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Rolling across the Red Planet:

Pathfinder and Sojourner Explore Mars



Pathfinder photograph of the rocky Martian field with twin peaks in the distance
The rocky Martian field around Pathfinder
with twin peaks in the distance

click NASA image to enlarge

more Pathfinder images

The Pathfinder interplanetary spacecraft from Earth ended its epic, 309-million-mile journey on July 4, 1997, by successfully delivering a pyramid-shaped Mars station, complete with camera, weather tower and instrument-laden rover named Sojourner, in an historic safe landing on the Martian surface at 1707 UTC or GMT

Making the first landing by a U.S. spacecraft on Mars in 21 years, Pathfinder bounced to a halt in the dry mouth of an ancient channel carved by flood waters. Shortly after it landed, scientists back on Earth were surprised to hear Pathfinder transmitting signals they had not expected to hear until four hours later in the day. The radio information told mission control at the Jet Propulsion Laboratory in Pasadena, California, the craft was in a stable condition on the Martian ground with its revolutionary airbags deflated as planned and its three unfolding steel and graphite petal-feet planted firmly on Martian soil.

Pathfinder sent out a 22-lb., remote-controlled buggy named Sojourner to explore the Red Planet and send back photographs of the rocky Martian landscape.


Mars Pathfinder was launched in December 1996 as one of NASA's new inexpensive Discovery-series spacecraft. By landing on the Red Planet, Pathfinder accomplished several firsts, including:
  • First spacecraft to land on a planet without orbiting it.

  • First to deploy a parachute at a supersonic speed of 1,000 mph.

  • First to use airbags to cushion the impact of a landing. The airbags were similar to, but larger than, those used in automobiles.
To salute the national achievement on July 4, 1997, the United States Postal Service issued a commemorative stamp on December 10, 1997.


The first pictures radioed back to Earth from the landing site in the Ares Valles showed many rocks of different sizes, set in a background of reddish soil, under a light brown sky.
  • Between the lander and the horizon is a rock-strewn Martian surface. The rocks are thought to be debris from ancient floods.

  • Between the rocks is brownish windblown soil. Furrows were produced in the soil as the lander's protective airbags were retracted after landing.

  • Large boulders, probably deposited by one of the ancient catastrophic floods that carved the ancient channel, are visible in the pictures

  • Two hills are seen in the distance.

  • The gray-tan hue of the sky above Pathfinder probably is caused by dust particles suspended in the Martian atmosphere.
NASA collection of Pathfinder images

The Landing

Two decades ago, the Viking-1 and Viking-2 landers used a computer-controlled liquid retro-rocket system to achieve a soft landing at about five miles per hour on Mars. This time around, a new system was used to drop the lander safely onto Mars.

The smaller tetrahedral-shaped Pathfinder lander used parachutes, three small solid-fuel rockets and four inflatable air bags constructed of a double-layered fabric surrounding the lander's exterior petals to perform the hard landing at about 35 miles per hour on Mars.
  • Like Viking, the Pathfinder lander arrived at Mars packaged inside a capsule-shaped entry vehicle. Hitting the thin upper atmosphere of Mars at more than 17,000 miles per hour, the entry vehicle's heat shield slowed the craft to 900 miles per hour in about two minutes.

  • A computer in the spacecraft sensed the slow-down in speed and deployed a large parachute. The parachute slowed the lander to 155 miles per hour in the rarified atmosphere of Mars, which is only 1/100th as dense as Earth's.

  • A radar altimeter inside the lander monitored the distance to the ground. At about 330 feet above the surface, the computer inflated the air bags.

  • Seconds later, three solid rocket motors inside the top half of the entry vehicle above the lander were fired. It took about two seconds for the rockets to bring the lander to a stop 40 feet above the Martian ground.

  • The parachute was released, and the lander, nestled inside its protective air-bag cocoon, fell to the ground, bouncing and rolling until it stopped.

On The Ground

Interestingly, Pathfinder landed right side up. Within an hour, the air bags were deflated and partially retracted toward the lander. Pathfinder then opened its three metallic petals. The micro-rover Sojourner, attached to the inside of one of the petals, was exposed to the Martian terrain for the first time.

After the lander camera snapped photos of its position on the Martian surface, engineers back on Earth sent additional instructions to pull an airbag away from blocking the rover. The engineers then worked to fix the radio communication system between the rover and the lander. Finally, the engineers instructed the rover to drive off and begin exploring the immediate surroundings, the ancient Ares Vallis flood plain.

The Pathfinder Lander

The Pathfinder lander represents very high technology:
  • The lander has three solar panels that supply up to 1,200 watt-hours of electrical power per day. At night, the lander operates on rechargeable silver-zinc batteries with a capacity of more than 40 amp-hours.

