opportunity will transmit essential spacecraft-status information throughout their atmospheric entry, descent and land-ing. On the surface of Mars, the rovers will be capable of communicating either direct-ly with Earth or through Mars orbiters acting as relays. The distance between Earth and Mars will increase by about 65 percent between mid-January and mid-April 2004, reducing the rate at which data can be transmitted across space. The Deep Space Network, which will be 40 years old on December 24, 2003, transmits and receives radio signals through large dish antennas at three sites spaced approxi-mately one-third of the way around the world from each other. This configuration ensures that spacecraft remain in view of one antenna complex or another as Earth rotates. The antenna complexes are at Goldstone in California's Mojave Desert; near Madrid, Spain; and near Canberra, Australia. Each complex is equipped with one antenna 70 meters (230 feet) in diameter, at least two antennas 34 meters (112 feet) in diameter, and smaller antennas. All three complexes communicate directly with the control hub at NASA's Jet Propulsion Laboratory, Pasadena, Calif. The network served more than 25 spacecraft in 2002. The network has been preparing to deal with an extraordinary level of demand for interplanetary communications in late 2003 and early 2004. Several missions besides Spirit and Opportunity will be conducting critical events. Among others, the European Space Agency's Mars Express will enter Mars orbit after dropping the Beagle 2 lander to the surface; Japan's Nozomi orbiter will be arriving at Mars; NASA's Stardust spacecraft will fly by a comet; and NASA's Cassini spacecraft will be nearing its mid-2004 arrival at Saturn. The Deep Space Network has upgraded the capabilities of its anten-nas at all three complexes and added a new 34-meter antenna at the Madrid complex. That new antenna alone adds about 70 hours of spacecraft-tracking time per week during the periods when Mars is in view of Madrid. During each Mars Exploration Rover mission's early cruise phase, a low-gain antenna mounted on the cruise stage provided the communications link with Earth. A low-gain antenna does not need to be pointed as precisely as a higher-gain antenna. During early cruise it would have been difficult to keep an antenna pointed at Earth and the solar panels oriented toward the Sun, due to the Sun-Earth angle at that stage of the mission. Later in the cruise toward Mars, the angle between the Sun and Earth shrank, making it possible for the spacecraft to switch to a more directional medium-gain antenna, also mounted on the cruise stage. Data transmission is most difficult during the critical sequence of atmospheric entry, descent and landing activities, but communication from the spacecraft is required during this period in order to diagnose any potential problems that may occur. Minutes before the spacecraft turns to point its heat shield forward in preparation for entering Mars' atmosphere, the cruise stage's low-gain antenna will take over again, which will reduce the data transmission rate to 10 bits per second, less than 2 percent of the mid-gain antenna's rate. Through this antenna and later through other low-gain antennas on the backshell, lander and rover, transmissions during the next hour or more will consist of simple signal tones coded to indicate the accomplishment of critical activities. For example, a change in tone might tell controllers when the spacecraft has successfully jettisoned its cruise stage about 15 minutes before hitting the atmosphere. During the descent through the atmosphere, about 36 ten-second signal tones will be transmitted. The signal-to-noise ratio for these tones is very low, especially as the spacecraft gets deeper into Mars' atmosphere. Some or all could be undetectable on Earth or require lengthy processing before they can be identified. Before its first night on the surface of Mars, each rover may deploy its high-gain antenna for use the following martian morning. Once a high-gain link is established, the rover may be able to communicate directly with Earth at transmission rates greater than 11,000 bits per second. About a minute before each lander drops to the martian surface, another important communication method -- relay through Mars orbiter spacecraft -- will begin to be used. An antenna mounted on each lander will transmit status information to the orbiting Mars Global Surveyor from the time the descending lander emerges from the backshell until ground impact. If that antenna survives the first bounce, it will continue to relay information for a few minutes as the lander bounces and rolls to a stop. The orbit of Mars Global Surveyor will be adjusted in preceding weeks to place it over the landing vicinity during those crucial minutes to receive the transmissions. The orbiter will later transmit the data to Earth. This first relay link is designed to provide information for later reconstruction of the events during the spacecraft's descent and landing, not to provide immediate play-by-play notice as they happen. Throughout each rover's surface mission, a rover-mounted antenna will be able to communicate with Mars Global Surveyor and Mars Odyssey for several minutes once or twice per sol while each of the two orbiters pass overhead via a UHF link at 128,000 bits per second. Plans call for using direct-to-Earth communications for transmissions critical to mission success, but about half the total data returned from the rovers could be relayed via the orbiters. One engineering goal for the project is to demonstrate relay capability at least once with the European Space Agency's Mars Express orbiter, which is due to begin circling Mars in December 2003. Planetary Protection Requirements In the study of whether Mars has had environments conducive to life, precautions are taken against introducing microbes from Earth. The United States is a signatory to an international treaty that stipulates that exploration must be conducted in a manner that avoids harmful contamination of celestial bodies. The primary strategy for preventing contamination of Mars with Earth organisms is to be sure that the hardware intended to reach the planet is clean. Each Mars Exploration Rover spacecraft complied with requirements to carry a total of no more than 300,000 bacterial spores on any surface from which the spores could get into the martian environment. Technicians assembling the spacecraft and preparing them for launch fre-quently cleaned surfaces by wiping them with an alcohol solution. The planetary pro-tection team carefully sampled the surfaces and performed microbiology tests to demonstrate that each spacecraft meets requirements for biological cleanliness. Components tolerant of high temperature, such as the parachute and thermal blanket-ing, were heated to 110 C (230 F) or hotter to kill microbes. The core box of each rover, containing the main computer and other key electronics, is sealed and vented through high-efficiency filters that keep any microbes inside. Some smaller electronics compartments are also isolated in this manner. Another type of precaution is to be sure that other hardware doesn't go to Mars accidentally. When the Delta launch vehicle's third stage separated from the spacecraft, the two objects were traveling on nearly identical trajectories. To prevent the possibility of the third stage hitting Mars, that shared course was deliberately set so that the space-craft would miss Mars if not for its first trajectory correction maneuver, about 10 days later. |
