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Why Shoot Down a Satellite? Analyzing an Analysis
Posted by
timothy
on Mon Aug 11, 2008 11:28 AM
from the burning-questions dept.
from the burning-questions dept.
A reader, name withheld by request, writes "Writing in the IEEE Spectrum, James Oberg analyzes whether there was, in fact a significant risk to humans from the satellite which the US military shot down on 21 February, purportedly 'to head off the possibility of its splashing a half ton of toxic hydrazine fuel somewhere on Earth.' Previous experts had 'scoffed' at the rationale put forth, pointing out that there was trivial possibility that any significant amount of toxic fuel would make it to the ground intact. Oberg's analysis, titled 'the inside story,' purports to debunk this, and claims that indeed it's possible, and even likely, that there could be a danger to the ground. Unfortunately, the analysis is full of flaws and lack of rigor — indeed, lacking any sort of numerical reasoning. It seems to be too much repeating official 'spin,' and could have used a hefty dose of skepticism — and could also use a little bit of actual analysis using numbers, rather than handwaving." Read on for the rest of an interesting analysis of a topic that suddenly seems more complex.
The submitter continues:
"Here's the first number that Oberg should have quoted: 32 Megajoules per kilogram. That's orbital energy, which is how much energy has to be removed by ablation or otherwise dissipated for the hydrazine tank to enter the atmosphere and hit the ground undamaged. For reference, TNT holds about 4.6 MJ/kg.
Oberg quotes 'Hydrazine requires a tremendous amount of energy to go from solid to liquid.' This energy is known as the heat of fusion, and for hydrazine it is just a little under 400 kJ/kg. That's about 1% of the energy released by entry heating. Hardly a 'tremendous' amount of energy, compared to the entry energy that's nearly a hundred times greater.
Oberg goes on to quote 'There is a widespread notion that meteorites falling to Earth arrive red hot.' He is correct here. In fact, meteorites falling through the atmosphere typically explode, shattering into dozens or hundreds of pieces; something that occurs at the point when the dynamic pressure on the leading face exceeds the yield stress of the material. This occurs for meteoroids of all compositions, including nickle-iron meteorites that are far more robust than hydrazine tanks. If the atmospheric entry of meteorites is relevant, it hardly bolsters the case that a tank will enter intact (and if it's not relevent, why did Oberg bring it up?)
Furthermore, if you look at a typical nickle-iron meteorite, you'll see a surface pitted and mottled with holes ranging from the size of golf balls up to pits the size of baseballs. These are known as regmaglypts; they are the areas ablated away by the entry plasma. Even a single such ablation pit would, of course, destroy a hydrazine tank.
The second number Oberg should have quoted is a number called ballistic coefficient, the mass divided by the area of the tank. Basically, the smaller the ballistic coefficient, the less stressful the entry will be. Unfortunately, a full hydrazine tank has a very high ballistic coefficient. It is an empty tank, not a full one, that is likely to enter intact. Talking about empty film canisters, or even empty fuel tanks, making it intact through atmospheric entry is really about as relevant as talking about dropping a piece of paper on the floor.
The article contains a quote from Andrew Higgins, with a link to (purportedly) the research done that contains the quote. Unfortunately the link does not actually contain the quote used in the article; in fact, it seems to be mostly a discussion of a side issue. Let me emphasize this: Higgins did not say what he is quoted as saying in the place he was reported as saying it. This may merely be sloppy journalism — maybe he said it somewhere else — but I am again left with the question: if I can't even trust the simplest things he says that can be easily checked, why should I trust anything else?
In short, Oberg's article is poorly thought out, avoids even simple back-of-the-envelope calculations, and accepts uncritically information that should have been aggressively questioned. He concludes that a well-defined and thoroughly researched technological hazard assessment — of a kind that someday, for better or worse, will be needed again — has wound up buried in obscurity and obfuscation. This may be true, but no well defined nor thoroughly researched technological hazard assessment was anywhere in evidence. The analysis he gives in the article is buried in obscurity and obfuscation.
(apologies for posting as Anonymous Coward. I work in the field.)"
