Philae and the Failure of Solar Power

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Huge news this week! Philae, the little lander that could, came back to life and started communicating with Earth. If you’ll recall, in November of last year the Rosetta mission successfully landed a robot on a comet for the first time ever. Or I should say, “somewhat successfully.” There were a few problems, as can be expected considering that the mission launched ten years prior, and in space no one can hear you scream “my thrusters aren’t working.”

The lander ended up experiencing a harder than expected landing on the comet due to the thrusters not firing, and then the anchors that were supposed to hold it to the comet also didn’t fire. As a result, Philae ended up bouncing a few times and landing in a less-than-desirable position.

Philae was meant to be powered by solar panels, but in the lander’s final position they couldn’t get enough sunlight to work. So, the engineers operating Philae had about 60 hours of life in the primary battery during which they gathered as much info as they could and tried to position the solar panels in a way that they might eventually be able to get enough light as the comet approaches the sun and begins losing mass.

Amazingly, that’s exactly what happened. They got extremely lucky with the way the comet started shedding its mass, and sure enough Philae came back online and said hello.

The weird thing is that the solar panels weren’t necessarily the ideal way to power the lander. That would have been the nuclear option: an RTG, or Radioisotope Thermoelectric Generator, which uses the heat from a decaying Plutonium isotope to provide just enough power.

NASA has used a lot of RTGs in the past, but the Rosetta Mission was sponsored by ESA, the European Space Agency. ESA doesn’t have access to RTG technology due to the perceived danger of putting radioactive material on top of what is essentially a giant bomb and because of treaties about trading that material AND because of the huge cost of producing the material.

RTGs have been in a number of failed missions in the past, and in the ‘60s they may have been responsible for distributing plutonium around Madagascar and Russia. But since then, the safety designs have improved drastically. Even Apollo 13’s RTG in 1970 made it to the bottom of the Pacific Ocean intact and with no environmental contamination, at least for the next 800 years or so. At that point, the human race will have evolved gills and/or invulnerability to nuclear waste, so it’ll probably be fine.

NASA and many other scientists and engineers believe that the risk is low enough to continue using RTGs in their missions, though the Juno mission, launched in 2011, will be the first mission to Jupiter that uses solar arrays instead of an RTG. NASA cited the increased cost of plutonium along with the improved function of solar arrays in extremely low light situations.

So RTGs aren’t necessarily the best for every mission, but it’s a shame that ESA’s hands are tied by political red tape, so they’ll need to make due with suboptimal technology to accomplish incredible goals. As for Philae, they were saved by the ingenuity of their crew and the serendipity of comet degradation, but they still lost 7 months of scientific research that they’ll never be able to get back.

Rebecca Watson

Rebecca is a writer, speaker, YouTube personality, and unrepentant science nerd. In addition to founding and continuing to run Skepchick, she hosts Quiz-o-Tron, a monthly science-themed quiz show and podcast that pits comedians against nerds. There is an asteroid named in her honor. Twitter @rebeccawatson Mastodon Instagram @actuallyrebeccawatson TikTok @actuallyrebeccawatson YouTube @rebeccawatson BlueSky

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  1. Very well explained, Rebecca! That’s an important aspect of the partial failure that I haven’t read about in the popular science articles about Philae.

    I wonder if 60s-type protests concerning anything nuclear has anything to do with the move away from RTG?

  2. One of the reason RTGs are no longer being considered for missions is because the US doesn’t have enough Plutonium. RTGs use Plutonium 238 (which is not the one used in atomic weapons). There are no natural sources of this isotope and producing it in sufficient quantities takes time. This coupled with the fact that NASA itself doesn’t produce Pu-238, that’s done by the DOE and the fact that some action by congress was required plus funding was needed means that NASA is running short on Pu-238. This Wired article paints the picture quite clearly. If I recall, NASA also mothballed its project to build the next generation ASRG (Advanced Stirling Radioisotope Generator) because of scarcity of Pu-238.

    I wonder if ESA doesn’t use RTGs because they don’t have sufficient Pu-238 either. I don’t know if Europe has the reactors needed to produce it.

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