Discarded pieces of rockets and old satellites that circulate in earth's orbit. The original life span of a satellite is on average seven years; there are hundreds of inoperable satellites circling earth.
Historical perspective: NASA estimates that there are more than 100,000 pieces of man-made space debris orbiting Earth. As reported in The Week in December 2013, the threat they pose to satellites and spacecraft has inspired researchers to focus on developing resilient new space materials capable of self-repair. Aerospace engineers at Concordia University in Montreal want to build an experimental satellite from a self-healing chemical composite. The satellite will be “constructed out of carbon fiber (to reinforce it) and an epoxy resin (for its matrix),” said Elizabeth Howell in UniverseToday.com. Whenever its structure is cracked or dinged, a “healing agent” stored in microcapsules will chemically repair the damage. “Prolonging the life of a spacecraft will decrease the required maintenance over its lifetime,” said the team. That would mean a welcome reduction in the number of dangerous space walks that astronauts are now required to take.