Oberg goes on to quote 'There is a widespread notion that meteorites falling to Earth arrive red hot.' He is correct here. In fact, meteorites falling through the atmosphere typically explode, shattering into dozens or hundreds of pieces; something that occurs at the point when the dynamic pressure on the leading face exceeds the yield stress of the material. This occurs for meteoroids of all compositions, including nickle-iron meteorites that are far more robust than hydrazine tanks. If the atmospheric entry of meteorites is relevant, it hardly bolsters the case that a tank will enter intact (and if it's not relevent, why did Oberg bring it up?)
Furthermore, if you look at a typical nickle-iron meteorite, you'll see a surface pitted and mottled with holes ranging from the size of golf balls up to pits the size of baseballs. These are known as regmaglypts; they are the areas ablated away by the entry plasma. Even a single such ablation pit would, of course, destroy a hydrazine tank.
The second number Oberg should have quoted is a number called ballistic coefficient, the mass divided by the area of the tank. Basically, the smaller the ballistic coefficient, the less stressful the entry will be. Unfortunately, a full hydrazine tank has a very high ballistic coefficient. It is an empty tank, not a full one, that is likely to enter intact. Talking about empty film canisters, or even empty fuel tanks, making it intact through atmospheric entry is really about as relevant as talking about dropping a piece of paper on the floor.
The article contains a quote from Andrew Higgins, with a link to (purportedly) the research done that contains the quote. Unfortunately the link does not actually contain the quote used in the article; in fact, it seems to be mostly a discussion of a side issue. Let me emphasize this: Higgins did not say what he is quoted as saying in the place he was reported as saying it. This may merely be sloppy journalism — maybe he said it somewhere else — but I am again left with the question: if I can't even trust the simplest things he says that can be easily checked, why should I trust anything else?
In short, Oberg's article is poorly thought out, avoids even simple back-of-the-envelope calculations, and accepts uncritically information that should have been aggressively questioned. He concludes that a well-defined and thoroughly researched technological hazard assessment — of a kind that someday, for better or worse, will be needed again — has wound up buried in obscurity and obfuscation. This may be true, but no well defined nor thoroughly researched technological hazard assessment was anywhere in evidence. The analysis he gives in the article is buried in obscurity and obfuscation.
(apologies for posting as Anonymous Coward. I work in the field.)"
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Oops... (Score:5, Informative)
Nice try on the anonymity, but there's your name on the Related Stories list with the original Firehose posting...
Re:Oops... (Score:5, Funny)
That wins the award 'Epic Fail of the Week'.
And a cookie.
Parent
Stupid Question... (Score:5, Insightful)
What was the risk of shooting it down? It seems close to "none." The missile used would surely have a mechanism to self-destruct in the event of a miss, and even if it didn't, I don't see how its falling could be any more dangerous than the hydrazine. Plus, it was probably a useful training exercise, should they ever need to shoot down a "really" dangerous satellite.
Of course, since it was done months ago, it's all hypothetical anyway.
Re:Stupid Question... (Score:5, Interesting)
The risk of shooting it down and, at least the way I see it, our big reason for having to justify it was that we had so recently criticized the Chinese for shooting down one of theirs.
Of course, the technology we used was wildly different than the technology that the Chinese used. And we didn't clutter up useful orbit space with a bunch of debris when we were done. But these things don't always matter to people just looking for a reason to US bash. There are a lot of folks out there that were calling the US hypocrites for shooting down their satellite after bashing the Chinese for doing "the same thing".
Parent
Re:Stupid Question... (Score:5, Insightful)
The end result is exactly the same and has nothing to do with US bashing.
The end result of the Chinese shot is a huge amount of space-junk cluttering up an otherwise useful orbit. The end result of the US shot was the relatively tidy destruction of a spy satellite with no risk of accidentally allowing sensitive components to be recovered.
I fail to understand how these are "exactly the same".
Parent
Re:Stupid Question... (Score:5, Funny)
If we did not shoot it down we would lose one of our missile sites or one of our 6 cities!
Loss of a city is not bad, but loss of the center missile site can make life a bitch as that satellite comes by..
Parent
This was a weapons demonstration, nothing more... (Score:5, Informative)
Re:This was a weapons demonstration, nothing more. (Score:5, Interesting)
That may be partially true, but the Chinese shot was way more difficult (albeit messy) than ours. We, of course, had no reason to get nearly as fancy as the Chinese did when they took theirs out and it would have been silly to even try (unless we just had a fancy satellite-killer that we just wanted to try). But, to the eyes of most of the world, I'd imagine you're right. They showed that they could do it, so we did too. Despite the fact that they were radically different shots and circumstances.
