Shooting down USA-193
Published on Feb 15, 2008 at 2:05 pm.
19 Comments.
Filed under politics, rockets.
Yesterday, the Pentagon announced that the US Navy would attempt to shoot down a US reconnaissance satellite that is out of control and spiraling back to Earth to an uncontrolled reentry into the atmosphere. The missile used to do the job is to be a modified Standard Missile 3, developed for use with the US ballistic missile defense program. The SM-3 in this configuration has a kinetic warhead (meaning that rather than the warhead exploding on contact, it is simply moving fast enough to smash the target by force of impact).
This comes almost a year after the US and the international community publicly decried China’s test of of an anti-satellite weapon to destroy one of their defunct weather satellites, resulting in a giant cloud of space debris. That debris cloud continues to spread, endangering any satellite or spacecraft passing through it. The speed at which orbits move means that a satellite or spacecraft encountering a piece of debris in a crossing orbit would have a huge closing speed relative to the debris, making an impact from even a tiny piece of debris devastating, damaging and possibly destroying the spacecraft.
Naturally, parallels are being drawn between that incident and this one. However, there are some crucial differences. The Chinese satellite was in a stable orbit that would not have reentered Earth’s atmosphere any time soon. The debris resulting from the satellite’s destruction created a huge orbiting cloud of debris, and most of that debris will continue to orbit for a very long time. The US satellite that is returning, USA-193 (NORAD 29651), is in a decaying orbit that is already dragging on the uppermost parts of Earth’s atmosphere. It’s orbit is decaying in excess of 700 meters per day. It should reenter Earth’s atmosphere of its own accord in early March, 2008. In principle, even if it is destroyed and produces a cloud of debris, most of that debris will also be in a very low orbit that is far too low to be a danger to orbiting spacecraft. That debris, too, would likely reenter Earth’s atmosphere in a reasonably short period of time.
So, if the satellite is going to come down on its own, why are we bothering to use up a very expensive missile to shoot the thing down? The blogosphere is awash in speculation. The official pronouncement is that the satellite is dangerous and that shooting it down is a public service done to protect human life and health. Some buy that statement. Others are convinced that there are other reasons to shoot down the satellite. Some people believe that the satellite may contain new surveillance technology that we don’t want to fall into the wrong hands. Others believe that this is simply an excuse to test the ballistic missile defense system again (prior tests have had mixed results, and this would be the first test against a target that had not already been planned in advance to follow a particular flight path). Others speculate that this may be an excuse to extend the capabilities of the missile defense system, testing its capabilities as a potential anti-satellite weapon system that could be used against hostile satellites in a future conflict with a nation having its own spy satellites. Personally, I can see all of these speculations as having some merit. And, of course, this could also simply be a political move aimed at defusing the poor public relations resulting from extensive media coverage of the errant satellite and potential dangers posed by the satellite coming down almost randomly somewhere on Earth.
Those potential dangers are what the Pentagon cited as the reason for shooting down the satellite. Those dangers are what have been in the news for some time, too. There are two possible dangers. First, and the first than that the media focussed on, is the possibility that someone might actually be struck by debris. I did a back-of-the-envelope calculation on that one. I found that I have probably less than one trillionth of a percent chance of being hit by debris. It seems more probable to win the state lottery twice in a row than to be hit by debris from the satellite. Of course, that is not impossible, but highly improbable. And, I am not the only person on the planet. The chances of anyone being hit are quite a bit higher than the chance of just me being hit. Still, the chances are vastly more likely that no one will be hit by debris. First of all, most of the Earth’s surface is covered in water. Most of the landmass of Earth is sparsely populated. Even in heavily populated cities, human beings are a small percentage of the surface area of the city, so the chances are that even if the debris fell in a city, the debris would most likely miss hitting a person.
But, the other danger posed by the satellite is toxic material on board. Most of the satellite will burn up in the atmosphere. Chunks, though, will likely survive to reach the ground. Some of those chunks will contain hazardous materials, primarily in the form of chemical toxicity. The chief danger listed is that of the satellite’s propellant, hydrazine (the subject of my very next posting). Hydrazine is quite toxic, and the stated fear is that people coming into contact with hydrazine can get sick (and perhaps seriously ill). This is the same warning issued to the public when the Space Shuttle Columbia broke apart over Texas. Some pieces of the shuttle landed fairly intact, including a apparently hydrazine tank. Hydrazine is, in fact, toxic, but it is also very volatile and unstable. It is unclear just how much of a danger the hydrazine poses. It breaks down readily, so even if it is spilled, there is probably little danger of long term environmental damage. If the tank ruptures during reentry, then the hydrazine is so volatile that it will be long since gone by the time the tank reaches the ground. If it ruptures on the ground, then the contamination is likely to be localized. As I told my students, if they see the thing fall, don’t go lick any of the pieces. 
