Built on Facts

Powering Spirit on Mars

I’m still not happy about NASA scrapping Ares and the manned lunar/Martian plans, but I’m less unhappy than I was. As long as unmanned planetary science picks up most of the slack I’ll grudgingly deal with it. The extra earth science is still stupid; if you want more of that, get NSF or NOAA to do it.

Mars exploration has been back in the news recently too, with the Spirit rover finally breaking down and getting stuck in place permanently. This is pretty impressive – it was originally designed to work for 90 days but here it is still functioning 6 years later. Even as a stationary platform it can continue to do interesting science. The main concern is keeping it sufficiently powered during the coming Martian winter. The rovers are solar powered, and solar energy is increasingly hard to come by as you move farther from the sun. It’s why probes that go to the outer planets are pretty much never solar powered.

Here’s the reason. The sun dumps its energy out evenly in all directions. By the time it gets to Earth, that energy is spread over a sphere with the same radius as the Earth’s orbit. The surface area of a sphere is 4*pi*r^2, so the sunlight energy flowing in through each square meter of sun-facing solar panel is the sun’s total output divided by that area. You can do the math, and it’ll turn out to be around 1300 watts per square meter.


Inverse Square Law, from Wikipedia.

But if you move to Mars, the radius that the sunlight is being spread over is much larger. Mars is about 1.5 times farther from the sun than the Earth is, so the power is reduced by a factor of (1.5/1)^2 = 2.25. That’s less than 600 watts per square meter in the ideal case, which a rover on the Martian surface assuredly is not.

What about the Cassini probe orbiting Saturn? Saturn is about 9.5 times farther from the sun than the Earth, so the sunlight there is reduced by (9.5/1)^2 = 90.25. That’s about 14 W/m^2 in the ideal case, which is why Cassini is nuclear-powered. The New Horizons mission to Pluto? By the time it gets there the available solar power will be reduced by a factor of around 900. It’s nuclear-powered as well.

Conversely, probes to Mercury and Venus have all the solar power they want. Unfortunately those planets present their own unique challenges, and adequate power is pretty far down the list.


  1. #1 IanW
    February 5, 2010

    So do you suppose that visitors to Mars will have more success in their explorations since they have a Spirit guide?!

  2. #2 rob
    February 5, 2010




  3. #3 Greg Laden
    February 5, 2010

    Now some guys in monk suits are going to come by and ‘collect’ spirit for scrap.

  4. #4 CCPhysicist
    February 5, 2010

    You are advocating that NSF and NOAA each start their own rocket development program?

  5. #5 Uncle Al
    February 6, 2010


    An entirely satisfactory moon base is ready to fly on existing boosters – admittedly for 1/3 the cost/gram of the Space Scuttle.

  6. #6 George Myers
    February 8, 2010

    And so the Planetary Society tried to launch a solar sail experiment with the help of the Russia submarine missile launcher that ended up in Kamchatka due to the lack of a missile’s missed upgrade. NASA had plans for one too. The Planetary Society is back on schedule for another with the help of JAXA, the Japanese space agency. Where’s the US’s? Whole lotta watts NEO!

  7. #7 Anonymous
    February 11, 2010

    Frankly, I am glad NASA does Earth science stuff. I think learning about our own planet is important and I don’t think NOAA can make and launch spacecrafts to investigate these things. I guess NASA and NOAA could work together in these things.

  8. #8 Abby Normal
    February 11, 2010

    I’ve been trying to resist saying something because it’s so inconsequential. But it’s bugging the hell out of me. So…

    That inverse square law picture is depicting at least 2 light sources. Look at the 9 light paths. They roughly form 3 rows of 3 beams each. It’s the middle row that is a problem. Lable the beams in that row A, B, C from left to right on each of the r planes. Following the paths from r to r2 everything works fine. But if you follow them from r2 to r3 you’ll see that the beam that had been A now intersects r3 at the B position vise verse on the B beam. Assuming they’re traveling in straight lines they must have had different origins. Otherwise the r3 B location would be in the lower right quadrant of its square and the A and B beams wouldn’t cross.

    Okay, I feel better now.

  9. #9 Nomen Nescio
    February 12, 2010

    You are advocating that NSF and NOAA each start their own rocket development program?

    what’s to stop them from purchasing launches from NASA, or even commercially (the way NASA might have to pretty soon, if they want to pretend to keep the ISS running much longer)?

  10. #10 Anonymous
    September 8, 2010

    hmmmmmmmm ha ha ha

  11. #11 Anonymous
    September 8, 2010


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