Parent
Monday morning quarterbacking the satelites (Score:5, Insightful)
I think they shot it down because they decided it was a good idea. What's the problem? The US is going to get criticized by someone for any choice it could possibly make, including doing nothing. This was probably the choice with the least uncertainty.
Dear submitter: (Score:5, Funny)
Thank you for debunking the debunking of the debunking.
Hydrazine conspiracy... (Score:5, Funny)
Actually, if there's a conspiracy, its this whole business of hydrazine being supposedly unsafe. Since I've been using hydrazine scalp cream, I've regained a full head of hair and my private assets have significantly increased in size. It's only because George Bush wants everyone to go bald, that the satellite was shot down.
Not so complex... (Score:5, Funny)
Most 8 year olds I know are good at making up a pretext for getting what they want.
A: I really wanna shoot down a satellite!
B: You can't do that, it'll make you look like a violent war provocateur.
A: But! But! But! What if it was a dangerous satellite. Like it was going to kill everyone or something. And we had to shoot it down to save everyone! And it had racing stripes and a turret on top and played the A-Team theme song!
B: Well.... Okay, but only if it's a dangerous satellite.
A: Yay! Mom! Dad says we can shoot down a satellite!
Chomsky explained it fine (Score:5, Interesting)
http://www.chomsky.info/talks/20011103.htm [chomsky.info]
You can't satisfy a conspiracy theorist (Score:5, Interesting)
I just love how someone can say "I work in the industry!", post as an AC, toss out a couple of buzzwords with no math to speak of, and scream "we're being lied to!". As the submitter of this story so clearly put it when posting his own "analysis":
Translation: "There is a conspiracy here! Trust no one! We're all being lied to!" If there's one thing I've learned over the years, there is nothing the government can say or do to convince someone who thinks like this.
Personally, I have little doubt that the satellite was shot down for exactly the official reason. We've had plenty of space junk hit the ground in recent years; as I remember, people were specifically warned not to handle debris from the space shuttle Columbia, because of concerns of hydrazine contamination. Clearly the shuttle's high ballistic coefficient didn't prevent that, did it? The hydrazine tank didn't have to reach the ground intact to cause concerns. And just imagine the headlines if nothing had been done, and debris from that spy satellite had eventually reached the ground. Russia still gets flack about the nuclear reactor debris that landed in Canada after the re-entry of COSMOS 954, and that was 30 years ago!
Of course, it was obviously an added bonus that the shoot-down was a nice demo of the military's capabilities. But if the U.S. military really wanted to test its ASAT technology, it would hardly need to hold a press conference beforehand, or issue a press release to China or Russia to inform them afterwards! China and Russia track our satellites the same as we do theirs. If one of our dead satellites conveniently "exploded", they would get the message quite clearly.
Oh come on... (Score:5, Interesting)
1) the amount of hydrazine fuel contained was infinitesimal compared to the amount of hydrazine that spills on humans every year. The F-16 uses hydrazine in its EPU, and you can trivially find stories of people practically bathing in it as a result of EPU problems and fuel dumps. The effects are generally less than the horrific outcomes presented in the stories surrounding the shoot-down. The idea that the hydrazine presented any sort of real risk is absolutely bogus, something the articles dance around and just won't address directly.
2) the chance of the debris coming down in a populated area is very close to zero. Although underreported (see http://imca.repetti.net/metinfo/metstruck.html), there are no recorded instances of anyone being killed by anything falling from space. Now of course a 1000 lb fuel tank is much deadlier than a small stone, but 1000 lb objects have fallen from space before, and we didn't bother shooting them down (http://www.space.com/news/spacehistory/dangerous_reentries_000602.html).
3) last time I checked, when heat shields fail the aluminum structure generally fails almost immediately thereafter (http://www.columbiassacrifice.com/$A_reentry.htm). I am not aware of any unprotected structure reaching the ground intact (although that could be ignorance) but I am very much aware of many unprotected structures breaking into small parts under the same conditions. This includes tanks with frozen volatiles inside. The only really large pieces of debris to reach the ground were the insulated tanks from Skylab.
