Above: A cutdrawing of an GPHS-RTG that are used for Galileo, Ulysses, Cassini-Huygens and New Horizons space probes (NASA)
Here on Earth, we have access to a wide variety of power sources. These include fossil fuels like coal and oil, as well as renewable options like wind and geothermal energy. But most of these energy sources aren’t available in space. So what are the options for fuelling extended space missions? Several different ones have been developed in recent decades, with even more ones set to come on line in the future.
Did you know? To date, all of the rockets used to launch spacecraft have been powered by chemical energy.Spacecraft are usually launched with the help of chemical energy, such as the energy found in the liquid (oxygen and hydrogen) and solid (mostly aluminum and an oxidizer) rocket boosters used with NASA’s Space Shuttles. And once they arrive in space, most spacecraft either coast to their destination or take advantage of gravity assist, using the gravity of a planet to gain additional speed.
However, a more efficient technology called electric propulsion was tested in 1998 on Deep Space 1, a mission to comet Borrelly. This spacecraft used a lens to focus sunlight onto advanced solar panels. A current mission, Dawn, is exploring the asteroid belt using a similar technology called ion propulsion. Instead of using sunlight, it expels charged particles at very high speeds to generate continuous low-level thrust very efficiently.
One measure of the efficiency of different fuel types is specific impulse. The higher the specific impulse, the more efficient the fuel source. Electric propulsion has a specific impulse of 10-20 times that of the chemical propulsion systems!
Solar panels are often used to power onboard instruments. For example, the International Space Station uses solar panels, as do most satellites and probes that explore the Solar System as far as Mars. However, for missions that travel farther from the Sun, solar power is insufficient. Likewise, alternate power sources are required for spacecraft that operate near the poles of a planet or moon, on a world that is covered by dust or clouds, or in night environments.
Did you know? The Voyager spacecraft were launched in 1977 and are expected to stay powered by an on-board radioisotope thermonuclear generator (RTG) until 2020.The newest Mars rover, the Mars Science Laboratory (MSL), uses a radioisotope thermonuclear generator (RTG). This is basically a nuclear battery that converts heat released by the radioactive decay of a man-made element, plutonium-238, into electricity.
The rover carries about 5kg of plutonium-238, which produces enough energy for the rover to use more than one instrument at a time and to operate during the night, as well as during Martian winter. Previous rovers had to “sleep” during these times.
Unlike nuclear reactors on Earth, which use fission, the naturally-decaying material used in RTGs does not emit harmful radiation. It can therefore be safely used aboard spacecraft. RTGs were also used for the Cassini mission, which studied Jupiter, Saturn, and some of their moons, as well as the New Horizons mission, which is scheduled to reach Pluto in 2015.
Looking to the future, space travel could rely on energy sources such as nuclear fission, fusion, and matter-anti-matter annihilation (yes, like the USS Enterprise from Star Trek!). These sources have specific impulses ranging from 20-4,000 times that of conventional chemical propulsion systems, meaning faster and more efficient space travel. It may all sound like science fiction, but it could very well become science fact as we continue to explore the solar system and beyond.
References Curiosity Rover: Power (NASA) Space Power (NASA) The Edge of Sunshine (NASA) Nuclear Power in Space Course Notes (G. L. Kulcinski, University of Wisconsin-Madison)