  • For communications, Pathfinder has a high-gain antenna for high-speed, 2,250-bps communications with NASA's Deep Space Network. It also has a low-gain antenna that sends information at a lower rates of 40 bps, but doesn't need to be actively pointed at Earth.

  • The "IMP" is Pathfinder's main eye on Mars. With its mast fully extended, the IMP (Imager for Mars Pathfinder) stands five feet above the ground. Using eleven individual geologic filters, four pairs of atmospheric filters, two pairs of stereo filters and one close-up lens, the IMP can take color and 3-D pictures of the surface. The close-up lens enables Pathfinder to snap high-resolution shots of magnetic wind-blown dust adhering to a special dust collector mounted on the IMP.

  • Pathfinder's weather tower (Atmospheric Structure Instrument/Meteorology Package or ASI/MET mast) collects atmospheric information from a variety of temperature, pressure and wind sensors. The ASI/MET mast holds temperature sensors at three different heights and uses windsocks to detect wind direction and speed. A wind sensor at the top of the mast senses temperature differences between six temperature sensors to measure wind speed and direction. A sensor on the base of the lander measures atmospheric pressure.

Sagan Memorial Station

Once it had touched down on the surface of Mars, NASA officially re-named the main Pathfinder lander the Carl Sagan Memorial Station after the late astronomer who had devoted his life to making the Universe less mysterious for millions. Sagan was a leading proponent of exploring the planets.

Sagan had served as a consultant for all NASA's spacecraft. His television show "Cosmos" retraced 15 billion years of cosmic evolution. Sagan, 62, died in December 1996 after suffering bone marrow disease for two years.

Louis Friedman, executive director of the Planetary Society said, "This is the first arrival of a U.S. spacecraft at Mars that Carl ever missed." Ann Druyan, Sagan's wife, said, "That he should touch another world so profoundly fills me with joy." The Sagan Memorial Station never will leave Mars.

In 1981, NASA had re-named the Viking I lander, which landed on Mars in 1976, for NASA administrator Thomas Mutch.

The Sojourner Rover

The landing of Pathfinder set the stage for an unprecedented exploration of the Red Planet by a tiny dune buggy named Sojourner (photo of the rover on the lander), driven by remote control from Earth on a survey to chart the composition and size of Martian rocks, dust and debris.
  • The six-wheeled "smart buggy" is 26 inches long by 7 inches tall. It's about the size of a small microwave oven and looks something like a giant radio-controlled toy car. Each of its wheels is about 5 inches high. As it explores the reddish surface of Mars, Sojourner is able to detect and avoid rocks bigger than itself.

  • Sojourner is powered by a 1.9 sq. ft. solar-panel array which can provide sufficient electricity to the rover for several hours of operation per day.

  • Sojourner communicates only with the Pathfinder lander, via UHF packet-radio link.

  • Most of Sojourner's electronic components are in the Warm Electronics Box which is the thermally-protected body of the rover. Temperature-controlling insulation is provided by a nearly weightless material called silica aerogel. Three heating units, each about the size of a C battery, contain small amounts of plutonium-238 each of which give off about one watt of heat to keep the rover's electronics warm.

  • For mobility, the rover's six-wheel chassis and suspension uses a rocker-bogie system, with joints that rotate and conform to the ground. Sojourner can lean as much as 45 degrees on a side without tipping over and is capable of climbing over boulders eight inches high. Remote-control operators on Earth choose targets for the rover to visit, and the rover autonomously decides how it will reach the target and perform its tasks.

  • For geology, the Alpha Proton X-ray Spectrometer (APXS) analyzes the elements that make up Martian rocks and soil by bombarding samples with charged particles and measuring how they interact with the sample. The APXS must be in contact with the sample, so the sensor head is mounted on a robot arm at the rear of Sojourner. A high-quality analysis takes about 10 hours.

  • A stereo camera with lasers at the front of the rover detects and avoids obstacles. The rover also has a color camera at the rear, next to the APXS.

Red, White and Blue Rocks

Pathfinder found red, white and blue rocks on Mars.

A University of Arizona scientist told the American Astronomical Society that Pathfinder pictures taken with different filters show a faint bluish tint on one side of exposed rocks, while the other side of the rocks is a typical Martian red. Some partially buried rocks are a whitish color and may actually be layers of a hard material called caliche, a calcium carbonate rock material common in Arizona.

The blue side is on the east side of the rocks, suggesting that the sometime-ferocious wind storms on Mars may have scoured off a layer, uncovering the color. The west side of the rocks, protected from the wind, are red, probably from layers of Martian soil.

