Mars Pathfinder and Sojourner

The Mars Pathfinder mission began as MESUR (Mars Environmental Survey), a 1991 proposal for a network of as many as 16 Mars landers to perform network science (meteorology and seismology on distributed sites) using nominally inexpensive landers. One prominent approach to reducing the unit cost of the landers was to use a semi-hard landing approach with airbags rather than a retrorocket system. The landing system proposed was sufficiently radical that a technology demonstration/flight validation was designed, originally MESUR Pathfinder, on which work formally began in 1993.

With the loss of Mars Observer and the onset of the Discovery programme in NASA, the Pathfinder concept was 'adopted' by the Discovery programme, and became the most widely cited example of the 'faster, better, cheaper' (FBC) approach (see McCurdy, 2001). NEAR technically was the first selected Discovery mission, but took rather longer to be built and reach its target. Note also that there were other FBC programmes within NASA, including the Small Explorer Earth orbiters, and the New Millenium technology validation programme. The success of some non-NASA projects like the Clementine moon orbiter, which came out of the Strategic Defense Initiative (the 'Star Wars' programme) also set the stage for the FBC era.

As an aside, one viewpoint of the background to the development of Pathfinder is described in Donna Shirley's book Managing Martians (1998). Andrew Mishkin's Sojourner (2004) gives a more detailed but narrower view, of the rover engineering development specifically. Since Pathfinder was fundamentally an engineering mission, the scientific payload was in fact rather modest, the principal aim being to demonstrate the successful deployment of the lander.

The stereo camera's capabilities were leveraged by some ancillary fixtures on the lander; conical metal windsocks were mounted at three positions on the ASI/ MET mast - these freely suspended structures hung at an equilibrium position determined by the windspeed and gravity. The onerous calibration of this experiment meant only a modest scientific return. A particularly fruitful investigation was the magnetic target that was imaged by IMP - over the duration of the mission, airborne dust particles progressively adhered to the target.

The entry protection system of Pathfinder (Wilcockson et al., 1999) comprised a Viking-heritage sphere-cone front shield geometry (2.65 m diameter 70° halfangle cone and a spherical nose cap of 1/4 radius to diameter ratio). Entry mass was 585.3 kg, cross-sectional area 5.51m . The ballistic coefficient quoted for

entry conditions is 63.2 kg m . As with Galileo, sensors embedded in the heat shield allowed its performance to be evaluated (Milos et al., 1999b).

The same entry protection material as Viking was used, a Martin Marietta (later Lockheed Martin) superlightweight ablator SLA-561. This is a mix of ground cork with silica and phenolic microspheres in a silicone binder. This mix is packed into a phenolic honeycomb structure. The material had to be requali-fied, since peak heating on the direct-entry trajectory for Pathfinder would approach 100 W cm—2, compared with the Viking heating rate (from orbit) of 30 W cm . A 1.9 cm thick layer of the ablator was applied to the front shield; the backshell received a 0.48 cm spray-on layer of a similar material (without the honeycomb); 1.5 h prior to entry, coolant was vented from the electronics boxes. Separation of the cruise stage occurred 30 minutes prior to crossing the entry interface at 130 km.

The entry speed (e.g. Braun et al., 1999a) was 7.26kms—1 (inertial frame) or 7.48 kms—1 (relative to the rotating planet), with a flight path angle of —13.6°. Peak deceleration was about 16 g. Parachute deployment was programmed to occur as close as possible to a dynamic pressure of 600 Nm—2; the deceleration was monitored and a time offset computed based on the deceleration sensed 12 s after a deceleration threshold of 5 g was exceeded. The deceleration record shows the actuation of the parachute mortar at 171.4 s after entry interface crossing; the parachute inflated in about 1.25 s, with snatch loads of about 6.5 g. The probe was at an altitude of 7.9 km, flying at Mach 1.8.

Ground proximity (e.g. Spencer et al., 1999) was sensed by a radar altimeter from 1.6 km down, which triggered the operation of braking rockets at an altitude of 98 m, 6.1 s prior to impact at a descent rate of 61 m s—1. The parachute bridle was cut 3.8 s prior to impact (while the rockets were still burning, ensuring that the parachutes would be carried away and not interfere with the deployment of the lander). Impact occurred with a vertical velocity of 14 m s— and an estimated horizontal velocity of 6ms— ; the loads on the first bounce were 18.7 g. It is estimated that the lander travelled about 1 km in the subsequent 2 minutes of bouncing - several initial bounces were captured by the accelerometers.

After the vehicle came to rest, the airbags were allowed to depressurize by opening vent patches in the bags by retraction of lanyards by electrical motor.

The vents were covered with a mesh scrim to retain particulates from the inflation pyros. The airbags were then retracted by operating electrical motors for some 72 minutes. Three sides of the tetrahedral lander were opened by high-torque electric motors, such that regardless of the landing attitude of the lander, its base would be placed on the ground and the three circumferential petals would be splayed flat around it, exposing the solar arrays to the sky.

