by Christopher Morrison
How do you provide power to spacecraft in the deep reaches of space far from the sun? For the past 50 years the United States has been using radioisotope thermal generators (RTG). Deep space missions using RTGs (including the Voyager Spacecraft that was launched in 1977) are currently still transmitting data from distances far beyond Pluto. More recently, the New Horizon’s spacecraft was launched in 2006 has just flown by Pluto offering the first images of the dwarf planet close up. RTGs were used on the moon during the Apollo mission with astronauts, the Curiosity rover, the Viking Mars Landers, and more. The United states has launched about 30 spacecraft and landers that have used RTGs for electricity and heat.
How do RTGs work? The RTGs have two main parts: The radioisotope and the thermal generator.
First is the radioisotope, which produces heat through radioactive decay. There are many different isotopes with different amount of heat that are generated from them. Plutoninum-238 (Pu-238) is the ideal choice for most space missions because it produces a steady amount of heat and a half life of 87 years. The half life is important because a radioisotope must last long long enough to complete the mission. A half life of 87 year is nearly perfect because it will take a few decades before the isotope begins to decrease much in power.
The video below shows an interview with Stacy McLaughlan, a scientist at Los Alamos National Laboratory, who worked on the New Horizons radioisotope power system. She talks more about Plutonium 238.
The Thermal Generator
The second piece of an RTG is the part that converts the heat from the radioisotope and turns it into electricity. Traditionally, we have used thermoelectric devices to turn that heat into power. In the video below, Jean-Pierrer from NASA JPL talks about have a thermoelectric device works.
NASA JPL works on the design of thermoelectric generators, which they call the Multi-mission RTGs because of the wide rage of operation conditions that they can be used in.
RTGs don’t produce a lot of power. RTGs provide approximately 500 watts per kilogram of thermal power. The RTG uses something called a “thermoelectric generator” that provides electricity by connecting a wire across two different metals. Both are at different temperatures inside the RTG. About 5 percent of that power is turned into electricity by the thermoelectric generator. The Mars Curiosity rover is powered on 110 watts of electricity, which is about the same as a laptop. Despite this seemingly low number, RTGs have been used successfully. Can you believe that the Voyager spacecraft is communicating from a distance three times further away than Pluto with only 300 watts (three laptops) of electricity?
To learn more about Radioisotope Power Systems, visit the NASA website: NASA Radioisotope Power Systems Website.
Chris is a space enthusiast currently in his fourth year of his Ph.D. pursuit from Rensselaer Polytechnic Institute and holds a B.S. from Embry Riddle Aeronautical University in Aerospace Engineering and Computer Science. Chris is also training for his senior reactor operator license, and works on an educational startup Learn-Blitz.com on weekends. He is currently on the Executive Committee (and webmaster) for the ANS Aerospace Division.