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MARS ROVERS: PAST, PRESENT AND FUTURE
Princeton Space Studies Institute's 20th Anniversary Conference
10 May 1997

Donna L. Shirley
Jet Propulsion Laboratory - California Institute of Technology
MS 264-255, 4800 Oak Grove Drive
Pasadena, California 91109
E-mail: dshirley @jpl.nasa.gov
Phone No.: [phone]
Fax No.: [phone]

Jacob R. Matijevic
Jet Propulsion Laboratory - California Institute of Technology
MS 230-235, 4800 Oak Grove Drive
Pasadena, California 91109
E-mail: Jacob. R.Matijevic @jpl.nasa.gov
Phone No.: [phone]

Abstract

Since the 1960's there have been efforts world-wide to develop robotic
mobile vehicles for traversing planetary surfaces. Two Lunakhods were
successfully operated on the Moon in the early 1970's, but since then
there have been no planetary rovers. Developments in mobility, navigation,
power, computation, and thermal control have now allowed a small, 11.5 kg
rover named Sojourner Truth to be heading for Mars. Sojourner will
explore an area within site of the Pathfinder Lander's camera, making
measurements of the surface properties, and imaging rocks and obtaining
their elemental composition. Future U.S., and perhaps Russian rovers are
planned to go to Mars in 2001, 2003, 2007 and 2011 to rove 10 kilometers
or so and collect samples for return to earth by missions launched in
2005, 2009 and 2013.

Introduction

On July 4, 1997, earthlings will invade Mars. Mars Pathfinder and its microwave
oven-sized rover, Sojourner, will land in an ancient flood channel and try to
find out what Mars is made out of. In March of 1998 Mars Global Surveyor will
begin a two earth-year mapping mission to study the surface and atmosphere of
the red planet. These missions will be followed every 26 months by additional
missions to search for water and clues to whether life ever began on Mars. 1,2

The Mars Exploration Program, which was initiated in 1994, is launching two
missions every 26 months to Mars to study it from orbit and in detail on the
surface. The program, managed by the Jet Propulsion Laboratory for NASA, is
currently funded at a total of about $150M per year - or per mission- which
is about the cost of a major motion picture. This is in contrast to the last
Mars mission, Mars Observer, which cost nearly $1 billion including the launch
vehicle and operations. Viking, in today's dollars, cost over $3.5 billion.

International participation is an important factor in the program, and
relationships are being established with Russia, Europe and Japan.

Since the discovery of possible signs of ancient life in the Martian meteorite
ALH84001 last August, excitement about Mars exploration has intensified. NASA
and JPL have developed a plan for an expanded program to include returning
carefully selected samples from the surface of Mars for analysis on earth.
Augmented funding for this program is included in the President's 1998 budget
request, but must be appropriated by Congress. Current information on the Mars
Exploration Program is available on the Intenet at http://www.jpl.nasa.gov/mars.

The ability to move about the Martian surface is key to the Mars Exploration
Program. The first U.S. robotic planetary rover is Sojourner, (the Mars
Pathfinder Microrover Flight Experiment). Sojourner is a NASA technology
flight experiment which is currently on its way to Mars.3


Rovers in the Past

Sojourner is the culmination of a long line of designs and test models for
planetary rovers. Beginning with the first successful planetary rovers,
Lunakhod, which roamed the lunar surface in the early 1970's, many countries
have been involved in rover research and development. An early focus was on
mobility. It was quickly found that six wheels were better than four, with
each wheel needing its own motor. U.S. researchers in the 1960's built
several versions of possible planetary mobility systems and tested them in
the field.

The problem of navigation and control was, however, harder to deal with in
the early days. Computers were large and slow and hard to program. Lunakhod,
for example, used simple teleoperation so that the operator could "joy stick"
the rover to direct it. But Lunakhod proved very difficult to drive: it had
only a monocular, black and white camera, and the 3 second round-trip radio
delay made driving very counterintuitive and exhausting.

Artificial intelligence was focused on rover navigation by DARPA in the 1980's
as part of the Automated Land Vehicle Program. Martin Marietta Corporation
and Carnegie Mellon University both built vehicles that could automatically
drive on roads and around obstacles. JPL outfitted a HMMV army vehicle to be
remotely controlled in ragged terrain.

