Adumbration

[a version of this post was previously made to my fb account]

When I saw this story, I had not yet heard the news about this collision, but knew for several months prior of the "defunct" satellite that was out of control and threatening other satellites.  Even when out of control, satellites obey orbital mechanics, and NORAD is tracking thousands of such things (most are very much smaller).

However, the behavior is not so easily described as two-body mechanics that we use for first order trajectory planning. Perturbations arise due to the moon, the sun, other planets, the fact that the Earth is not a perfect sphere (I think there's a joke there somewhere in my past), incident solar radiation, and the very thin atmosphere that rises above the surface of our planet, which in low earth orbits can become the dominant term, more so if the exposed area is large with respect to the mass. So even if we could get a perfect fix on position and velocity (Heisenberg writ large), we would still need regular updates to maintain a good prediction of the object path.

No one wants uncontrolled chunks of stuff floating around where one is trying to do work (communicate, survey, ...). Usually when spacecraft are dying, the controllers know well enough in advance that they can command them to an out of the way place in space (or drive them to re-entry), but this one must have been a surprise failure of two redundant paths, happening in some sequence to quickly to react.

The problems of space junk like this has been a concern all of my professional life. When I was working on the Space Station I learned some things specifically about this concern. Mostly in that environment the requirements were concerned with the effect of collisions with micrometeoroids, as opposed to space debris like this represents - tiny grains of fragmentary asteroids or moon dust spit up from a crater, as opposed to what might be substantially larger pieces of who knows what. Traveling at a closing velocity of 14 km/s is pretty darned fast, remembering that kinetic energy is a velocity squared function. Space station has taken evasive action in the past to avoid much smaller objects than this one satellite represents; there is a famous case of a fleck of paint causing a large crack in the windshield of the Space Shuttle.

And now it's collided with another satellite, most likely both being broken up into a larger number of smaller chunks, each now of which need to be tracked until they too decay into the atmosphere. The article says their orbit is around 790 km altitude. Space Station is around 400 km, so there is a lot of time for all those little pieces to drop down far enough that their orbits intersect, subject to orbit ellipticity; it might be a couple years or longer before that point, but when we do start seeing this, it will be like a rain of objects to monitor and maneuver around.

When micrometeoroids collide with the space station, there is a protective shield surrounding the pressure vessel in which the astronauts/cosmonauts live. The collision with the shield causes the micrometeoroid to vaporize, so what hits the main wall of the living space is a high pressure wave created by the vaporized combination of micrometeoroid and what once was shield material. As I recall, the material composition makes a difference in how effective this works, which means the fragments will more likely be vaporized if they are similar to aluminum (which I think is the shield material - it's been a while since I worked this every day).

The victim in this encounter was an Iridium satellite. This was one of 60-70 in a constellation that provides global telecommunication services to people in remote outposts, and various government agents. All of those travel at about the same orbit, phased around the earth to provide service worldwide. That means the debris field that is expanding around the collision is centered in the same altitude as all those other satellites.

My guess is that the Iridium controllers are more than a little worried.