In the summer of 1977, two separate launches occurred off of the NASA launchpad in Cape Canaveral, Florida. The spacecraft probes were tasked with traveling to Jupiter and Saturn. The initial plan for the two Voyager probes was a 5-year mission to survey these two distant planets. The initial mission was successful, and the probes were able to travel impossibly farther to Uranus and Neptune for additional flybys. And NASA kept the probes going, farther and farther out until they both eventually reached interstellar space (Voyager 1 in 2012 and Voyager 2 in 2018). What NASA originally intended to be a 5-year mission has now been going for 43 years. As they move out of our solar system, the Voyager probes continue to take measurements and send data back to Earth, giving us a unique opportunity to learn about the more distant reaches of space. You might be wondering what has been keeping the probes going for over 40 years. Technically speaking, the probes require very little power to keep moving forward—in space, where there is no drag, inertia can carry you forward indefinitely—but the probes do use power to make course corrections and run the instruments. This far out into space, in the dark regions beyond our Sun’s reach, solar panels cannot provide sufficient power to keep the probes going. Instead, the Voyager probes employ radioisotope thermoelectric generators (RTGs) that take advantage of the unique properties of radioactive materials to produce gradual and enduring power.
Every element on Earth, and beyond, is made of a configuration of minuscule atoms. An atom is made up of a nucleus of packed-together protons and neutrons (which are themselves composed of even smaller particles called quarks) and a cloud of tiny, whizzing electrons. The protons are positively charged, the neutrons are neutral, and the electrons are negatively charged. Atoms can accrue charge (either negative or positive) by gaining or losing electrons from their electron cloud. This is also how atoms combine to create larger, more complex molecules—they share electrons with other atoms. The defining characteristic of an atom—that dictates what element it makes up—is its number of protons (atomic number). Hydrogen has one proton, helium has two protons, carbon has six protons, oxygen has eight protons, etc.
The number of neutrons in an atom’s nucleus does not affect its charge, although it does add to the atomic mass. Atoms with the same number of protons but different numbers of neutrons are called isotopes and they are named by their unique atomic masses. Carbon, the primary element of living systems, normally has six protons and six neutrons for an atomic mass of twelve (electrons are so insignificant in size that they do not contribute to atomic mass). Carbon-14, a common isotope of carbon, containing six protons and eight neutrons, arises naturally from atmospheric nitrogen (seven protons, seven neutrons) bombarded by neutrons excited by cosmic rays. The nitrogen atom accepts the extra neutron and sheds a proton, converting it into carbon-14. These special carbon atoms are, for the most part, indistinguishable from normal carbon, and they get easily absorbed into the life cycle on Earth. But carbon-14 is technically an unstable form for carbon to take, so atoms of carbon-14 slowly decay over a long span of time. This is the basis of carbon dating—the process by which fossils of organic material can be dated to their approximate time of death based on the remaining amount of carbon-14 present.
Some isotopes, like carbon-14, shed particles or energy to regain their nuclear stability—as such, they are considered radioactive isotopes (or radioisotope). A nucleus is unstable when the strength of the electrical repulsion between the protons (the positive charges repel each other, just like a magnet) exceeds the strength of the nuclear forces holding the atom together. You can think about it like the cohesiveness of a drop of water. Water dropped on a surface defies the forces of gravity, forming a dome-like drop rather than spreading out on the surface. This is because the attractive force between the water atoms exceeds the pull of gravity. But the more water you add to the drop, the heavier it gets. Eventually, gravity wins, and the dome breaks. Similarly, nuclear stability is defined by the ratio of neutrons to protons. In lighter elements (less than 20 protons), an equal number of protons and neutrons is preferred. As atoms get heavier and heavier, they need more neutrons to hold their nuclei together.
A radioisotope can decay by emitting bundles of protons and neutrons (alpha particles), electrons (beta particles), individual neutrons (neutron emissions), or high energy radiation (gamma rays). Each radioisotope has its own unique half-life, which is defined as the amount of time it takes for half of any given sample to decay into its stable form. The half-life of carbon-14 is about 5,700 years, which is why it is effective for dating fossils. Other radioisotopes with shorter half-lives can be used in medical imaging and treatments.
The radioisotope that powers the Voyager probes is plutonium-238. Plutonium has an atomic number of 94, and as a heavier atom, it tends to be highly unstable. The most stable atomic mass of Plutonium is 244 (94 protons and 150 neutrons), but even this isotope decays into uranium-240 over the scale of 80-million years. The isotope plutonium-239 was first created as part of the Manhattan Project, and it was eventually used in the core of the atomic bomb dropped on Nagasaki (the bomb dropped on Hiroshima had a uranium core). When plutonium-239 is hit with a neutron, a rapid chain reaction of nuclear fission—where the atom is hewn into two smaller atoms—is activated creating enough energy to decimate a city (or power one through a nuclear reactor).
On the other hand, plutonium-238 cannot decay through nuclear fission. Instead, it decays primarily through alpha particle emission (two protons and two neutrons—essentially a helium atom nucleus). The kinetic energy of the released alpha particles creates heat that is translated into usable energy by something called a thermocouple. A thermocouple takes advantage of the heat differential between the plutonium-238 core and cold vacuum outside the probe. Just like we discussed in the post on entropy, heat naturally tries to disperse. So the heat moves from the hot plutonium core out to the cold exterior, and in a thermocouple, this movement of thermal energy is used to create a closed circuit that generates electrical energy.
Plutonium-238 has a half-life of about 88 years, meaning that not even half of the plutonium aboard the Voyager probes has yet decayed—half the atoms will be decayed by 2065. The power output of the RTG (radioisotope thermoelectric generator) declines as the atoms decay, so the probes have to conserve more energy the longer they travel. In the beginning, an RTG containing about 10 pounds of plutonium can generate 2,000 watts of thermal power and 120 watts of electrical power. This makes it an ideal solution for long-distance probes and Mars rovers.
But the energy output may not be high enough or consistent enough for a long-term Mars colony. NASA estimates that roughly 40 kilowatts (40,000 watts) of continuous power would be needed for a permanent colony. That kind of power generation requires a power system that is more sustainable and easily scalable. Next week, I’ll talk about NASA’s newest power endeavor, the Kilopower nuclear fission reactor, and how it may just be the key to sustainable space power.
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