Mars Pathfinder will be the first U.S. spacecraft to land on Mars since the two Vikings in 1976. A unique feature of Mars Pathfinder will be its microrover, Sojourner. Sojourner is a small vehicle which can range up to a few tens of meters away from the spacecraft and examine the composition of surrounding rocks and soils.
Mars Pathfinder is planned for launch aboard a Delta II rocket sometime between December 1996 and January 1997. The spacecraft will be on a direct course from the Kennedy Space Center in Florida to the martian surface. Landing is planned for July 4, 1997, nearly 21 years after the Viking landings.
Mars Pathfinder will land during the Martian night. Just before the spacecraft impacts the surface, giant airbags will inflate to cushion the landing. After the spacecraft comes to rest on the surface, the airbags will deflate and three solar panels will unfold. These panels are arranged in a way that will allow the spacecraft to be flipped over if it should land upside-down. The solar panels will begin providing power to the spacecraft as soon as the sun comes up that first morning on Mars.
Mars Pathfinder received new start funding from NASA and Congress in October 1993. The project is required to have a cost of less than $150 Million, have a fast schedule (less than three years from new start to launch), and achieve a set of significant but focused engineering, science, and technology objectives.
Science. The scientific objectives include examination of the composition of rocks and soils in the vicinity of the lander. The mission will characterize surface morphology and geology, acquire elemental composition information, and obtain atmospheric measurements such as temperature, pressure, and wind velocity.
The Deceleration Subsystems are used to reduce Mars Pathfinder's velocity and allow a safe landing on Mars. These subsystems consist of an aeroshell, or heat shield to protect the spacecraft as it comes into the atmosphere, a parachute to slow the descent, small solid rockets to further slow the descent, a radar altimeter to indicate proximity of the surface, and airbags which inflate to protect against impact. The parachute and aeroshell are derived from technology used successfully in the Viking landings. The airbags are a new feature which are basically designed like the airbags which protect motorists during automobile crashes.
The lander is shaped like a tetrahedron. The tetrahedron consists of four similarly-shaped triangular panels. All lander equipment except the solar arrays and rover are attached to a single center panel. The other three panels are attached to the edges of the center panel.
Microrover, Sojourner. The microrover, Sojourner, has 6 wheels and a mass of about 10 kg. It is about 65 cm long by 45 cm wide and 32 cm high (about the size of a typical office laser printer) . The rover will allow scientists to examine rocks that would otherwise be out of reach from a fixed lander. The microrover carries the Alpha Proton X-ray Spectrometer (APXS) experiment (described below). It also has two small cameras which act as "eyes" for navigating the rover around the landscape. These imagers can see objects about one millimeter in size and may allow scientists a close-up view of the texture of rocks found at the landing site. The rover is expected to function for a minimum of 7 days, but hopefully will last 30 or more days on the martian surface.
The name Sojourner was announced in July 1995 after a year-long, worldwide competition, initiated by The Planetary Society of Pasadena, California, in cooperation with NASA's Jet Propulsion Laboratory. Students up to 18 years old were invited to select a heroine and submit an essay about her historical accomplishments. The students were asked to address how a rover named for their heroine would translate these accomplishments to the martian environment. Valerie Ambroise, 12, of Bridgeport, Conn., submitted the winning essay about Sojourner Truth, an African-American reformist who lived during the tumultuous era of the U.S. Civil War. An abolitionist and champion of women's rights, Sojourner Truth, whose legal name was Isabella Van Wagener, made it her mission to advocate the abolition of slavery and the rights of women to participate fully in society. The name Sojourner was selected because it means "traveler." The second place prize winner was Deepti Rohatgi, 18, of Rockville, MD, who proposed naming the rover after Marie Curie, a Polish-born chemist who won the Nobel Prize in 1911 for her discovery of the elements radium and polonium. The third place prize went to Adam Sheedy, 16, of Round Rock, Texas, who chose the late astronaut Judith Resnik for the rover. Other popular names included Sacajewea, who explored North America with Lewis and Clark; Amelia Earhart, one of the first female aviators; Athena, the Greek goddess of wisdom; Harriet Tubman, a 19th-century African-American writer and political reformist; Greek goddesses Minerva and Atlanta; and Thumbelina, the tiny fairy tale character created by Hans Christian Andersen.
