Powerful technology carries New Horizons through the Solar System
It is no easy feat to send a spacecraft billions of miles, to the outer, frozen edges of the Solar System. The recent successes of the New Horizons’ team would seem to disprove this notion. However, their efforts were, in large part, made possible by the technologies and techniques that were employed.
Due to the Plutonian system’s distance from the Sun, solar power was never an option for the spacecraft visiting it. Solar panels, which convert sunlight into electricity, would not receive enough of the Sun’s light from a distance of three billion miles.
The fastest spacecraft ever launched, New Horizons needed a different power system, which Lockheed Martin gave it in the form of a Radioisotope Thermoelectric Generator (RTG).
Tim Hoye, who worked on New Horizons’ RTG as well as on the power systems for the Galileo and Cassini orbiters, explained, “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.”
Whereas New Horizons is approximately the size of a baby grand piano, the RTG is roughly the size of a piano bench at 42 inches (107 cm) long with a diameter of 17 inches (43 cm).
RTGs generate power through the decay of radioisotopes, a process that produces heat. Through thermoelectric couples, that heat is converted into electricity.
New Horizons’ RTG is powered by plutonium-238, which has a long half-life of 87.7 years, making it capable of powering the vehicle for several decades. However, in addition to the diminishing power output of 0.787 percent per year due to radioactive decay, further power loss results from the degrading properties of the bimetallic thermocouples used to convert thermal energy into electrical energy.
For instance, in the year 2000, the RTGs used by the Voyager probes, 23 years after production, the radioactive material inside the RTGs had decreased in power by 16.6 percent, but the RTGs were working at about 67 percent of their total original capacity instead of the expected 83 percent.
The U.S. Navy was the first to launch an RTG into space aboard the Transit 4A satellite on June 29, 1961. Now, more than fifty years later, that similar but more advanced RTG technology powers the New Horizons spacecraft into the blackness of deep space.
“New Horizons and other deep space exploration missions to the outer Solar System would not be possible without the development of RTGs pioneered by Transit 4A,” says Alan Stern of the Southwest Research Institute, principal investigator for New Horizons. “This is a key area of U.S. space leadership that’s tremendously benefited both unmanned and manned exploration of the Moon and planets.”
“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.”
Only seven percent of the thermal energy produced in the RTG is converted into electricity. Because of this low-efficiency level, Lockheed Martin was working on a new power system called the Stirling conversion system, based on an advanced Stirling radioisotope generator (ASRG), but the project was cancelled toward the end of 2013 due to NASA budget cuts.
The new system is about four times more efficient than the RTG. If the project had continued, it could have, someday, powered missions farther into the Kuiper Belt – the Solar System’s third zone – as well as manned missions into deep space.
New Horizons successfully completed its closest approach past Pluto on July 14, providing the first detailed imagery from the distant world. The spacecraft also captured photos of Pluto’s largest moon – Charon.
The mission’s Principal Investigator, Alan Stern, discussed the spacecraft’s next target, which has not yet been determined, with SpaceFlight Insider during an exclusive interview. Given that the spacecraft has already traversed the 4.67 billion miles (7.5 billion kilometers) between Earth and Pluto, it is likely that the technologies that have allowed New Horizons to fly for nine years will grant it the ability to fly for an additional four.
The 3-D animation below shows the main components of the Advance Stirling Radioisotope Generator – a different type of radioisotope generator that was previously considered by NASA to provide power for some missions that explore the Solar System. Although the ASRG is no longer in development for spaceflight at this time, NASA continues to study the Stirling converter technology for potential use on future space missions.
Video courtesy of NASA
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.