JPL's technique involved an operator viewing stereo pictures taken by the
vehicle and then plotting the vehicle's path and radioing it to the vehicle.
The vehicle would then automatically follow that path using a compass and
its own vision system. This system was known as "Computer Aided Remote
Driving" or CARD.

With improvements in computers came the ability to investigate legs for mobility.
Ohio State University built a legged vehicle for the army. Its gait was automated,
but it required a driver to direct it. Carnegie Mellon University built several
walkers, the most successful being Dante, which descended into the inferno of
Mount Spur. Martin Marietta built a model of a simple "beam walker" which
involved raising and lowering legs which slid along a beam. Tiny, legged rovers
were built by MIT and IS Robotics.

Navigation systems also became more complex. At JPL a "little blue rover" about
6 feet long was used as a navigation test bed. In the late 1980's a computer
that could handle autonomous navigation was too big to be carried by the rover,
so it dragged cables connected back to a VAX computer in the lab.

In the late 1980's the Mars Rover Sample Return (MRSR) mission studies gave
hope to the rover community that at last a rover would go to Mars. The mission
was designed to land a pickup-truck sized rover to go hundreds of kilometers,
collecting a wide variety of rock samples, and returning them to a sample
return vehicle to bring back to earth. In order to navigate these rovers high
resolution orbital imaging was planned. These pictures would be used for an
operator to plan a basic course for the rover, and for the rover to automatically
locate itself within the resulting map. This was known as "Semi-Autonomous
Navigation" or SAN. Because the computers even in the early 1990's were pretty
bulky, the test vehicle for SAN was "Robby" which featured six, one-meter
wheels and a large, ragged body to carry its "brain".

In 1988 JPL, Martin Marietta, and FMC Corporation developed three independent
designs for integrated control/mobility systems for large rovers, and then
compared them to develop     4,5 requirements for the sample collecting rover.

Naturally, MRSR required a large sample return vehicle to bring back the 10
kilograms or so of sample that the large rovers would collect. And, also
naturally, because so much was invested in each vehicle there should be two of
everything to prevent total mission failure caused by any one element: orbiter,
lander, rover, ascent vehicle, earth return vehicle. The total price tag for
the mission began to approach $10B, and in 1991, when Congress slashed all
funding for Mars missions- human OR robotic, MRSR was canceled. Rover designs
which could "scout" and "survey" sites for human missions were also being
studied in the early 1990's. Again, these were large rovers.6,7,8,9

In the meantime, work on small rovers was going on. Rodney Brooks at MIT
created what he called a "subsumption architecture" for navigation, based on
insect behavior.10 Simple behaviors could be implemented in a simple,
light-weight and inexpensive computer. David Miller and his team at JPL built
small rovers based on this technique, the first of which was "Tooth". Don
Bickler of JPL developed a six-wheeled mobility system which by hooking the
wheels to levers rather than directly to the body allowed a stable platform,
suitable for mounting instruments and solar arrays. Don built small models
intended to be 1/8 scale for MRSR rovers. They were known as "Rocky", short
for "rocker bogie". When "Tooth's" brain was added to Rocky's body a small,
autonomous rover was born.

"Rocky    3"    demonstrated a fully autonomous traverse and sample collection
and return to a simulated sample return vehicle in 1990. In June of 1992 a
team led by Lonne Lane of JPL demonstrated "Rocky 4", a small rover which
carried out many of the functions which would be required of an actual flight
rover: mobility, instrument operation, and goal setting by an earth operator
but autonomous path execution and hazard avoidance. Rocky 4 was the starting
point for the Mars Pathfinder Microrover Flight Experiment which resulted in
Sojourner. 11,12,13

Sojourner Truth, the Rover

Sojourner, named as a result of a children's essay contest, is a NASA technology
demonstration flight experiment which is integrated with the Mars Pathfinder
(MPF) lander and was launched on December 4, 1996. After landing on Mars
July 4, 1997, the MFEX rover will deploy from the lander and conduct a series
of experiments which will validate technologies for an autonomous mobile vehicle.
In addition, Sojourner will deploy its science instrument, an alpha proton
X-Ray spectrometer (APXS), on rocks and soil to determine the elemental
composition. Lastly, the rover will image the lander as part of an engineering
assessment after landing. 1 15

