Built on Facts

The Boston Globe has a beautiful retrospective on some of the most visually stunning photographs the Cassini probe has taken from the vicinity of Saturn. All of them are beautiful and I highly recommend the link so you can see all the images. This image is of Dione, Saturn’s 4th largest moon. It’s absolutely surreal.

This particular photograph was taken from just a few hundred kilometers above the moon’s surface. That human beings can send such an exquisite piece of instrumentation to fly just above another world a billion miles away is mind blowing. In some sense we can comprehend what earthly places are like even if we haven’t been there. “Dione, moon of Saturn” seems a little unreal, a small blurry dot in a book. A picture like this dispels those vague impressions and makes it seem like a solidly real place our children or their children might one day stand and run their gloved fingers through the Dioneian dust in silent contemplation.

I wouldn’t personally want to be an astronomer for a living. I like my experiments to fit comfortably on a tabletop. But I’m very glad there are astronomers so I can see the beautiful end results of their missions.

Which isn’t to say there’s not a lot of physics on these planetary science missions. There’s a reason many universities have a combined Department of Physics and Astronomy. Even the design of the spacecraft sometimes has both the physics and the PR problems of physics to deal with. For instance: Saturn is roughly 1.4 billion kilometers from the sun. The earth is about 149 million kilometers from the sun. Take the ratio, square it, and you’ll find that there’s only about 1/93 of the solar power per square meter at the distance of Saturn as there is at the distance of the earth. Cassini needs a lot of power, and there’s not a lot of solar power to be had way out there. The solution developed was the use of radioisotope thermoelectric generators, or RTGs. This is essentially a very small scale nuclear reactor. The radioactive material produces heat, outer space provides the cold, and a thermoelectric material turns this temperature difference into electrical energy. It’s reliable and stores lots of power. In the case of Cassini, it’s hundreds of watts for the entire decades-long duration of the mission.

The problem is of course that it’s radioactive, and so the plan generated some protest. If the launch vehicle were to blow up or reenter the atmosphere during a gravitational assist the radioactive material could be dispersed into populated areas. While a nuclear explosion is totally impossible due to the nature of the material, the radiation by itself is not insignificant. Fortunately, exhaustive analysis indicated that the risk was extremely minimal. Basically there weren’t any plausible ways for the material to dangerously disperse even if the rocket exploded, and even if so the Pu-238 fuel is an alpha emitter which is the easiest form of radiation to shield against. A t-shirt will block it, though it’s still dangerous upon direct skin contact, inhalation, or ingestion.

Not perfectly safe, but as a matter of probabilities it’s a lot more safe than the radiation your local hospital uses to treat cancer. Hopefully the continued progress in solar panel efficiency, cost, and weight will make it unnecessary in the future. Personally I hope that the safety and success of Cassini might be one more thing in the public consciousness to help improve the acceptance of modern and safe nuclear power as one pillar of the fight to rely less on fossil fuels. It’s not as though coal power isn’t radioactive, after all.

Cassini wasn’t exactly cheap either, at a cost to the US of around $2.6 billion. But somehow I can’t get really worked up about this, as the federal government spends that much roughly every 8 hours. The Cassini mission will produce about 6 years of exploration of the Saturn system while Congress has spent $17 billion on pork projects in FY 2008 alone. I know which I’d rather see more of my money supporting.