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Rovers and their power

Energy and power · level 4, lesson 6 of 6

A question to start

Perseverance drives, drills rocks, fires a laser, takes pictures and talks to Earth. A home on Earth runs a fridge, lights and a TV. Which has more energy to spend each day?

The idea

Perseverance gets its power from an MMRTG, a power source that turns the heat of decaying plutonium into electricity. It gives a steady 110 W or so, day and night. Two rechargeable batteries store energy so the rover can use more than 110 W for short bursts, such as driving or drilling.

Over one sol, the MMRTG makes:

\[110 \text{ W} \times 24.66 \text{ h} = 2{,}700 \text{ Wh} = 2.7 \text{ kWh}.\]
Energy for a day Mars Earth
A machine Perseverance: about 2.7 kWh a sol A home in the United States: about 30 kWh
One adult's food about 2.3 kWh about 2.3 kWh

The lander InSight, which ran on solar panels, got about 5 kWh a sol from them soon after it landed, before dust settled on them.

So Perseverance, a car-sized laboratory, has about a tenth of the energy of an American home each day, and only a little more than a person eats. Every activity is planned to fit.

US home, EarthInSight, MarsPerseverancea person's food 30 kWh52.72.3

The helicopter Ingenuity needed about 350 W while flying, more than three times the MMRTG's output, so it flew only a minute or two at a time on its batteries, then spent the rest of the sol recharging from its small solar panel.

Worked examples

1. A drive. Suppose driving uses about 200 W more than resting. An hour of driving: \(0.2 \times 1 = 0.2\) kWh, about a fourteenth of the sol's budget.

2. Ingenuity's flight. 350 W for 2 minutes (1/30 h): \(350 \div 30 \approx 12\) Wh. Its panel could make that in a few hours of sunlight.

Try it

  1. Perseverance's MMRTG gives 110 W. How much energy is that in a whole sol, in kWh?
  2. Why does the rover need batteries, if the MMRTG never stops?
  3. A science instrument uses 40 W for 3 hours. What share of a 2.7 kWh sol is that?
  4. Why does a home on Earth have so much more energy to spend?
Answers
  1. \(110 \times 24.66 = 2{,}713\) Wh, about 2.7 kWh.
  2. Some jobs need more than 110 W at once; the batteries store energy for those bursts.
  3. \(40 \times 3 = 120\) Wh, about 4.4% of 2,700 Wh.
  4. It is connected to a grid of big power stations.

Common mistake

"110 W is too little to do anything." As a steady supply all day and night, 110 W adds up to 2.7 kWh a sol. With batteries to save it up, that is enough for years of driving and science.

Where it leads