The 384-pound golf-cart-sized Mars Exploration Rover Spirit, the first of two identical robot geologists, launched on June 10, 2003 from Cape Canaveral, safely landed on Mars at 11:35 p.m., Eastern Standard Time, Saturday, January 3, 2004, 200 meters from target in the Gusev Crater, ending a 302.6-million-mile trip through space and a one-mile bounce in airbags on the Martian surface. Its twin, Mars Exploration Rover Opportunity, was launched July 7, 2003, and is on course for a landing in Meridiana Planum on the opposite side of Mars on January 25 (Universal Time and EST; 9:05 p.m. on January 24, PST). Both rovers are part of an $820-million Mars exploration project to find indications of life on the planet. The area in the vast flatland of the Gusev Crater where Spirit landed this weekend will be called the Columbia Memorial Station. Over the past few days, Spirit has been sending extraordinary images of its new surroundings, among them an image of a memorial plaque placed on the spacecraft to Columbia's astronauts and the STS-107 mission who perished Februrary 1, 2003, on the return trip when their shuttle exploded over Texas. The plaque is mounted on the back of Spirit's high-gain antenna, a disc-shaped tool used for communicating directly with Earth.
The first signals from Spirit were received by Deep Space Network (DSN) antennas in Australia and California. The DSN is an international network of antenna complexes, spaced about 120° apart, designed to support interplanetary space explorations and deep space radio astronomy observations with continuous communication. The first one, the Goldstone complex, is located in California's Mojave Desert, the Australian complex is located 40 kilometers southwest of Canberra, and the Spanish complex is 60 kilometers west of Madrid. Each complex consists of at least four deep space stations equipped with ultrasensitive receiving systems and large parabolic dish antennas: One 34-meter diameter High Efficiency antenna, one 34-meter Beam Waveguide antenna (three at the Goldstone Complex), one 26-meter antenna., and one 70-meter antenna.
To achieve such a precise landing on Mars, the navigator team at the Jet Propulsion Laboratory (JPL) had to calculate the exact speeds of a rotating Earth, a rotating Mars, and a rotating spacecraft while they all are spinning in their own orbits around the Sun. The navigation was so accurate that the navigators canceled two trajectory correction maneuvers scheduled to correct the spacecraft's flight path by firing rocket thrusters. According to JPL, navigators ran up to 1,000 different location accuracy solutions several times every day to cover the full range of possible answers. The team also used a tracking technique called spacecraft-quasar delta differential one-way range or DDOR (pronounced "Delta Door"), which utilized their knowledge of locations of quasars to a few billionths of a degree to help locate the spacecraft's motion in the "up or down" direction in the sky. A small 5-cm error in the measurement of the location of any of the antennas on earth is tranlated over a distance of 90,000,000 miles to a 450-meter error on Mars.
Spirit's landing site, within Gusev Crater south of the equator, was chosen based on evidence from Mars orbiters that this crater may have held a lake long ago, fed by a long, deep river. Gusev Crater, a basin the size of Connecticut created by an asteroid or comet impact early in Mars' history, has few boulders and not much dust accumulation. Spirit's task is to spend the next 90 days exploring for clues in rocks and soil about ancient signs of life-sustaining water. Inside the crater, researchers expect to find sediments, which may be nearly 3,000 feet thick. These sediments, which researchers hope were deposited by water, may have been covered by dust and sand that has blown into the crater over the past two billion years. It is believed that if there was once water in Gusev, its signature should still be there.
After the spacecraft carrying Spirit entered the thin Martian atmosphere, it started sending tones back to earth via the Mars Global Surveyor and Mars Odyssey orbiters flying by. It took six minutes (dubbed "six minutes of terror"for the suspense and stress created by complete silence in the landing sequence) for the spacecraft to slow from 12,000 miles to 0 miles. During this time, the spacecraft was on its own. The parachute was deployed, retrorockets fired, the airbags inflated, and the lander separated from the parachute to an airbag-protected landing, all controlled by onboard instruments. Bouncing to a stop the lander maintained a suspenseful moment of silence. Then at 11:51 p.m., EST, Spirit called home, to the relief of all who witnessed the historic moment at JPL mission control.
