Dating interstellar objects with the Webb Space Telescope

The data collected on `Oumuamua in late 2017 was not sufficient to conclusively test whether it was a technological artifact.

The Vera C. Rubin Observatory in Chile will be our “dating app” for finding the next anomalous interstellar object after `Oumuamua. Its 3.2-billion-pixel camera will survey the southern sky every four days and is forecasted to find new interstellar objects every few months. Together with my postdoc, Richard Cloete, we are currently developing software that will allow us to identify and characterize interstellar objects from the data stream of the Legacy Survey of Space and Time (LSST) planned with this camera. Through most of the survey, we will “swipe to the left” when noticing an ordinary comet or asteroid, but on occasion we might discover another interstellar object worthy of attention because its anomalies might reflect an extraterrestrial technological origin.

The data collected on `Oumuamua in late 2017 was not sufficient to conclusively test whether it was a technological artifact, but it was sufficient to indicate that `Oumuamua did not resemble familiar asteroids or comets in the solar system. As `Oumuamua was tumbling every eight hours, the flux of sunlight reflected from it changed by a factor of 10, whereas for solar system asteroids the variation level is at most a factor of three. This suggested an extreme geometric shape, and a detailed model by astrophysicist Sergey Mashchenko implied that `Oumuamua was disk-like at 91 percent confidence.

Moreover, `Oumuamua did not show any hint for a cometary tail while being pushed away from the sun by a mysterious non-gravitational force that declined smoothly in inverse proportion to the square of the distance from the sun. In 2018, it occurred to me that this non-gravitational force could result from the reflection of sunlight. In order for radiation pressure to be effective, `Oumuamua had to be a thin membrane. While we know of no natural astrophysical process that produces disk-shaped membranes, a technological civilization could have manufactured it. In a recent paper, I suggested that a membrane of this type might be space trash, possibly a piece of a broken Dyson sphere that was originally composed of thin tiles of light sails.

In September 2020, the Pan-STARRS telescope in Hawaii that discovered `Oumuamua, identified another object pushed away by reflecting sunlight with no cometary tail. It was later identified as a rocket booster launched by NASA in 1966 with thin walls made of stainless steel. This second object, named 2020 SO, was definitely artificial because NASA produced it. The question is: Who produced `Oumuamua?

Incidentally, NASA never launched a spacecraft as big as `Oumuamua - the size of a football field. It is, therefore, likely that smaller technological relics are far more abundant. However, their detection is more challenging because they reflect less sunlight and can therefore be noticed only at smaller distances — where they move faster across our sky. Nevertheless, in another recent paper, coauthored with  my student Carson Ezell, we calculated that LSST could find interstellar objects that are a few times smaller than `Oumuamua every couple of years.

The most significant instrumentation advance since `Oumuamua’s discovery is the launch of the Webb Space Telescope in 2021. The Webb telescope is located 1 million miles away from Earth. Simultaneous observations by Webb and Earth-based telescopes would allow a precise distance measurement for interstellar objects passing within the Earth-sun separation. (This parallax method is the reason that animals with two eyes survived better than single-eyes competitors, in gauging the distance to a threat.) The Webb and Earth-based telescopes could determine the three-dimensional trajectory of an interstellar object to exquisite precision. This would allow us to discern whether the object possesses propulsion — such as the rocket effect from gas emission, in excess of the sun’s gravitational acceleration.

The Webb telescope offers additional advantages. The surface temperature of an object near-Earth would be hundreds of degrees Kelvin above absolute zero, with thermal emission centered at a peak wavelength of tens of micrometers and detectable by the Webb telescope. Measuring both the surface temperature and the emitted flux of the object in addition to its parallax distance would fix its surface area. As the object spins, its apparent area will evolve periodically in time. Combining these measurements with the flux of sunlight reflected from the same surface, will enable the ability to determine the reflectance coefficient, or albedo, of the object’s surface.

Here’s hoping that dating the next `Oumuamua will be as intriguing as our original encounter with `Oumuamua. If the Webb telescope will find indisputable evidence that an interstellar object was manufactured by an extraterrestrial technological civilization, we will know that we are not alone. This will constitute an opportunity to learn about our cosmic neighborhood, just like finding a tennis ball thrown by a neighbor among the rocks that usually populate our backyard.

The new scientific knowledge about our neighbor might provide a new meaning to our cosmic existence, which had been pointless until then. There is nothing more important for singles than to find a partner. Such a finding is definitely worth the $10 billion invested in our matchmaker — the Webb space telescope.

Avi Loeb is the head of Harvard University’s Galileo Project, founding director of Harvard’s Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, as well as the former chair of Harvard’s Department of Astronomy (2011-2020). He chairs the advisory board for the Breakthrough Starshot project, and 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. His new book, titled “Interstellar,” is scheduled for publication in August 2023.