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Why Shoot Down a Satellite? Analyzing an Analysis
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.)"
If the satellite was left in the sky, it might still be around when the democrats get into office. That would be hard evidence of all the (domestic) spying that went on. A low-flying satellite would be able to provide some hi-res pics. I don't know much about orbits, but maybe it was an opportune time, and they weren't sure if they'd get another shot later. It also allowed us to flex some military might. There certainly isn't a constructive case for shooting it down.
[1] choose your conspiracy, there are plenty to go around - you're probably closer to the truth than this article, but you already knew that.
politicians are like babies' nappies: they should both be changed regularly and for the same reasons
Brand new, state of the art spy satellite stops working. Are you going to risk parts of it falling and being salvageable, or are you going to blow it up?
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".
He's getting rather old, but he's a good mouse.
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.
He's getting rather old, but he's a good mouse.
http://www.chomsky.info/talks/20011103.htm
Wait, are you talking about the Chinese shot or the US shot? The debris field from the US shot was centered around 130 miles up (209 Km) in a rapidly decaying orbit, where the Chinese shot was about 500 miles up (804 Km) in a stable polar orbit (IIRC). The US debris field has disappeared, burned up in the atmosphere, while the Chinese debris field is still out there and will remain so for many, many years.
Si hoc legere scis nimium eruditionis hebes
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.
See my blog http://ilovecookes.blogspot.com/ for light hearted technical information.
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.
I don't see how you can claim that the Chinese shot was more difficult since the shot that the Chinese made was at a target at a greater distance, and a slower velocity. Also the point of the matter is that the United States used a mobile platform to launch. To me this would be a test to prove that we could shoot down an ICBM in route to a destination at a low orbit. Correct me if I am wrong but it seems to me that the farther the distance you have and the slower the velocity of the projectile, there is greater chance of correcting your trajectory.
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.
There is no "I disagree" mod for a reason. Flamebait, Troll, and Overrated are not substitutes.
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
I'm posting AC and I can't say who for obvious reasons, but I know someone involved in the project and I said to them "sooo hydrazine eh?" and they laughed.
All of this analysis is unneeded the reality is they did it whether it was needed or not, because it allowed them to show that they could and it allowed them to do it in a much safer way that did not result in a debris cloud in orbit.
I'm sure the submitter had a reason to want to post this anonymously. He made a mistake, but his intention was clear. There was no reason to be an asshole and repost his name here, just in case it's removed by one of the editors.
Also, it's interesting that all the people posting his name here are posting as AC. Perhaps they weren't quite as sure about this as they claim.
EagerEyes.org: Visualization and Visual Communication
I have also heard - though don't quite believe - another reason that may have influenced it: the Air Force supposedly has recently become very interested in "smart crowbars" - orbital kinetic munitions.
And suddenly the Navy demonstrates ASAT capability.
Hasn't one of the ideological doctrines of the US Armed Forces, from day one, been that any rogue branch can be dealt with by the other two?
You're making me think. You won't like me when I'm thinking.
I have a great deal of respect for Jim O , but he knows that he and I differ in our opinions regarding the destruction of 193. As one of those observers who followed this bird through 2007 and as the unfortunate soul who was , seemingly , the only person to obtain high power images of it on a number of occassions , I formed my own opinion , based on what I saw and the fact that there was no way the Pentagon was going to allow this satellite to fall any lower , let alone , re -enter. Its quite simple , at 300 km I could see the basic outline of the satellite using back yard equipment.At 200 km I would have seen "much" more detail had I had the opportunity. If I could do that , then the opposition , using much more expensive kit , and adaptive optics would have been able to discriminate the exact shape and proportions of the spacecraft , from which it could then glean a great deal of intelligence. ( See my recent images of Persona ) As I stated back in mid December 2007 "The Pentagon will not allow this bird to come much lower and will probably destroy it on orbit." Those words were , at the time , met with derision but proved to be correct. The chances of the tank making ground in tact in a densly populated area were infinitesimal. We will probably never know the truth , but for my part , I'm convinced that the toxic ice cube theory was simply fabricated to mask the real reasons for the satellite's destruction. More details on both Persona and USA 193 at http://satcom.website.orange.co.uk/