The first 3-D picture from Spirit showed a panoramic view of the Martian surface. Spirit's main antenna is oriented towards Earth for a high-speed transmission of pictures and data from Mars, and commands and software changes from the mission control at the Jet Propulsion Laboratory to Spirit. More high-resolution pictures have been released by NASA. By Tuesday, 6 Janurary 2004, 15 terabytes of pictures have been downloaded from NASA sites, equivalent to 20,000 CDs, which reach over 100 feet in height if stacked without sleeves.
For a week to nine days or longer if necessary, the flight team expects to run a series of tests of all systems on Spirit, and direct the rover through a series of steps in unfolding, standing up and other preparations necessary before the rover rolls off its lander platform to get its wheels onto the ground. Meanwhile, Spirit's cameras and a mineral-identifying infrared instrument will begin examining the surrounding terrain. That information will help engineers and scientists decide which direction to send the rover first. On January 5, scientists are examining a dust-filled depression, about 9 meters (30 feet) across and about 12 meters (40 feet) north of the lander, dubbed "Sleepy Hollow," as a possible first target for exploration.
The four science goals of Spirit are to determine whether life ever arose on Mars, to characterize the climate of Mars, to characterize the geology of Mars, and to prepare for human exploration
The rover's six wheels rest on a suspension system called "rocker-bogie" system similar to the Sojourner rover on the Pathfinder mission. A bogie is a train undercarriage with six wheels that can swivel to curve along a track. The term "rocker" designates the design of the differential, which keeps the rover body balanced, enabling it to "rock" up or down depending on the various positions of the multiple wheels. When one side of the rover goes up, the differential or rocker in the suspension system automatically makes the other side go down to even out the weight load on the six wheels. This system causes the rover body to go through only half of the range of motion that the wheels could potentially experience without a "rocker-bogie" suspension system. Though designed to withstand a tilt of 45 degrees in any direction without overturning, the rover is programmed through its "fault protection limits" in its hazard avoidance software to avoid exceeding tilts of 30 degrees during its traverses.
The rover has a top speed on flat hard ground of 5 centimeters (2 inches) per second. However, in order to ensure a safe drive, the rover is equipped with hazard avoidance software that causes the rover to stop and reassess its location every few seconds. So, over time, the vehicle achieves an average speed of 1 centimeter per second. The rover is programmed to drive for roughly 10 seconds, then stop to observe and understand the terrain it has driven into for 20 seconds, before moving safely onward for another 10 seconds.
The rover body is called the warm electronics box, or "WEB". The rover body is a strong and temperature-controlled outer layer that protects the rover´s computer, electronics, and batteries (the rover´s brains and heart). Its gold-painted, insulated walls also keep heat in when the night temperatures on Mars can drop to -96 degrees Celsius. The warm electronics box is closed on the top by a triangular piece called the Rover Equipment Deck (RED), allowing a place for the rover mast to raise the cameras 1.4 meters in the Martian air.
The rover computer is inside a module called the Rover Electronics Module (REM) inside the rover body. The communication interface that enables the main computer to exchange data with the rover´s instruments and sensors is a Versa Module Europa bus or VME, an industry standard interface bus to communicate with and control all of the rover motors, science instruments, and communication functions. The computer is composed of equipment comparable to a high-end, powerful laptop computer. It contains special memory to tolerate the extreme radiation environment from space and to safeguard against power-off cycles so the programs and data will be safeguarded against accidental erasure when the rover shuts down at night. On-board memory includes 128 MB of DRAM with error detection and correction and 3 MB of EEPROM, roughly the equivalent memory of a standard home computer. Still this onboard memory is roughly 1000 more than the Sojourner rover from the Pathfinder mission had.
