No one cares about nuclear power once it’s in space.  Only on it’s way to space.

Radioactive Contamination

Nuclear power in space comes from an RTG (Radioisotope Thermoelectric Generator).  The RTG is powered by a radioactive isotope.  The isotope decays and produces heat in its decay.  That heat produces electricity through the use of thermocouples.

For space harware, the main threat from an RTG is that explosive destruction of the hardware containing the radioactive material would spread radioactive material.  The RTG containers are designed to survive this type of event so the odds of contamination are small.

What happens when the container is destroyed…

If the container were to break up the estimated odds of contamination are 1 in 10.  The alpha radiation that the most common RTG fuel cannot penetrate the skin but if it were inhaled it would do serious damage to internal organs.

There were protests when Cassini launched…

People were scared that if the Cassini launch failed then the radioactive fuel would find its way into the atmosphere and people would inhale it.  While this shouldn’t be taken lightly the launches are generally conducted over the ocean.  The winds in the atmosphere may disperse the radioactive material.

Obviously if the launch vehicle travels through all of the atmosphere it is unlikely that we can wait for a launch window where all of the prevailing winds are blowing out to sea.  This dispersion shouldn’t be taken lightly but understand that the fuel most US satellites use is only a problem when inhaled or ingested.  If the particles fall in the sea or on the land then it poses no threat - to people.

Basics of an RTG

An RTG is a Radioisotope Thermoelectric Generator.  Essentially these radioactive isotopes decay to more stable atoms.  As the isotopes decay they give off heat and that heat is converted to electricity.  The conversion to electricity is done using thermocouples.

An RTG is used on space craft instead of solar panels when the mission is very long or to an outer planet where the light energy provided by the sun is too weak (low density).  The Voyager and Cassini missions used RTGs.

Benefits of an RTG

For space the benfits of an RTG are obvious.

1. Long term power (in the range of hundreds of watts)
2. No moving parts - like solar arrays that have open up after launch
3. Degradation of output power happens but much slower than with solar panels

These advantages lead to a stable power source capable of operating for decades in extreme environments.

Fuel Source

From Wikipedia:

Plutonium-238, curium-244 and strontium-90 are the most often cited candidate isotopes, but other isotopes such as polonium-210, promethium-147, caesium-137, cerium-144, ruthenium-106, cobalt-60, curium-242and thulium isotopes have also been studied. Of the above, ²³⁸Pu has the lowest shielding requirements and longest half-life. Only three candidate isotopes meet the last criterion (not all are listed above) and need less than 25 mm of lead shielding to control unwanted radiation. ²³⁸Pu (the best of these three) needs less than 2.5 mm, and in many cases no shielding is needed in a ²³⁸Pu RTG, as the casing itself is adequate.

²³⁸Pu has become the most widely used fuel for RTGs, in the form of plutonium(IV) oxide (PuO₂). ²³⁸Pu has a half-life of 87.7 years, reasonable energy density and exceptionally low gamma and neutron radiation levels.

The criteria leading these choices of fuels are

1. Must have a half-life short enough that its decay produces substantial heat
2. Must ahve a half-life long enough that the heat generated is relatively stable for decades
3. For space, the energy output per density must be high
4. The radiation produced should be high energy with low penetration - preferably alpha-radiation - so that shielding can be minimal

Next post: Why people are scared.

This time Phoenix stays dead…

Where the Phoenix lander is on Mars it’s winter.  Phoenix ran out of power recently.  The 2 rovers, Spirit and Opportunity, have to fight for their own survival every Martian winter.  Why?  Because they use solar arrays.  Solar arrays that degrade with time.  Solar arrays that degrade with the deposition of dust.  Solar arrays that don’t produce much power when the Sun isn’t very high in the sky.

Power constraints have hampered interplanetary probes in the past.  The problem is that once a probe gets out near Jupiter there just isn’t enough sun light to do much.  Even large arrays can’t entirely overcome the problem of “not enough light”.

We have $100 million missions that end because of power

We have missions to Mars and Jupiter and beyond.  Some like Cassini use nuclear power.  Others use solar power.  Some missions, like Phoenix, have ended not because of mechanical failure, communications failure, or a lack of good science that still needs doing but because they have too little power.

A lack of power has not stopped the Voyager missions decades after their launch.  A lack of power doesn’t need to kill or maim anymore Mars missions.  A Radioisotope Thermoelectric Generator (RTG) is the answer.

Next post: What is an RTG?

Next Next post: Why people are scared.