Empty Trash Bag Objects (ETBOs) are fragments of space debris in near-Earth orbits that are pushed around by solar radiation pressure, similarly to an empty lightweight trash bag tumbling in the wind. ETBOs are often found to orbit the Earth and considered to be broken parts of rockets or satellites. The largest ETBOs, often meters in size, are cataloged online.
ETBOs are characterized by their high area-to-mass ratio. Some are composed of aluminized plastic insulation and show non-gravitational acceleration from solar radiation pressure. Most of the known ETBOs are in highly elliptical orbits with a period of about one day, one had been tracked to have a 2.2-day orbit for seven years, and few have been discovered in even higher orbits. These orbits suggest that most near-Earth ETBOs are human made.
The name ETBO reflects the fact that solar radiation pressure pushes these objects mostly away from the Sun, but sometimes with a time-dependent or sideways force owing to their complicated structure and tumbling motion, like an empty trash bag being blown down the street. These characteristics make it difficult to predict their motion far in advance.
In 2019, a strange ETBO labeled A10bMLz was noticed in an elongated orbit that carried it to a distance of half a million kilometers, 1.3 times the Earth-Moon separation, with an orbital period of about two weeks. The specific origin of this ETBO is unknown as it zig-zags in unpredictable ways as a result of huge non-gravitational accelerations.
The first recognized interstellar object 1I/`Oumuamua exhibited a tumbling motion with a net non-gravitational acceleration that declined consistently with an inverse-square law from the Sun, as expected for solar radiation pressure. A few months after this observational data was reported, I suggested in a peer-reviewed paper with my then-postdoc Shmuel Bialy that `Oumuamua had a large area-to-mass ratio, effectively behaving like an interstellar ETBO. Since interstellar objects originate from outside the solar system, they cannot be human made. In fact, interstellar ETBOs could be generic signatures of extraterrestrial technological civilizations. The light curve of reflected sunlight from `Oumuamua exhibited extreme brightness variations, most consistent with a flat, disk-like object.
As soon as the public and the media paid attention to the suggestion that `Oumuamua may be artificial in origin, mainstream comet experts wrote numerous papers arguing that `Oumuamua is a comet with an invisible tail. Today, `Oumuamua is defined by these comet experts as a dark comet. But what if it is instead an interstellar ETBO? What would be a possible source for it?
One origin for interstellar ETBOs is debris from broken interstellar probes. After billions of years of travel through the interstellar medium, the steady bombardment by micrometeorites, dust particles, cosmic-rays and gas particles would naturally erode and possibly break an aging interstellar spacecraft like our own Voyager 1, Voyager 2, Pioneer 10, Pioneer 11 and New Horizons. The surface layers of a billion-year-old technological probe could be torn apart to pieces, many of which represent surface layers with a large area-to-mass ratio. Some of the pieces may have been pushed out from habitable Earth-like planets, after the host stars became bright giants or supergiants, illuminating their surroundings with a radiation pressure that is up to hundreds of thousand times larger than that of the Sun.
Alternatively, I suggested in a 2023 publication that a megastructure like a Dyson sphere which is composed of light sails hovering over a Sun-like star, would be broken into pieces once the host star evolves to a red giant. Here again, the pieces would have a large area per unit mass similar to empty trash bags.
For these reasons, I maintained over the past 7 years the argument that “The emperor (`Oumuamua) has no clothes (cometary tail),” like the statement of the child in the folktale written by the Danish author Hans Christian Andersen. I refused to behave like the adults in the room.
My reasoning is simple. Why would comet experts not define the ETBO A10bMLz as a dark comet? In fact, the same Pan-STARRS telescope in Hawaii that discovered `Oumuamua also found three years later an object 2020 SO which was pushed by solar radiation pressure and later identified as the Centaur upper stage of the Surveyor 2 lunar spacecraft, made of stainless steel.
The 2.3 gigapixel camera of the Rubin Observatory in Chile will start operations later this year and could discover many interstellar objects like `Oumuamua. The most exciting question it could answer is whether the family of `Oumuamua-like objects which exhibit anomalous non-gravitational acceleration are dark comets with an invisible tail or interstellar ETBOs of extraterrestrial technological origin.
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.
