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A SETI Institute preprint proposes searching moon dust for alien industrial debris
Lewis J. Pinault of the SETI Institute and his co-authors argue that the airless, tectonically dead moon has spent a billion years collecting and burying interstellar grains that no radio search could ever hear.
The Scientist · Science desk
What happened
- Lewis J. Pinault, an associate researcher at the SETI Institute, and his co-authors posted a preprint, submitted to the International Journal of Astrobiology, proposing a search for alien physical artifacts in lunar soil.
- Their starting complaint is synchronicity: radio listening only works if a civilization is transmitting in our era, and the Milky Way has had about 13 billion years in which it might instead have transmitted.
- The paper names two target classes, the first being Arkhipov particles, unintentional industrial debris such as fragments of a decaying Dyson swarm, after the Ukrainian astronomer who raised the idea in the 1990s.
- The second class is Bracewell particles, deliberately sent to other star systems as smart dust, named for the physicist Ronald Bracewell.
- The moon is offered as the archive because it has no atmosphere, plate tectonics or water cycle, and because micrometeoroid churning can bury microscopic material metres deep, out of reach of cosmic rays.
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Why it matters
- decision Anyone designing a regolith sampling campaign with this objective in mind has to drill rather than scoop, since the grains that kept their structure are the ones that got buried.
- constraint An empty core would settle very little. It bounds the abundance of one narrow class of grain, and tells you nothing usable about whether anybody ever built a megastructure.
- capability A buried grain waits. Unlike a transmission, it can be pulled out and re-analysed whenever laboratory methods improve. That removes the timing requirement that makes listening a gamble.
A grain falling in toward the sun gathers speed as it comes, and by 1 AU the paper puts its relative velocity at about 42 km/s [10]. Anything striking the lunar surface faster than 5 km/s is vaporized on contact [11]. So a grain that survives as a recognizable object has to arrive at under roughly 12 percent of its infall speed [2], its kinetic energy per unit mass cut by a factor of about 71 [3]. The brake the authors propose is radiation pressure from the sun, and it acts only on grains within a particular range of size and density [12].
The brake is a selection on geometry and mass. The composition estimate is a selection on chemistry, since only refractory grains are credited with surviving the transit at all [9].
The time budget is the part of the argument that improves with scale. The sun circles the Milky Way about once every 230 million years [6], which over 13 billion years works out to roughly 56 circuits [1]. The recoverable record is far shorter than the delivery history: a grain that lasts a billion years in transit has covered about four of those circuits [4], or around 8 percent of galactic history [5].
The two classes of particle would also not be equivalent evidence: unintentional debris would indicate that someone had industry, while deliberately dispatched dust would indicate that someone had intent [8].
The account of the preprint gives no expected abundance of such grains per kilogram of regolith, and names no mission that would collect them [13].
What to watch
- Whether reviewers at the International Journal of Astrobiology accept the radiation-pressure braking calculation for realistic grain sizes and densities.
- Whether the final paper specifies how a laboratory would tell an engineered refractory grain from an ordinary micrometeorite fragment.
- Any published estimate of expected grain abundance per kilogram of regolith. That number would turn the proposal into a search with a denominator.