Craterside Classic: Off-Roading on Mars, 20th Anniversary Edition (Part 1 of 3)

Image: NASA

To date, NASA and the Jet Propulsion Laboratory (JPL), in California, have launched three successful mobile exploratory missions to the surface of Mars. A total of four wheeled rovers now grace the crust of the long-mysterious red planet. The Pathfinder project was the first of NASA’s Discovery Program missions to reach Mars’ surface. Pathfinder, planned in an era of constricted space budgets, cost less than $150 million and was developed in less than three years. (Both were stated targets at the mission’s outset.) Mars Pathfinder was originally designed as way to demonstrate how to deliver both an instrumented lander and a free-ranging robotic rover to Mars’ surface. The mission not only accomplished this goal but also returned a stunning volume of data, with both the lander and rover far outliving their projected lifespans.

Image: Palm Beach Daily News

Launched on a Delta II rocket from NASA’s Kennedy Space Center on December 4, 1996, the Pathfinder mission touched down on Mars’ surface on July 5, 1997. The mission used a unique landing method. A descent package directly entered the Martian atmosphere from space, then used a large parachute to slow its descent through the thin Martian atmosphere, and finally employed a system of giant airbags to cushion the impact. (I swear that this contemporary cartoon was the clearest visualization of the process that I could find. –ed.)

Photo: NASA/JPL

Here, prior to the mission, JPL scientists inflate a full-scale version of the airbags in what appears to be a spot that is perilously close to a wire-topped fence. In fairness, the mocked-up rockscape identifies this as JPL’s Mars simulation field. Once the bags deflated, a pyramidal structure unfolded to reveal the static, instrumented base station, (now the Carl Sagan Memorial Station, after the well-known popularizer of space science who died just before Christmas, 1996) and its Sojourner rover, named for the early US civil-rights pioneer Sojourner Truth.

Image: NASA/JPL

JPL’s astrophysicists chose the landing site, an ancient flood plain in Mars’ northern hemisphere known as Ares Vallis, because it held few steep craters while still featuring a wide variety of rocks that were likely deposited during a catastrophic flood. Under its own solar+battery power, the Sojourner rover drove off of the lander, effectively circling its base for 83 Martian days. A panoramic camera atop the Sagan station pieced together the plot of the rover’s journey, during which Sojourner conducted at least 15 chemical analyses of rocks and soil. Other experiments tested soil conditions, temperature fluctuations, and telemetry solutions to be used in planning further missions.

Image: NASA/JPL

Sojourner had six wheels and could move at speeds up to two feet per minute. During the course of a day on Mars the rover could travel up to ten feet. Sojourner never strayed more than ten yards away from the lander. The rover’s wheels and suspension used a springless rocker-bogie system. Instead of springs (and, presumably, shocks), its joints rotated and conformed to the contours of the terrain, providing a high degree of stability for moving over rocky, uneven surfaces. According to JPL, six-wheeled vehicles can overcome obstacles three times larger than those passable by four-wheeled vehicles. In fact, Sojourner could tip as much as 45 degrees from level before rolling over (likely an unrecoverable condition). Three motion sensors along the rover’s frame detected any excessive tilt and intervened before the tipping point. Each 5-inch aluminum wheel was fitted with stainless steel treads and cleats and could move up and down independently of all the others. Sojourner could scale a vertical obstacle of more than 8 inches before high-centering.

Image: NASA/JPL

From touchdown until its last data transmissions on September 27, 1997, Mars Pathfinder returned 2.3 billion bits of information, including more than 16,500 images from the Sagan station, and 550 images from the Sojourner rover. Onboard instruments also reported on wind and other weather factors. In fact, higher-than expected winds regularly blew dust away from solar panels on both the rover and lander, greatly extending their useful lives. Once compiled, mission data suggested that Mars was once both warm and wet, with free liquid water beneath a thicker atmosphere. Sojourner employed an onboard 2MHz CPU with 512 KB of RAM. Due to distance, weather, position, and communication technologies available in 1996, data streamed in sequential bursts of 2 kilobytes at a time. The final ’90s-style web page for the Pathfinder mission lives here: https://mars.nasa.gov/MPF/index1.html

Photo: NASA/JPL

Following several flybys and orbits by Mariner spacecraft during the 1960s, NASA’s Viking 1 and Viking 2 (in summer 1976) became the first landers to touch down safely on the surface of Mars. (Dr. Carl Sagan poses above with a Viking lander mockup in California’s Death Valley, probably in 1975.) Two identical spacecraft, each consisting of a lander and an orbiter,  launched atop Titan IIIE/Centaur rockets from Cape Canaveral, Florida. Each orbiter-lander pair flew together and entered Mars orbit; the landers then separated and descended to the planet’s surface. However, no rovers were aboard either craft. Besides taking photographs and collecting other science data on the Martian surface, the two landers conducted three biology experiments designed to look for possible signs of life. Perhaps disappointingly, Viking 1 and 2 provided no solid evidence of living organisms, past or present.

Image: NASA

Earlier in 1996, just prior to the Pathfinder launch, Mars Global Surveyor began streaming back data from orbit, and continued to do so for the next seven years.