Sojourner Description

Sojourner (see Fig. 1) is a 6-wheeled vehicle, 10.5 kg in mass (including payload),
and 65 cm long, 48 cm wide and 30 cm tall in its deployed configuration
(neglecting the height of the UHF antenna). The rover is stowed on a lander petal
for launch and during the cruise-to-Mars phase of the Pathfinder mission. In
this stowed configuration, the rover height is reduced to 19 cm. In this
configuration, the rover has been tested and shown to withstand static loads of
66g, consistent (with margin) with the less than 40g expected at impact upon landing
on Mars. At deployment, the lander fires cable cutting pyros, releasing tie-downs
which restrain the rover to the stowed configuration. Under command, the rover
drives its wheels, locking the bogeys and deploying the antenna so that the
deployed configuration is achieved.

In the deployed configuration, the rover has ground clearance of 15 cm. The
distribution of mass on the vehicle has been arranged so that the center of mass
is slightly aft of the center of the body (the Warm Electronics Box (WEB))and at
a height at the base of the WEB. Due to the innovative design of the rear
differential which transfers wheel angle to half-angle tilt of the WEB the
vehicle could withstand a tilt of 45 degrees in any direction without over-turning,
although fault protection limits prevent the vehicle from exceeding tilts of 35
degrees during traverses.

The rover is of a rocker bogie design16 which allows the traverse of obstacles of
up to 17 cm (more than a wheel diameter (13 cm) in size). Each wheel has cleats
and is independently actuated and geared providing for climbing in soft sand and
scrambling over rocks. The front and rear wheels are independently steered,
allowing the vehicle to turn in place. The vehicle has a top speed of 0.4rn/minute
and can turn at a rate of 7 deg/sec.


The rover is powered by a 0.22 sqm solar panel comprised of 13 strings of 18, 5.5
mil GaAs cells each. The solar panel is backed up by 9 LiSOCL2 D-cell sized
primary batteries, providing up to 150W-hr of energy. The combined panel and
battery system allows the rover to draw up to 30W of peak power while the peak
panel production is 16W. The normal driving power requirement is 10W.

Rover components not designed to survive ambient Mars temperatures (-800 deg C
during a Martian night at the Pathfinder landing site) are contained in the warm
electronics box (WEB). The WEB is insulated with solid silica aerogel, has an
exterior finish of gold-coated kapton, and is heated under computer control during
the day. This design has been verified in both stand-alone and integrated (with
the lander) environment tests showing that the WEB maintains components between-40
deg C and +40 deg C during all mission phases including Mars landed operations.
These test have led to the development of a thermal model of the WEB with performance
under a variety of environment conditions.17

Computer control is implemented by an integrated set of computing and power
distribution electronics. The computer is an 80C85 with a 2 MHz clock rated
at 100 Kips which uses, in a 16 Kbyte page swapping fashion the memory
provided in 4 different chip types.

At boot up or upon reset the computer begins execution from the PROM.
The programming stored in PROM loads programs into the RAM (IBM 2568 chip set)
from non-volatile RAM (SEEQ chip set). Program execution proceeds from the
RAM. As commands are executed, other programming in non-volatile RAM is required
and then swapped into the RAM for execution. To prevent excessive thrashing,
some programs are executed from non-volatile RAM. While programs are executed,
data is stored in temporary RAM storage area (Micron chip set). Telemetry
generated during program execution is regularly transmitted to the lander for
relay to earth. If communication with the lander is not available, telemetry
is stored in a FIFO rotating buffer, reserved in the temporary RAM storage area.
At boot up during night operations, provision for data storage in non-volatile
RAM is provided. Such data is sent as telemetry to the lander when a
communication opportunity permits.

The remainder of the electronics supports switching, power conditioning,
and I/O channels.

