"Well, we don't quite have a crystal ball to predict slippage of the rover wheels," said Randy Lindemann, rover mobility lead, "but our team of mobility experts and rover drivers can make predictions using a rather nice piece of paper with a curved line on a standard plot."
Image right: Mobility experts tested the rover engineering model in a sandbox while the real rovers were cruising to Mars. Credit: NASA/JPL.
How well does this prediction on paper work? It's all about prior testing. While Opportunity was cruising through deep space on the way to Mars, engineers on Earth tested the rover's mobility using an engineering model of the rover of the same weight and size with identical wheels. Engineers affectionately named it the "SSTB-lite rover." SSTB-lite stands for Surface System TestBed, and the lite means that this rover doesn't have any of the appendages, such as the robotic arm, high-gain antenna, or panoramic camera mast assembly.
"Our sandbox was a variable-tilt platform covered with 6 inches of dry, loose sand similar to what is used with construction cement. Throughout the weeklong test, we plotted how much the test rover slipped at different angles going up, down and across the simulated martian terrain," explained Lindemann.
"We never expected the incredible result that the rover's slipping behavior at the Meridiani site would be almost identical to its behavior on dry and loose sand. The reason that this surprised us so much is that the soil at Meridiani is nothing like dry beach sand from the Earth in terms of the minerals that it is made of or even how it was formed. What we have learned is that the primary characteristic of any loose soil in terms of how the rover will drive on it is determined by the characteristics of the friction between all of the tiny grains -- pretty much no matter what they are made of," said Lindemann.
How Far Will Rovers Slip and Slide?
Image left: Engineers' trustworthy mobility plots predict slips up and down the slopes of Mars. Credit: NASA/JPL.
The test created a set of trustworthy mobility plots. The plots show rover drivers that, at a 15-degree angle facing down, the rover will slide an extra 25 centimeters downward for every meter it is trying to go. The chart is not a straight, even line. At a 20-degree angle downward, the rover will slide 55 centimeters (1.8 feet) down, whereas at 20-degree angle upward, the rover will slip 90 centimeters (2.9 feet) in place and only move forward 10 centimeters (3.9 inches) out of a drive of 100 centimeters (3.3 feet).
For the type of sand Opportunity is in, the dead-end point where the rover simply can't climb upwards anymore, regardless of how many times the wheels turn, is 25 degrees," explained Baumgartner. "We've already gone up slopes as steep as 22 degrees on Opportunity Ledge. Luckily, the Long Term Planning team has found places around the crater with slopes lower than 25 degrees, so we can eventually get out of this crater where we landed," said Baumgartner.