Wednesday, February 20, 2008

The dangers of shooting down satellites

Several news accounts suggest that the U.S. military is going to try and "shoot down" a non-functional spy satellite in the next few days. I think this is a dangerous idea, both from a scientific and a political standpoint.

Last month, I blogged about how I thought the chance of the satellite endangering a human was small. I still think that is the case.

Make no bones about it, the hydrazine fuel that is being used as a justification for shooting down the satellite is very nasty stuff. Hydrazine has properties that are great for rocket thruster fuel, but it is a very hazardous chemical. However, it is also quite volatile, which is why many people suspect the fuel tank on the satellite would not survive re-entry, but would explode due to the heat of re-entry. Even if the tank were to survive intact, the chances of it hitting a populated area are amazingly tiny.

If NASA or the military were simply launching an operating thruster that could attach itself to the spy satellite and purposefully de-orbit the satellite into the ocean, I would have no qualms with the operation. We de-orbit satellites like this a lot, and an ocean splashdown would not only protect humans, but it would successfully hide any sensitive equipment that we do not want our enemies to have.

But what it appears the military is doing is actually launching a missile to collide with the satellite. To some degree, this will also serve the purpose. The collision will slow the spy satellite, causing it to de-orbit. If the collision is timed properly, the satellite would fall safely into the ocean.

However, if you've ever been in a car wreck, or seen wrecks on TV, you know what happens when two big things collide. Yes, the main bodies of the cars pretty much stop, but small pieces go flying in every direction, some at very high speed. Now, image that the collision is at thousands of miles per hour. The same thing will happen, with small pieces flying off at even faster speeds.

In space, some of those pieces will stay in orbit, forming an ever-expanding cloud of debris. If some of the small chunks of debris, even something as small as a bolt that we cannot detect with radar, were to hit the space shuttle or space station, it could puncture the spacecraft and endanger the entire crew, not to mention millions of dollars of investments. Or, the debris can hit commercial satellites in low-earth orbit, including many different communications systems.

Last year, the Chinese shot down one of their aging weather satellites to test anti-satellite capabilities. The resulting debris field now threatens satellites of every space-faring nation.

The problem is worse than it looks. The more debris that is up in space, the more likely that some chunk of it will collide with another dead satellite that can't move out of the way. That collision creates more debris, increasing the likelihood of debris collisions, and threatening a runaway reaction that could clog the space near Earth with shrapnel and make low-Earth orbit too dangerous for satellites or people.

I don't want to be too alarmist here -- we are unsure how much debris it takes for such a runaway to occur. Some scientists think we are near that point, others don't. If there is one saving grace, it is that most of the debris in low-Earth orbit will slowly fall out of orbit in the next several years. But if the debris is replenished at a rate faster than it falls, the problem slowly gets worse.

In my opinion (and let me stress that this is an opinion; I have no information on this than what is in the news), this shoot-down is not as advertised. It is not an attempt to protect people from the fuel or other hazardous components of the satellite. It's not even an attempt to ensure that our enemies don't get their hands on the satellite. There are other ways of meeting these goals without creating an expanding field of sometimes-undetectable debris that threatens both our astronauts and our vital commercial satellites.

No, I think the obvious purpose of this test is to demonstrate that the U.S. can shoot down satellites. We've proven this in the past -- both the U.S. and the Soviet Union performed several such tests through the mid-1980s. The technology required for a "shoot-down" doesn't really change, so I am skeptical that any test is necessary. In other words, we're just proving to other countries that may have recently shot down satellites that we can do the same (though we've proven that many times in the past), and at the same time we are risking setting off yet another space arms race, all the while endandering our astronauts and multi-million dollar investments in space hardware. The gain is small, and the risk is large.

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