An old technology is the key to a new method in the search for extraterrestrial intelligence.
The simple mirror can act as a powerful beacon when it is reflecting direct sunlight. Unlike the surfaces of most objects in the Solar System, which reflect light diffusely in many directions, mirrors display specular reflection. Light that hits a flat mirror retains its directivity, instead of going every which way. Since sunlight comes from the narrow-angled disk of the Sun on the sky, an alien probe with a mirror-like surface would reflect sunlight into a narrow cone, and the reflection would shine very brightly. These glints are visible from the smooth surfaces on our own satellites, most famously in the Iridium flare phenomenon. An alien satellite, located not in Earth orbit, but deep in the Solar System, should glint in the same way.
In my new paper, I calculated the observable properties of these glints. Human satellites in Earth orbit glint for only a few seconds, but a probe in interplanetary space would glint for hours at a time, before seeming to vanish when it goes out of alignment with the Sun and the Earth. It would be a unique form of transient event in the upcoming imaging surveys by telescopes like the Large Synoptic Space Telescope. The glints can be very bright – even a square meter mirror might be seen from hundreds of millions of kilometers away, depending on whether it is spinning or not. The brilliance of these events comes at a cost, though: because sunlight is reflected into only a narrow angle, we have to get lucky in order to see a glint, unless extraterrestrials are deliberately trying to signal us. According to the estimates of the new paper, even after years of observations, there would have to be thousands of mirrors in the Solar System to catch even a single glint.
Mirrors are interesting from a SETI point of view, because unlike most previously hypothesized beacons, like radio emitters and lasers, mirrors shine passively. An extraterrestrial intelligence passing through the Solar System might have come and gone thousands of years ago. By using the mirrors to reflect sunlight, they could draw our attention after all this time without worrying about maintaining complicated electronics or power systems. A simple, low tech solution might answer one of the profound and deepest questions of our science: are we alone?
The new paper has been accepted to the journal Publications of the Astronomical Society of the Pacific, and is also available on the arXiv preprint server.