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To Open The Sky

The Front Pages of Christopher P. Winter

The Cassini Mission to Saturn

A personal view

The Risks of Cassini

This scientific mission to Saturn has risks very similar to those of any space mission. To wit:

Titan IV failure   The Titan IV-B booster fails at liftoff, or a little later.
This would most likely mean it explodes.
Centaur failure   Since the spacecraft is in Earth orbit by this time, two outcomes
are possible. The Centaur might fail to ignite, or it might burn
incorrectly — wrong timing, wrong direction, wrong thrust.
Spacecraft bus failure   Once Cassini is headed away from Earth, it is possible that a failure
in the control system, the communications gear, or the thrusters might
cause loss of the mission.

Of course, Cassini is not just any space mission. It carries Plutonium on board. The great worry, therefore, is that one of the failures I list above could result in the release of plutonium into the atmosphere of Earth.

Come, let us reason together about the possibilities.


If the Titan IV-B explodes at liftoff, the explosion is unlikely to be strong enough long enough to break the protection around that plutonium. In other words, the RTGs are designed to withstand a launch explosion, and the design has been verified by a great deal of testing. All this is explained in great detail on NASA's Web sites and elsewhere.


If the Titan fails partway to orbit, Cassini will fall back to Earth somewhere — probably in the sea. It may or may not have left the atmosphere before falling back. Again, the RTGs are designed to survive such a re-entry intact. Also, similar designs have actually undergone re-entry. And history records that they did survive.


As mentioned, two outcomes are possible if the Centaur upper stage fails. Either Cassini will orbit the Earth for a time, and eventually re-enter, or it will careen off into space, to wind up in a random orbit of the sun. In the first case, it will encounter conditions quite similar to the re-entry due to a Titan failure, and the RTGs will almost certainly survive. In the second case, Cassini will probably never be seen again by anyone now alive. It will drift in space for at least hundreds of years, during which time its payload of plutonium-238 (half-life 88 years) will decay to harmlessness.


The last possibility is the most worrisome. It arises because we have no way to boost Cassini directly out to Saturn. We get around this by using the Gravity Assist maneuver; we send the spacecraft swinging close to a planet. The planet slows down by a tiny amount; the spacecraft gets a big boost in its velocity relative to the sun. Gravity Assist lets us do missions that would otherwise be impossible. It's been used since the 1960s. The most notable example is Voyager 2, which used it to tour the four gas giant planets in this system. NASA has a very good track record of placing the spacecraft exactly where they want it for such maneuvers.

Nevertheless, it is conceiveable that some combination of failures might put Cassini on a course to hit Earth instead of swinging by it as intended in August 1999. And this re-entry would be the most dangerous for the RTGs, for Cassini would be moving much faster than orbital speed. The temperature during re-entry, and perhaps its duration, would be higher and might (as NASA acknowledges) result in release of plutonium.

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The smarter opponents of Cassini's launch home in on this last case as the most credible threat of plutonium release. I agree, which is why I'm spending more time on it. The other three cases I dismiss because the engineering and the flight history of RTGs convince me that Cassini's RTGs will in fact come through those accidents without releasing plutonium.

Since everyone agrees that plutonium could be released if Cassini re-enters instead of swinging by Earth, the next question is what would cause re-entry to occur. It would take a chain of events. First, during most of the trip from Venus to Earth (June-August 1999), Cassini is aimed to miss Earth by a wide margin. So then, thrusters would have to fire without being commanded to do so. Second, NASA would have to either not be able to detect this, or not be able to turn the thrusters off. Third, this random thruster burn would have to meet very tight limits in order to set up a re-entry. In fact, the precision required for the burn, plus the backups built into Cassini's systems, make such a chain of events extremely improbable.

A week before swingby, NASA will fire Cassini's thrusters to aim it at a point 500 miles above Earth. If the burn lasts too long, the aim-point distance might drop to zero. But, again, the key factor is the precision required. The burn must last just a little too long. If it lasts way too long, Cassini just misses Earth on the other side. Then the other failures have to come into play, so what is required is a burn overlong by a precise amount, plus a breakdown in either telemetry or command functions — and those circuits have backups. Multiple failures must happen in a small time window (less than a week) in a spacecraft that has performed complex maneuvers perfectly during months of operation. And you can be sure that NASA will be paying close attention during this week. It is not going to overlook something.

Another possibility exists during this week. If the planned burn goes OK, putting the craft on track for its 500-mile miss, a subsequent accidental burn could also aim Cassini at Earth. But now, extreme precision in both timing and direction is required. Does the accidental burn send Cassini toward Earth, or away? This is not just a fifty-fifty chance; we are working in three dimensions, remember. Roughly speaking, it's like hitting the bullseye on a dart board that is several hundred feet away — with a dart that your opponent can steer by remote control. Even if you have superhuman coordination, you won't have a prayer of scoring unless the opponent (NASA) loses that remote-control capability.

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Finally, should this remote possibility come about, how much of a health risk is it to have 72.3 extra pounds of plutonium in our atmosphere? A negligible one, in my opinion. Here's why I believe so.

Consider the fallout from open-air nuclear bomb tests. That burden remains, although the hottest isotopes have decayed since such tests were abolished in 1963 by the test-ban treaty. [Note: According to data from the Natural Resources Defense Council, China conducted the last atmospheric test in 1980.]

Comparing fallout's effect on the population in general with the hypothetical dispersal of Cassini's plutonium is not as easy as calculating the amount of plutonium already present. Still, two things are clear: that fallout is not a dramatically deadly danger to the populace at large (as opposed to specific groups like the Downwinders); and its quantity is significantly greater than the amount of radioactivity Cassini carries. Therefore, I conclude that Cassini's plutonium is a minor health risk, even if the worst happens.

Leaving aside the man-made radioactivity already present in the environment, there are natural sources of several kinds. Even more important, there are many other risks to health. Indeed, most people knowingly accept greater risks every day. Now I will grant that it is generally unfair to criticize someone's concern over one risk, however miniscule, by citing a greater risk — especially if he understands and accepts the latter, but not the former. Yet, considering the facts I've laid out here, I find myself unwilling to get my knickers in a knot over the plutonium aboard Cassini.

NOTE: The opinions expressed here are solely my own.


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Copyright © 1997-2015 Christopher P. Winter. All rights reserved.
This page was created in 1997. Its contents were last modified on 25 October 2015.