4) A nuclear reactor is MUCH more robust than this fuel tank, yet when Cosmos 954 fell to Earth it's 50 by 35 cm reactor shattered and spewed its contents over 600 km (http://gsc.nrcan.gc.ca/gamma/ml_e.php). Yes, the shaping is critical in terms of shock generation and aerodynamic loading, and it's definitely easier for a sphere to re-enter than a cylinder, but still... bologna.
5) the article Oberg's is based on claims ~8 gee loading. Again, bologna; that's what you get on a carefully controlled re-entry, uncontrolled will cause much greater loadings (again, http://www.columbiassacrifice.com/$A_reentry.htm)
6) The article links to several others that are essentially dismissive of the "publicity" cover-story angle as a conspiracy theory. However, we're talking about an administration who's history shows a well-recorded "shoot first" policy based on extremely inflated data. This case fits the pattern to a T, and I see no reason to believe it differs in any way.
I call BS. Sorry James, but the argument remains specious in my books.
Maury
Landis fails not only in anonymity: (Score:5, Funny)
Dr. Landis implies that Andrew Higgins didn't say the quoted item.
In fact, it's easily found in the link given.
In the linked text, Higgins gives a hyperlink back to a previous letter which was in The Space Review which contains that very quote and in the context Oberg said.
Landis snipes at Oberg for poor journalism, but apparently can't follow a bloody hyperlink. Why, even Cowboy Neal could do that and on a bad day to boot.
Secondly, Landis is an expert in solar cells and solid state devices. He apparently also works on elements of spacecraft electrical power systems, lander design and operation and writes articles on a variety of subjects. Impressive, but not directly in the area.
On the other hand, Andrew Higgins is a principle investigator and an expert in the behavior of materials under extreme hypersonic conditions and computer simulation of the same. His work on materials and combustion in hypersonic ram accelerators leaves him very well equipped to comment on the dynamics of reentry and the behavior of spacecraft materials and fuels under such extreme conditions.
Landis seems to be using the very sloppy and misleading tactics that he accuses Oberg of. Pot. Kettle. Black.
(Mild disclaimer. Andy Higgins is a friend going back to undergrad days (and believe me, it's been a while). I was mildly nettled that Landis invokes the name of an old friend and then becomes selectively blind when Andy gave the link to the very quoted item in the letter that Landis read.)
Re:Yeah, lets talk about numbers and credibility (Score:5, Insightful)
What I find so amusing is that people on Slashdot take one look at the qualifications of the author or the IEEE spectrum article which is.
"James Oberg is a veteran NASA mission-control engineer living in Houston. He is now a news consultant, lecturer, author, and occasional tour guide of Russian space centers."
Then decide some bozo posting on Slashdot is more creditable.
Parent
Re:Yeah, lets talk about numbers and credibility (Score:5, Interesting)
I for one will admit to not being an expert on hypersonic aerodynamic heating and reentry dynamics but the AC is only included on bit of math.
"32 Megajoules per kilogram. That's orbital energy, which is how much energy has to be removed by ablation or otherwise dissipated for the hydrazine tank to enter the atmosphere and hit the ground undamaged."
This would hold true of the fuel tank was reentering by it's self.
But that fuel tank was inside a rather large satellite. Now you have to take in account all the energy of that would be absorbed by destroying the rest of the satellite around the fuel tank. Then of course there is the simple fact that no transfer of energy is 100% efficient. Not all 32 MJ per KG will be transferred as heat into the fuel tank. Some of it will be transferred into the atmosphere and some of it will be radiated away from the tank as it reenters.
So the AC post is at best a very simple High School physics look at the problem. It assumes a 100% energy transfer to the fuel tank and totally ignores the rest of the structure surrounding the fuel tank. So should I put more value in the qualified author in a subject that I have only a limited knowledge of or some AC on Slashdot's overly simplistic criticism? Frankly after seeing what survived breakup of Columbia I think that the AC is probably just as I said. Some Bozo on Slashdot.
