NASA's Mars rover Curiosity used the Navigation Camera (Navcam) on its mast to record this westward look on the 347th Martian day, or sol, of the rover's work on Mars. The rover had completed a southwestward drive of 60.1 meters on that sol.The prominent rock in the right foreground, informally named "East Bull Rock," is about 20 inches (half a meter) high. The rock-studded local rise dominating the image is called "Elsie Mountain." A distant portion of the rim of Gale Crater is visible in the upper portion of the view.
Courtesy of NASA/JPL-Caltech
Curiosity has started its journey to Mount Sharp, a 3.4-mile-high mountain rising from the center of Gale Crater.
Mars-orbiting spacecraft have found layered deposits of clays and sulfates that will better tell us the history of the crater, including the presence of any water in the past.
Out for a spin
It's not the same as driving the actual Mars rover, but the Jet Propulsion Laboratory has a simulation that you can give a try here. You'll likely need to install a Web player that is available at the site. More interactives and features can be found here.
But the base of Mount Sharp is about five miles from Curiosity’s landing site. How do we drive a rover for five miles on a planet that is currently over 200 million miles away from Earth?
We cannot drive the rover in real time. It takes light about 20 minutes to travel this distance, and the radio signals carrying the commands travel at the same speed. This means that the images taken by Curiosity are at least 20 minutes old when we see them.
Commands that are sent back take an additional 20 minutes to reach Curiosity.
Because of variations in the orbits of the Earth and Mars, the total time between sending an image and receiving commands could be even greater than 40 minutes.
NASA has therefore developed two ways to move the rover on Mars. The fastest and safest way is directed driving. Curiosity has two cameras, called Navcams, mounted about six feet above the surface. It uses these cameras to take 3D images of the terrain ahead and send them to Earth.
The rover drivers use the images to chart a path for Curiosity to follow the next day. They then convert the plan into a set of instructions that reads something like: drive forward 1 meter, turn right 5 degrees, drive forward 2 meters, take pictures, etc.
The command list is then tested with simulation software called the Rover Simulation and Visualization Program. When the list passes the simulation, it is ready to send to Curiosity.
We communicate with the rover roughly twice a day. Each Martian morning a list of commands to complete that day is sent to Curiosity.
At about 4 p.m. Mars time, the rover sends back the results of what it did that day by telemetry and pictures. Then the rover goes to sleep for the Martian night.
While the rover sleeps, the drivers develop a plan for the next day. This cycle repeats based on Martian time, so the rover drivers have to live on Mars time, not Earth time.
In directed driving mode, Curiosity can travel at a maximum speed of 150 yards per hour, but it is driving blind. We tell the rover to “trust us, there are no big rocks in front of you.”
Normally we can only see about 40 yards ahead of the rover with the Navcam images, so we can only drive in directed mode for 20 to 30 yards a day. At that rate it would take a very long time to get to Mount Sharp.
Curiosity therefore has another mode for driving on Mars: the autonomous mode. In autonomous mode, the rover opens its eyes and uses the Navcams and other instruments to find its own path and keep itself safe.
In autonomous mode, the rover drivers tell it the destination but let Curiosity find its own route. To accomplish this, the rover takes images with the Navcams, evaluates the terrain, and locates any obstacles to determine the best route.
But with the relatively slow processor on the rover, this process takes a lot of time. In autonomous mode Curiosity travels much slower, at about 40 yards per hour, less than one-third as fast as in directed mode.
To travel the maximum distance in a day, Curiosity therefore uses both driving modes. Each day’s drive starts in directed mode, with the rover traveling 20 to 30 yards following the instructions sent that morning. Once it goes beyond the Navcam horizon, it switches to autonomous mode, with navigation handled by the rover.
By combining the faster directed mode with the slower autonomous mode, it is hoped that Curiosity will routinely be able to drive distances of around 100 yards each day.
Traveling at a football field a day means that even without science stops, it will take some time to reach Mount Sharp. But every journey starts with a single step, even if it’s a small one.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.