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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.)"
The submitter is debunking an article written in IEEE Spectrum, a civilian magazine. To debunk an article written by a non-expert says very little about whether a shoot-down was actually warranted.
Nice try on the anonymity, but there's your name on the Related Stories list with the original Firehose posting...
The reason we shot it down was because China had just shot down one of theirs in a weapons demonstration. China was using it as propaganda about how great they were. So we chopped them down a notch by showing them that we can do it as well. And not only that, we can do it from a mobile platform (i.e. a cruiser at sea), not just from a land based stationary platform. This was simply an international pissing match. Nothing more, nothing less.
We were all warned a long time ago that MS products sucked, remember the Magic 8 Ball said, "Outlook not so good"
Oberg had an earlier analysis (March 2008) on the same topic in The Space Review that covers many of the same points with a little more detail than this article.
http://www.thespacereview.com/article/1073/1
That would be hard evidence of all the (domestic) spying that went on.
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.
Don't disappoint your bird dog. Go to the range.
There are plenty of good reasons to question the timing and urgency of the U.S. decision to destroy US 193, but the last thing they needed to do was prove to China they could shoot down a satellite. Both the U.S. and Russia proved this capability many times over until they were satisfied they had it down back in the Cold War with the U.S. even shooting down a satellite with an F-15 in 1985 [http://www.spacedebate.org/evidence/1245/] (arguably a way cooler stunt than the sea-based shot).
The U.S. probably felt compelled to shoot it down because of its legal obligations under international space law to deal with potentially hazardous satellites (albeit a diminished risk and maybe not as urgent as they claimed). Its also very likely they were concerned that an intact satellite re-entering in an unfriendly region could compromise intelligence sources and methods. However, its not likely that they were doing this as a demonstration / warning shot to China or to test out new technology. They already have better methods for taking out a satellite if they wanted to and this shot was a one-time hack [http://www.spacedebate.org/evidence/3269].
Shooting down objects in high orbit requires a 2-stage attack - They're too difficult to hit directly in spite of your greater correction time. First, a carrier rocket is sent up to get as close as practical to the target. It then launches a kill-vehicle that will track and destroy the satellite.
The US did not require that much sophistication. Since the satellite was on its way down, the attack was more akin to shooting down an ICBM when its location and velocity are both known well in advance.
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?)
Perhaps he brings it up because that widespread notion is dead wrong. In fact, the parts of meteorites which make it all the way to the ground arrive quite cold, way below 'zero'. That's because of ablation. The outer part of the meteorite gets superheated by friction with the atmosphere, but before any significant portion of that heat can conduct to the inner part, the superheated part loses structural integrity and is torn away from the rest. However, the part torn away has, up until that moment, shielded the inner part from absorbing any direct friction heat.
Rinse and repeat. The end result is that whatever part does make it all the way to the ground is still at substantially the same temperature as it was when it entered the atmosphere.
Moderating "-1, Disagree" is simple censorship. Have the guts to post your opinion.
Are you joking? The SM-3 the US used is a four stage interceptor! Booster, main stage, dual pulse third stage, kill vehicle.
The submitter wrote:
I'm not sure why the submitter seems to have only partially quoted Oberg here (apparently out of context), and ignored the point Oberg was trying to make. Although meteorites entering the atmosphere generate a fireball, there seems to be ample evidence that the objects themselves remain cold even upon impact.
I'll cite a few articles here:
From this NASA page titled "Hot Meteors and Cold Meteorites," under the section titled "Meteorites Don't Pop Corn," we have this salient paragraph:
A slightly less assertive article on Howstuffworks is a little more reserved in its claims:
A more nuanced perspective is provided by this amateur astronomer who specializes in the study of meteors (specifically meteor spectroscopy). It's a short read, but a little too long to block quote here. Suffice it to say, there are numerous factors, including the composition and albedo (reflectivity) of the object, whether it was camping out in the Earth's shadow prior to impact (and for how long), the trajectory and velocity upon entering the atmosphere, etc.
Seems to me that the submitter is conflating two separate thoughts. Oberg brings up meteorites because they can and do impact the Earth while still cold, or only moderately warm. That's as far as the analogy goes -- he apparently wants to make the case that an object can remain cold enough that hydrazine fuel inside the container in question might not vaporize prior to impact. Whether we want to extend the analogy to the question of whether the container will shatter during re-entry is a question best asked of metallurgists or material scientists, and that I suspect depends entirely on the composition and manufacture of the fuel tank. (For that matter, whether a meteorite shatters upon re-entry would seem to be a function of the composition of the object, as well as the stresses it encounters -- and those stresses would be a function of speed and trajectory, as well as shape, would they not?)