The US military just destroyed a spy satellite with a missile strike. Let's forget, how unlikely it was that the satellite would have hit near anyone if they had just let it reenter the atmosphere on its own. Let's also forget that this strike was a simple way to make a flawed missile defense system look good (I am sure that the path of the satellite was much more predictable than any real incoming ordinance). Let's also forget about the worry that the strike could contribute to the militarization of space. What I want to concentrate on is the possible effect of all the junk that the explosion produced, and the resulting danger to satellites and astronauts.
Friday, February 22, 2008
Spy Satellite and Space Junk
The photo above shows about 10,000 objects of baseball size or larger in low Earth orbit tracked by NASA. There are perhaps 600,000 objects larger than a centimeter which are too small to track.
Now if that doesn't sound worrying to you, note that objects in such orbits move around the Earth at more than 17,000 mph (27,000 kph), and the energy of an object goes as the velocity squared. That means being slammed with a 1 kg object at orbital speed involves as much energy as a 60,000 kg 18-wheeled truck crashing into you at 70 mph (113 kph), except that in the former case the energy is concentrated into a much smaller object. This is what being hit by a tiny object going 17,000 mph looks like:
So how many pieces of space junk did the spy satellite strike create? Well, it is estimated that the Chinese destruction of one of their satellites in 2007 increased the amount of space junk by about 30%.
[It is fair to note, as Rampant Clam's Comment points out, that the Chinese satellite was in a more stable orbit than the US satellite, so what the Chinese did was far worse because it created much more long-lived debris.]
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5 comments:
Whoa there, Eyesopen, you didn't do your research!
The Chinese Fengyun 1C satellite was in a stable polar orbit, 851x883 km high, while USA 193 (aka NROL-21) was was originally in a 349x365 km orbit at 58.48 deg inclination, which decayed to 244x268 km x 58.5 degree inclination by Feb 19 - and was rapidly decaying. It was coming down, not staying up there like the Chineses sat. So any debris created by destroying the sat is coming down too.
We can debate why the DoD took a shot at it, but not about orbital debris.
Thanks for the comment Clam. I quoted the figure I have seen for the Chinese satellite but did not give an estimate for the US satellite. The reason is that there is a great uncertainty: the distribution of velocities introduced onto the satellite debris by the explosion. While the US satellite was in a decaying orbit, some of the pieces of its destruction will surely be boosted into more stable orbits by the force of the explosion (I am thinking of elliptical orbits for which the satellite explosion point is the perigee).
Also, the explosion could add a velocity component in any direction, which I would think would increase the relative velocity of any collision of a given piece of debris with a satellite in a normal orbit, relative to normal debris.
It may well be that the number of long-term pieces of debris is substantially smaller than from the Chinese satellite, and that the lifetime of the "more stable orbits" is not that long. But I stand by my basic point that it is a bad idea to blow up Earth-orbiting satellites because it is likely to add to the debris problem, at least in the near term.
There will be no* long-term debris from the US explosion. Orbits are closed: every fragment will repeatedly pass through the same spot as the explosion. In the worst cases the velocities have zero component parellel to the radius and a positive component in the direction of the orbit. For these, the explosion point will be the perigee and the fragment will spend less time at that altitude than the satellite would have, but will still decay rapidly. In all other cases, no matter how large the relative velocity, the perigee will be lower, (in most cases, inside the earth). The decay of these will be very fast.
To get a sense of the scale, imagine tweaking the orbit of something orbiting a quarter inch above a basketball.
That said, I agree blowing up the satellite was a stupid idea.
*I'm ignoring hyberbolic orbits, and secondary collisions, as I don't believe those are significant. If you think they are, data would be helpful.
Thanks Jack, but I was aware of all that, which is why I said in response to the last comment, "I am thinking of elliptical orbits for which the satellite explosion point is the perigee."
Now I'm not sure how long such orbits last, but given that atmospheric pressure falls exponentially with altitude, the lifetime of such an orbit will rise with eccentricity--perhaps exponentially. Think of something in an orbit that spends only a small fraction of its orbital period near the "basketball".
Anyway, my main point was that space junk is a real problem and that, if nothing else, blowing up a satellite sends the wrong message.
i find this subject to be very interesting. i hope there will be more blogs about space and the universe.
John Allen
CEO
Cosmetic Dentist Houston
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