And that is no accident. The chemical letters Pu were given on purpose, as
a joke, in 1941 by the element's discoverer, Glenn Seaborg.
Even so, nuclear energy's re-entry into space, if it occurs, would likely involve
a new breed of spacecraft propulsion called nuclear electric power, a plutonium-based
system that engineers say is dramatically safer than conventional notions of
nuclear energy.
The low-power nuclear generators now called radioisotope thermoelectric
generators, or RTGs, were developed in the 1950s. Nowadays, RTGs usually
rely on plutonium-238 (Pu-238), which has a half-life of 87.7 years, meaning
that after 87.7 years, half of the original fuel remains. This coupled with
the fact that RTGs contain no moving parts makes the setup a reliable alternative
for long-term missions.
The first RTG in space generated 2.7 watts aboard a U.S. Navy satellite launched
in 1961. Spacecraft powered by RTGs have landed on the Moon and Mars, and still
today explore the outer planets via the Pioneer, Voyager, Galileo, and Ulysses
missions.
Only practical way
Most engineers say RTGs are the only practical way to provide electricity and
heat for a spacecraft in the frigid outer reaches of the solar system, where
temperatures can dip to -400 degrees Fahrenheit and where using solar panels
to tap energy from the distant Sun becomes increasingly difficult.
And they say it is safe.
Antinuclear activists warn that if ingested or inhaled, plutonium is deadly.
Few question this. But when prepared for space flight, plutonium is not in a
form that can be ingested or inhaled even if a launch accident pulverized the
rest of a rocket and its spacecraft, supporters say.
As it decays, Pu-238 emits radiation mainly in the form of alpha particles,
which have a very low penetrating power compared to other potential fuel sources.
Only lightweight shielding is necessary because alpha particles cannot penetrate
a sheet of paper. Still, the material is housed inside several protective layers,
including an outer heat-resistant shell, to prevent leaks even in a launch or
re-entry accident.
And inside those layers, plutonium is packed into ceramic pellets, four per
capsule, each about the size of a marshmallow and flexible to resist breaking
apart. The pellets cannot dissolve in water.
NASA officials say the grade of plutonium used in RTGs, even were it to be
released, would have no more effect on people than mammograms or high-altitude
airline flights.
Cassini, focus of criticism
As part of a pre-launch Environmental Impact Statement that NASA was required
to file for the Cassini mission, the space agency issued this statement:
"In the unlikely case of a launch accident that resulted in the release of
plutonium, the risk to public health is assessed to be negligible. In the even
less likely event of an inadvertent atmospheric reentry of Cassini during its
Earth swingby, there is a small potential for public health effects, according
to studies reported in the [Draft Environmental Impact Statement]. However,
NASA is designing the spacecraft and mission to ensure that the probability
of an Earth reentry, after the spacecraft has left Earth orbit, is less than
one in one million."
Further, Cassini scientists and many health experts say, even if plutonium
were vaporized and spread into the atmosphere, it is unlikely that very many
people would be affected.
One Cassini scientists likened this hurdle to that of spreading the common
cold: "There are enough germs in one sneeze to give a billion people a cold,"
he said, "It's the distribution problem that stops this from happening."
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