There are three major science experiments aboard Mars Pathfinder, and each has a variety of scientific goals. For example, the imaging system can obtain multispectral images of the surface and atmosphere, thus allowing estimation of how much dust is in the air and what types of rocks might be present. The imaging system will also look at a wind sock experiment, allowing determination of wind velocity above the surface. The imaging system may also be able to monitor changes in weather, particularly cloud cover, and can also be used to plan the work of the microrover.
The experiments aboard Mars Pathfinder promise to tell us all something new about Mars. In particular, these instruments will provide information about the element abundance, iron-bearing minerals, and atmospheric properties during descent and on the surface. These instruments and the science objectives they represent will provide us with a new, fresh view of the Red Planet from its surface.
The imaging system on the Mars Pathfinder lander can look at the landing site in stereo and in both black-and-white and color modes. The IMP is being developed by a team under the direction of Dr. Peter Smith at the University of Arizona in Tucson.
Each of the IMP's two cameras (two in order to get stereo images) has 24 filters between wavelengths of 0.4 and 1.0 microns. These filters were carefully selected to allow determination of certain atmosphere and surface properties. For example, a band at 0.425 microns can look up at the sun through the atmosphere, and thereby provide an estimate of how much dust or cloud material is suspended in above the landing site. A band at about 1.0 microns will allow determination of the presence of pyroxene, a key mineral expected to be present in the rocks.
Besides studying the surface and atmosphere, IMP will assist in navigation for the microrover, monitor wind conditions, and examine the magnetic constituents of the soil. A series of magnetic surfaces will be placed on the lander. The magnetic targets, provided by a team under the direction of Dr. Jens Martin Knudsen of the University of Copenhagen, Denmark, will collect magnetic dust settled out of the atmosphere. Images of these magnetic materials will help determine their mineral composition and the magnetic strength of these minerals.
The IMP wind sock experiment promises to be very exciting. Like the IMP itself, the wind sock experiment has a unique Arizona connection. It is being developed by Dr. Ronald Greeley and Dr. Robert Sullivan of Arizona State University in Tempe. Small flags will be placed on one or two masts up to a height of about 0.5 meters. As the wind blows, the flags will point in the direction the wind is going.
The Alpha Proton X-ray Spectrometer (APXS) experiment is mounted on the microrover and constitutes the rover's main scientific objective. The purpose of APXS is to determine the abundance of major chemical elements in the rocks and soil near the lander. The APXS has a long history, going back to a similar instrument included on the Lunar Surveyor landers in the mid-1960's. APXS experiments were also included on the Soviet Venera Venus landers and provided critical data on the composition of Venusian rocks.
Because the APXS is mounted on a rover for the very first time, scientists will be able to move around an area and sample the composition of a number of rocks and surfaces that would otherwise be inaccessible on a fixed lander. When collecting data, the APXS is placed on the surface to be examined. It must sit on that surface for 10 hours to collect data in the Alpha Proton mode, and 1 hour for the X-ray mode. The surfaces to be examined by the APXS will also be imaged by the microrover's two small cameras. The surface being examined is exposed to a radioactive source (Cm - 244) which provides the alpha particles. The detectors determine the energy of alpha particles, protons, and x-rays emitted off the surface being bombarded with alpha particles. Most important chemical elements (e.g., C, O, Mg, Al, Si, Ca, K, Fe, Ni) can be identified using this instrument. This will allow for investigation of rock and soil compositions.
The ASI/MET is a combined package of a number of instruments designed to investigate the properties of the martian atmosphere as the spacecraft descends and after it has landed. The package is based upon similar experiments used on the 1976 Viking landers. The instruments are used to determine temperature, pressure, and density of the atmosphere.
