One positive side effect of the Cold War was the discovery of plutonium-238, a non-weapons-grade isotope of the element that has served as the power source for a handful of space probes over the last 50 years. This year, after a quarter-century hiatus, the United States resumed producing the element for the express purpose of powering future deep space missions, as NASA’s supply has been dwindling to the point that the plutonium on board the rover Curiosity had to be purchased from Russia. But not all plutonium launched into space stayed there. Most famously, the 8.5 pounds of plutonium that launched to the Moon on board Apollo 13 returned to Earth with the crew, which is potentially a big problem, if it were to somehow land in your backyard.
Plutonium-238 in Spacecraft
Plutonium was discovered in December of 1940 by a University of California Berkeley team led by radiochemist Glenn Seaborg. While bombarding uranium-238 with alpha particles (nuclei of an isotope of hydrogen), they produced a completely new element, neptunium-238, which decayed in just two days. The decay yielded a new element: plutonium. Within months of this initial discovery, they had confirmed plutonium, which is chemically similar to uranium, as the 94th element. Uranium, neptunium, and plutonium are of course named for the planets Uranus, Neptune, and Pluto, reflecting the order in which they were discovered.
Before long, Seaborg and his team had successfully isolated plutonium-239, an isotope of plutonium that became the fuel for nuclear bombs. But something else came from the process that yielded this weapons-grade plutonium: the "hot" waste material plutonium-238. In the 1950s, scientists started researching how this isotope could be used to power spacecraft: though the space age hadn’t formally begun, it was on the near horizon, and practical issues like heating and powering machines in the frigid vacuum of space were already under consideration.
The method by which plutonium powers spacecraft is fairly simple. Plutonium naturally produces heat as it decays, and that heat can be converted into energy by passing through thermoelectric converters. The device that harnesses and facilitates this process, turning radioactive plutonium into a power source, is called a radioisotope thermoelectric generator, or RTG. The advantages of using RTGs is that they're simple, reliable, and can last a very, very long time, especially in cases where solar power isn't effective.
The first RTG launched into space in 1961; two of these units supplied some of the power to the Navy navigational satellites Transit 4A and 4B. In 1963, RTGs began flying as the sole power sources of spacecraft, and by the end of the decade plutonium was powering the sophisticated barrage of experiments Apollo astronauts took to the lunar surface.
Apollo 13 Commander Jim Lovell practices carrying the ALSEP before launch.
ALSEP
The Apollo Lunar Surface Experiment Package (ALSEP) was a suite of scientific instruments designed to stay on the lunar surface gathering data long after the crew had returned to Earth. When Apollo 13 launch on April 11, 1970, it carried with it the second complete ALSEP. Apollo 11 had carried an Early Apollo Scientific Experiments Package (EASEP) in July, and Apollo 12 carried the first actual ALSEP in November, both in 1969. These first two ALSEPs had returned a wealth of data, yielding more than one billion scientific and engineering data points relating to the Moon’s surface and lunar phenomena. So, NASA had understandably high hopes for Apollo 13’s ALSEP results, particularly since this mission would be landing in a geologically interesting site at Fra Mauro.
Apollo 13’s ALSEP had four distinct instruments with individual aims. Four seismic elements made up the passive seismic sensor, a unit identical to that left behind by Apollo 12 designed to gather a second set of seismic data that could be correlated to the first. A cold cathode ion gauge was designed to measure the pressure of the Moon’s scant atmosphere. Two charged particle physical analyzers were on board to identify and quantify charged particles reaching the lunar surface. Finally, a pair of sensitive temperature probes that were to be embedded ten feet into the lunar surface were designed to measure thermal conductivity of the surface and heat flow during a full lunar day.
These four experiments, which the crew would set up west of their landing site, were connected to a Central Station by heat-resistant wire ribbons. This central station was ALSEP’s communications centre that contained the receiver, data processor, transmitter, and power controls for all four experiments. Its power source, and by extension the four ALSEP experiments’ power source, was an RTG containing Eight pounds and 96 ounces of plutonium inside a SNAP-27 (System for Nuclear Auxiliary Power) generator, with the plutonium itself locked up in a graphite and ceramic cask.
Aquarius Comes Back Home
Apollo 13, of course, never landed on the Moon. After one of the oxygen tanks in the service module exploded, NASA canceled the landing and the mission became a fight to get the astronauts home alive.
NASA launched its lunar missions such that there were ample abort options. The translunar injection burn that sent the crew out of the Earth’s orbit and on a path to the Moon actually put the spacecraft free return trajectory; without an additional correction, they would whip around the Moon’s far side and comeback to Earth. On Apollo 13, the crew didn’t make the adjustment burn that would put them on a path to land on the surface. Skipping this burn put them on free return trajectory, but one that had them landing in Africa. Oops.
