Sojourner

NASA/JPL. Public domain (NASA / US government work).

Overview

Sojourner was delivered by the Mars Pathfinder lander on 4 July 1997, the first rover to operate on Mars [1]. Planned for seven sols, it returned data and images for 83 sols.

Specifications

Parameter	Value	Source
Mass	10.6 kg	[1], [9]
Height	about 30 cm	[9]
Wheels	six, rocker-bogie suspension	[1]
Instruments	alpha proton X-ray spectrometer, three cameras	[9]

Mission profile

Parameter	Value	Source
Launch	4 December 1996 UTC, Cape Canaveral	[9]
Landing	4 July 1997, Ares Vallis	[1], [9]
Planned mission	seven sols	[1]
Surface operations duration	83 sols
Final data transmission from Pathfinder	27 September 1997	[9]
Linear distance traversed	approximately 100 m	[1]
Surface area explored	approximately 250 m2
Commanded rover maneuvers	230
APXS in-situ elemental analyses	16
Rover camera images	550	[1], [9]
Lander camera images	16,500
Environmental measurements	approximately 8.5 million	[1]
Total mission data returned	approximately 2.3 Gbit	[1], [9]

The mission page counts more than 15 chemical analyses of rocks and soil [9] where the rover engineering report gives 16 APXS in-situ analyses [1].

Pathfinder entered the atmosphere directly, decelerated by heat shield, parachute and retrorockets, and landed inside an airbag cocoon [1]. Airbag landing is viable at this mass because the atmosphere, at a mean surface density near 0.020 kg/m3, decelerates a light entry vehicle adequately under a parachute alone [4].

Mobility

The rover masses 10.6 kg on six wheels in a rocker-bogie suspension [1]. The Wheel Abrasion Experiment occupied the right center wheel of the six. The linkage keeps all six wheels loaded on uneven ground without springs or active levelling, distributing load passively as the terrain changes, and allows the vehicle to climb obstacles taller than its wheel diameter. The same geometry has been used on every subsequent NASA Mars rover.

Wheel surface speed during instrumented operation was 1 cm/s at approximately 1.2 rpm [1], a rate low enough that wheel-soil interaction is quasi-static and inertial effects in the soil do not enter. Wheel dig-in tests give the only direct in-situ measurement of surface bearing behavior from this mission: in softer soil the test wheel sank 0.6 to 0.7 cm in the first two revolutions and reached 0.8 to 1.2 cm after seven, while in harder material it reached 1.5 cm in two revolutions. Ground tests in analog soils produced depths over 3 cm [1]. The spread reflects the strength range of Martian surface materials, which run from sand at cohesion at or below 1 kPa to indurated soils near 11 kPa [3].

Power and energy

A solar array on the top surface supplies about 16 W, with non-rechargeable lithium batteries for short night operations [1]. Atmospheric opacity at the landing site was 0.5 during surface operations, with suspended particles of approximately 1 µm [1]. That opacity sits at the top of the background range for the aphelion half of the Martian year, so array output over the 83 sols was near its seasonal floor rather than depressed by a storm [4].

Hazard sensing and the Earth link

Hazard detection uses two binocular cameras and a laser ranging device [1]. Both sense geometry; neither senses soil strength, and cohesion across Martian surface types varies from at or below 1 kPa for sand to 11 kPa for indurated soil with no corresponding change in relief [3]. The rover has no direct Earth link; all communication relays through the Pathfinder lander over UHF, with telemetry at 40 bps. One-way Earth-Mars signal delay during the mission was at least ten minutes, which is what forces onboard hazard handling rather than direct teleoperation.

Autonomy

Operators sent waypoint goals rather than driving directly. Between waypoints the rover used laser stripe projection and a forward camera to detect and avoid hazards [1]. The dig-in results show why that is only a partial safeguard: the same seven wheel revolutions produced 0.8 to 1.2 cm of sinkage in one material and 1.5 cm in two revolutions in another, a difference in bearing behavior that no geometric sensor resolves [3]. The relay-only link also confined the rover to about 10 m from the lander.

Payload and instruments

The alpha proton X-ray spectrometer was placed against targets by backing the rover into them, returning the first in-situ measurements of Martian rock composition; 16 analyses were completed [1].

Wheel Abrasion Experiment

The WAE is the reason Sojourner produced a quantitative result about the abrasiveness of Martian surface material rather than only about its composition.

Parameter	Value
Location	right center wheel
Substrate	black anodized, dye-impregnated aluminum strips, 25 x 120 x 0.25 mm
Metals	aluminum, nickel, platinum, resistively evaporation-deposited
Film thicknesses on one strip	200, 300, 450, 700, 1000 Å
Brinell hardness of the three metals	Al 16, Pt 64, Ni 100
Sensor	photovoltaic reflectance detector
Typical run	2 revolutions acquiring, 3 not, 2 acquiring; seven total
Readings per sample per revolution	five, central reading taken as peak
Runs conducted	11, totaling 39 wheel revolutions

Values from [1].