The rover computer registers signs of health, temperature, and other features that keep the rover "alive." The software in the main computer of the rover changes modes once the spacecraft begins to enter the Martian atmosphere. At this time, the software executes a control loop that monitors the "health" and status of the vehicle. It checks for the presence of commands to execute, and performs communication functions. The software does similar health checks in a third mode once the rover emerges from the lander.
This main control loop essentially keeps the rover "alive" by constantly checking itself to ensure that it is both able to communicate throughout the surface mission and that it remains thermally stable at all times. It does so by periodically checking temperatures, particularly in the rover body, and responding to potential overheating conditions, recording power generation and power storage data throughout the Mars sol (a Martian day), and scheduling and preparing for communication sessions.
The neck and head part of the rover, called Pancam Mast Assembly, stands 1.4 meters in height to serve both as a periscope for the Mini-TES and as a tripod to give the cameras a "human geologist's" perspective with a wide field of view. One motor for the entire Pancam Mast Assembly head turns the cameras and Mini-TES 360º in the horizontal plane. A separate elevation motor can point the cameras 90º above the horizon and 90º below the horizon. A third motor for the Mini-TES elevation, enables the Mini-TES to point up to 30º over the horizon and 50º below the horizon
The pictures sent by Spirit are taken by a pair of panoramic cameras called Pancams, a high-resolution color stereo pair of charged-couple devices (CCD) cameras perched on the camera bar on top of the Pancam Mast Assembly (PMA) capable of revolving 360°. The camera's "eye" has a filter wheel that gives Pancam its multispectral imaging capabilities. Images taken at various wavelengths can help scientists learn more about the minerals found in Martian rocks and soils. Blue and infrared solar filters allow the camera to image the sun. These data, along with images of the sky at a variety of wavelengths, will help to determine the orientation of the rover and will provide information about the dust in the atmosphere of Mars. The Pancam is also part of the rover's navigation system. With the solar filter in place, the Pancam will be pointed at the Sun and therefore will be used as an absolute heading sensor. Like a sophisticated compass, the direction of the Sun combined with the time of day tells the flight team exactly which way the rover is facing . The Pancams have the highest resolution of any camera sent to Mars.
The rover is equipped with a robotic arm (called the instrument deployment device, or IDD) with a 350° turning range that carries four science instruments to get close-up images of rocks. The Microscopic Imager is a combination of a microscope and a CCD camera located on the arm of the rover. Its field of view is 1024 x 1024 pixels in size and it has a single, broad-band filter rendering images in black and white. The imager will provide information on the small-scale features of Martian rocks and soils, and complement the findings of other science instruments by producing close-up views of surface materials. Some of those materials will be in their natural state while others may be views of fresh surfaces exposed by the Rock Abrasion Tool. Microscopic imaging will be used to analyze the size and shape of grains in sedimentary rocks, which is important for determining whether water may have existed in the past.
Another instrument mounted on the turret at the end of the rover arm, the Mössbauer Spectrometer is an instrument specially designed to study iron-bearing minerals. It can determine the composition and abundance of these minerals to a high level of accuracy. This ability can also help scientists understand the magnetic properties of surface materials. Its sensor head is small enough to fit in the palm of your hand. A measurement, which takes 12 hours, is made by placing the sensor head against the rock or soil sample.
The Alpha Particle X-Ray Spectrometer (APXS), also on the rover arm, is designed to study the alpha particles and x-rays emitted by rocks and soils in order to determine their elemental chemistry. Alpha particles are emitted during radioactive decay. The elemental composition of Martian rocks provides information about the formation of the planet's crust as well as any weathering that has taken place. Most APXS measurements will be taken at night and will require at least 10 hours of accumulation time, although just x-ray alone will only require a few hours.