Of course, hydrazine can also enter through the skin, so touching the pieces is bad, too, and hydrazine has a high enough vapor pressure than sniffing the debris is also a bad idea.
But, why is USA-193 even falling out of the sky in the first place? Media reports clearly state that the satellite is out of control. Does that mean that it can’t fire its engines to keep itself up there? It is a bit more complicated than that. I remember a couple of episodes in the original Star Trek series in which the story line involved a spacecraft whose engines were off line and was in danger of falling into the atmosphere of a planet. In one case, a shuttlecraft was out of fuel and could not maintain orbit. In another, the Enterprise itself was orbiting a planet with its engines off line and was spiraling downward. There is a kernel of truth in these stories, but most of it is wrong. You don’t need engines to stay in orbit. Traveling in space is not like moving on the surface of the Earth or through the atmosphere. If an aircraft loses engine power, the drag forces of the air slow it down and it loses lift and can not remain flying. If a boat cuts its engine, viscous drag from the water slows the craft and it stops. If a motor vehicle is driving along and the engine stops, friction in the wheels and air resistance slow the vehicle and it stops. So, in all of those cases, you need an engine to overcome the resistive forces trying to stop the vehicle. But, in space, there is nothing to run into (more on that later). So, you just keep going. In fact, the shuttle, satellites, and the space station keep orbiting without engine thrust. They just keep going. So, an out of control satellite should just keep orbiting, and most do just that.
However, the Earth’s atmosphere is a gas. A gas does not have a sharp edge. So, the atmosphere does not just end like in the cute little drawing that I made for the top of this posting. If it did, then the satellite would continue to orbit without issue. Instead, though, the atmosphere just gets thinner and thinner with altitude. Most of the Earth’s atmosphere is in its lowest layer, the Troposphere. Above that is a much thinner layer called the Stratosphere. Above that is the Mesosphere, and above the Mesosphere is the Thermosphere. Above the thermosphere are still some occasional atoms and molecules flying around, and this is the Exosphere. Each region is thinner than what came before. The mesosphere, itself, is so thin that most of us would think of that as near vacuum. The thermosphere is extremely thin, indeed. In fact, the thermosphere is thinner than the best vacuum pumped by the best vacuum pump on the surface of the Earth. Objects moving through the thermosphere encounter almost no atmospheric resistance because the air is so thin. The Space Shuttle, International Space Station, and many low Earth satellites are technically still orbiting in the thermosphere. The effects of the thermosphere on their orbits is pretty minimal. But, minimal is not none. There is a tiny effect. Atmospheric resistance does slow the craft a tiny bit. That tiny bit is hardly enough to matter in an orbit, or two, or a hundred. But, over time, that tiny effect adds up. The craft moves slower, and that causes it to drift to a lower orbit. But, the lower the orbit, the less thin the atmosphere, and so the bigger effect. The orbit drifts lower and lower over time. We call this a decaying orbit. Eventually, the effect of the atmosphere becomes so great that the craft starts falling faster and it eventually enters the upper part of the mesosphere where atmospheric effects are bigger. The fast moving craft starts to push the air ahead of it, compressing the air. This had two effects. First, the compressed air increases air resistance, causing the craft to spiral down even faster. Secondly, the compressed air heats up. These heated gasses can get quite hot, and that heat can begin to have a bad effect on the spacecraft compressing the gasses. The material of the spacecraft begins to get hot and soften. Eventually, unless there is some heat shield on the craft, it gets hot enough to begin to break apart. The craft disintegrates and burns up, except for the most durable parts of it or pieces shielded by more durable parts. That is the fate of uncontrolled reentries. To keep that from happening, space craft in low Earth orbit need to be occasionally boosted to higher orbit. Obviously, when a craft is out of control, it can’t boost itself. USA-193 was launched in December 2006. Now, a bit over a year later, it has spiraled down so low that it is in imminent danger of uncontrolled reentry.
So, whatever the reason for the loss of control of the satellite, and whatever reason for the decision to shoot it down, this satellite, which has never actually done anything because control was lost immediately after launch, is about to come to an end. But, you might get a chance to see the satellite before it comes to its end. The web site Heavens Above gives data on observing satellites for just about any location on Earth. According to their information, USA-193 is passing fairly high in the sky from Fort Worth, Texas, Saturday and Sunday nights. If it is clear, I might go out and take a look. Tomorrow night is expected to be cloudy, but I might see if I can see it Sunday night. Enter your location, and they’ll calculate when you might be able to see the satellite, too.
-Astroprof
Seeking Solace on February 15, 2008 at 3:15 pm: 1
This made me think of the episode of the Simpsons where a missile was shot to save the town from an incoming comet.