The ASI/MET package is critical to meeting the engineering objectives of this mission. The ASI/MET will obtain data about the atmosphere as the spacecraft is descending toward the surface for landing. Data acquired during the entry and descent of Mars Pathfinder will allow the reconstruction of profiles of atmospheric density, temperature, and pressure from an altitude of over 100 km down to the surface.
The hardware basically consists of an accelerometer and a number of temperature and pressure sensors mounted at several locations on the lander. Once the spacecraft has landed, these instruments will provide information about the day-to-day variations of weather on the martian surface.
Mars Pathfinder must land somewhere that is safe and that is at the appropriate latitude on Mars to ensure maximum sunlight to power the solar panels. It is equally important that the latitude of this landing must have Earth above the horizon long enough each day to transmit data.
The engineers designing Mars Pathfinder determined that the best latitude for a July 4, 1997 landing will be 15 degrees North, plus or minus about 5 degrees. The landing will occur during Northern Hemisphere Summer. The sun will be up the longest at 15 degrees North during the early part of July 1997. Likewise, Earth will be best seen from around this latitude.
The engineers also determined that they could predict Mars Pathfinder's landing spot within an elliptical area about 100 km wide by 200 km long along an axis that trends N 74 E. Thus, the Mars scientific community was asked to come up with a landing site that occurs near 15 degrees North and can have as much as 150 km uncertainty in the exact landing spot. Because of the difficulty of landing in the thin Martian atmosphere, scientists were asked to find a landing site below 0 km elevation. This would provide a thicker atmosphere through which to descend.
The Project Scientist for Mars Pathfinder, Dr. Matthew Golombek of the Jet Propulsion Laboratory, convened a workshop at the Lunar and Planetary Institute in Houston, Texas, on April 18-19, 1994, so that members of the scientific community could suggest landing sites. In mid-June, 1994, the Mars Pathfinder scientist teams met and narrowed the list of possible landing sites down to four key locations, then they took a vote and finally picked Ares Vallis. The site chosen is at 19.5 degrees North latitude and 32.8 degrees West longitude, at an elevation of -2 km. The Ares site was chosen mainly because it is likely to offer an abundance of rocks brought down by the channels. Sampling the chemistry of these rocks using the APXS on the microrover is expected to provide considerable new information about martian crustal composition.
Mars Pathfinder will be launched from the Kennedy Space Center, Florida, on a McDonnell Douglas Delta II rocket. There is a 30-day launch window between December 5, 1996, and January 2, 1997. Landing is fixed at July 4, 1997. The 6 to 7 month cruise will be a relatively quiescent period during which the spacecraft will not do much. The primary cruise activities include periodic maneuvers to keep the radio link pointed at Earth and maneuvers to ensure safe entry into the martian atmosphere once the landing date approaches. No science investigations are planned for the cruise phase.
When Mars Pathfinder reaches Mars, it will enter the atmosphere at a velocity of 7.65 km per second. The lander's velocity will slow through a sequence of air-braking maneuvers utilizing the lander's heat shield. Eventually a parachute is deployed to slow the craft. Finally, small solid rockets will also fire to slow the lander, and then giant airbags will inflate to cushion the impact. Important engineering data will be collected and radioed to Earth during entry and landing, because the details of this landing and how the system performs are the main objective of Mars Pathfinder's mission. The entry will last about 5 minutes. As the spacecraft touches down, it will still be nighttime at the landing site. Earth will have just risen above the local horizon, and sunrise will occur about 4 hours later.
After landing, the highest priority activities on the first day are to make sure the lander is in an upright position, radio all of the engineering data collected during the landing back to Earth, acquire a panoramic image of the surrounding terrain, and deploy the micro-rover. It will be extremely important to move the rover off of the solar panel upon which it sits during delivery to Mars. The rover must be moved off the solar panel within the first day or two to ensure that the lander can collect enough solar energy to keep functioning. The Primary Mission will last about 30 days. An Extended Mission phase will occur indefinitely as long as the lander survives past the first 30 days.