The fourth instrument located on the rover arm, which is mportant for a close-up examination of rocks and soils, is the 720-gram Rock Abrasion Tool (RAT), a powerful grinder able to create a hole 45 millimeters (about 2 inches) in diameter and 5 millimeters (0.2 inches) deep into a rock on the Martian surface. It uses three electric motors to drive two grinding wheels at high speeds. These wheels also rotate around each other at a much slower speed so that the two grinding wheels sweep the entire cutting area. The RAT is able to grind through hard volcanic rock in about two hours. Since the interior of a rock may be very different from its exterior, that difference may reveal how the rock was formed and the environmental conditions in which it was altered. A rock sitting on the surface of Mars may become covered with dust and will weather, or change in chemical composition from contact with the atmosphere.
Four black-and-white Engineering Hazcams (Hazard Avoidance Cameras) mounted on the lower portion of the front and rear of the rover use visible light to capture 3-D images. This imagery safeguards against the rover getting lost or inadvertently crashing into unexpected obstacles, and works in tandem with software that allows the rover to make its own safety choices and to "think on its own." The cameras each have a wide field of view of about 120 degrees. The rover uses pairs of Hazcam images to map out the shape of the terrain as far as 3 meters (10 feet) in front of it, in a "wedge" shape that is over 4 meters wide at the farthest distance. It needs to see far to either side because the Hazcam cameras cannot move independently being mounted directly to the rover body.
Two black-and-white Engineering Navcams (Navigation Cameras) mounted on the mast (the rover's "neck and head") use visible light to gather panoramic, three-dimensional (3D) imagery. The Navcam is a stereo pair of cameras, each with a 45-degree field of view to support ground navigation planning by scientists and engineers. They work in cooperation with the Hazcams by providing a complementary view of the terrain
The Miniature Thermal Emission Spectrometer (Mini-TES) is located in the body of the rover at the bottom of the "rover neck," known as the Pancam Mast Assembly (PMA). Its scanning mirrors lodged in the Pancam Mast Assembly act like a periscope to send light down to the instrument. This structure allows Mini-TES to see the terrain around the rover from the same vantage point as Pancam. Mini-TES looks one way, and the Pancams looks the other way. Mini-TES is an infrared spectrometer that can determine the mineralogy of rocks and soils from a distance by detecting their patterns of thermal radiation. All warm objects emit heat, but different objects emit heat differently. This variation in thermal radiation can help scientists identify the minerals on Mars. Mini-TES will record the spectra of various rocks and soils. These spectra can be studied to determine the type of minerals and their abundances at selected locations. One particular goal will be to search for minerals that were formed by the action of water, such as carbonates and clays. Mini-TES will also look at the atmosphere of Mars and gather data on temperature, water vapor, and the abundance of dust.
When fully illuminated, the rover solar arrays generate about 140 watts of power for up to four hours per sol (a Martian day). The rover needs about 100 watts to drive. Comparatively, the Sojourner rover´s solar arrays provided the 1997 Pathfinder mission with around 16 watts of power (that of an oven light) at noon on Mars. This extra power will potentially enable the Spirit rover to conduct more science.
The power system for the Mars Exploration Rover includes two rechargeable batteries that provide energy for the rover when the sun is not shining, or at night. Over time, the batteries will degrade and will not be able to recharge to full power capacity. Also, by the end of the 90-sol mission, the capability of the solar arrays to generate power will likely be reduced to about 50 watts due to anticipated dust coverage on the solar arrays (as seen on Sojourner/Mars Pathfinder), as well as the change in season. Mars will drift farther from the sun as it continues on its yearly elliptical orbit, and because of the distance, the sun will not shine as brightly onto the solar arrays. Additionally, Mars is tilted on its axis just like Earth is, giving Mars seasonal changes. Later in the mission, the seasonal changes at the landing site and the lower position of the Sun in the sky at noon than in the beginning of the mission will mean less energy on the solar panels.
