The collision of two satellites on Tuesday was the first of its kind. Questions immediately swirled: how did this happen? Who was at fault? And what will the consequences be?
The satellites were orbiting at about 790 km above the Earth. Objects at that altitude travel at about 7.5 kilometers a second, or more than 16,700 miles per hour. (For a good backgrounder on orbital physics, I recommend this excellent guide, especially Sections 4 and 5). Objects traveling at these speeds carry a tremendous amount of kinetic energy; when they collided, the satellites shattered like glass. However, the resulting fragments don't just fall back to the ground - they stay in orbit until they come close enough to the dense part of the atmosphere to burn up. Because the only thing that can slow these fragments down is atmospheric drag, which this far above the Earth is extremely weak, much of this orbital debris will remain in orbit for centuries. Here it can be as lethal as a bomb - if one of these pieces of debris strikes another satellite, it could be damaged or destroyed, and the secondary collision could produce even more debris.
Since the consequences of a collision are so dire, why wasn't the collision avoided? While the details at this point are still sketchy, it appears that no one knew the collision was imminent - though the data was probably there, had anyone looked. The United States maintains a catalog of the 18,000 man-made objects in Earth orbit that can be tracked, though only 900 of these are operational satellites. As you might imagine, the amount of work required to determine how close each of these objects will come to all the others each orbit is immense, especially considering that natural forces cause their orbits to shift over time. US Strategic Command, which is responsible for tracking space objects, doesn't have the computing power or manpower to perform "conjunction" calculations for every space object. Due to the uncertainty inherent to predicting the positions of so many objects traveling at such incredible speeds, it is impossible to give a definite yes or no answer as to whether any single close approach will result in a collision. The best that can be done is to calculate the probability of collision for a given situation.
While STRATCOM does ensure orbits are clear for the International Space Station and some vital military, intelligence, and NASA satellites, commercial operators (such as Iridium, a provider of satellite phone services which operated one of the satellites involved in Tuesday's incident) are largely left to their own devices. When a potential collision is identified, current operating procedures give satellite operators the responsibility make their own decision on whether or not to conduct a collision avoidance maneuver. In certain circumstances, satellite operators can ask STRATCOM for more detailed data than is normally made public to help make that decision, although one such request by an operator earlier this year was rebuffed. Furthermore, while other countries like France and Russia also track space objects and produce partial satellite catalogs, these catalogs are generally not shared, partly due to concerns about disclosing the locations of sensitive intelligence satellites and revealing details about satellite tracking capabilities.
The final exacerbating factor is that an entire category of objects that can cause serious damage cannot be tracked with any regularity by existing facilities - the size threshold for reliable tracking is about 10 cm in diameter, but objects as small as 3 cm in diameter can cause a catastrophic failure to a satellite if they collide. Researchers at NASA and the European Space Agency estimate that there are several hundred thousand objects between 1 and 10 cm wide in Earth orbit that are not currently tracked.
What is to be done? There is a near-consensus that "space situational awareness," the military jargon for tracking space objects, needs to be improved. If more objects could be screened for potential collisions further in advance, Iridium could have moved its satellite away from a collision course with the dead Russian satellite. Improved situational awareness might also help reduce fears about attacks on US satellites by removing some of the ambiguity from space operations.
Improved international cooperation might also help. Right now, there are no "rules of the road" for operating in space. When ships at sea pass close by each other, they follow certain rules to avoid a collision. The same could be done in space, if countries were willing to hash out answers to legitimate questions about what actions are responsible and what information can be withheld. Such rules could also clarify and streamline procedures for using data from different sources to help avoid collisions.
Initial reports indicate that at least 500 pieces of debris were created by the collision. This number will grow as the debris field spreads and it becomes easier to distinguish between individual pieces. The real question is, can this incident propel us towards making changes that are needed to conduct operations in space in a safe and sustainable manner?
(Cross-posted at Black's Bailiwick).