Space Trash on Our Way to the Stars

Avi Loeb
5 min readJan 27, 2024
Image credit: The European Space Agency.

Humanity is trashing the space near Earth. Here are some numbers.

In October 1957, Sputnik-1 was launched as the first artificial satellite of Earth. By now, sixty six years later, 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 number of debris objects regularly tracked by Space Surveillance Networks is 35,150. The total mass of all space objects in Earth orbit is more than 11,500 tons.

For example, on February 10, 2009, an accidental collision at a speed above 10 kilometers per second occurred between two communication satellites: the active commercial satellite, Iridium 33, and the defunct Russian military satellite, Kosmos 2251. The collision created thousands of debris pieces larger than 10 centimeters, some of which still orbit Earth today. Many pieces followed a decaying orbit towards Earth, and burned up in the atmosphere within a couple of years.

By now, there are of order 130 million pieces of debris that are too small to be tracked, in the size range of millimeter to centimeter. Bigger pieces are rarer. In particular, there are a million orbiting objects in the range 1–10 centimeters and 36,500 pieces larger than 10 centimeters. These fragments orbit the Earth at a typical speed of about 8 kilometers per second, carrying 50 times more kinetic energy than the fastest rifle bullets with the same mass. Collisions between this space debris and functioning satellites on their orbital highway, can cause significant damage. Satellites can be warned of dangerous collisions. The International Space Station maneuvers to avoid such debris.

The concerns about space trash extends also to astronomy. The chance that one out of 130 million fragments larger than a millimeter would cross a single point in the sky within an orbital time of hours is of order a percent. According to my calculations, sunlight reflected from a perfect reflector of millimeter-size translates to an AB magnitude of 25 at a distance comparable to the radius of the Earth. This resembles the brightness of distant galaxies or faint supernovae at the edge of the Universe. A centimeter-scale fragment would appear as a 20 AB magnitude source and a 10 centimeters object would be a bright 15 AB magnitude source.

As these fragments move across the sky, their glint produces a flare that typically lasts a fraction of a second at a favorable orientation relative to the Sun. Due to the motion of the objects during the telescope integration time, typically tens of seconds, these flares would be smeared across a streak on the sky.

The forthcoming Rubin Observatory in Chile will use a 3.2-billions pixel camera to survey the entire Southern sky every four days, starting in 2025. It could employ a novel strategy which sacrifices a small fraction of its observing time in order to avoid large commercial satellites and reduce the occurrence of satellite streaks in its images. In a new paper, I showed that hundreds of thousands of fragments larger than a few centimeter at low Earth orbits would contaminate the Rubin Observatory’s images. Larger objects would be glinting even brighter at orientations that would create Unidentified Anomalous Phenomena (UAPs) in other sky surveys. The Galileo Project Observatory at Harvard University is sensitive to large communication satellites.

If space debris of this type is generically created by advanced technological civilizations, we might find the imprint of a debris cloud surrounding their home exoplanet in transit light-curves of stars.

The above numbers summarize humanity’s record in trashing the terrestrial neighborhood so far. But what does our future hold? A recent report from the United Nations forecasts that the number of satellite collisions will increase dramatically after 2025. In particular, the report projects a `tipping point’ to tens of catastrophic collisions within the next decade, which could trigger a dramatic growth in the population of small fragments. The resulting debris would add artificial lights on the sky and provide astronomers with an added incentive for building telescopes on the Moon or in space, where human-made space trash is nonexistent as of now.

Establishing astronomical observatories beyond Earth could be part of humanity’s plan to become a multi-platform civilization. In the long run, what matters the most for continuing our scientific heritage of observing the Universe is survival. Many scientists argue that we are unique and special, but it is better to spread many copies of ourselves beyond Earth than to praise one copy for being exceptionally rare.

This incentive for duplication reminds me of a story about a research team that spent a year with a group of chimps in a jungle. The researchers noticed that the alpha-male leader spends a lot of time fighting other males in an attempt to maintain its societal status, so that it can get access to the females and reproduce. But at the same time, a lower-ranked peaceful male had more offspring because it did not spend time on fighting or recovering from bruises. Here lies an important lesson as we contemplate how to escape from our cloud of debris after a long history of international conflicts, and move peacefully into the pristine space that surrounds this broken world.


Image credit: Chris Michel (October 2023)

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 chairs the advisory board for the Breakthrough Starshot project, and 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”, was published in August 2023.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".