Shot Down in Midair!

3/2/08 - 3/8/08
by C. Zaitz

I’m not usually picky about why people get interested in science. But when my spouse came to me excited about the fact that the US Navy had shot down one of our own satellites with an unarmed missile launched from a ship out in the middle of the ocean, I felt smug. “Oh that’s nothing” said I. “We went to the moon six times, when we had to launch incredibly huge Saturn V rockets, figure out how to slip three astronauts into orbit, plunge two of them down to the surface to play, then scoot them back into a tiny aluminum foil launch vehicle, blast them off the moon, rendezvous with the orbiting vehicle, and have them burn themselves back into a tricky return to earth, with just the right trajectory to ensure they neither would skip off the atmosphere nor burn up in re-entry. No easy feat!” He rolled his eyes.

Meanwhile, I was just being cynical, and he was right, the destruction of the satellite was pretty interesting. Launching a missile from a rocking, moving ship is no easy feat, and hitting a fast-moving target is trickier still. Shooting down satellites is a controversial affair, due to China’s demolishing of a weather satellite last year. Nations get nervous when other nations send missiles shooting into the sky. But they did it, and so did we. How? Why?

The “why” of shooting down our own satellite was apparently due to the tank of hydrazine on board. Hydrazine is a toxic substance which once gained fame as being a product of a compound called Alar which was sprayed on apples to keep them on the tree to ripen them. The fear of this substance falling to earth as a cold slush, possibly killing or injuring people within 30 feet of it, was reason enough for the US to destroy it before reentry. The satellite had died, leaving no energy to keep the hydrazine warm, and hydrazine has a freezing point above water, so the threat of it surviving reentry as a half-frozen substance was real. Some say we did it just to see if it could be done, which leads up to the “how” of hitting a satellite moving at 17,000 km/second.

Breaking up a satellite before reentry poses problems. We have a pretty good understanding of orbits and trajectories of objects, once we know their mass and velocity. But when conditions like weather are uncertain, or change rapidly, problems of math and physics become very complex. So we use guiding systems which can track infrared, or heat signals. Unfortunately the satellite was dead so it wasn’t producing much heat. When China blew up one of its spy satellites, a cloud of debris was left behind in orbit that still exists and will remain for years, creating hazards for other orbiting bodies. We needed to hit the satellite in the hydrazine tank, which upon impact, even without explosives, would smash both the missile and the satellite, the pieces of which would reenter the atmosphere and burn up within weeks. And apparently that’s what happened.

Without knowing all the military details, but knowing that we are target practicing, as are other countries, I’m kind of glad that some of us are very interested in science.

Until next week, my friends, enjoy the view.