One of the petals carried the rover vehicle, initially referred to as the Microrover Flight Experiment (MFEX), subsequently named 'Sojourner'. The Pathfinder lander itself received the designation 'Sagan Memorial Station'. The rover had a size of 62 X 47 X 32 cm3, roughly the size of a typical mid 1990s laser printer. Its mass was 10.5 kg.

Sojourner featured a 'rocker bogie' design, wherein the three wheels on each side were arranged in a 'tree', with two wheels on a bogie that could articulate at one end of an arm that held the other wheel (see the entry that follows on MER and Figure 27.3). This arrangement permitted the vehicle to traverse larger obstacles than a simpler configuration with similar-sized wheels. Wheels were 13 cm diameter, 7 cm wide, driven by independent motors with maximum torque, via 2000:1 reduction gearing, of 3-4.5 Nm. The motors were driven with 15.5 V, having a 10 mA no-load current and a torque slope (from which soil mechanical properties were deduced by sensing motor current) of approximately 4NmmA~1. The vehicle could move forward at a maximum speed of 0.4 m per minute, and turn at 7° s_1 (the front and rear wheels could be independently steered). Steering position was sensed by potentiometer, while wheel rotation positions were sensed by optical encoder.

Sojourner navigation was accomplished principally by ground command using the Pathfinder camera to determine Sojourner's location and status. The vehicle itself had a number of small cameras, and a structured-light obstacle-detection system.

Manoeuvres were in general commanded directly, although some 20 high-level commands (to move to a specified location in X-Y space around the lander) were sent, and two higher-level (move up to the rock at a specified X-Y position) were also used. Operations were complicated by the difference between the day lengths on Earth and Mars, such that the lander schedule shifted by 37 minutes every day.

Power was supplied by a 0.22 m2 (16.5 W Mars Noon - 45 W AM0 at Earth) solar array (13 strings of 18 GaAs on Ge cells), and a lithium thionyl chloride (LiSOCl2) primary battery. The latter, comprising nine D-cells providing 150 W-hr, was important in that the principal scientific instrument on Sojourner, an alpha-X-ray backscatter spectrometer, required long integration times (5-16 h, i.e. overnight) to obtain statistically useful numbers of counts. Deposition of dust from the atmosphere resulted in the partial obscuration of the solar array and a steady drop in output power of 0.2% per day.

The small Sojourner vehicle was more susceptible to diurnal temperature changes than the large lander. One measure adopted to mitigate the night-time ambient temperature drop (to —110 °C) was the incorporation of silica aerogel insulation. The rover also had three 1 W radioisotope heater units to limit the low temperatures reached at night in the warm electronics box (WEB) at its core.

Sojourner was controlled by an 80C85 processor running at 105 instructions per second, accessing 576 KB of RAM and 176 KB of PROM. The computer controlled or read some 70 sensor and actuator channels. Half-duplex communications between the lander and the rover were conducted via an off-the-shelf UHF radio modem.

Experiments were conducted with the rover to determine the adhesion properties of the soil and the abrasion resistance of surface materials. These tests also indicated some triboelectric charging of the surface dust.

Downlink of data from Pathfinder was at up to 9 kbps to the 70 m DSN stations. After September 27, communications became unreliable - the last data were received on October 7; subsequent attempts to regain contact persisted until March 1998. It is believed that component failure in the communication system due to the deep thermal cycling was responsible for the end of the mission.

Sojourner travelled some 104 m in total, always within 12 m of the lander. It is possible that Sojourner may have episodically operated after communications between Earth and Pathfinder were lost, but of course there is no evidence of this.

A large number of images were acquired by the Imager for Mars Pathfinder (IMP) experiment, a stereo CCD camera mounted in a turret on a mast that was extended vertically soon after landing. The individual camera frames were quite small, but could be mosaiced together by tilting and panning the turret to form large composite panoramic images. The generation of stereo image products greatly facilitated scientific interpretation and public appreciation of the scene observed by the lander.

Among the main scientific results of Pathfinder, the site was confirmed by the lander imagery to be what had been suspected from orbit, namely an outwash plain created by fluid flow that transported rocks across the surface. The imager also detected several dust devils. A couple of anomalously bright spots (1-3 pixels across) in distant images are interpreted to be the entry shell and back cover 1-2 km away.

Significant effort was devoted to attempts to identify the minerals present in local rocks, using imaging through various filters to build up a reflectance spectrum. These analyses were somewhat impaired in the sense that most rocks were covered to a greater or lesser extent with surface dust. An additional complication was that the diffuse illumination from the sky had an intensity and colour distribution that depended significantly on the viewing geometry and time of day. The APXS instrument on the rover also suffered challenges due to poor collimation - the particles were somewhat scattered in the Martian atmosphere. Nonetheless some worthwhile mineralogical studies were made by both instruments.

The lander's meteorology package also recorded winds, temperatures and pressures; some signatures of dust devils were also noted in the latter data. Measurements of windspeed were made in two ways - by a hot-wire anemometer, and by a windsock experiment wherein weighted conical vanes were suspended from a mast such that their orientation depended on the windspeed.

The total data return was some 2.3 Gbit, including 16 500 lander images, 564 rover images, 16 APXS measurements and 8.5 million pressure, temperature and windspeed records.

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