Earth is surrounded by space trash. The European Space Agency reports that there were 6,500 successful rocket launches of nearly 17,000 satellites into Earth orbit. Out of these, about 11,500 satellites are still in space and about 9,000 of them are functioning. But there are many more objects orbiting Earth as a result of broken satellites or discarded rockets. Altogether, there were more than 640 break-ups, explosions, collisions, or anomalous events that resulted in fragmentation. The total mass of all space objects in Earth orbit is more than 11,500 tons. Bigger fragments are rarer. There are 10 million orbiting objects in the range 1–10 centimeters and 36,500 pieces larger than 10 centimeters, with the two populations carrying a comparable amount of total mass per logarithm of individual fragment mass. A recent report from the United Nations forecasts that the number of satellite collisions will increase dramatically after 2025.
Humanity will eventually be polluting interstellar space. Our spacecraft Voyager 1 & 2, Pioneer 10 & 11 and New Horizons, will exit the Oort Cloud’s boundary of the Solar system in about 10,000 years, merely a millionth of the age of the Milky-Way galaxy. At that time, these technological objects will be defunct and serve as archaeological artifacts for interstellar historians who might study humanity from a distance. In the future, any additional spacecraft that we launch at a faster speed than these historic rockets, capped at a few tens of kilometers per second, could reach interstellar space earlier. As long as these technological objects do not exceed the local escape speed from the Milky-Way, 500 kilometers per second or 0.17% of the speed of light, they will accumulate in interstellar space like plastics in the ocean, bound to our galaxy by gravity. In a billion years, Voyager 1 will reach the opposite side of the Milky-Way disk relative to the Sun and flag humanity’s first century of space exploration. Where humanity will be at that time remains to be seen.
The reservoir of interstellar space trash reflects the cumulative contributions from technological civilizations over the past billions of years of Milky-Way history. There are hundreds of billions of Milky-Way stars which formed billions of years before the Sun, allowing for plenty of space trash in interstellar space. Most of the defunct artifacts may have been broken into pieces during their tumultuous journey between the stars. Impacts by cosmic-rays, dust, micrometeorites and gas particles, as well as damage from the shock waves triggered by stellar explosions or from high-energy radiation emitted by black holes or neutron stars, could have resulted in a broad mass spectrum of fragments. The larger the speed, the bigger is the damage inflicted by the surrounding interstellar medium.
Space trash could also be produced by the breakup of technological megastructure around stars, as a result of the natural evolution of these stars. Stars more massive than the Sun die on a timescale shorter than the age of the Universe. Their death is naturally followed by the ejection of a substantial fraction of their envelope mass, which could kick any technological infrastructure that existed around the parent stars to interstellar space. In particular, the interstellar object `Oumuamua could have been a broken piece of a megastructure — like Dyson sphere, as I suggested in a recently published research note.
The simplest assumption to make is that there is no special spatial scale in the break-up process of space trash. This would imply an equal amount of total mass in fragments per logarithm of individual fragment mass. As it turns out, the limited data on the abundance of interstellar objects is consistent with this assumption. Considering a representative volume of the local interstellar medium, the estimated number of `Oumuamua-like objects on the 100-meter size scale is about a thousand times smaller than the number of IM1 or IM2-like objects on the 1-meter scale, which is about a trillion times smaller than the number of interstellar dust particles on the micrometer scale. Since fragment mass scales as size cubed, this tentatively suggests, with large uncertainties, that each of these components carries a similar amount of mass per logarithm of fragment mass.
The non-gravitational acceleration of `Oumuamua declined roughly inversely with distance squared from the Sun, as expected from solar radiation pressure. This does not imply that `Oumuamua was a light sail. It could have been pushed by radiation pressure in the same way that the object 2020 SO, discovered by Pan STARRS three years later, was found to be pushed by sunlight. As it turns out, 2020 SO was a rocket booster from a 1966 launch by NASA, namely our own space trash.
The Rubin Observatory in Chile will likely revolutionize our understanding of various populations of interstellar objects, starting in 2025. The fundamental question is whether among the natural icy rocks from interstellar space, we might find evidence for space trash produced by other technological civilizations. If so, their trash will be our treasure.
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.