Syzdekistan

Today, the news media reported that the Pentagon has decided to “shoot down” an spy satellite that failed to reach orbit. The satellite is in a very low orbit and is hitting the tenuous upper atmosphere enough to cause it to come down on its own in a few weeks. The problem is, that no one knows where or when it will come down. Ostensibly, the Pentagon is concerned about the threat to human safety posed by the propellent hydrazine. Not much is known about the satellite but presumably it has a large hydrazine tank on board that could survive reentry and then harm people on the ground. Hydrazine is dangerous, but not terribly so. It is similar to ammonia and would oxidize rapidly at ground level.

I have some problems with this explanation. The possibility of the spy satellite coming down in an inhabited area is pretty remote. While a strong high-pressure tank could easily survive reentry, it could probably survive impact with the earth without rupturing. I just don’t see the danger as being significant.

There must be another explanation. I can think of a few possibilities:

1. This failed spy satellite has some really cool technology that could survive reentry and that we don’t want this to fall into unfriendly hands.
2. The spy satellite has a radioisotope thermoelectric generator (I have no idea if it does or not but they are often used in space) and we don’t want the plutonium fuel to fall into unfriendly hands.
3. Let’s show up the Chinese! The Chinese recently blew up one of their own malfunctioning weather satellites which spewed a huge amount of debris dangerous to other spacecraft. Now we can do the same in the name of protecting human health and safety. I think this is the most likely explanation. It would be an unique test of the SM-3 missile which is designed as a ballistic defense missile. The orbit of the target is so low that any debris generated would be in a similar low orbit (fears that debris would be kicked up into higher orbits and endanger other spacecraft) and would decay to earth rapidly. This test would show the Chinese that we have the capability to take out a satellite.

Of course, this will set an ugly precedent. I wonder which country will next have to shoot down one of their “malfunctioning” satellites? India? Iran? I don’t like where this could lead. Furthermore, this test is done at nearly the maximum range of the SM-3. We couldn’t use this system to shoot down most satellites that are in usable orbits. This is just political posturing and not even a realistic test.

But at least the physics are cool (link):

1. The intercept will occur at 240 kilometers (130 nautical miles)
2. The mass of the satellite is 2,300 kg (5,000 pounds)
3. The mass of the interceptor is 20 kg. (From CBO)
4. The closing velocity will be 9.8 km/s (22,000 mph), suggesting a virtually head-on collision.

Other pertinent observations. At 240 km, the satellite should be traveling 7.8 km/s; the SM-3 has a burnout velocity of 3 km/s.

If you want to see the broken spy satellite with its cargo of hydrazine pass over your town, check out this website. They also have some good information on the process of orbital decay and the difficulty in predicting reentry. We’ve got a 1.3 magnitude pass (it’s bright because it’s low and big) at 6:35 PM tomorrow night. Should be fun to see the center of all this fuss….