Major Discoveries During the First Year on Mars

Here's how Pathfinder lander and Sojourner rover made out during their first year on Mars.
  • Landing
    Pathfinder's mobile rover Sojourner was the first semi-autonomous roving vehicle, capable of conducting science experiments and taking pictures, to be safely delivered onto the surface of another planet. The most important elements of Pathfinder's assignment simply was to prove that a spacecraft could fly straight into the Martian atmosphere, use airbags to land on the planet's surface, bounce across that surface, and then deploy a rover. Pathfinder and Sojourner did it.

  • Water
    Astronomers once thought a great flood had covered the surface of Mars long ago. Pathfinder landed in what was thought to be a former water channel. The probe's Sojourner rover recorded and transmitted to Earth sampling data and pictures of worn rocks and dune-shaped deposits that seemed to indicate the presence of sand. Researchers said sand is evidence that weather processes and flowing water helped create the surface of Mars. In fact, a younger Mars may have been like Earth.

  • Dust
    Pathfinder confirmed that dust absorbs solar radiation in the planet's atmosphere, partially blocking the transmission of solar energy to the Martian surface.

  • Clouds
    The Viking missions discovered that visibility is redcued as a Martian day begins. Was it clouds or fog obscuring that morning view? Pathfinder sent back the answer -- clouds.

  • Temperature
    Pathfinder found large day-night temperature variations on the surface of Mars. In fact, it sent back a temperature profile of the planet different from one made by Hubble Space Telescope observers.

  • Color
    Mars is referred to as the Red Planet. While the Viking landers found evidence the planet surface wasn't exactly red, Pathfinder confirmed it. The surface of Mars is more of a butterscotch color.

  • Rocks
    Prior to Pathfinder, what we knew about rocks on Mars came from meteorites found on Earth. Pathfinder data showed that the chemistry of rocks inspected at its Mars landing site may be different from the chemistry of rocks from Mars found on Earth.

  • Core
    Using Doppler radio signal tracking, two-way radio ranging, and knoweldge of Pathfinder's stationary position, NASA was able to measure the rotation of the planet Mars. Scientists were able to calculate closer measurements of the rate of change in Mars' spin axis. Using the new measurements, researchers now will be able to learn more about Mars' interior and whether it has a solid or liquid core.

  • Driving
    Sojourner was driven around the Martian surface by remote control from Earth. NASA engineers had a brief problem at the outset when they tried to drive Sojourner down its ramp from the spacecraft. The Pathfinder lander airbags got in the way. Later, Sojourner stalled on a rock while navigating across the surface of the planet. However, the overall drivearound was very informative. NASA says the next time it will give a rover a tool to brush dust off rocks.

  • Photography
    The ultra-wide panoramic view around the Pathfinder landing site -- which took NASA several days to assemble frame by frame -- is the clearest picture of another planet ever transmitted to Earth to date.

  • Data
    Pathfinder and Sojourner returned:

    • 16 chemical analyses of rocks and soil;
    • 16,500 pictures of the Martian surface;
    • 8.5 million measurements of the planet's temperature and pressure; and
    • a total of 2.3 billion bits of data about the Red Planet.

  • Now and Then
    Compare the Pathfinder mission with the two Viking missions to Mars in the 1970s:

    • Cost -- Pathfinder cost less than $250 million for launch, lander and rover. The Vikings cost $3 billion in 1997 dollars for two launches, two orbiters and two landers.

    • Development -- NASA took four years to develop Pathfinder. It needed eight years to develop the Vikings spacecraft.

    • Lifespan -- The Sojourner rover worked for a month. The Pathfinder lander lasted a year. The Vikings lasted six years.

Mars Pathfinder on a U.S. Postage Stamp

Mars Pathfinder on a U.S. Postage Stamp
In honor of Mars Pathfinder, the U.S. Postal Service issued a $3 Priority Mail stamp commemorating the historic mission to the Red Planet.

The stamp was officially dedicated on December 10, 1997, in a ceremony at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.

Fifteen million copies of the stamps displayed the first image received from Mars Pathfinder after its landing on the Martian surface July 4, 1997. The picture showed the Sojourner rover resting on the Pathfinder with a panoramic view of the Ares Vallis region of Mars in the background. Informational text about the Pathfinder mission was printed on the reverse of the stamp sheet.

Mars Pathfinder was the third U.S. stamp with hidden images to prevent counterfeiting. The hidden text -- including the words Mars Pathfinder, July 4, 1997 and the letters USPS -- was not visible to the naked eye, but could be seen by using a USPS decoder lens. Earlier 1997 stamps commemorating the U.S. Air Force and Classic Movie Monsters also had hidden images.

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