Vehicle motion control is accomplished through the on/off switching of the
drive or steering motors. An average of motor encoder (drive) or potentiometer
(steering) readings determines when to switch off the motors. When motors are
off, the computer conducts a proximity and hazard detection function, using
its laser striping and camera system to determine the presence of obstacles
in its path. The vehicle is steered autonomously to avoid obstacles but
continues to achieve the commanded goal location. While stopped, the computer
also updates its measurement of distance traveled and heading using the
average of the number of turns of the wheel motors and an on-board gyro.
This provides an estimate of progress to the goal location.

Command and telemetry is provided by UHF radio modems on the rover and lander.
The modems are capable of 9.6 Kbaud transmission. Overhead associated with
the protocol in data transmission and the effective link performance results
in 2 Kbps transmission. Estimates of the amount of telemetry data transmitted
by the rover during its operations is given below. In general, most of
engineering data collected by the rover supports. technology experiments. The
amount of experiment data listed is aggregates for all science, technology and
mission experiments, although not all experiments are scheduled to be performed
each sol. The available data volume between the lander and rover is 14.4 Mbit
and is based on 2 hour of continuous transmission during a given sol. In these
estimates no 'overhead' data (e.g., header, frame protection, re-transmission
of frames) is included.

Sojouner's Mission

During the day, the rover regularly requests transmission of any commands
sent from earth and stored on the lander. When commands are not available,
the rover transmits any telemetry collected during the last interval between
communication sessions. The telemetry received by the lander is stored and
forwarded to the earth. In addition, the communication system is used    to
provide a 'heartbeat' signal during vehicle driving. While stopped the rover
sends a signal to the lander. Once acknowledged by the lander, the rover
proceeds to the next stopping point along its traverse.

The rover's mission consists of: (1) conducting a series of experiments which
validate technologies for an autonomous mobile vehicle, (2) deploying an alpha
proton X-Ray spectrometer (APXS), on rocks and soil, and (3) imaging the
lander as part of an engineering assessment after landing. The rover's mission
plan for the first week on Mars is shown in Fig. 2.

Fig. 2. Mars Pathfinder lander and rover: nominal 7 sol scenario

The first sol on Mars will be devoted to the release and deployment of
Sojourner from the lander, and the acquisition of an AP-XS measurement on
soil. If possible, the end of sol imaging performed by the rover will
include a portion of the lander. The next two (as necessary) sols will be
used to acquire an APXS measurement of some rock in the vicinity of the lander.
Along the traverse to the rock, a soil mechanics technology experiment will
be performed. With success, sols 4-6 will be devoted to positioning the
APXS for measurement of a specific rock: the rock selected through an
evaluation of the panoramic image of the landing site taken by the lander
camera. Along the traverse to this rock, another soil mechanics technology
experiment will be performed, in a second soil type. In addition, images
from the rover will be taken of rocks, soil and terrain to assist in the
terrain reconstruction at the landing site. Images of the lander will be
taken in an attempt to complete a full survey. The remaining sol in the
7 sol nominal mission will be devoted to ensuring data has been collected
for all the technology experiments while Sojourner is traversing to
another rock or other objectives in the extended mission.

Technology Experiments

The primary function of Sojourner is to demonstrate that small rovers can
actually operate on Mars. Until at least 1995 there was considerable skepticism
by the science community that the capabilities of such a small rover to
collect science were worth the $25M investment in the rover. Therefore,
Sojourner will perform a number of experiments to evaluate its performance
as a guide to the design of future rovers. The performance of these
experiments on Mars will assist in verifying engineering capabilities for
future Mars rovers, completing a data set which includes environment and
performance testing conducted with Sojourner prior to launch and with the
spare rover (Marie Curie) on earth in parallel with Sojourner's experiments
on Mars. The technology experiments are listed below:

╖ Mars Terrain Geometry Reconstruction from Imagery- Each sol, images are
taken by the rover and lander as a means of planning the next sol of
operations. As a collection -these images will be used to construct a map
of the landing site.

╖ Basic Soil Mechanics- In a soil sample, as a single front or rear wheel
is turned in place, the motor current is measured and an estimate of
torque is derived.

╖ Dead Reckoning Sensor Performance and Path Reconstruction/Recovery - The
telemetry logged by the rover during traverses provides a means of
reconstructing the path traversed by the vehicle.

