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Why rovers drive slowly

Forces and motion · level 2, lesson 6 of 6

A question to start

The rover Perseverance cost billions of dollars and has six motors in its wheels. Yet its top speed is slower than a toddler crawling. Why don't they build it to go faster?

The idea

Mars: Perseverance at top speed Earth: a person walking
Speed about 150 m in an hour about 5,000 m in an hour
Time to cross a football pitch about 40 minutes about 1 minute

There are good reasons to go slowly:

  • Nobody can help. If a rover tips over or gets stuck, there is no one on Mars to fix it. A slow rover has time to stop before trouble.
  • Rocks and sand. The ground is full of sharp rocks and soft sand. Hitting a rock fast could break a wheel; at slow speed the rover just climbs over.
  • Messages take minutes. A radio message takes between about 3 and 22 minutes to get from Earth to Mars. Nobody can steer a rover live like a toy car. The rover has to look, think and drive carefully on its own.
  • Little power. The rover's power source gives a steady 110 watts or so, about as much as one bright old-style light bulb, topped up by batteries. The motors are geared down to turn slowly but strongly.
a message: 3 to 22 minutesslow and careful

So the rovers spend most of their time standing still: taking pictures, studying rocks, and planning. On a good day, Perseverance can drive itself a few hundred metres, using its cameras to pick a safe path as it goes.

Worked examples

1. A football pitch. About 100 m long. At 150 m an hour, Perseverance would take about 40 minutes to drive it.

2. Thinking as it drives. Perseverance's computer can map the ground ahead and choose a path while it keeps moving. Earlier rovers had to stop, look, think, and only then drive on.

Try it

  1. Give two reasons why rovers drive slowly.
  2. About how long does a message take to get from Earth to Mars?
  3. Why can't someone on Earth drive the rover live, like a toy car?
  4. About how far can Perseverance drive in an hour at top speed?
Answers
  1. For example: no one can fix it; rocks and sand; messages take minutes; little power.
  2. Between about 3 and 22 minutes, depending on where the planets are.
  3. By the time a message arrives, the rover would already have moved on. It has to drive itself carefully.
  4. About 150 m.

Common mistake

"Rovers are slow because Mars gravity makes them heavy." Mars gravity is weaker than Earth's. Rovers are slow on purpose, to be safe and careful far from any help.

Where it leads