For communications, the rover has direct-to-Earth low-gain and high-gain antennas that serve as both its "voice" and its "ears". They are located on the rover equipment deck (its "back").
The omni-directional low-gain antenna transmits radio waves at a low rate to the Deep Space Network (DSN) antennas on Earth. The high-gain antenna can send a "beam" of information in a specific direction and it is steerable, so the antenna can move to point itself directly to any antenna on Earth. The benefit of having a steerable antenna is that the entire rover doesn´t have to change positions to talk to Earth; it saves energy by moving only the antenna.
Not only can the rovers send messages directly to Earth, but they can uplink information to other spacecraft orbiting Mars, utilizing the 2001 Mars Odyssey and Mars Global Surveyor orbiters as messengers who can pass along news to Earth for the rovers. The orbiters can also send messages to the rovers. The benefits of using the orbiting spacecraft are that the orbiters are closer to the rovers than the Deep Space Network antennas on Earth and the orbiters have Earth in their field of view for much longer time periods than the rovers on the ground.
The radio waves to and from the rover are sent through the Mars Global Surveyor and 2001 Mars Odyssey orbiters using UHF antennas, which are close-range antennas which are like walky-talkies compared to the long range of the low-gain and high-gain antennas. One UHF antenna is on the rover and one is on the petal of the lander to aid in gaining information during the critical landing event. The Mars Global Surveyor was in the appropriate location above Mars to track the landing process. (2001 Mars Odyssey was not in the vicinity.)
A Busy Mission
When all tests and preparations are finished at mission control, the rover Spirit begins to explore the planet's surface on command from earth.
The rover is designed to travel up to 100 meters (about 328 feet) across the Martian surface each Martian day. While a complete Martian day (called a sol) is about 24 hours and 37.5 minutes long, the Sun can only provide enough power for driving during a four-hour window around high noon. That means the rover has to be able to move quickly and effectively.
Moving safely from rock to rock or location to location is a major challenge because of the communication time delay between Earth and Mars, which is about 20 minutes on average. The drivers of the rover on Mars cannot instantly see what is happening to a rover at any given moment and they cannot send quick commands to prevent the rover from running into a rock or falling off a cliff.
During surface operations on Mars, the rover receives a new set of instructions at the beginning of each sol. Sent from the scientists and engineers on Earth, the command sequence tells the rover what targets to go to and what science experiments to perform on Mars. The rover is expected to move over a given distance, precisely position itself with respect to a target, and deploy its instruments to take close-up pictures and analyze the minerals or elements of rocks and soil.
Scientists and engineers have to figure out how far the rover has traveled, use hazard avoidance software for a safe journey, create maps to help guide the rover , keep the rover right side up and balanced, understand which direction the rover is facing and traverse far and well .
The solar-powered rover's typical workday is from sunrise to sunset. A typical rover day begins with a morning wake-up that is triggered by an on-board alarm clock. Commands are received from Earth via the rover´s high-gain antenna. The commands are "uplinked" to the rover and become its master sequence for the day. These are tasks that it will complete during the current Martian day as well as part of the next. This overlap is necessary so that the rover will know what to do in the hours between wake-up and uplink on each subsequent sol.
In the afternoon, the communication between Earth and the rover is reversed. Data that have been gathered are transferred to scientists and engineers via a "downlink" through the high-gain antenna. This information is used to determine the rover's condition and assess results of science experiments. The rover's UHF antenna is also used for the return of science and engineering information via two orbiters the Mars Global Surveyor and the Mars Odyssey. Data are analyzed and the next sol's activities are planned. A sequence of commands is created to uplink to the rover the next morning.
