Three Shovels for Interstellar Archaeology
The new NSF-DOE Rubin Observatory in Chile is the most effective discovery machine ever built for interstellar objects. Over the past year, I collaborated with Richard Cloete and Peter Veres to develop Machine-Learning software that assigns a likelihood for the interstellar origin of objects detected by the Rubin Observatory.
Since each field of view is visited in two consecutive 15-second exposures, the Rubin data alone might not be sufficient for a robust identification. Once likely interstellar candidates are flagged, potentially on a monthly basis, follow-up observations with other ground-based telescopes could take additional measurements to infer the precise orbital parameters of these candidates and verify whether they exceed the escape speed from the Solar system. While doing so, it will become clear from the reflection of sunlight off these objects whether any of them shares the extreme pancake shape inferred for the first reported interstellar object, `Oumuamua.
The most unusual anomaly of `Oumuamua was its non-gravitational acceleration away from the Sun, with no evidence for a familiar cometary tail that could cause it through the rocket effect. `Oumuamua was not a familiar comet, nor a familiar asteroid. What was it? To answer this question, we need to collect more information on `Oumuamua-analogs in future Rubin data.
Observing Rubin’s interstellar objects also with the Webb telescope, located a million miles away from an Earth-based telescope, will allow us to detect via parallax their non-gravitational acceleration to exquisite precision, as shown in a recent paper I wrote with my student Sriram Elango. A scaling of the non-gravitational acceleration inversely with the square of the distance from the Sun, would suggest radiation pressure as its origin. This was my suggestion for the origin of the non-gravitational acceleration of `Oumuamua. Human-made space trash, like 2020 SO or “Empty Trash Bag Objects” near Earth, exhibit the same radiation pressure push.
Spectral features in the emitted and reflected spectrum of an interstellar object can be used to infer its surface composition. If the non-gravitational acceleration results from evaporation of exotic gases like hydrogen or nitrogen — as speculated for `Oumuamua, it would be possible to identify the spectral features of these gases around the object. `Oumuamua had to lose about a tenth of its mass, given its measured level of non-gravitational acceleration. Substantial evaporation of this magnitude cannot go unnoticed in detailed observations of new interstellar objects.
The Webb telescope can also measure the emitted infrared flux and surface temperature of an interstellar object — which when combined with its known parallax distance, can be used to infer its surface area. If the object is tumbling, it would be possible to use the evolution of the surface area projected along the line-of-sight in order to map the shape of the object in three dimensions. The known size and shape and the reflected flux of sunlight from the object will allow us to infer its surface albedo (reflection coefficient) for sunlight. We did not have direct measurements of the area, surface temperature or albedo for `Oumuamua. At their typical distances from Earth, it is impossible to resolve interstellar objects in the images of ground-based telescopes. They appear as points of light.
If the interstellar object does not come close enough to an imaging telescope, the telescope must come close to the object. This is possible if an interstellar object as anomalous as `Oumuamua is identified early enough, a year in advance of its closest approach to Earth. A dedicated space mission could then get close to intercept the expected path of the object and take a close-up photograph of it. This would conclusively probe whether the object is natural or artificial in origin. The parameters of such a rendezvous mission were outlined in a paper written by the Galileo Project research team in 2022. ESA’s comet interceptor is not up to the task of catching up with the high speeds associated with interstellar objects.
The cost of a space mission to rendezvous with an interstellar object is on the scale of billions of dollars. One can save a factor of a thousand in mission cost by discovering interstellar meteors and recovering their materials on the surface of Earth. These rare interstellar objects cross the path of Earth around the Sun. The high speed of their fireball in the Earth’s atmosphere can be used to flag their interstellar origin, as was the case for the meteor identified by U.S. Government satellites on January 8, 2014. In 2023, I led a Galileo Project expedition to retrieve materials of this meteor and we plan similar expeditions in the future.
Yet, the cheapest approach for studying interstellar objects is to search for a signature of their interstellar origin in existing meteorite collections. I am currently collaborating with Stein Jacobsen and Eugenia Hyung in measuring the abundances of rare isotopes in a special class of anomalous meteorites. Our goal is to check whether these isotope ratios lie outside the range of solar system materials. If successful, this will be the simplest method for seeking materials from interstellar objects and inferring their origin. In addition to telescope data and a rendezvous mission, it constitutes the third shovel for interstellar archaeology.
With a wealth of data collected by these three shovels, it would be impossible to hide any stubborn anomalies under the carpet of traditional thinking. Whether we received a natural rock or a well-designed package through interstellar delivery services from a distant star, will be determined by data and not the opinions of people.
And even if all interstellar objects are rocks, it would still be interesting to figure out whether any of them contain the building blocks of life-as-we-know-it, like the amino acids found in the materials retrieved from the asteroid Bennu. If extraterrestrial life is abundant in interstellar space, we will soon find it.
Such a finding will, no doubt, make our life on Earth far more interesting.
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.