USA 193 « Lamentations on Chemistry on February 16, 2008 at 10:47 am: 2
[…] February 16, 2008 Posted by gaussling in Uncategorized. trackback There has been considerable buzz lately that the launch and deployment of USA 193 has somehowgone terribly wrong. Reports say that the package failed to deploy properly and it is presently in a rapidly decaying orbit that will bring it back into the atmosphere for an uncontrolled reentry. […]
CCPhysicist on February 16, 2008 at 10:19 pm: 3
Thanks for the observation pointer.
Question: They say this is a radar satellite, but no mention of a high power source like an RTG?
Comment: I share the suspicions about the true motivation for shooting it down. Lots of fairly big bits of lightweight stuff came down with Columbia. They recovered a video camera, right? I think they are worried about what might survive reentry, and not just a 1000 pound tank of hydrazine. Could the overall design concept of the system be much more interesting than what those sketches show, and be exposed just by the kind of bits that fall from the sky?
Astroprof’s Page » Hydrazine on February 17, 2008 at 1:17 pm: 4
[…] « Shooting down USA-193 […]
River on February 19, 2008 at 5:21 pm: 5
Seems obvious to me that typical Bush Adminisration Spin is in play here. The danger of hyrdrazine seems minimal at best, but it is an advantageous way to advertise a US demonstration in real world conditions of the US anti-ballistic missile capability. China demonstrated their ability, and the billions the US has pissed away to date on this hideous wafare escalation path. Yes, there may be a chip onboard that would add to the desirability to kill it, but ultimately investment in weapon testing is where the current administration sees this as a payoff. (wouldn’t it be sweet if the test fails, the sattelite crashes with zero damage, and the cost of the test and embarassment breaks the back of the program and the rest of the industrial war complex crashes significantly?) …soap box dismounted.
KSoldier on February 20, 2008 at 11:34 am: 6
River-
The only thing “typical” here is your ignorant Liberal American self-loathing.
Wouldn’t it be sweeter if everything worked as planned and the freedom which you so eagerly abuse was safely protected?
…soap box dismounted.
Mel on February 20, 2008 at 9:10 pm: 7
Tee-Hee! Liberal American self-loathing? Are you serious? You still like the Bush Administration after eight years of Iraq, whe it was Alquida that bombed us? And what does soap box dismounted mean? just thought I’d ask…
Mel on February 20, 2008 at 9:11 pm: 8
huh?! Liberal American self-loathing? Are you serious? You still like the Bush Administration after eight years of Iraq, whe it was Alquida that bombed us? And what does soap box dismounted mean? just thought I’d ask…
Mel on February 20, 2008 at 9:12 pm: 9
oh sorry, didn’t mean to post that twice
Phil on February 21, 2008 at 3:27 am: 10
Will any of the debris, sent upwards by this explosion, end up captured in higher orbits? Aren’t these pieces liable to damage other orbitors?
(I, too, think this is part of an ‘astral pissing contest’)
KSoldier on February 21, 2008 at 8:35 am: 11
Mel, I was in Iraq for 2 of those 5 (where did you get 8?) years. Trust me, it looks a lot different while viewed from your livingroom sofa.
As for the “self-loathing” response. Well, apparently it was more important for that imbecile to enjoy an American failure in security than it was to enjoy the security which he takes for granted.
I believed he called the idea of failure “sweet.”
The soap box dismounted reference was also a quote from that individual’s previous message. I do wish to apologize for bringing a political response into this forum. I consider it off topic, but apparently others couldn’t resist either and I wasn’t gonna let it slide.
As for the topic itself. This morning I learned that there was a direct hit.
Sweet.
MountainMike on February 24, 2008 at 1:24 pm: 12
Phil said: “Will any of the debris, sent upwards by this explosion, end up captured in higher orbits? Aren’t these pieces liable to damage other orbitors?”
This is one of the more intelligent comments made regarding the long-term effects of such a stupid “pissing contest”. No one seems to understand any of Newton’s laws of motion here! That junk that was created by smashing the Satellite will cause future problems to any low-earth orbit satellite or, ironically, the Space Shuttle. It will not be coming out of orbit any time soon, particularly the junk that is “knocked into” a higher, more stable, orbit.
Sorry about the reality here, “KSoldier”, we are acting just as stupidly as the Chinese. It has NOTHING to do with “security”, despite what CNN tells you!
It shows that the present administration:
1.) Lies for the real reason of a so-called “shootdown”. If it really “came down” out of orbit, that would be great - but that’s not the reality of the laws of physics and nature.
2.)Hydrazine as a hazard? I do not think the tank would have been likely to survive re-entry, probably in late March, 2008 - it would have ruptured and all the hydrazine would have vaporized LONG BEFORE the container reached our planet. Even if the hydrazine reached our planet, the chances of it being anywhere near a human being is infinitesimally SMALL.
