The Rubin Observatory Could Discover Corner Reflectors or Artificial Lights in the Outer Solar System
A week ago, the Rubin Observatory initiated the release of its much-anticipated flood of data from its 3.2 gigapixel camera (as announced here).
Solar-system astronomers are accustomed to discovering asteroids by detecting the reflection of sunlight from their surface. However, a class of objects with the same surface area are potentially detectable to much greater distances in the outer solar system. They could be discovered, for the first time, by the Rubin Observatory.
Natural objects which reflect sunlight get dimmer inversely with distance to the 4th power at distances that are much larger than the Earth-Sun separation. The reason is simple. The flux of sunlight impinging on their surface declines inversely with distance squared and the observed flux as a result of reflection off their surface declines by another factor of inverse distance squared. Combining these factors implies that asteroids or the nuclei of interstellar objects get dimmer inversely with heliocentric distance to the 4th power.
On the other hand, a source that generates its own light, like a spacecraft or a city, would brighten inversely with distance squared as it approaches us from the outer solar system. This resembles the way that a lamppost brightens as we approach it from a dark street.
The difference between scaling with distance to the 4th and 2nd powers can distinguish between a natural and a technological object. In 2012, I published a paper here with Ed Turner from Princeton University, which showed that existing optical telescopes and surveys can detect artificially-illuminated objects comparable in total brightness to a major terrestrial city out to the outskirts of the Solar System. Since orbital parameters of Kuiper belt objects are routinely measured to exquisite precisions, we proposed to measure the variation of the observed flux from such objects as a function of their changing orbital distances. This idea is particularly feasible now with the Rubin Observatory. If objects with an inverse-square brightening law are found, follow-up observations can measure their spectra to determine whether they are illuminated by artificial lighting. The search can also be extended beyond the Solar System with future telescopes, which would be capable of detecting phase modulation due to very strong artificial illumination on the night-side of planets as they orbit their parent stars.
Another interesting class of objects which would follow the inverse-square brightening law are corner reflectors along the Sun-Earth axis. A corner reflector is a passive retroreflector consisting of three mutually perpendicular, intersecting flat surfaces that reflect waves directly back toward their source, regardless of the incidence angle. They are essential for enhancing radar visibility, satellite tracking, and laser ranging.
Corner reflectors were particularly helpful in measuring the distance between the surfaces of the Earth and the Moon using lasers. The distance is calculated from the round-trip time of laser pulses propagating at the speed of light, which are reflected back to Earth by the Moon’s surface or by reflectors on the Moon. Three reflectors were installed by the United States’ Apollo program, two by the Soviet Lunokhod 1 and 2 missions, and one by India’s Chandrayaan-3 mission. The precise distance measurements so far imply that Newton’s constant does not change by more than a part in ten trillion per year (as reported here and here).
Now, consider a corner reflector created by another technological civilization. If that corner reflector happens to be aligned with the Sun-Earth axis, it will follow the inverse-square brightening law because it will reflect rays of sunlight back to where they came from, allowing an observer on Earth to detect them. The shadow of the Earth has a negligible effect at large distances, because the surface area of Earth is 12,000 times smaller than that of the Sun. Our situation resembles a fly hovering over a lamppost and observing the reflection of light from a corner reflector in a dark street.
As long as the Earth is observed to transit the Sun from the vantage point of the corner reflector, we would observe the corner reflector to follow the inverse-square brightening law. This favorable geometry selects a swath that covers 0.47% of the sky, the ratio between the radius of the Sun and the Earth-Sun separation. In other words, only one out of 214 corner-reflectors that are randomly distributed around the Sun will follow the inverse-square brightening law from the Earth’s vantage point. Gladly, these corner reflectors will all be detectable at nighttime when the body of the Earth blocks the sunlight and allows a ground-based telescope to search for them. They would all appear in opposition to the Sun within a cone of 0.25 degrees, corresponding to the angular radius of the Sun from Earth. The Rubin observatory or other survey telescopes resemble the eyes of the fly hovering near the lamppost and looking away for reflected light from objects in the dark street.
A corner reflector can be distinguished from a source of light by its spectrum matching that of sunlight and by its position in the sky being in opposition to the Sun. Here’s hoping that the Rubin Observatory will show evidence for either sources of artificial light or a corner reflector. Any such detection will surely make our life on Earth far more exciting.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
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