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Orbital debris is defined as any man-made object orbiting our planet that no longer serves a useful function. It can include nonfunctional spacecraft, abandoned launch vehicle stages, and other junk produced by space missions.

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From abandoned satellites, spacecraft that broke apart, and other missions, 170 million pieces of debris smaller than 1 cm (0.4 in), about 670,000 pieces of debris 1–10 cm, and approximately 29,000 larger pieces of debris are in orbit. Debris, or "space junk," is tracked as it orbits the Earth, and it poses a serious threat to humanity’s continuing efforts to explore space. The possible consequences are unthinkable. Just a few uncontrolled space crashes could generate enough debris to set off a runaway cascade of fragments, rendering near-Earth space unusable and threatening other spacecraft. "If we go on like this, we will reach a point of no return," says Caroline Frueh, an astrodynamics researcher at Purdue University in West Lafayette, Indiana.

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She cites the large constellations, or fleets of internet satellites, that companies like SpaceX, Amazon, and OneWeb are planning to launch. In total, the companies plan to launch more than 100,000 satellites by the end of the decade. SpaceX has already rocketed nearly 900 new satellites into Earth's orbit since May 2019 of its Starlink internet satellites to low Earth orbit, and it has permission to loft about 12,000 of the craft…

A fixture of human spaceflight will be avoiding debris that could collide with your space station. We've seen orbital collisions already.

This scenario, known as the Kessler Syndrome, could make it difficult to operate in Earth orbit if things get bad enough. The spaceflight community should therefore start taking mitigation measures now, many exploration advocates say.