Getting Apollo 13 to splashdown someplace other than Africa took some work. The lunar module’s evaporator cooler was venting gas, pushing the spacecraft off its path. The compensate, NASA had the crew line up the optical device on the lunar module’s window with the Earth’s terminator (the line that separates the light day side from the dark night side) and fire the LM’s descent engine again. It was a maneuver NASA had come up with during Apollo 8; recalling it now played no small part in getting the crew home. Similar burns continually nudged the spacecraft back when it drifted from its reentry trajectory.
A SNAP-27 RTG on the Moon.
Getting Ready to Reenter
Getting the spacecraft lined up for reentry was only half the battle. After an optimal lunar landing mission, the crew would return to Earth with just a working command and service module, and the service module would be jettisoned. The worst case scenario was for the two modules to reenter together but unattached, allowing for a mid-air collision during descent. On Apollo 13, mission controllers had all three modules returning to Earth, and the service module was dead. Isolating the command module, the crew's reentry vehicle, was a separate challenge.
The crew ditched their service module first using a clever maneuver retro office John Llewelyn had come up with: burn forward slightly, jettison the module, then back up to regain the correct trajectory. Mission Control had the crew thrust forward with the LM, separate the Service Module, then thrust back. It put them on the right trajectory with one less piece of their spacecraft.
The second challenge was figuring out how to ditch the lunar module. A normal mission dictated that this spacecraft be jettisoned after leaving the Moon and sent into a solar orbit. Bringing it back home was unprecedented, and without a working command module, the crew didn’t have a way of separating the two spacecraft. Again, the men in mission control thought back to a maneuver from a previous mission, this time Apollo 10. When it had come time for this crew to separate their lunar module from their command module on this lunar landing dress rehearsal mission, they hadn't completely depressurized the tunnel linking the two vehicles. Upon separation, explosive decompression blasted the two spacecraft apart. Deliberately leaving the tunnel slightly pressurized on Apollo 13 allowed the crew to jettison and gain distance from their now exhausted lunar module.
But mission control’s work with the lunar module didn’t end once it was jettisoned. The ALSEP was still on board with its 8.5 pounds of plutonium, and NASA couldn’t just let it fall randomly to Earth. Somehow the agency had to make sure the dangerous, radioactive material landed safely away from any populated areas.
Apollo 13's lunar module Aquarius just after it was jettisoned by the crew.
Aquarius’ Final Resting Place
Though it was a first for Apollo, this mission was not the very first time plutonium destined for deep space had come back to Earth. In 1964, the Transit-5-BN-3 mission was aborted after a launch vehicle failure, and the RTG on board allowed plutonium to leak out as it reentered the atmosphere. The design of RTGs was subsequently changed to ensure survival of a plutonium cask during atmospheric reentry. The SNAP-27 RTG on board Apollo 13 reflected these design changes. In another stroke of good fortune, part of the agency's preparation for Apollo 13’s flight had been developing contingency procedures for a non-nominal mission, and one scenario mission controllers had worked out was what to do in the event that a lunar module came home with its cask of radioactive plutonium intact.
Retrofire officer Chuck Deiterich had worked out these possible plutonium cask reentry scenarios with Bill Remini from the Atomic Energy Commission (AEC) and a group of men from the Mission Planning and Analysis Division who calculated trajectories. They had worked out the aerodynamics for the plutonium cask and figured out what to do with it should it come back to Earth, and the solution was to drop it in a deep part of the ocean. It worked out that Apollo 13’s return trajectory, after all the burns to correct for errors, had both the command and the lunar modules landing in the Pacific Ocean. Nevertheless, reentry was tense for AEC representatives. They were listening and watching what was happening, and ready with airborne planes to check for any radioactive fallout as the lunar module reentered the atmosphere.
The AEC surveyed the area where the lunar module landed and found no traces of radioactive material. The cask of plutonium performed as designed and didn’t allow any radiation to leak out. Deiterich and Robert McAdams, one of the scientists from MPAD, received a letter from the Chairman of the AEC thanking thanking them and their colleagues for having appropriate safety measures in place.
In light of NASA’s recent plutonium shortage, people have wondered whether the space agency would ever recover the plutonium from Aquarius’ resting place. It’s unlikely. Not only has no recovery mission been mounted before now, with the US’ plutonium production restarted there’s less reason to recover lost material. Instead, the SNAP-27 RTG resting at the bottom of the Pacific Ocean will likely continue to stand as evidence that NASA’s Apollo-era nuclear safety program worked. LINK
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