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Appeared in The Year in Computing, an annual review

Computers on Mars: The 1997 Pathfinder Mission

by Stephen Morrill


The 1997 Mars Pathfinder mission, and its lander/rover experiment, was an example of NASA thinking that it should be possible to perform unmanned planetary exploration with low-cost, relatively low-tech equipment. The equipment had to be either operated remotely from Earth or had to be able to perform on its own, using instructions sent from Earth on a daily basis or, in some instances, pre-loaded into computer memory.

Having no human repairman along meant the systems on the Mars mission had to be tough and either able to operate independently or over a very long radio link, with an 11-minute (more or less) time-lag. Computer systems needed to be able to withstand the shock of launch from Earth and of landing on Mars.

They had to contend with extreme cold, and with being bombarded by assorted types of radiation. On top of all that they had to be cheap; the entire mission was budgeted for $171 million, about one twentieth of the cost of the 1975 Mars Viking mission , NASA's last visit to the red-ink planet.

The Jet Propulsion Laboratory in Pasadena, California is NASA's unmanned space exploration headquarters. JPL looked to the commercial market for equipment that could be made to work, rather than using the traditional route of designing everything from scratch. Planners used a straightforward set of parameters:

• Should we make the equipment ourselves, or buy it from an outside supplier who can build them to our requirements?

• If we buy equipment, should we buy commercial grade or military grade? (Military grade may be more rugged, may have rugged cases, or may have redundancies built-in to protect from errors.)

• If we buy commercial grade, can we reliably fly it to Mars?

• If we buy military grade, can we carry the heavier weight and provide the larger power supply needs--if there are such?

• What kinds of modifications and tests do we need to perform on the hardware to prove their reliability?

• If we make the equipment, will we have enough time and money?

In the end, hardware and software was off-the-shelf and commercial grade in most instances, with modifications at sensitive points for the hazards of spaceflight.

Lloyd Keith, Cognizant Engineer for the Mars Pathfinder Flight Computer and Operating System, explained the advantages: "By having both real time operating system and hardware that came out of the commercial world, with millions of hours of operations on them, we had products that were very solid from the beginning. Anything anybody could have done to them, they already had done. That equipment had been through the wars."

For the transit from Earth to Mars, for example, "We used VxWorks," (by Wind River Systems) said Glenn Rogers, Flight Software Cognizant Engineer for the project. "This gave us a lot of flexibility in testing. The fidelity wasn't perfect since our equipment ran it slightly faster than the flight computers did. But it allowed us to cheaply set up real environments for every developer on the software team."

" With the typical space program, you build a computer, then build the operating system, then after that you build your applications code," Said Keith. "You get only a limited set of tools at each level because there is no time or money to build what you would like to have." But with this project, "We could set up our software development immediately, in a tool-rich environment. And, using commercial products, we not only get a wider range of tools, but we get better code-writing and debugging. We're actually sharing the cost of development with all those commercial users."

The Lander used a derivative of a commercial IBM R6000 computer with VME bus, 22 MIPS (million instructions per second), 128 MB of DRAM (dynamic RAM, used in place of a standard hard drive). The DRAM stored photo images and data for later transmission to Earth.

The Rover computer was trickier to adapt, given that the entire vehicle was the size of a microwave oven. "We used a fairly antiquated processor," said Jacob Matijevic, Rover Manager. The result was an Intel 80C85 CPU (central processing unit)with a multiplexed address/data bus with an 8-bit address bus and an 8-bit data bus. Designed for very low power consumption, the unit operated on 3 to 6 volts. Total memory was 672 KB, of which 160 KB was EEPROM (electrically-erasable, programmable ROM). Of the remaining 532 KB of useable memory, 48 KB was hardened against radiation to protect it from crashes caused by passing cosmic rays. This is where the operating system was stored. The rover too had no hard disk but stored images and data in RAM until it could transmit those to the Lander.

Surprisingly, the Rover's CPU ran at a snail-paced 2 MHz, about 1/100th the speed of today's laptops. But the Rover was no Indy-500 car, and the controls were more than adequate.

" The principal software language is C," Matijevic said. "We used a compiler, and did some assembly language development in areas where we had time-dependent I/O for select components, primarily for the cameras. 'C' has some common characteristics in the sense of there being an expertise across the programming community. It has good structure/programming characteristics, allowing for a natural language instruction set. And it tends to document itself readily."

The main software programs were loaded into the Lander and Rover before launch, but programmers could send short scripts or "sequences" at need. A sequence might cover a day or less (when the Sojourner was busy on the surface of Mars) or as long as a month (when the spacecraft was en route to Mars).


