Geek Trivia: (Nu)clear to the moon

How many nuclear power sources have operated on the moon?

Editor's note: The Trivia Geek is once again lying about on vacation, but he phoned in this Classic Geek, which originally ran on July 20, 2005, to tide you over.

More than 37 years ago, Apollo 11 touched down on the surface of the moon, marking the first time human beings set foot on a celestial body other than planet Earth. There is scarcely a man or woman left in the industrialized world who hasn't at least once heard astronaut Neil Armstrong's famous quotation, commemorating the moment he became the first person to set foot on another world: "That's one small step for man, one giant leap for mankind."

Still, for all the cultural and historical significance of the Apollo 11 mission, Armstrong and fellow moonwalker Edwin "Buzz" Aldrin spent a mere two hours and (almost) 32 minutes walking the surface of the moon outside the "Eagle" Lunar Module. Yet, in that time, the duo of intrepid explorers accomplished a staggering number of technical objectives, many of which continue to produce tangible scientific benefits today.

When Armstrong and Aldrin blasted off from Tranquility Base on July 21, 1969—having spent less than 22 hours on the moon—they left more than just their famous footprints behind. One of Apollo 11's primary mission objectives was to set up the Early Apollo Scientific Experiments Package (EASEP), a modest collection of automated experiments, which would continue to gather data after the astronauts returned to Earth.

These experiments included the Solar Wind Composition (SWC) sensor, a laser-ranging retroreflector (LRRR), and a passive seismic experiments package (PSEP). While each of these devices was notable simply by its presence on the moon, the PSEP was doubly unusual—thanks to its internal heating system, the Apollo Lunar Radioisotopic Heater (ALRH). The ALRH embodied the first major use of nuclear energy in a manned spaceflight—and the first use of nuclear energy on the moon.

However, the ALRH was neither a nuclear reactor nor a power generator. It simply used the decay of plutonium 238 to generate heat and keep the otherwise solar-powered PSEP's delicate internal components warm and functional during the long, cold lunar nights.

That said, more than one nuclear power source has operated on the moon, all of them close cousins to the trailblazing ALRH.


How many nuclear power sources have operated on the moon, all of them close cousins to the Apollo Lunar Radioisotopic Heater (ALRH), which used nuclear decay to keep Apollo 11's Passive Seismic Experiment Package (PSEP) from freezing during the lunar night?

Five nuclear SNAP-27 radioisotope thermoelectric generators (RTGs) have operated on the moon, each providing centralized power for the array of experiments left on the lunar surface by Apollo missions 12, 14, 15, 16, and 17. Like the ALRH, natural nuclear decay—rather than nuclear fission—is the source of power for RTGs.

As radioisotopes decay, they emit a predictable amount of heat, which the ALRH focused to keep seismic sensors warm and functional. In contrast, an RTG converts radioisotope decay heat directly into electricity using thermocouples. As such, RTGs are ideal power sources for space probes and spacecraft, as they can provide years of stable electrical energy with no risk of a runaway chain reaction.

After Apollo 11's successful deployment of a radioisotope heat source, subsequent Apollo missions brought along an Apollo Lunar Surface Experiments Package (ALSEP), which included an RTG power source that connected by cable to the individual sensors and devices left behind by the NASA astronauts. The ALSEP RTGs continued to provide power to these lunar experiments for years after the Apollo moon program ended, and NASA remotely shut down the devices in the late 1970s.

The only lunar RTG that one could consider a failure was the ALSEP power source that reentered the Earth's atmosphere following the return of the aborted Apollo 13 mission. The Apollo 13 RTG crashed into the South Pacific Ocean, but official investigations determined that it never compromised its nuclear containment or released any harmful radiation.

RTGs continue to serve as viable power sources in numerous spacecraft, notably communication and navigation satellites. However, despite their sturdiness and stability, RTGs do suffer two notable drawbacks, which preclude their use on Earth.

First, RTGs are typically powered by plutonium 238, which isn't exactly an over-the-counter fuel source—and certainly not one that governments want to see proliferate. Second, RTGs are hideously inefficient, returning far less power from the radioisotopes than controlled fission could offer, consigning these ingenious devices to work only in the far reaches of space—and the inner depths of Geek Trivia.

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The Quibble of the Week

If you uncover a questionable fact or debatable aspect of this week's Geek Trivia, just post it in the discussion area of the article. Every week, yours truly will choose the best post from the assembled masses and discuss it in the next edition of Geek Trivia. (To see the original quibble from this article, see Listing A.)

This week's quibble comes from the August 2 edition of Geek Trivia, "Strange (water)bedfellows." TechRepublic member VirtualGardener busted me for poor sentence construction.

"You state: 'Hall originally set out to build a super-comfortable chair, and he began by filling a 300-pound vinyl bag with cornstarch.' Boy, that's a heavy bag. How much cornstarch did he put in that 300-pound bag?"

You're right, dear reader, that should have read: Hall originally set out to build a super-comfortable chair, and he began by filling a vinyl bag with 300 pounds of cornstarch. Thanks for the grammar lesson, and keep those quibbles coming.

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The Trivia Geek, also known as Jay Garmon, is a former advertising copywriter and Web developer who's duped TechRepublic into underwriting his affinity for movies, sci-fi, comic books, technology, and all things geekish or subcultural.