A Rendezvous with Pluto: The Technology that is Powering a Probe for 3 Billion Miles

When the New Horizons spacecraft was launched from Cape Canaveral in 2006, Tim Hoye was eagerly watching nearby from Kennedy Space Center’s New Horizons Control Center.

For most of his career, he had worked on developing and testing technologies that power spacecraft, like Galileo, Ulysses and Cassini, on exploration missions into deep space.

This mission was different, though. The spacecraft soaring into the clouds before him was the fastest ever launched—embarking on the longest ever, 3-billion-mile journey to rendezvous with its primary objective, the ninth planet in our solar system, Pluto.
Flash forward nine and a half years and a lot has changed. For one, Pluto is no longer considered a planet but rather a dwarf planet. For another, New Horizons is providing us with near real-time images of its approach to the dwarf planet through its own Twitter account.

During its July 14 closest flyby, New Horizons will reveal to the world the very best images of Pluto we have ever seen and will complete the initial exploration of the ninth body in the classical solar system.
And as it has since launch day, the power system Hoye and his Lockheed Martin teammates worked on will continue to power the probe for this historic flyby, as well as whatever may come next.

WHY NOT SOLAR POWER?

Most spacecraft use solar arrays for power. This technology converts sunlight into electricity. However, for a mission destined for Pluto, the sun is so far away that solar energy provides very little power. In fact, the sun probably looks like just a bright star from way out there.

“Instead, a radioisotope power system can take you places far away from the sun or to a remote location where solar power isn’t feasible, such as locations beyond Jupiter’s orbit or parts of the moon where the sun does not reach,” Hoye said.

This radioisotope power source onboard New Horizons is called a Radioisotope Thermoelectric Generator or RTG. The New Horizons spacecraft is approximately the size of a baby grand piano, the RTG is around the size of a piano bench—just 17 inches in diameter and 42 inches long.

HOW IT WORKS

Scientists developed systems that could generate electricity from the decay heat of radioisotopes. The heat that is produced is converted into electricity via thermoelectric couples. Specifically, plutonium-238 was selected because of its high specific power and mechanical and chemical properties in the oxide form.
The major benefit to Pu-238 is that it has a very long half-life (87.7 years)—allowing it to produce usable power levels for multiple decades.
The RTG technology has been used to power select space missions since the early 1960s because it is extremely reliable. The RTG design powering New Horizons was developed in the early 1980s.
“The very desirable quality of RTGs is that they are solid state, meaning they have no moving parts,” Hoye said. “You can install the plutonium fuel and it goes. There is nothing to wear out. It has very high reliability and the design has a demonstrated long life.”

BEYOND RTGs

While RTGs like the one on board New Horizons are extremely reliable, they also operate at only around 7 percent efficiency—that is, only 7 percent of the thermal energy produced gets converted into electrical energy.
The next evolution in radioisotope space power systems may be a Stirling conversion system that Lockheed Martin has developed based on an advanced Stirling radioisotope generator (ASRG).

The Stirling cycle engine—named after the thermodynamic cycle engine’s developer—is a heat engine that drives an oscillating piston. The piston is coupled to a linear alternator, which generates alternating current power.
This technology results in a system far more efficient than thermoelectric conversion. In fact, the ASRG is about four times more efficient than the RTG. This also cuts down on the amount of fuel needed and the expenses associated with the manufacture of Pu-238.

WHAT’S NEXT?

Scientists know that beyond Pluto, there is another class of celestial objects collectively known as Kuiper belt objects, many of which are very small.

While these can be found billions of miles beyond New Horizons’ original destination, Hoye believes that the RTG can keep producing plenty of power to take us there.

A mission extension would allow New Horizons to explore the deeper cosmos in ways we haven’t been able to before. Not to mention, the next wave of power generators, like the ASRG is opening the door to even greater possibilities, including human exploration.