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Through a dust storm

Energy and power · level 7, lesson 6 of 6

A question to start

The 2018 dust storm covered all of Mars for months. Suppose a solar-powered base sat under it. What would it take to keep the air, heat and lights going?

The idea

A night is short and comes every sol, so a store can be sized for it. A storm is different: it is long and deep, and it comes only every few Mars years. The rovers show how hard it is:

  • In the 2007 storms, Opportunity's panels fell to 128 Wh a sol. All science was stopped to save power, and it survived.
  • At the end of 2008, Spirit's panels made about 89 Wh a sol, against a normal 700 Wh.
  • In June 2018, with \(\tau\) about 10.8, Opportunity went silent. It never woke again.

A base has four choices, and real plans mix them:

  1. Store enough energy for the whole storm.
  2. Oversize the panels, so that even dim light gives enough.
  3. Cut back: pause what can wait, keep only what keeps people alive.
  4. Add a source that ignores the sky, such as a nuclear reactor.
demandfrom the storestorm: weeks to monthssolar

Here is the store alone, sized for a base that needs 10 kW, against a long spell of bad weather on Earth:

Suppose Mars Earth
Dark spell 60 sols at 5% of usual sunlight 7 days of thick cloud at 20%
Energy the base needs a day 247 kWh 240 kWh
Shortfall over the spell about 14,000 kWh about 1,340 kWh
Lithium-ion cells at 200 Wh/kg about 70 tonnes about 6.7 tonnes

Worked examples

1. The Mars shortfall. A sol is 24.66 h, so the base needs \(10 \times 24.66 = 247\) kWh a sol. The panels give 5% of that, so the store gives \(0.95 \times 247 \times 60 \approx 14{,}000\) kWh.

2. In hydrogen. With a 50% fuel cell, the hydrogen must hold 28,000 kWh: \(28{,}000/39 \approx 720\) kg of hydrogen and 5.7 tonnes of oxygen. Lighter than 70 tonnes of cells, though tanks add mass.

3. Cutting back. If the base drops to 4 kW for life support only, the shortfall becomes \(0.95 \times 4 \times 24.66 \times 60 = 5{,}600\) kWh: still 28 tonnes of cells.

Try it

  1. A base uses 6 kW. A storm lasts 40 sols, with solar power at 10% of usual. Find the shortfall.
  2. What mass of 250 Wh/kg cells would cover it?
  3. Why would twenty times as many panels be a poor answer?
  4. What did Opportunity's team do to get through the 2007 storms?
Answers
  1. \(6 \times 24.66 = 148\) kWh a sol; \(0.9 \times 148 \times 40 \approx 5{,}300\) kWh.
  2. \(5{,}300{,}000/250 \approx 21{,}000\) kg, 21 tonnes.
  3. They would sit mostly idle in clear weather, they would need cleaning, and a deeper storm would still beat them.
  4. They stopped all science and used as little power as they could.

Common mistake

Sizing the store for a night. A store for one night is a few hundred kilograms of cells; a store for a storm is tens of tonnes. A plan that works every sol of the year can still fail in the one month that matters.

Where it leads