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Dust on the panels

Energy and power · level 6, lesson 5 of 6

A question to start

The rovers Spirit and Opportunity were built to last 90 sols. Spirit lasted six years and Opportunity more than 14. Both ran on solar panels, in a dusty place. How did the panels keep working?

The idea

Fine Mars dust is always settling out of the air. A thin layer on a solar panel blocks some of the light, so its power drops a little every sol. That sounds small, but over a few hundred sols it adds up. Spirit's panels normally made about 700 Wh a sol; by the end of 2008, with dusty panels and a winter storm, they made about 89 Wh.

Two things helped the rovers:

  • Cleaning events: gusts of wind and passing dust devils blew the dust off. Each time, the power jumped back up, sometimes almost to what it was at landing.
  • Careful planning: in dusty seasons, the team used less power, and parked the rover on slopes tilted towards the Sun in winter.
solspowercleaning eventanother

The lander InSight was less lucky. Its panels made about 5,000 Wh a sol just after landing in 2018. No cleaning event ever cleared them properly, and by mid-2022 they were down to about 500 Wh a sol. InSight fell silent in December 2022.

For a city, the answer is simpler: people or robots can sweep or blow the panels clean. Some research has looked at panels that shake or repel dust using electric charges.

Worked examples

1. A slow loss. Suppose a panel loses 0.25% of its power each sol. After 200 sols, the power is \(0.9975^{200} = 0.61\) of the start, a loss of nearly 40%.

2. InSight. From 5,000 Wh to 500 Wh is a factor of 10: 90% of the power lost, mostly to dust.

3. Why tilting helps. In winter the Sun is low. Tilting the rover towards it can raise the panels' power by tens of percent.

Try it

  1. Why does a Mars solar panel slowly lose power?
  2. What is a cleaning event?
  3. A panel loses 0.3% a sol. Roughly what fraction of its power is left after 100 sols?
  4. Why would a city not have InSight's problem?
Answers
  1. Dust settles on it from the air and blocks some light.
  2. A gust of wind or a dust devil that blows the dust off the panels.
  3. \(0.997^{100} = 0.74\), about three-quarters.
  4. People or robots could clean the panels.

Common mistake

Adding the losses as a straight line. 0.25% a sol for 200 sols is not a 50% loss. Each sol takes 0.25% of what is left, so the losses multiply: \(0.9975^{200} = 0.61\).

Where it leads