Harvesting Numerous Interstellar Objects with a Dedicated Space Telescope
It is easiest to search for missing keys under the lamppost. Our nearest lamppost is the Sun. Our biggest scientific question is “Are we alone?” One way to address this question is to search for empty trash bags from extraterrestrial civilizations that entered our backyard in the Solar system from interstellar space. We can most easily find them near the Sun. This is the focus of a new paper that I finished writing overnight and submitted for publication before my routine morning jog at sunrise.
In a recent 632nm Podcast interview, I was challenged by MIT’s Mike Dubrovsky. When I had mentioned that billions of dollars should be allocated to the search for extraterrestrial technological space trash near the Sun, Mike asked: “Let us assume that I find an investor that gives you that money. What would you do with it?” I replied that there are numerous interstellar objects within the orbit of the Earth around the Sun and I can design a dedicated space telescope to look for them. A day later, I wrote a new scientific paper -accessible here, about my vision.
My new paper shows that a dedicated space telescope with a meter-size aperture can detect numerous interstellar objects, 10-meter in diameter, that pass within 20 degrees from the Sun. Separating the emitted thermal radiation from the reflection of sunlight would allow us to measure the surface temperature, area and reflectance of these objects. Spectroscopic observations of any evaporated material at the expected temperature of 600 degrees Kelvin would provide important clues about the nature and birth sites of interstellar objects. Most importantly, this future telescope would allow us to separate technological space trash from natural asteroids or comets.
The discovery of interstellar objects, such as `Oumuamua, Borisov and IM1, opened a new path for studying planetary systems around other stars. Within our own Solar system, a new Nature paper, led by MIT’s Artem Burdanov (who contacted me with interest in the Galileo Project), reported recent results from observations by the Webb telescope of the main asteroid belt.
The first reported interstellar object, `Oumuamua, had an effective diameter of order 100 meters. Adopting Webb’s power-law distribution for the number of Solar system asteroids with a diameter below 100 meters, I estimated the number per unit volume of 10-meter interstellar objects based on the discovery of `Oumuamua.
Starting this year, the Legacy Survey of Space and Time of the Vera C. Rubin Observatory will be able to detect interstellar objects with a diameter of 100 meters out to a distance of twice the Earth-Sun separation. The observed flux of reflected sunlight from objects of a given diameter scales in proportion to the diameter squared and inversely with the square of the product of the distance from the Sun and the distance from Earth. Therefore, the detectability of 10-meter objects within a third of the Earth-Sun separation — similar to the orbital radius of Mercury — is better than that of 100-meter objects at twice the Earth-Sun separation.
Taking account of gravitational focusing by the Sun, I calculated that the arrival and departure rate of 10-meter interstellar objects within Mercury’s orbit is roughly once per 5.5 hours! There are plenty of them out there.
Gravitational acceleration increases the characteristic speed of these close-in interstellar objects to a value of about 80 kilometers per second. These objects traverse in one second the distance that a car on the highway traverses in one hour. They cross the orbit of Mercury in a couple of weeks.
The observed flux of reflected sunlight from `Oumuamua implies that a 10-meter object at Mercury’s orbital radius would be detectable with a signal-to-noise ratio of 10 in a three-hour exposure by a space telescope with a meter-size aperture. The existence of a cometary tail of dust or gas which scatters sunlight around the object, as observed for the interstellar comet Borisov, would improve the detectability prospects of even smaller cores of interstellar comets.
Unfortunately, the Hubble Space Telescope has a Sun-avoidance region of 50 degrees. However, a future space telescope with a meter-size aperture can be designed to withstand the excess Solar heat, in the spirit of the more extreme examples of the space-based Parker Solar Probe or the ground-based Inouye Solar Telescope. A novel space telescope could optimize a search strategy that maximizes the discovery rate of new interstellar objects which are even closer to the Sun than Mercury is. Near-twilight observations by the Rubin Observatory might also detect close-in interstellar objects. The observing circle of 20 degrees around the Sun extends to 75 times the radius of the Sun and is well outside the Solar corona. In principle, rarer but brighter Sun-divers could also be identified as I discussed in a previous paper with my former postdoc, John Forbes.
Interstellar objects can be distinguished from Solar system asteroids and comets by their orbital speed. Their speed exceeds the escape speed from the Sun’s gravity, and its measured value can be used to infer their interstellar speed relative to the Sun outside the Solar system.
At the orbital radius of Mercury, the average Sun-facing temperature of the interstellar objects is about 600 degrees Kelvin. This results in thermal emission at a peak wavelength of about 4,800 nanometers (nm). This wavelength is 7.6 times larger than the label of the 632nm Podcast that inspired my paper.
Separating the two components of reflected sunlight and emitted thermal radiation would allow to determine the diameter of the interstellar objects, based on their surface temperature and total emitted luminosity from the hotter Sun-facing hemisphere. Knowing their surface area and reflected sunlight flux would determine their reflectance (albedo) for sunlight.
At a surface temperature of 600 degrees Kelvin, some interstellar objects will evaporate and disintegrate. Spectroscopy of their cometary tail and non-gravitational acceleration would reveal their chemical composition and provide important clues about their nature and potential birth sites. These diagnostics would clarify whether the anomalous non-gravitational acceleration of interstellar objects like 1I/`Oumuamua resembles that of Solar system comets or that of empty trash bags carried by Solar radiation pressure.
If Elon Musk will accept my new bet with him — accessible here, we will have sufficient funds to pursue a state-of-the-art space telescope to harvest numerous interstellar objects. Within the next decade, this novel telescope could potentially find empty trash bags or other broken pieces of extraterrestrial technological objects among the icy rocks from exoplanetary systems.
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.