╖ Sinkage in Each Martian Soil Type - At the end of selected wheel rotations
performed during the soil mechanics experiment, images of the resulting rut
are taken.

╖ Logging/Trending of Vehicle Performance Data -During vehicle operations,
engineering measurements are taken regularly which will help to verify
rover performance.

╖ Rover Thermal Characterization - The rover has 7 temperature sensors
internal to the WEB and 6 external sensors. These sensors will be sampled
during both day and night each sol in tracking the thermal characteristics
of the vehicle.

╖ Rover Imaging Sensor Performance - Engineering telemetry gathered during
traverses are the primary means for the reconstruction of paths taken by
the rover across the terrain and evaluation of the navigation and hazard
avoidance systems.

╖ UHF Link Effectiveness- The rover routinely communicates with the lander,
transmitting telemetry and receiving commands. Data transfer errors will be
logged to develop a model of the UHF link effectiveness.

╖ Material Abrasion - Wheel material wear can be correlated to the amount
of abrasion caused by Martian soil per distance of wheel travel.

╖ Material Adherence - Power from a "clean" solar cell will be compared with
that from a "dusty" cell. The correlation between the amount of dust and
cell output measure the effect of dust on solar panel performance during
the mission.

╖ APXS- In addition to the technology experiments the Alpha Proton X-Ray
Spectrometer and the visible and near infrared filters on the lander
imaging system will determine the elemental composition and constrain the
mineralogy of rocks and other surface materials at the landing site.

╖ Lander Assessment - During the mission, the rover cameras will be used
to image portions of the lander.

After landing, Sojourner will be deployed from the lander and begin a nominal
7 sol (1 sol = 1 Martian day) mission to conduct it's experiments. This mission
is conducted under the constraints of a once-per-sol opportunity for command
and telemetry transmissions between the lander and earth operators. As such,
Sojourner must be capable of carrying out its mission with a form of
supervised autonomous control, in which, for example, goal locations are
commanded and the rover navigates and safely traverses to these locations.

Sojourner's Operation

The operation flow for the rover is driven by a daily command load from earth
via the lander. These commands are generated at the rover control station, a
silicon graphics workstation which is a part of Pathfinder's ground control
operation. At the

end of each sol of rover traverse, the camera system on the lander takes a
stereo image of the rover vehicle in the terrain. Those images, portions of
a terrain panorama and supporting images from the rover cameras are also
displayed at the control station. The operator is able to designate on the
displayed image(s) points in the terrain which will serve as goal locations
for rover traverses.18 The coordinates of these points are transferred into
a file containing the commands for execution by the rover on the next sol.
This command file is incorporated into the lander command stream and is
sent by Pathfinder ground control to the lander for transmission to the rover.

Engineering telemetry which is transmitted from the rover to the lander is
transmitted by the lander back to earth on a priority basis, including lander
images and rover position data needed to develop the command sequence for
execution on the next sol. Analysis of this telemetry is conducted through
the rover engineering team's workstations.

In this telemetry analysis an engineering go/no-go decision is reached concerning
the execution of a nominal "next sol" sequence of rover activities (e.g., see
Fig. 2 for the first 7 sols). In the presence of a "go", a brief review of the
mission objectives of the next sol of rover operation is performed by members of
the experiment teams. Any modification of targets of opportunity (e.g.,
locations for soil mechanics experiments, rock selected for eventual placement
of the APXS) based on a review of the images from the prior sol is evaluated as
part of a trafficability and mission time assessment performed by engineering
personnel and the rover operator. An agreement on the targets of opportunity
results in an update (perhaps) of the sequence of rover activities. This update
is used by the rover operator to prepare the command sequence for submission to
the rover. A review of the sequence (in a human readable form) by experiment
and engineering personnel both for the rover and the Pathfinder mission
represents the final check (and edit) before transmission.

Autonomous Control

In order to accomplish the mission objectives, the rover must traverse extended
distances within the vicinity of the lander. The once-per-sol commanding strategy
of the mission require that the rover perform these traverses essentially
unsupervised by earth-based mission control. The rover uses a strategy of
on-board autonomous control which allows the commanding of high-level, goal-oriented
commands supported by a hazard avoidance system.19,20 In this system the
rover attempts to determine where to go, drive toward the location, avoid
obstacles along the route and decide when (or if) it has made sufficient
progress to the goal.

