On Tuesday, at approximately 5pm GMT, two satellites made history. They became the first artificial satellites ever to collide accidentally in low-Earth orbit. The event happened between a defunct Russian satellite (Cosmos 2251, launched in 1993) and an active commercial Iridium communications satellite (Iridium 33, launched in 1997), destroying the pair. Now there’s a mess up there, pieces of debris threatening other satellites, possibly even the International Space Station…
The collision happened at an altitude of 790 km over northern Siberia and the US space surveillance network has already detected 600 pieces of debris. As time goes on, this number will grow as the debris cloud expands and any object 9.9 cm in diameter or greater can be singled out by detection systems.
The first thing that comes to mind is the safety of the International Space Station orbiting at an altitude of just over 350 km. As time goes on, the impact debris will slowly fall into orbits closer to the Earth, eventually burning up in the atmosphere. Larger debris may take years or decades to do this, smaller pieces will drop a lot quicker.
“There are two issues: the immediate threat and a longer-term threat,” said Nicholas Johnson, NASA’s chief scientist for orbital debris at the Johnson Space Center in Houston. “It turns out, when you have a collision like this the debris is thrown very energetically both to higher orbits and to lower orbits. So there are actually debris from this event which we believe are going through the space station’s altitude already. Most of it is not, most of it is still clustered up where the event took place. But a small number are going through station’s altitude.
“Yesterday, we did an assessment of what the risk might be to station and we found it’s going to be very, very small. As time goes on, those debris will (come down) some over months, most over years and decades and as the big ones come down they’ll be tracked, we’ll see them and the worst-case scenario, we’ll just dodge them if we have to. It’s the small things you can’t see are the ones that can do you harm.”
At the end of the day, although it may be getting crowded up there in low-Earth orbit, there is still a lot of room to fly a spaceship. These kind of collisions will remain rare for a long time to come. However, once you have one high energy collision like Tuesday’s accident, more debris is created, expanding the collisional cross-section of the mass of two satellites. This may cause problems for other satellites, and in a worst case scenario, the space station. Depending on the direction of travel, space junk can travel very fast, with severe consequences.
For example, in 1996 a French spy satellite was hit by a shard of space debris from a previous Arianne rocket launch (ironic, as Arianne is a France-based company). The Cerise satellite had its gravity-gradient stabilization boom ripped clean off (sounds painful) by the debris travelling at over 3 km per second. The Cerise event was the first of its kind, where a small piece of junk caused a huge amount of damage.
However, that pales into insignificance when compared with the Cosmos 2251/Iridium satellite impact. After all, Iridium 33 has a mass of over 670 kg and Cosmos is likely to be a fairly weighty vehicle itself, the collisional energy (depending on the angle they collided, it may not have been a head-on impact) will have been huge. Bad Astronomer Phil Plait even made an estimate from the available data as to how energetic the mid-space crash may have been. The energy of the impact could have been as large as detonating a tonne of TNT. Not such a shabby day for the satellite derby after all.
Lets just hope all that debris doesn’t cause issues for other satellites in the orbital paths of ex-Iridium 33 and ex-Cosmos 2251, and I especially hope it wont cause disruption to ISS operations.
Sources: Spaceflight Now