From: dadams@netcom.com (Dean Adams)
At present most of the scientists associated with the Mars Pathfinder mission are busy building the three science instruments (Imager for Mars Pathfinder, Alpha Proton X-ray Spectrometer, and the Atmospheric Structure/Meteorology Package) for the spacecraft and rover. Over the next year the instruments will be built, tested and calibrated before and after integration on the spacecraft. The progress and status of the instruments will be reported in a future status report. However, in addition to building the instruments a smaller number of scientists are attempting to learn everything possible about the place we plan to land the spacecraft on the martian surface. This status report will focus on this topic.
As previously announced, the project has decided to land where Ares Vallis opens into Chryse Planitia on Mars (19.5 degrees North latitude, 32.8 degrees West longitude). The rationale for choosing this location is that it represents a place where there is the potential for sampling a wide variety of different rocks that make up the planet Mars in the small area accessible to the rover (a few tens of meters). One of the areas of greatest scientific return possible from the Pathfinder mission is in its ability to learn about the mineralogy and chemical composition of the various materials at the landing site. Imaging rocks and other materials at the landing site with a variety of spectral filters should allow discrimination of different pyroxene and iron oxide minerals. The rover would then be directed to different interesting rocks, where the alpha proton x-ray spectrometer would be used to determine their elemental composition. Close-up images of the rocks would also be taken by the color and monochrome cameras on board the rover, allowing discrimination of any millimeter-sized crystals present in the rock. Using all these data together, scientists will attempt to determine the mineralogy of the rock. If the mineralogy can be determined, then a tremendous amount of information can be inferred with regard to the processes and environment in which the rock formed. The greater the number of different rocks that can be studied at the landing site, the more that can be learned about the geologic history of the planet. Because the Ares Vallis floods drained from the ancient highlands, which include some of the planet's oldest rocks, Pathfinder may enable scientists to determine how the planet differentiated into a crust and mantle and whether early Mars was both warmer and wetter than at present (and thus more like the early Earth).
Images of the surface of Mars at the meter scale (of interest to a lander) currently exist only at the two Viking landing sites. The reason for this is that images of Mars taken from orbit are at a scale insufficient to resolve lander-sized objects. At the Ares site, for example, the highest-resolution Viking Orbiter images are at about 40 meters per picture element (pixel). That means that when Pathfinder lands on Mars, virtually all of its observations will be taken within a single pixel of the orbiter images. Predicting what the surface will look like at the meter scale is virtually impossible from such coarse-resolution images.
For this reason, other remote sensing techniques must be used to attempt to understand the nature of the surface. Such techniques include Earth-based radar, which can give information about the roughness of the surface at a scale of 10-100 meters. In addition, Viking Infrared Thermal Mapper (IRTM) observations can be used to infer the relative abundance of rocks on the surface (as opposed to dust) and the albedo and color of the Viking images can be used to infer the relative amounts of bright martian dust and dark rocks. Scientists studying Mars are actively involved in using these data sets to learn as much as possible about the surface at the landing site. In addition, because Mars is presently near opposition with the Earth, additional Earth-based radar data are being acquired of areas of interest.
Another way to learn about Ares Vallis is to study similar features here on Earth. Ares Vallis is one of the largest outflow channel on Mars, which form when enormous quantities of water flow over the surface during a short period of time. A well-known similar feature on the Earth can be found in the Channeled Scablands of Washington State. Large streamlined islands and channels were carved when ice that dammed a large glacial lake (Lake Missoula - about the size of Lake Ontario) ruptured and the water drained to the Pacific Ocean over a period of about two weeks. Ares Vallis flood involved substantially greater quantities of water (roughly the entire volume of all five Great Lakes) draining into the martian northern lowlands. Another group of scientists is using knowledge gained from studying the Channeled Scablands on Earth to better understand similar large-scale features visible in the orbiter images of Mars.
The Mission Design and Navigation Office in the Mars Pathfinder Project is responsible for designing the interplanetary trajectory from Earth to Mars, designing the Mars atmospheric entry trajectory, defining the high-level sequence of events performed during the mission, and developing the software and processes required to navigate the spacecraft to Mars.
The detailed design of the interplanetary and Mars atmospheric entry trajectories is essentially complete. Mars Pathfinder plans to use a direct transfer from the Earth to Mars with a total flight time of 6-7 months. Launch is currently scheduled to occur sometime between December 5 and December 25, 1996 from Cape Canaveral. The launch vehicle is a three-stage Delta manufactured by McDonnell Douglas Aerospace. Recent trajectory analysis has assessed the possibility of launching earlier, potentially as early as December 2. The advantage of this earlier date is that it allows a longer launch window, which improves the overall odds of launch. As a note, the next available launch opportunity to Mars should Pathfinder miss the 1996 opportunity is in late 1998.
The spacecraft arrives at Mars on July 4, 1997 and enters the Martian atmosphere directly from the approach hyperbola. The speed at entry is approximately 7.6 km/s, or about 17,000 mph. The trajectory which the spacecraft follows through the atmosphere depends on this initial speed, plus the spacecraft mass and the entry angle. The entry angle is angle between local horizontal and the spacecraft's direction of travel at entry. If this angle is steep, the spacecraft descends very quickly through the atmosphere and experiences high heating and deceleration forces. If the angle is too shallow, the atmosphere will not slow down the spacecraft enough to keep it from flying back out of the atmosphere and escaping. The spacecraft must be carefully navigated to make sure that this entry angle is within an acceptable range. In the case of Mars Pathfinder, this means knowing where the spacecraft is to within 50 kilometers when it is more than 190 million kilometers from Earth. The navigation engineers on Mars Pathfinder use the observed Doppler shift in the communications signals coming from the spacecraft in order to determine where it is.
The Mission Design and Navigation Office, in conjunction with other project elements, is responsible for developing the overall mission plan summarizing the key activities performed during the mission. The mission is essentially divided into four major phases. The launch phase is relatively short, extending from launch through the point when the spacecraft separates from the upper stage. This 75-minute period is crucial, however, in that the launch vehicle must perform a specific series of activities to send the spacecraft to Mars. The cruise phase of the mission starts at separation and ends just before the spacecraft enters the Martian atmosphere. Cruise is a relatively quiet period, except for periodic maneuvers which target the spacecraft to the correct entry trajectory. The spacecraft also communicates to the Earth in this period to make sure that all components are still functioning correctly.
The entry, descent, and landing phase of the mission occurs when the spacecraft enters the Martian atmosphere. This is also a relatively short period, but it is definitely the most critical to successfully completing the mission. The spacecraft performs a series of activities to descend through the atmosphere, land safely on the surface, and establish the correct surface configuration. The surface phase starts when the spacecraft is safely on the surface of Mars and continues indefinitely. The spacecraft is required to operate for 30 Martian days, but should operate for much longer. The surface phase is when most of the key science and technology objectives are accomplished. The microrover is deployed soon after landing and proceeds to explore the area around the lander. The science instruments gather data in order to provide scientists with a better understanding of the geologic and atmospheric characteristics of Mars.