For many years, the data that the early DDEs sent back to Earth was thought to be missing or lost. Since its surprise rediscovery in 2006, those in the inner circle of outer space activities have slowly begun to realize that O'Brien's unassuming detectors have a lot more to tell us about moondust than anyone could have imagined—except, of course, for O'Brien himself. Now 85, still sprightly and living in Perth, he's been waiting half a century for the chance to share with the world what he knows about one of the solar system's most baffling substances.

O'Brien always had an affinity for extreme environments. He took up spelunking as a teenager and once got stuck in the depths of Australia's Yarrangobilly Caves for 79 hours. The experience was traumatizing—his lamp ran out of fuel, and the only sound, according to a contemporary newspaper account of his rescue, was the “bats above his head and the feel of their tiny skeletons under his boots”—but it didn't put him off caving. A few years later, while exploring a crystal grotto, he met his future wife, Avril Searle.

By the age of 23, O'Brien had completed a PhD in physics at the University of Sydney and been appointed deputy chief physicist for the Commonwealth Antarctic Division. He was assigned to the icebreaker Magga Dan and found himself gazing in wonderment at the aurora australis rippling in reds, purples, and greens across the polar sky. This was in 1958, a year after the Russians launched Sputnik and the same year NASA was founded. O'Brien began to dream of putting a satellite into orbit to study how energized protons and electrons gave rise to the southern lights. He got his chance the following year, when James Van Allen, discoverer of the Van Allen belts, got him a job at the University of Iowa. O'Brien and a few students built a satellite from scratch in five months. Other launches followed, and in 1963 O'Brien was offered a post in Rice University's new space science department.

Not long after O'Brien and his family moved to Houston, he got a call from NASA. The agency hoped to hire him as an astronaut instructor, but it also invited him to submit a proposal for a science experiment to go to the moon. He suggested a device that would measure the energy spectra of charged particles as they rained down on the lunar surface. From a field of 90 submissions, his was one of seven that got the green light. NASA told him that, as a matter of policy, the experiment should include a dust cover, basically a sophisticated strip of plastic. No one knew at this stage just how pesky moondust would be, but O'Brien figured that if the agency was going to the trouble of installing dust covers, it should also include a dust detector.

At first, NASA and its private contractors balked. It would be too difficult, they believed, to construct a detector that was light enough to meet the mission specs and simple enough that it wouldn't take up any of the astronauts' limited time and attention. On the moon, distractions could be deadly. O'Brien thought their resistance was “bloody stupid” and, with the help of that cocktail napkin, came up with a design to allay their concerns. It consisted of three tiny solar cells mounted on a box, which was painted white to reflect sunlight. As dust settled on the cells, their power output would drop, providing a clear record of accumulation over time. O'Brien threw in a few temperature sensors for good measure, bringing the experiment's total weight to a dainty 10 ounces. Because the DDE was so small, it could be bolted onto the seismometer that Aldrin and Neil Armstrong were setting up to measure moonquakes. Upon hearing all this, NASA relented: The DDE could go to the moon. Once there, it would feed its data to the seismometer, whose antenna would transmit the readings back to Earth. They'd be stored on reels of magnetic tape for further analysis.

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O'Brien, Avril, and their three children moved back to Sydney in 1968, so he made arrangements to have the tapes shipped to him. He can't quite remember now where he was on the morning in late July 1969 when the Apollo 11 Lunar Module alighted on the moon. He thinks he listened to the radio broadcast between interviews with various Australian news outlets. Yet he does remember, vividly, the moment Aldrin said the module was “kicking up some dust” as it came in to land, as well as Armstrong's observation, just before he stepped off the ladder, that the surface was “almost like a powder.” With a spike of excitement, O'Brien realized his DDE might very well prove its worth.