Imagine driving your car to work by sitting in your chair in front of your home computer.
You'd type in the detailed route directions, including right and left turns, then enter the desired speeds and terrain information to reach your office.
Finally, you'd upload the data wirelessly to your car, which would then follow your directions without your having to touch the steering wheel, gas pedal or brake pedal.
If you can imagine all of this, then maybe you can understand what it's like to be one of NASA's Mars rover drivers.
From 100 million miles away, about 14 workers at NASA's Jet Propulsion Laboratory in Pasadena, California, take turns daily planning and plotting the routes, work schedules and areas to be explored by the two Mars rovers, Spirit and Opportunity, as they make their way across the Martian surface.
The NASA engineers don't sit at a console and operate the rovers remotely using a joystick. Because of the vast distance between Earth and Mars, there's a lag time between when the data is sent to the rovers and when it is received. That means that operations in real time, or at the split second a command is given, aren't possible.
Scott Maxwell, 36, one of the original members of the rover driver team, said the time lag is about four minutes each way when the planets are on the same side of the sun, but can lag as much as 20 minutes each way when the sun moves between Mars and Earth as they orbit. The distance between the Earth and Mars can vary from approximately 36 million to 250 million miles, depending on their locations as they orbit around the sun.
"So if you try to drive it like a radio-controlled car or a slot car ... nothing would happen on the rover for at least four minutes" until the commands reached Mars, he said. "Because of that delay, by the time you see a cliff coming, you've already driven over it because what you see already happened in the past. As a result, we don't drive them that way."
NASA instead uses "once-a-day commanding," where two or more rover drivers work together to plan the rover activities for the following day, Maxwell said.
At night on Mars, when the sun fades, each of the rovers goes into sleep mode to save electrical power, while the rover drivers on Earth are working in daylight to create their next rover mission plans. The drivers analyze the most recent data and photographic images downloaded by the rovers, which give the current state of the missions and the planet's exploration, then they plug it into a 3-D simulator and formulate what the next day's mission will include.
After creating the detailed daily program commands used to put the rover through its paces in a simulator -- which runs on a Linux PC -- the team checks and double-checks its work. They then send the commands to the rovers, which will follow the mission plan the next day on Mars.
"When we get the simulated rover doing what we want the real rover to do, we send those commands up from the simulator and we go home to go to sleep," Maxwell said. "Basically, every Martian night we're planning out the next Martian day for the rovers. I sometimes refer to it as the Martian night shift."
Because one bad command can harm a rover, the drivers check each other's work for errors. The summer dust storms on Mars, which for several months caused NASA to put the rovers into low-activity modes to save power, have begun easing. That means the rovers can get back to their missions of exploring the Martian terrain and looking for traces of life and water on the planet.
In the past several weeks, the rover Opportunity has been sent into Victoria Crater, giving scientists more detailed views of the surface.
Driving into such craters is "fun, but they're actually dangerous," Maxwell said, because steep slopes make it risky for the rovers to make their way through the terrain. "You can't hit a button and stop it" if something goes wrong," he said. "If something does go wrong, we're not going to know until the next day."