The rover performs a traverse by executing a "go-to-waypoint". During the traverse,
the rover updates its position relative to the lander to determine (at a minimum)
if it has reached the goal location. This update is accomplished using the
encoder reading on the wheel actuators. The counts accumulated on each of the
six wheels are averaged to determine a change in the odometer. This averaged
value is used to update the estimated vehicle position in the lander-centered
coordinate system.

To change heading the rover executes a command to "turn. The four outside wheels
are cocked to a 'steer-in-place' orientation through driving the steering
actuators to the appropriate position as measured by the potentiometers on
each actuator, and the wheels are driven. Once the commanded orientation is
achieved, the integrated angular measurement from the gyro is used to update
the vehicle heading reference.    The "go-to-waypoint" and "turn" commands are
developed by the rover operator using the once-each-sol stereo image of the
rover taken by the lander camera.

In achieving the goal locations of its traverse commands, the rover must determine
a safe path for traverse at any distance from the lander. The onboard hazard
detection system provided by the front camera system and laser light stripers
illuminate a part of the region in front of the vehicle and the results are
correlated to develop a sparse map of obstacle distances and heights in front
of the vehicle.

The map is then assessed to determine vehicle trafficability. If a hazard is
detected as a result of this assessment, the rover autonomously turns. The hazard
detection and assessment is then repeated until a clear path is identified. The
rover is then autonomously driven past the hazard. The goal location again becomes
the objective of the traverse and the rover turns back to the proper heading.
The rover also measures tilt to avoid slope hazards.

Each 'go-to-waypoint' command executed in a traverse has as a parameter a time
value for execution. During the execution of any command and hazard avoidance
activity the rover updates position and orientation. An assessment of progress
to the goal location is performed. When the onboard estimates come within a
threshold of the goal location, the rover stops, sends telemetry collected
during the command execution and proceeds to execute the next command in
the operative sequence.

During testing with the vehicle system, the combined navigation and hazard
avoidance function has been shown to accumulate an error of approximately 7%
of the range of travel. By traversing a sufficient distance MFEX can miss rock
destinations. However, a combination of correction in position using lander
camera images with a nominal 2-sol (at least) mission plan to reach a specific
rock should allow imaging and deployment of the APXS at specific sites.

The successful conclusion of Sojourner's mission will not only result in a
great increase in our knowledge of the composition and characteristics of
the Martian surface, but will provide insight for the design of future rovers.

Future Mars Rovers

In January 1999 the U.S. will launch a lander with a robotic arm to land near
the south pole. While this mission has no rover the arm will determine a lot
about the soil characteristics in a new region by digging a trench. In 2001 a
rover much more capable than Sojourner will be sent to Mars. The 2001 rover
will be capable of traversing "over the horizon" perhaps 10 kilometers or more,
to explore a region of Mars which might contain evidence of past life, and to
collect rock and soil samples. The 2001 rover is still very mass, power, volume
and cost constrained. It is anticipated that the rover and its payload will
mass less than 50 kg and cost no more than $45 M. In order to achieve this
great increase in performance for a very modest increase in cost the 2001 rover
will be based on technology work currently going on at JPL.

There are two "branches" to this technology development: "Rocky 7" being
developed by a team led by Samad Hayati,21 which is primarily a navigation and
control development platform: and the Lightweight Survivable Rover (LSR) which
is primarily a platform for developing advanced mobility, thermal control and
mechanisms. The LSR team is led by Paul Schenker. In FY98 the two platforms
will be brought together to demonstrate a robust, relatively long range mobility
and navigation capability with the capability for the rover to survive long
enough to travel a long distance over many months. Sampling capabilities will
also be
demonstrated. An  "Announcement o f Opportunity" for selection of an integrated
payload for the 2001 rover is in preparation and will be released in the
summer of 1997.

