Getting your car stuck is irritating enough, but what do you do when your vehicle is dug into the sand of another planet and the nearest auto club is 180 million miles away?
If you’re the Jet Propulsion Laboratory, you bring Mars down to Earth.
In May, the Mars rover Spirit became embedded in a patch of fluffy Martian soil, the worst such incident in the more than five years that Spirit and its twin, Opportunity, have been exploring the planet’s equatorial region.
Since then, engineers have been trying to figure out the best way to extricate the rover, a project that has come to be known at the La Canada Flintridge laboratory as the “Free Spirit” program. T-shirts are being made to memorialize the effort to liberate the rover.
This week, scientists finished replicating the situation on Mars in a 30-foot-square workroom, mixing together 5,400 pounds of diatomaceous earth and clay to produce a fine, powdery mixture the color of creme brulee and as fluffy and light as flour. Now comes the task of trying to free a test vehicle, the so-called Earth Rover, to figure out what might work for Spirit.
“This isn’t the same as we have on Mars,” cautioned Paolo Bellutta, a rover team member, about the powder. “Diatomaceous earth is made of fossils, and we have no evidence of fossils on Mars.”
But it is the closest thing to the soil on Mars, rover project manager John Callas said.
The engineers drove the Earth Rover, about 5 feet tall by 7 feet wide, into an 8-by-12-foot sandbox. By Wednesday, it was stuck -- its six wheels embedded in 5 inches of the Martian soil analogue. After sloping the sand so that the test rover was pitched on its side to match Spirit’s predicament, the engineering team pronounced itself satisfied that it had succeeded in marooning two rovers on two planets.
The trick now, Callas said, is to put together a series of maneuvers on Earth that can be applied on Mars.
The first technique, which engineers intend to apply next week, is to ask the rover to simply drive forward. Because it has a gimpy right front wheel, Spirit has been driving backward in recent years, dragging the wheel along. Commanding the rover forward would amount to backing out of the fluffy sand patch onto surer ground nearby.
“We think terrain we can drive on is just a few feet away,” Callas said.
He said that although Spirit is embedded, he doesn’t believe it is truly stuck. This may sound like a subtle difference, but Callas says it’s crucial to the outcome of the Free Spirit effort.
“This is not like getting your car stuck in the mud,” he said. When last commanded to move, “the rover made a tiny bit of progress, which we intend to exploit to get the rover out.”
The primary goal of the Earth-based testing is to make sure the situation doesn’t get worse and leave Spirit irrevocably trapped.
But even if the worst happens and Spirit is entombed where it sits, that won’t be the end of its mission, Callas said. Instead of a rover, it would become a stationary lander of the sort NASA has sent to Mars repeatedly over the years. It could still do science from its new, permanent home.
Getting embedded has already yielded some surprising science, Callas said. By digging so deeply into the soil in one spot, the rover has turned up evidence of ferric sulfate, calcium sulfate and silica, which Callas said could be evidence that snow or ice was present at the equatorial regions of Mars in the fairly recent past. These days, water ice on the Martian surface is believed to be located only at the poles. This discovery is more proof that, where the rovers are concerned, “when Mars hands us lemons, we make lemonade,” Callas said.
While Spirit is parked, Opportunity, on the other side of the planet, is continuing to head for a crater called Endeavour. The twin rovers have far exceeded their expected lifetimes, and Callas declined to speculate how much longer they could last.
“These are phenomenal vehicles,” he said. “They go through horrendous thermal cycles, about 150 degrees Fahrenheit each day. That’s very stressing on a vehicle.”
Though freeing Spirit is a challenge, Callas said this event is not yet the worst threat to the survival of the rovers. The greatest danger comes from periodic global dust storms that can blot out the sun, robbing the rovers’ solar panels of power. In November, a storm that hit the area where Spirit was exploring was so severe that JPL managers called workers in on the weekend to send commands to the rover to save energy.
“It was like having a child lost in the wilderness,” Callas said. “On that one, I was losing sleep.”