The Mars Pathfinder launched on December 4, 1996 atop a Delta II rocket. The objective was a part of the Ares Vallis on Mars. Given the distance and long flight time, five "burns" or trajectory corrections were planned for the mission, the ultimate objective being to arrive at a specific point above the atmosphere of Mars on July 4, 1997. NASA likened this to a golf game where you have just five strokes to reach a hole that is more than 497 million kilometers (309 million miles) from the tee. (The distance is along the curving flight path.)

After a seven-month low-speed, low fuel consumption flight, the Pathfinder arrived on target after just four correcting maneuvers, scoring a "birdie" in NASA's golf game.

The Lander arrived within 20 km (13 miles) of its target center. It slowed its descent with a parachute and retro rocket and cushioned the final impact with surrounding airbags. The Lander hit the surface at about 40 mph, bounced 50 feet into the air, and bounced fifteen more times before finally rolling to a stop 2.5 minutes later and 1 km from the initial impact. Though designed to "right" itself no matter which of its four sides ended up face down, the Lander happened to come to rest on its base.

The Mars Pathfinder's Rover rolled onto Mars' surface at 0540 UT (1:40 AM EDT) on July 6. The little robot was an instant hit with the public and scientists alike. Named *Sojourner* after 19th century black abolitionist and women's rights advocate Sojourner Truth, the little Rover's daily adventures made nightly news as it drove around poking at rocks, testing the soil underfoot, and measuring temperatures.

Back in Pasadena, California, programmers at JPL planned each move of Sojourner by testing it first on a sample "test bed" made up to look like the landing site. A spare rover, this one named Marie Curie and an exact replica of Sojourner, would perform each instruction first to see if it was feasible. Only then would the new instruction set be transmitted to the Lander for relay to Sojourner. Sojourner's radio, with a signal similar to a small walkie-talkie, had a limited range. Programmers also took care not to send the Rover behind a rock or down into any depression that might take the Rover's antenna out of line-of-sight to the Lander.

Each day (or "Sol" as scientists call the 24.5-hour Martian day) the Rover would wake up when the solar panels began to function, then check in with the Lander. Programmers would upload the entire day's instruction at one time. The 11-minute communications delay (almost 23 minutes round-trip) meant the Rover had to act autonomously, using command sequences stored in computer RAM.

Software, running on a Silicon Graphics Onyx2 graphics supercomputer, permitted the uplink team to generate each day's command sequences for the Rover, using a graphical user interface of clickable buttons, sliders, and some text input boxes. The team would select each Rover command from prepared lists, then input the command parameters manually. Complex sequences could be built up in this fashion, each command (or a sequence) being first tested on Marie Curie.

One of the Lander's first jobs had been to transmit sufficient imagery back to Earth to permit technicians to built a virtual 3-D model of the landing site. Now, the "Rover driver" could see the Martian landscape, on his computer, using 3-D goggles. Using a joystick he would "drive" a virtual Rover across the Martian terrain while the computer made precise measurements of waypoints along the intended track, digitizing these as "go to here" commands to incorporate into the next day's command sequence. Lest anyone think the programmers at JPL have no sense of humor, they used images of giant "lawn darts" to mark each Rover waypoint on the 3-D computer picture.

Cold-sensitive computer and radio components were inside a Warm Electronics Box (WEB) in the Rover, and a battery provided a trickle charge to this area for heating through the Martian nights when the Rover sat idle. The box was effective, keeping the Rover's internal temperature at a toasty +40 to -22 degrees centigrade, while outside the temperature dropped to as low as -88 degrees centigrade. Power to operate the Rover was provided by solar panels, and the battery was not rechargeable. Planners decided against a rechargeable battery because it would have required some sort of trickle charge itself during those seven long months the Rover was snuggled inside the spacecraft en route to Mars.

The combination Rover/Lander had been planned to operate on the Martian surface for seven days. If they were still functional, scientists could then go to an "extended mission" of 30 days. This was done, but on September 28, 1997 , after 85 days, the Lander failed to check in with Earth and all subsequent attempts to communicate have failed. Most likely reason for the loss of signal is the effects of extreme cold on the batteries. When the batteries died the clocks lost their time hack, and then the Lander simply didn't know when it was time to phone home. The Rover had been designed to operate on solar power even after its batteries had expired, but of course it cannot talk to Earth directly.

But the mission was an unqualified success. Both Lander and Rover had worked as planned, and both had worked far beyond their designed life span. In the process they gathered much information about Martian soils, rocks, atmospherics, water vapor, and more.

" This mission cost $175-million, less than the cost of some Hollywood movies," Lloyd Keith noted. "And I have yet to hear anyone say, 'I didn't get my $8 worth watching Mars Pathfinder on July 4, 1997.'"

— end —



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