Parent
Re:Yeah, lets talk about numbers and credibility (Score:5, Insightful)
What "math"? There is no math in the rebuttal, besides a number for orbital energy. No equations, no calculated results, no nothing. I truly hope that Dr. Landis is not the person who submitted the story, because if so my respect for him has taken a hit. A real scientist knows better.
Tell you what. Why don't you post the complete mathematical analysis that proves Oberg wrong? It should take you 3 minutes to complete, and maybe 15 minutes to post. And while you're at it, provide some math to explain why hazardous debris from COSMOS 954 and the shuttle Columbia somehow did reach the ground, despite their obviously comparable ballistic coefficients.
Parent
Re:Why so hard? (Score:5, Funny)
Parent
Re:Destroying evidence (Score:5, Informative)
Right, because they could have flown the U.S.S. Michelle Obama (a special UFO-technology-based ship that the Evil Republicans have been hiding at Area 51) up to the wrongly-orbited sattelite, and... what? Retrieved the film? Any "domestic spying" done from orbit comes in the form data hauled down to earth... you know, satellite imagery? You're a few decades late for the satellite itself to have evidence of something like that. Anything worth talking about is on disk drives and tapes right here on the ground.
Parent
Re:IEEE Spectrum is not a government source (Score:5, Insightful)
Except that James Oberg [jamesoberg.com] is an expert [jamesoberg.com] (or at least damn close to one, read the resumes linked off the second link.) - having been a mission controller for NASA and a professional space engineer, analyst, writer, and journalist for decades. Synchronicity at work - as part of a research project I'm working on, Jim's 1982 book Mission to Mars sits right beside my coffee cup even as I type this...
That being said - the debunking is full of errors as well. The AC provides us with a wonderful handwaving smoke and mirrors show, but fails to acknowledge the role of the structure of the tank itself (which is insulated and has to be accounted for before the Hydrazine starts to vaporize). He also fails to acknowledge the role the structure of the satellite plays, as it too will act as shielding (and a drag brake!) for the tank. (I know Jim is aware of these factors because I've discussed them with him.)
In short, what the AC claims is a debunking is actually closer to being a partial rationale for conducting the shootdown.
I don't know what 'field' the AC works in, but to this knowledgeable non expert he doesn't sound like an expert at all - but rather sounds like someone with an axe to grind. If he is an expert, he has allowed bias to supplant analysis.
Parent
Re:IEEE Spectrum is not a government source (Score:5, Informative)
There's a number of good papers out there on how this is analyzed, if someone is seriously interested I'll post some citations, I'm away from the office today.
Parent
Re:We all know it was an excuse (Score:5, Interesting)
To test our new weapons and show the Chinese how we roll.
That's half of it. While there was a possibility of hydrazine rain, I think we also have to consider that this was state of the art spy satellite which was part of a much larger network of spy satellites. If this thing were to make it to the ground even partially intact, it would be a treasure trove of information concerning the US spy satellite ability and could possibly show a way to counter the shiny new spy network we spent so much money deploying.
Parent
The situation is even less simple than you think.. (Score:5, Insightful)
Or, it's probably a combination of factors, and not just a dick-measuring competition.
Consider the following:
1. A very expensive intelligence satellite is stranded in low orbit, useless. Said satellite contains expensive, highly-classefied equipment; there's a finite chance that some of this equipment might reach the surface relatively intact (see Skylab, Columbia). Obviously, certain groups (China and Russia, especially) would love to get hold of anything that survived and analyze it. It would be nice to ensure that this equipment is rendered unusable and worthless.
2. The hydrazine tank mentioned. Yes, I know the odds of it hitting anything were very, very small... but not zero. And the public outcry had it hurt or killed anyone would have been loud and swift. The decision-makers probably figured it would be better to face the inevitable international grumbling by shooting down the satellite than to face the very small (but potentially devastating) risk of impact in a populated area.
3. The Navy ABM system is going operational, and someone realizes it has the capability to shoot down low satellites. Someone probably figured "hell, we have this satellite problem; it's going to reenter soon anyways so it's a nice convenient test target. We might as well try it while we have the chance". Besides, the additional cost is a drop in the proverbial bucket.
4. At the top levels, there probably was a bit of "let's show them" going on. But I suspect it was as much a coincidence as anything else, with #1 above leading the "rational" reasons list. The hydrazine tank story just made for the best PR.
Parent