Departing Earth

NASA’s MESSENGER mission is a space probe launched in August, 2004. Its planned itinerary includes no fewer than six planetary flybys (one of Earth, two of Venus, and three of Mercury) before it enters orbit around Mercury in March, 2011.

In the past I’ve sometimes griped about Earth flybys of space probes powered by plutonium RTGs; not so much to argue that the risks of a high-altitude vaporization and subsequent release of plutonium aren’t worth it, but to call for a more-honest discussion of the risks than has sometimes been offerred by mission supporters.

But MESSENGER doesn’t represent a problem in that area; its destination in the inner solar system means that it will have plenty of Sun power from its solar panels, and so it apparently is plutonium-free.

And the Earth flyby already happened, anyway, on August 2, 2005. I talked in PhotosFromTheSpaceShuttleColumbia about the importance of having human eyes in space to deliver perspective-changing images, but even if a robot can’t be as good at catching opportunistic snapshots as a human being, it can still deliver some amazing views.

Like the one on display here: Earth departure movie. The official site’s description:

The Mercury-bound MESSENGER spacecraft captured several stunning images of Earth during a gravity assist swingby of its home planet on Aug. 2, 2005. Several hundred images, taken with the wide-angle camera in MESSENGER’s Mercury Dual Imaging System (MDIS), were sequenced into a movie documenting the view from MESSENGER as it departed Earth.

Comprising 358 frames taken over 24 hours, the movie follows Earth through one complete rotation. The spacecraft was 40,761 miles (65,598 kilometers) above South America when the camera started rolling on Aug. 2. It was 270,847 miles (435,885 kilometers) away from Earth – farther than the Moon’s orbit – when it snapped the last image on Aug. 3.

4 Responses to “Departing Earth”

  1. ymatt Says:

    Out of curiosity, what was your basis for being critical of RTGs? Admittedly I grew up around the space business to an extent so I was biased against the critics.

    But RTG units use plutonium pellets something like the size of a golfball, which is vastly below critical mass of course. And it’s only a poison if ingested or inhaled, and even then it’s on the order of the deadliness of other chemical weaponry (you can handle it bare-handed). Which of course isn’t particularly safe, but you’re talking about a spacecraft with a very small quantity of the stuff with the (very very low) risk if missing its trajectory and burning up in the atmosphere, scattering the stuff over such a large area and from such a height that the danger to humans is zero. The nasty bioweaponry out there depends on targetted deployment to be at all effective — this is the exact opposite.

  2. jbc Says:

    Yeah, your comments are actually fairly similar-sounding to stuff that bugged me about the “public debate” that took place in connection with the Galileo probe’s RTGs at the time of its launch. There was a lot of focus placed on those areas where the risk was demonstrably low. But then the area where the risk was higher got glossed over really fast (at least in one radio-show discussion of it on some public station in LA that I listened to). The impression this made in my mind was that people who had already reassured themselves that the mission’s risks were worth enduring were trying to massage the public discussion of the risks to ensure the “right” decision was reached. That smacked of bad science, and bad public policy, to me.

    Yes, stipulated: NASA’s RTG containment housings are probably sufficient to guard against the release of plutonium due to a launch accident. The RTGs posed no threat of undergoing a chain reaction or exploding or anything like that.

    But at least based on the limited public discussion that took place about this at the time of the Galileo launch, it didn’t sound to me like NASA could be as reassuring about the scenario of a malfunction or human error or sabotage leading to an inadvertant re-entry during the near-Earth flyby. That flyby would be at much higher speed, and the spacecraft, including the RTG containment structure, would be destroyed in the upper atmosphere. And unlike what you say, I don’t believe that taking a bunch of plutonium pellets the size of golfballs and turning them into millions of teency little particles in the upper atmosphere poses “zero” risk to humans.

    Even a single teency particle of plutonium inhaled by a human is sufficient to cause cancer. Turning a chunk of plutonium into little teency particles and spreading it through the upper atmosphere is probably the worst you could do in terms of its potential health damage to humans.

    Realistically, I was interested in the figures on the number of worldwide cancer deaths that could be predicted for the scenario where the RTGs were destroyed in the upper atmosphere during a flyby. Were the estimated fatalities in the tens? In the hundreds? In the thousands? How many would be acceptable in return for the benefit to humankind of being able to power deep-space probes with a particularly efficient, if dangerous, form of energy?

    That discussion seemed to get glossed over in the public debate about RTGs, and I found that unfortunate. Because NASA engineers were, in effect, making those choices on behalf of the people of the world who were bearing that risk. And I’m not sure they made those decisions in a particular transparent or accountable manner.

  3. ymatt Says:

    Well, you seem to be suggesting that since there was little said about it, that there must be something they were hiding. It might just be that… there was nothing to talk about.

    I’d be interested in any information you have about the real dangers it could pose. Again, the only information I’m going on is that the toxicity is on the order of bioweapons agents which just don’t do anything unless deployed in close proximity. And the shuttle launches over the ocean, away from people. This makes it seem to me like demanding a debate about those kind of risks is like debating with a cab driver what might happen if he decided to drive blindfolded. He’s not going to, so who cares.

  4. jbc Says:

    We’re not talking about biological weapons. The analogy isn’t informative. We’re not talking about the shuttle, or releases as a result of launch accidents, so again, not relevant.

    Most of the web discussion of this seems to center on Cassini; I may have mis-remembered what probe I originally had my concerns about.

    Most of that discussion seems weighted toward the arguments of the pro-Cassini camp. On the other side of the issue, a few minutes searching turned up this:

    Again, the issue for me here is the accidental release of vaporized plutonium in the upper atmosphere during flyby. I’m not assuming it’s dangerous. I’m expecting that before people do stuff like that, with consequences potentially consisting of tens of thousands of additional worldwide cancer deaths, they be required to perform a coldly rational analysis of the potential risks, and use that analysis for some sort of open, accountable decision-making process that assesses whether the risks are worth the rewards.

    I’m not very confident that that actually happened in this case. I think there’s a significant chance that what we got instead was a bureaucratic advocacy proceeding where the “correct” answer was predetermined based on an opaque, non-accountable political process, and then that correct answer was sold to the public. I’m aware that stuff like that happens all the time, and most of the time the politicians rolling the dice with other people’s lives get away with it.

    Sometimes they don’t, and we get a big stirred-up-hornet’s-nest of public recriminations afterward. Like, for example, the recriminations over the Katrina aftermath that we’re going through now.

    The way to avoid the need for the recriminations afterward is to be honest and open with your decision-making process before the bad thing happens. When propaganda is being used to sell one side or the other of the risk-assessment decision, by definition you’ve fallen short of an honest and open process.

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