Each sol may be designated to focus on different kind of task. One might involve a taking panoramic data with Pancam and Mini-TES that can be used by scientists to select targets for further study. Another might involve measurements with the science tools the rover carries on its arm (called the Instrument Deployment Device or IDD). Others might be to drive as great a distance as possible, or to approach a rock target that has been identified. The rover is commanded to travel from point A to point B and is intelligent enough to maneuver through a Martian landscape littered with boulders and rocks. An approach sol is used to position the rover so that its multi-jointed arm can reach a selected target. Hazard avoidance cameras, or Hazcams located under the solar panel deck capture images of the work area to make sure it is clear. Only then is the arm deployed.
A typical scenario for using the science tools on the arm of the rover might be to deploy the Microscopic Imager to collect close-up views of a selected Martian rock. The arm then rotates to bring the Rock Abrasion Tool (RAT) into position to grind into the target´s surface. The Microscopic Imager is repositioned to collect images of these freshly exposed layers. The Alpha-Particle-X-ray-Spectrometer (APXS) then may be used to gather information on the elemental make-up of the rock, or the Mössbauer Spectrometer may be brought into position so that scientists can learn the composition of the iron-bearing minerals in the selected target. The arm is then returned to its stowed position before another drive.
The challenges facing the rover are enormous. Each instrument has different energy needs and the position of the sun affects the availability of solar power. Various pieces of equipment have thermal requirements and the entire rover must be kept warm while it sleeps during the cold Martian nights. In addition, the position of the Earth and the location of the two Mars orbiters affect telecommunication, and must be continuously known by the rover. The rover itself needs to know where it is too. If navigation runs into an arduous situation, the rover may lose track of where it is. For example, if the rover wheels bog down in a sandy spot and keep spinning without moving, the odometer may register progress when in fact the rover merely digs itself in. This complicates the scientists' task of "driving" the rover.
Eventual End of Mission
Toward the end of the surface phase for the mission, both power and telecom capabilities will be decreasing, as the Earth and the Sun become more distant from Mars, dust falls on the solar panels, the batteries lose capacity, and the Sun moves further North past the landing site latitude. Eventually, somewhere near Sol 91 it is expected that the rover will be unable to store up enough thermal or battery energy to prevent its components´ overnight temperatures from falling below flight allowable levels. That will sooner or later result in failure of one or more of those components, silencing the rover forever.
Problem with Spirit
On January 21, 2004. six days after Spirit rolled off its lander to explore its environs, it was examining a nearby rocked named Adirondack, when it suddenly fell silent. No pictures were received during scheduled sessions from Mars Global Surveyor orbiter or Mars Odyssey orbiter. Spirit merely signaled via direct link on command from misison control that it was still alive. However its computer went into repeated cycles of rebooting. Mission controllers feared that a catastrophic failure in its power system, antenna or heating systems might have damaged the rover beyond repair. Initially stumping the controllers, the problem now appears to be a fixable flaw in the computer program that controls the rover's flash memory, a storage device equivalent to the hard disk on the personal computer. Flash memory stores data safely so that during poweroff periods it will not be lost
On January 28, Spirit successfully communicated via its high-speed antenna and sent back the first photograph since the malfunction, taken by the hazard identifaction camera, showing its robotic arm extending the Mossbauer spectrometer against the rock nicknamed Adirondack. Everything seems to be the same as when the rover's troubles began during this session a little over a week earlier.
After deleting 1,700 files from Spirit's flash memory and rebooting its computer, engineers were finally able to get Spirit working again. More pictures have been sent back, among which the first-ever microcopic image taken of a rock on Mars. Preliminary data show that Adirondack is made of olivine-rich basalt, a very common type of volcanic rock found on the surface of Earth. Spirit's next potential targets are two light-colored rocks named Cake and Blanco.
In a separate development, on January 27, 2004, NASA named the three hills seen in the distance at the Spirit's landing site after the Apollo 1 crew members — Virgil I. Grissom, Edward H. White 2nd and Roger B. Chaffee — who were killed in a fire that broke out during a test on the launching pad at Cape Canaveral on January 27, 1967.
Most of the information herein contained came from NASA/JPL site.
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