Mission Timeline: Surface Operations

Figuring out how far the rover has traveled

The rover has a difficult time knowing exactly how far it has traveled, where it has been and where it is.

For example, if the flight team asks the rover to move forward 100 centimeters, turn right, then extend its robotic arm and analyze a rock, the rover will follow the commands in reference to its current location. What would happen if the rover couldn´t see, and had to rely just on its wheels to tell where it had moved?

Like a car on Earth, the rover uses its odometer to click off the distance it has traveled. If one revolution of the rover wheel equals 25 cm, then after the wheels have revolved four times, the rover should technically have moved forward 100 centimeters (25 cm X 4 = 100 cm). But, unlike cars on Earth, the rover doesn´t drive on smooth, paved roads. The rover moves on rocky and sandy martian terrain.

The rover wheels might have a hard time grasping onto the loose-gravel ground. The wheels could spin in place before they actually gain tracking. So if the wheels spin four times before they find firm footing, the odometer will read 100 centimeters, and the rover will stop. Thus, the rover will believe it has moved forward 100 centimeters, when in reality, it hasn´t moved at all and may have dug itself into a rut instead. Without other safety checks it might then turn and bang its wide solar panel wing into a rock behind it (that wouldn´t have been in the area if the rover had moved forward). The rover would then continue to follow the chain of commands and extend its robotic arm, hoping to meet the rock 100 centimeters from where the rover began its "trek." The lonely robotic arm, alas, would be flailing in the wind, never finding its appointed rock - or worse: without camera-enabled safety checks the arm could again firmly crash into another rock it should have safely passed by along its 100-centimeter journey.

Imagine yourself being given a command to walk from your bedroom to your kitchen, and the only way to get there was to follow these rules:

1. Scan the area and the route in front of you.
2. Close your eyes and wait at least 20 minutes.
3. Keeping your eyes closed, walk to the kitchen without hitting anything.

Now imagine how much easier it would be to get from your bedroom to your kitchen if you could open your eyes every 30 centimeters (1 foot) to reassess the situation.

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