Sixteen sols after landing on Mars, Curiosity made its first drive. It was, like any rover’s leaving its first tracks on another planet, an historic mission event. Since Curiosity landed right on its wheels, its first drive was a relatively simple one; after wiggling the wheels and checking the systems, engineers were ready to drive the rover a cautious few feet.
But for Sojourner, the first rover to land on Mars in 1997, had a much more complicated first drive. It had to get from a landing platform to the surface before leaving history’s first tracks on Mars.
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The Mars Pathfinder mission took a suite of scientific instruments and a camera to Mars inside a pyramid-shaped landing vehicle. It successfully landed patriotically on July 4.
During its decent, a little under 1,000 feet from the Martian surface, an array of airbags inflated around the pyramid-shaped lander, protecting its contents as it bounced and rolled to a stop in Ares Vallis. The airbags deflated, the pyramid walls unfolded like flower petals, exposing the main scientific payload to the Martian environment.
The tiny rover Sojourner was attached inside one of the petals. At just two feet long, a foot and a half wide, a foot high, and weighing just 23 pounds, the little rover carried a lot of hope. It was the first time a robotic rover would travel around the surface of another planet, and engineers from NASA’s Jet Propulsion Laboratory were anxious to prove that rovers were a viable technology for planetary exploration.
As a proof of concept rover, Sojourner’s science payload was limited: it carried an alpha proton x-ray spectrometer and three cameras.
But for Sojourner to do any science, the team behind the rover would have to get it down to the surface. Mission scientists had to engineer a path for Sojourner.
The basic idea was to put two ramps on the petal, one fore and one aft of the rover so it would have two options to drive down. The ramps were made of a lightweight open weave Kevlar mesh to keep any stray airbag fabric from snagging the rover’s wheels and strengthened with stainless steel battens. Two steel tracks on either side of the ramp’s surface guided the rover on its drive.
The challenge was designing ramps that could be stored and remotely deployed while staying under the weight limit imposed by the mission. Atmospheric and temperature considerations influenced the design, too.
The team behind Sojourner’s egress method determined that ramps rolled up for storage could be easily and reliably deployed once on Mars. Each 3.2-foot long ramp was rolled into a tight cylindrical spools three inches in diameter and 16.5 inches in length for the cruise. The original design used cables to keep the rolled ramps in place. On Mars, pyrotechnic cutters would free the ramps, leaving them to unfurl and lock into place.
But there were problems with this design. The team found that tight cables were needed to keep the ramps in place, but releasing those tight cables resulted in a violent ramp deployment. In some tests, the ramps shot forcefully upwards before hitting the surface. They needed a way to control the deployment.
So they added Velcro. A strip of Velcro along the edges of the ramp slowed the deployment and had the added benefit of keeping the ramps rolled tightly in place, negating the need for pyrotechnic cutters. Standard nylon Velcro worked extremely well, until the team simulated Mars’ cold atmosphere. It turns out that at temperatures below –40º C (which is also –40ºF) Velcro doesn’t peel apart; it gets stuck. So the team created a stronger hybrid Velcro that used nylon loops and steel hooks. It had the same properties as traditional Velcro without the sensitivity to temperature.
Midway through the ramps’ development, engineers ran into another problem: whether Sojourner might tip over if the ramps unfurled at a steeper angle than expected. There was some thought. Tests showed that if the rover tilted beyond 30 degrees, its center of gravity moved enough to send it tumbling down. The solution was another fairly simple one. The ramps were made flexible with integral hinges near the bases. These let the ramps extend and settle under their own weight with the end safely stable on the surface.
Two sols (Martian days) after landing safely on Mars, Pathfinder’s petals were open, the ramps unfurled, and Sojourner made its slow way down towards the surface. The drive started a little before 11 p.m. Pacific Time on July 5 (2 a.m. Eastern time July 6). When the rover reached the surface it stopped. Mission manager Richard Cook made the historic announcement: “The rover is on the surface of Mars. We’ve got some great images back, and all the scientists are in heaven.”
The first picture offering incontrovertible visual evidence that all six wheels were on the surface came shortly thereafter. The camera on the Pathfinder lander snapped the rover a few inches beyond the ramp. For the first time, mission scientists could see the site of remote analysis; the soil underneath the rover was the site of its first spectroscopic measurements. Sojourner looked around it’s new home. It sent images to Earth taken with two cameras so engineers would have a sense of perspective when planning the rover’s path. The little rover didn’t have the sophisticated autonomy of Spirit, Opportunity, or Curiosity so relied almost entirely on its Earthbound driver.
Sojourner’s primary mission was just seven days long, though the team hoped it would last at least a month. It lasted longer, more than 80 days, and covered over 330 feet moving at the near glacial pace of just 1.3 feet per minute.
NASA lost communications with its first ever planetary rover on September 27, 1997.