Over 27,000 pieces of orbital debris, “space junk,” are tracked by the Department of Defence’s global Space Surveillance Network (SSN) sensors. Much more debris — too small to be tracked, but big enough to threaten human spaceflight and robotic missions — exists in the near-Earth space environment. Since both the debris and spacecraft are traveling at extremely high speeds (approximately 15,700 mph in low Earth orbit), an impact of even a tiny piece of orbital debris with a spacecraft could create massive problems.
The rising population of space debris increases the potential danger to all space vehicles, including to the International Space Station and other spacecraft with humans aboard, such as SpaceX’s Dragon Crew.
NASA takes the threat of collisions with space debris seriously and has a long-standing set of guidelines on how to deal with each potential collision threat to the space station. These guidelines, part of a larger body of decision-making aids known as flight rules, specify when the expected proximity of a piece of debris increases the probability of a collision enough that evasive action or other precautions to ensure the safety of the crew are needed.
Orbital Debris
Space debris encompasses both natural meteoroid and (human-made) orbital debris. Meteoroids are in orbit about the sun, while most artificial debris is in orbit about the Earth (hence the term “orbital” debris).
Orbital debris is any human-made object in orbit about the Earth that no longer serves a useful function. Such debris includes non-functional spacecraft, abandoned launch vehicle stages, mission-related debris, and fragmentation debris.
There are approximately 23,000 pieces of debris larger than a softball orbiting the Earth. They travel at speeds up to 17,500 mph, fast enough for a relatively small piece of orbital debris to damage a satellite or a spacecraft. There are half a million pieces of debris the size of a marble or larger (up to 0.4 inches, or 1 centimetre) or larger, and approximately 100 million pieces of debris about .04 inches (or one millimetre) and larger. There is even smaller micrometre-sized (0.000039 of an inch in diameter) debris.
Even tiny paint flecks can damage a spacecraft when traveling at these velocities. A number of space shuttle windows were replaced because of damage caused by material that was analysed and shown to be paint flecks. In fact, millimetre-sized orbital debris represents the highest mission-ending risk to most robotic spacecraft operating in low Earth orbit.
In 1996, a French satellite was hit and damaged by debris from a French rocket that had exploded a decade earlier.
On Feb. 10, 2009, a defunct Russian spacecraft collided with and destroyed a functioning U.S. Iridium commercial spacecraft. The collision added more than 2,300 pieces of large, trackable debris and many smaller debris to the inventory of space junk.
China’s 2007 anti-satellite test, which used a missile to destroy an old weather satellite, added more than 3,500 pieces of large, trackable debris and many smaller debris to the debris problem.
Tracking Debris
The Department of Defence maintains a highly accurate satellite catalogue on objects in Earth orbit. Most of the catalogued objects are larger than a softball (approximately 10 centimetres).
NASA and the DoD cooperate and share responsibilities for characterizing the satellite (including orbital debris) environment. DoD’s Space Surveillance Network tracks discrete objects as small as 2 inches (5 centimetres) in diameter in low-Earth orbit and about 1 yard (1 meter) in geosynchronous orbit. Currently, about 27,000 officially catalogued objects are still in orbit and most of them are 10 cm and larger. Using special ground-based sensors and inspections of returned satellite surfaces, NASA statistically determines the extent of the population for objects less than 4 inches (10 centimetres) in diameter.
Collision risks are divided into three categories depending upon size of threat. For objects 4 inches (10 centimetres) and larger, conjunction assessments and collision avoidance manoeuvres are effective in countering objects which can be tracked by the Space Surveillance Network. Objects smaller than this usually are too small to track for conjunction assessments and collision avoidance. Debris shields can be effective in withstanding impacts of particles smaller than half an inch (1 centimetre) for the U.S. modules on the International Space Station.