Emily Lakdawalla • Feb 14, 2008
Nonfunctioning descending U.S. spy satellite to be blown up
Various news outlets are reporting today that the United States plans to deal directly with a potentially harmful situation (here's the Reuters and Associated Press stories). A large spy satellite that was launched in 2006 but failed after reaching a low orbit is now descending slowly but inexorably toward Earth. Because the spacecraft is nonfunctioning, there's no way to control where it comes down. Worse, it's extremely difficult even to predict where (or when) it will come down, because the rate of its orbital decay is influenced by such complex and time-variable things as the solar and Earth magnetic fields -- as they flutter in and out of activity, the upper reaches of Earth's atmosphere puff up and get damped down, constantly changing the force of drag on the spacecraft.
Unlike a spacecraft that has served its operational lifetime and then decayed from orbit, the USA 193 satellite still has on board the entire tank of hydrazine fuel that it would have been using to maintain its orbit, had it become operational. Hydrazine is a monopropellant -- unlike gasoline or hydrogen, which require a different, oxidizing chemical to be present before burning can begin, hydrazine serves as its own oxidizer, so it's terribly flammable, and it's toxic in all kinds of ways. Apparently, modeling of the spacecraft's descent suggests that the hydrazine tank would survive intact until the crash, when the stuff would be released, possibly pretty violently, and spread over a fairly wide area.
So the U.S. has announced that it plans to strike the satellite with a modified tactical missile, the type designed to shoot down ballistic missiles, with the goal of blowing it apart (and presumably exploding and/or dispersing all that hydrazine) before it descends into the atmosphere. The news outlets are calling this "shooting down" the spacecraft but that's not really accurate; the spacecraft is coming down anyway. Blowing it up will make it come down in many more, smaller pieces, most of which should hopefully burn up upon descent. Of course, the proposed action brings to mind the Chinese destruction by missile of a defunct weather satellite a year ago, an action that was universally condemned because of the resulting massive cloud of space junk that the destruction of that satellite produced, space junk that is hazardous to other functioning spacecraft up there. Will the U.S. now be adding to that problem?
I don't have a quantitative answer to that question, but I can tell you that the Chinese satellite was orbiting stably at a relatively high altitude of 865 kilometers when it was blown up, while the American one has already descended to under 270 kilometers and is spiraling down about a kilometer per day, a rate that will accelerate with time. (For comparison, the ISS orbits at between 300 and 400 kilometers; stuff entering the atmosphere usually begins burning up at altitudes below 100 kilometers.) Even if some chunks of the American satellite get tossed away from the explosion in a manner that raises them to much higher altitudes, I think they'd be destined to fall to Earth, because part of their orbit must pass through the altitude at which the satellite was destroyed.
My inner cynic can't quite take the proffered explanation for the necessity of blowing up the spacecraft entirely at face value. It was a spy satellite, after all, and the U.S. government probably don't want anyone getting their hands on any of the component technology that might survive intact to the ground. But I also think that this potentially hazardous descending satellite is an opportunity that the American government just can't pass up. I'm sure the U.S. government would love to be able to practice blowing up satellites, just to prove to themselves and demonstrate to others that they have the capability. The unfortunate loss and decay of USA 193 allows the government to blow up a satellite under ethically defensible circumstances. It's not a perfect test, because the missile that will be used to blow up USA 193 can't reach the altitude of active satellites, but intercepting this fast-moving thing from space is a huge technical challenge.
The falling satellite is being tracked by amateur observers all over the world. Here's a very interesting account from one Ted Molczan about his observations and his predictions of its current (as of Monday) orbit and the rate of its decay. An especially interesting detail is that there have lately been unusually high levels of geomagnetic activity, which has resulted in increased atmospheric drag on the spacecraft. Molczan predicts that, if left alone, the date of decay from orbit is March 18, with an uncertainty of "at least one week."
The AP story says that the Navy plans to fire a missile at the spacecraft sometime during a window of opportunity that "will open in the next three or four days and last for seven or eight days.
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