Also in 2001 there is an opportunity for a joint U.S.-Russian mission to land
a Marsokhod (a fairly large, 6 wheeled rover). The mission would be developed,
launched and operated by the Russians with communication being provided
through U.S. orbiter relays. In 2003 a U.S. rover will be flown, essentially
a copy of the 2001 rover with some opportunity for technology upgrades.
Upgrades will be minimal within the strict funding limitations of the program.


In 2005 it is currently envisioned that the sample return vehicle would land
close to one or the other of the 2001 or 2003 rovers with their caches of
samples. There would be a small, short-range rover carried by the sample
return lander to retrieve the cache of samples and bring it back to the return
vehicle. The program concept includes at least two more rounds of rovers and
sample returns to acquire three distinct samples from different areas of Mars
to give a good chance of detecting signs of past or present life. Since
sample return is a 3 year round-trip, collecting three samples will not be
accomplished, until about 2016. Rovers would fly in 2007 and 2011, with sample
returns in 2009 and 2013.

Rover Design and Technology Needs

As the Mars Exploration Program progresses the need for more sample diversity
and the selection of more specific samples will require continuous improvements
in rover mobility, navigation, and sampling capability. To enable roving
within the tight cost constraints of the program will require low mass, extreme
power efficiency, and great resistance to low temperatures. Therefore,
ongoing advances in rover technology are required.

Some of these technology needs are:

╖ Efficient Mass/Volume
-    For Low Cost Delivery
-    For Maximizing Payload

╖ Tens of Kilometers Travel Capability (In Weeks or Months)
-    Continuous Mobility
-     Improved Navigation (Sensors/Software) - Efficient Communication With
      Orbiters - Physical Robustness
╖ All Season Survival
-    Thermal Protection
-    Efficient Energy Generation/Storage/
Management
-    Robustness to Environment

╖ Sample Selection/Collection
-     Instrument Support (Power/Volume/Thermal/Data)
-    Improved Instruments
-     Manipulation/Deployment
-     Sample Caching
╖ Planetary Protection
-    Forward and Backward Contamination Protection

Applicable technology is being developed worldwide. But to make the best
use of this technology its development must be directed by rover system
design and evaluation. In 1994 Kenton Leitzau of MIT based his master's
thesis in Systems Engineering on the development of a process for rover
design and evaluation.22 He stressed the need for standard metrics and
evaluation techniques to guide rover technology development and design.

Examples of parameters which need to be considered in rover design and
research include:

╖ Mobility
-    Reduced Gravity Operations
-    Traction
-    Turning Radius
-    Tipping Resistance
-    Slipping Resistance
-    Slope Capability
-    Obstacle Handling
-    Energy Consumption
-    Speed
╖ Navigation & Control
-    Computation
-    Sensors
-    Obstacle/Hazard Avoidance
-    Speed
-    State Knowledge
-    Energy Consumption
-    Safety

╖ Science Support
-    Operational Scenario Execution
-    Experiment/Instrument Support

╖ Autonomy
-     Autonomous Navigation - Prioritization/Scheduling - Health Monitoring
    -     Fault Avoidance/Response
    -     Energy Consumption
-     Required Human Interaction

╖ Environmental Stress Resistance/Robustness, e.g.
-    Vibration
-    Shock
-    Pressure Variation
-    Electromagnetic Interference
    -     Vacuum
    -     Thermal Cycling
    -     Temperature Extremes
    -     Radiation

There is a tendency in rover research, as in all research, to focus on
individual technologies, e.g. mobility OR navigation OR sample collection.
But in order to support the pace of rover development needed to support
the Mars Exploration Program a systems approach to rover R&D is required.
Mobility techniques which look good by themselves may founder when mated
with the needs of navigation or science investigations. And demonstrations
which ignore the exigencies of the constraints of actual planetary missions
are not very useful.

It is unlikely in the next couple of decades that funds will become available
anywhere in the world to fly large rovers to Mars. Therefore, rover research
should focus on cost effective, mass and power efficient, high performance
machines which are capable of withstanding large g-forces and great extremes
of temperature, and operating reliably in dangerous and unknown terrains.

Acknowledgment

The research described in this paper was carried out by the Jet Propulsion
Laboratory, California Institute of Technology, under a contract with the
National Aeronautics and Space Administration.

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