Mars Pathfinder

Spacecraft: Lander with Microrover

Launch: December 1996

Arrival: July 1997

Primary Mission: 30 days

Contents:

Introduction

 Pathfinder will be the first U.S. spacecraft to land on Mars since the two Vikings arrived in 1976. Pathfinder is likely to be joined on the Mars surface in 1997 by two landers and two penetrators of the Russian Mars 96 mission. A unique feature of Pathfinder will be its Microrover, 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.

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.

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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 upsidedown. The solar panels will begin providing power to the spacecraft as soon as the sun comes up that first morning on Mars.

The Mars Pathfinder Project received a new start 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.

Pathfinder Objectives

 There are two sets of objectives for the Pathfinder mission: Engineering and Science.

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The primary objective for Pathfinder is to demonstrate a low-cost approach for cruise, descent, and an upright landing systems to place a payload on the Martian surface in a safe and operational configuration. The technologies pioneered by Pathfinder have the potential for use in future Mars missions. The lander also carries the microrover which will test mobility for small rovers on Mars. The microrover will also examine the condition and configuration of the Pathfinder landing craft to assess how well the landing system performed.

The scientific objectives include examination of the composition of rocks and soils in the vicinity of the lander. A landing site is being chosen in a way that will hopefully maximize the scientific information returned. The mission will characterized surface morphology and geology, acquire elemental composition information, and obtain atmospheric measurements such as temperature, pressure, and wind velocity.

Pathfinder Spacecraft

 The Pathfinder flight system has three main features: the Cruise Stage, the Deceleration Subsystems, and the lander. The lander contains all of the science instruments and the microrover. The spacecraft mass will be about 710 kg at launch, including 25 kg of payload (lander, rover).

The Cruise Stage will separate from the launch vehicle in space. The cruise stage has a solar panel for power, a medium-gain antenna for communication with Earth, and rockets and sensors for adjusting the trajectory on the trip out to Mars. The cruise stage will be jettisoned just before Pathfinder enters the Martian atmosphere.

The Deceleration Subsystems are used to reduce 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.

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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. These three panels can move in a way that would flip the lander into an upright position if it should land upside-down. The lander is designed to last a minimum of 30 days on the Martian surface. It's instruments are described futher below.

The microrover has 6 wheels and a mass of about 10 kg. It is about 65 cm long by 45 cm wide and 32 cm high. The rover will allow scientists to examine rocks that would otherwise be out of reach from a fixed lander. The rover will also be able to look at the lander and check out its condition.

The microrover carries the Alpha Proton X-ray Spectrometer (APXS) experiment (described below). It also has two small monochrome 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 two cameras provide operators on Earth with a stereo view of the area in front of the rover. The rover is commanded from Earth, but most of its activity relies on capabilities programed into the onboard computer. The 6 wheels can respond independently to conditions in rough terrain.

Pathfinder Science Instruments

Summary

 There are three major science experiments aboard 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 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.

Imager for Mars Pathfinder (IMP)

 The imaging system on the 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. The specific capabilities of IMP are:

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Each of the IMP's two cameras (two in order to get stereo images) each has 12 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. Current plans (April 1994) call for having two targets, one near the surface level and one at a height of 0.5 meters. 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 up the length of one or two of Pathfinder's antenna masts, from heights near the surface up to about 0.5 meters. As the wind blows, the flags will point in the direction the wind is going. Both direction and wind speed as a function of height above the surface will be determined. Besides the microrover, these flags are the only features expected to be seen to have moved during the mission. The wind sock experiment complements similar wind measurements that will be made by the Atmospheric Structure Instrument/Meteorology Package (ASI/MET) experiments, described below.

The IMP system is being developed at the University of Arizona with contributions from the Martin Marietta Astronautics Group, the Max Planck Institute for Aeronomy in Germany, the University of Braunschweig in Germany, and the Neils Bohr Institute in Denmark.

Alpha Proton X-ray Spectrometer (APXS)

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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. Duplicate APXS's will be aboard the landers planned for the Russian Mars 96 and Mars 98 missions.

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 Alpha and Proton Spectrometer portion of the instrument are provided by the Max Planck Institute of Mainz, Germany; the X-ray spectrometer is provided by the University of Chicago in Illinois.

The APXS consists of alpha particle sources and detectors for (a) back-scattered alpha particles, (b) protons, and (c) x-rays. The APXS determines elemental chemistry of surface materials for most major elements except hydrogen. The process takes advantage of three kinds of interactions between alpha particles and matter: (1) elastic scattering of alpha particles by atomic nuclei, (2) alpha-proton nuclear reactions induced in some of the lighter elements, and (3) the excitation of atoms by alpha particle bombardment leads to the emission of characteristic x-rays.

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 on Mars.

Atmospheric Structure Instrument/Meteorology Package (ASI/MET)

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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 through the atmosphere and after it has landed. This package does not have a principal investigator, but instead is known as a facility instrument under direction of the Jet Propulsion Laboratory (Pasadena, CA). The package is based upon similar experiments used on the 1976 Viking landers. The ASI/MET package does not examine the composition of atmospheric gases. Instead, these 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 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 critical data about the day-to-day variations of weather on the Martian surface. The ASI/MET will be able to determine atmospheric pressure and temperature, plus provide information about wind speeds that will complement the IMP wind sock observations.

Pathfinder Landing Site Selection

 Pathfinder must land somewhere that is safe for the landing 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 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 25 degrees North. The best compromise is 15 degrees North because it ensures that Pathfinder's solar arrays will generate enough power that the onboard computers will operate through each night (when no solar power is available).

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The engineers also determined that they could predict 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 Pathfinder, Dr. Matthew Golombek of NASA's 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 come and suggest landing sites. In mid-June, 1994, the Pathfinder science team met and narrowed the list of possible landing sites down to four key locations. They also listed these sites in order of preference.

To see how the landing site was selected, click HERE to read the letter sent out by M. Golombek. The landing site chosen is in the Ares-Tiu Valles system.

Above: Artist Conception of Pathfinder Landing Site. White object in upper left is the parachute. Note white material beneath the solar panels-- these are the deflated airbags. Rover is in lower left corner. (Courtesy NASA / Jet Propulsion Laboratory, 1993)

Pathfinder Launch and Mission Operations

 Pathfinder will be launched on a Type-1 Earth-Mars transfer trajectory from a McDonnell Douglas Delta II rocket. Current planning (April 1994) assumes a 30-day launch window between December 5, 1996, and January 3, 1997. Landing is fixed at July 4, 1997.

Pathfinder mission operations at the Jet Propulsion Laboratory (JPL) will be supported by NASA's Deep Space Network (DSN). The DSN antennas and facilities are located in Pasadena and Goldstone, California; Canberra, Australia; and Madrid, Spain.

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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 Pathfinder reaches Mars, it will enter the atmosphere directly 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 Pathfinder's mission. The entry will last about 5 minutes.

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Landing will occur at a site near 15 degrees (plus or minus 5 degrees) North Latitude on July 4, 1997. 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.

Sources consulted for this report:

Authorship Credits

 Text written by: K. S. Edgett

Figures scanned into .gif format by: S. Schmidt

Figures provided by: NASA / Jet Propulsion Laboratory, Pasadena, California

Some Line Drawings Modified by: K. S. Edgett

Arizona Mars K-12 Education Program / K.S. Edgett