The metals bracket a hardness range: material that abrades aluminum but not nickel has a hardness between Brinell 16 and 100 [1]. In flight, aluminum wore most and wore inversely with film thickness, while platinum and nickel wore little. The limits placed on Martian dust are therefore harder than aluminum and softer than nickel.

Two experiment modes were run [1]. Rolling wear accumulated during normal driving. At intervals all wheels except the test wheel were locked to hold the rover stationary while the test wheel alone spun into the regolith, producing wear more severe than rolling and the dig-in depths quoted above.

The experiment returned a second result it was not designed for. Electrostatic charging of the wheels attracted dust onto the WAE wheel, which made reflectance interpretation problematic on many runs [1]. That the dust was attracted electrostatically constrains its grain size to somewhat smaller than 40 µm, and the observation is the first detection of electrostatic charging in the Martian environment.

WAE photodetector

Parameter	Value
Detector	three GaAs-on-Ge solar cells, 2 x 4 mm each
Package	1 cm cube, 10 g mass budget
Design / actual operating temperature	-50 C / -30 C
Load resistor	1.26 kΩ at 0.1 percent, against a calculated maximum of 1.275 kΩ
Output voltage limit	250 mV
Peak response	5 pA·cm²/mW at 0 C
Short-circuit current at 40 mW/cm²	196 pA
Isc temperature coefficient	0.067 percent per C
Pointing tolerance	±15 deg
Standoff, sample center to detector face	19.2 mm
Imaged sample area	1.5 x 2.5 cm
Landing load qualification	60 g
Low temperature qualification	-110 C, 1 h dwell
Bakeout	110 C minimum, 50 h
Vacuum test	below 1 torr, 24 h at 40 C

Values from [2].

Detector output feeds the reflectance record from which the metal film wear is read [1]. The load resistor is the design constraint that decides the whole measurement. Photocurrent at maximum illumination is 196 pA, so the resistor must be large enough to develop a readable voltage and small enough that the cell stays near short circuit; 1.26 kΩ against a calculated 1.275 kΩ maximum leaves no margin above, and the 250 mV output ceiling is what sets it [2]. Detector temperature was specified at -50 C and the flight units ran at -30 C, over which range the short-circuit current moves 0.067 percent per degree.

The detector was qualified to 60 g of landing load, to -110 C with a one hour dwell, and through a 110 C bakeout of at least 50 hours plus 24 h below 1 torr at 40 C [2]. Its viewing geometry is fixed at 19.2 mm standoff over a 1.5 by 2.5 cm sample area with a ±15 degree pointing tolerance, so alignment error translates directly into apparent reflectance change and had to be held mechanically rather than calibrated out.

Surface conditions measured

Quantity	Value
Mean atmospheric pressure	6.7 mbar, varying 0.2 to 0.3 mbar daily
Atmospheric opacity	0.5
Suspended dust particle size	approximately 1 µm
Precipitable water vapor	0.01 mm
Maximum daytime / minimum night temperature	-9 C / -76 C
Temperature sensor heights on the lander mast	25, 50, 100 cm

Values from [1].

Winds were light and variable relative to the Viking sites: steady from the south at night, rotating clockwise south to west to north to east through the day [1]. Later compilation across all landed sites puts daytime wind maxima and a clockwise diurnal rotation of wind direction as the general low-latitude pattern, driven by convective mixing [4]. Dust devils were detected repeatedly from mid-morning through late afternoon, with marked temperature and pressure signatures as they passed the lander. Those figures sit inside the ranges later compiled across all landed sites, where measured air temperature spans 140 to 300 K and background column dust optical depth stays below 1 outside storm season [4].

Background column dust optical depth over the aphelion half of the Martian year sits below 1, and the opacity of 0.5 measured through these 83 sols is within that range rather than elevated by regional activity [4].

Ground operations

The mission returned 83 percent of a projected objective across 230 commanded maneuvers [1]. The command cycle was one uplinked plan per sol executed against onboard hazard handling, because the one-way light time exceeded ten minutes. Eleven Wheel Abrasion Experiments were run inside that cycle, each costing seven wheel revolutions and a stationary interval with five wheels locked, which is why the total across the mission is only 39 revolutions of instrumented wear [1].

The WAE strips carried five film thicknesses of each metal on one substrate, so a single run returns a wear-versus-thickness curve rather than a single wear datum, and the inverse relationship observed for aluminum is what identifies abrasion rather than contamination as the cause of the reflectance change [1].

Technologies developed

Rocker-bogie mobility, onboard hazard avoidance from stereo plus laser ranging, and the sol-by-sol command cycle were all first flown here and are carried by every later Mars surface mission [1]. The dig-in depths it measured, 0.6 to 1.5 cm depending on material, are the earliest in-situ constraint on the bearing behavior later quantified as cohesion at or below 1 kPa for sand and up to 11 kPa for indurated soils [3].

The Wheel Abrasion Experiment established a method that has not been repeated: using the rover’s own wheel as a tribometer, with a graded set of metal films as the calibrated reference, to bracket the hardness of the surface material. It cost 10 g of detector and one modified wheel [2]. Its accidental detection of electrostatic charging, and the grain size limit that follows from it, is a result no imaging or spectroscopic instrument on the vehicle could have produced [1].