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.