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A Billion Miles Under Nuclear Energy

An exceptionally successful twenty-year nuclear-powered mission is about to enter its grand finale.

The Cassini-Huygens mission will begin its final phase on April 26, when the Cassini probe now orbiting Saturn makes a course adjustment to swing through the 1,500-mile gap between the planet and its rings, an area not yet explored.

Presuming it survives in that space without colliding with anything else orbiting Saturn, then this fall, it will dive a little deeper and plunge toward Saturn’s surface, where it will transmit data until it burns up in the planet’s atmosphere. A NASA video explains.

The dramatic end is deliberate; it will eliminate the possibility that the probe could crash on a planet that could support any form of life that stowed away on the spacecraft.

Cassini took seven years to get to Saturn, and then transmitted dramatic pictures of the rings. It confirmed an ocean of water on the moon Enceladus, raising the possibility of life there. It found new rings. Its findings advanced our understanding about the origin of the rings and the nature of the planet.

And it carried the Huygens probe, which was the first man-made object to land on a world in the outer solar system. Huygens landed on the surface of Titan on December 25, 2004.

Cassini above the rings of Saturn.
Cassini above the rings of Saturn.

Nuclear power made it possible. Cassini carries 72.3 pounds of plutonium-238, in a Radioisotope Thermoelectric Generator (RTG). When it was launched on its billion-mile trip, on October 15, 1997, the plutonium put out enough heat to be converted into 878 watts of electricity – about two-thirds of what a hair dryer draws, but sufficient to run the instruments and radios. Twenty years later, it is still putting out about 600 watts, but mission controllers are ending the flight because spacecraft is running out of the chemical propellants used to adjust its orbit, mono-methyl hydrazine and nitrogen tetroxide.


Plutonium-238 was originally produced as a by-product of creating plutonium-239 for nuclear weapons. After the United States stopped making new weapons fuel, it bought some plutonium-238 from Russia. There are several efforts now for making supplies for future space probes.

Sunlight on Saturn is only about 1/80th as strong as on earth, so solar panels are not helpful. Mars is the approximate limit of useful sunlight; some of the rovers on Mars used photo-voltaic panels, but Curiosity used a radio-thermal generator like the one on Cassini. The New Horizons probe that visited Pluto in the summer of 2015 used a similar generator. That generator is still running, as the probe prepares to visit an object in the Kuiper Belt on January 1, 2019.

After Cassini goes, the stunning photos will still be here.

The above is a guest post from Matt Wald, senior communications advisor at NEI. Follow Matt on Twitter at @MattLWald.

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