3.) Has a total disregard for the lives of future astronauts, who may be threatened by all the garbage created by this misadventure.
As to the OPINION that the technology was designed to be destroyed upon re-entry, not so! Any nation that was able to gather the pieces together, MIGHT be able to “figure out” the technology and configuration. That is, if they can get the pieces. This is possibly the core reason that the administration wanted the satellite “destroyed” - the pieces could probably never be put together by any one nation. Remember Humpty-Dumpty?
Frido on February 25, 2008 at 2:10 pm: 13
Remember Humpty-Dumpty?
Lols
Professor.
You can never now really if its going to hit someone. They wont take any chances.
And would you let any country steal your technology. By some ungratefull countries non the less that got rich out of you guys.
And i think it was both of the three.
A pissing contest.
The hydrazine.
And the technology stealing.
The pissing contest i liked this one because now china is asking america for some data on how they did because they said ”
Within hours of the reported hit, China said it was on the alert for possible harmful fallout from the shootdown and urged Washington to promptly release data on the action.”
I think its a crock of BS just to get
the info it needed.
“China requests the U.S. to fulfill its international obligations in real earnest and provide to the international community necessary information and relevant data in a timely and prompt way so that relevant countries can take precautions.”
Now they see the U.S as someone that needed to fulfill its international duty. Since when ?
If i was gates i wouldnt have given those people anything or maybe just a
trailer of the hit.
But it has some disadvantages too, that china isnt going to give theirs if anything happened back. ok give them a peek
Prof this is a great site.
just found it. thnx allot.
Damn i shouldnt be here. im only 18.
Cant talk with the big guns. XD
Interstellar Fellow on February 25, 2008 at 6:12 pm: 14
Ksoldier, I must interject your mini rant, old man River merely uttered the words ‘typical Bush administration spin’ and you go off on one spouting an attack on freedom and security, calling people imbeciles?
WAKE UP AND SMELL THE COFFEE! If you face your PC out of FOX?CNN America for a few minutes you will find the rest of the world out here knows all too well the modus operandi of this Deceitful Bush Administration, and is still collectively suffering for it! And as a British ex-Special Forces soldier of the French Foreign Legion who still sees the same conflicts in the world that he fought in 25 years ago, I find it hard to believe you spent two years in any army, let alone Iraq, not with an attitude like that, certainly not front line anyway, not so much Captain America more like Kept in America, maybe that’s what the K stands for… Freedom means freedom FOR EVERYONE to criticise the state, any state, without fear of retribution, especially ill-informed amateur retribution, oh, back to Bush again…
Sorry this has gone off topic, but like the Iraq War, you allegedly started it.
Anyway professor, latest indications are the main debris field is now 210 miles up and ascending from 150 miles up at impact, does this mean there is now a spread`of debris in that particular region and what future for any other satellites or manned craft entering that area? Subsequent tit for tat ‘rescue missions’ testing increasingly more powerful technology is surely on the agenda, with Russia another player ready to join the table.
Surely then,the Chinese are entitled to ask exactly the same questions as the US did when the Chinese shot a satellite down, after all, this is a pissing contest, quid pro quo as the Romans would say…
What it does show is that neither Superpower has any regard to life on this Planet other than their own short-term narrow, petty agendas. Apart from the damage to the Planets surface by these great Industries adding more and more space junk only complicates matters for the future of the world as a whole. The future of Science is assured though, money always flows when an Arms
Race is on!
Conclusion? Humpty Dumpty goes to pieces whilst we all sit on the wall waiting to be next..
your soapbox I believe…
Interstellar Fellow on February 25, 2008 at 6:36 pm: 15
CORRECTION: the main debris field was 210 ‘km’ up at the time, not miles, its current track varies in a wave formation between 120 and 160 odd miles up, not as sated in the previous post.
For those with a true interest try the link to a site which has Real Live Tracking of USA 193, amongst other military, civilian and alien craft…
http://www.n2yo.com/?s=29651
I lied…
there are no alien craft,
the Bush Administration made me do it
but it is a really good site, called ‘REAL TIME SATELLITE TRACKING’ try the ‘whats up’ page to find out when USA 193 passes over you…
Astroprof’s Page » Standard Missile-3 on February 27, 2008 at 6:33 pm: 16
[…] Last Wednesday night, while a lot of us were watching the lunar eclipse, the guided missile cruiser USS Lake Erie (CG-70) fired a missile to shoot down the defunct USA-193 satellite. The missile’s kinetic warhead struck the satellite at an altitude of 133 nautical miles, shattering the satellite. A cloud of expanding gas from the shattered satellite indicated that the hydrazine tank had been ruptured, which was the stated goal of the exercise. […]
KSoldier on March 3, 2008 at 12:24 pm: 17
Oh for heaven’s sake, stick to science and leave reality to the rest of us who live in it. I do not question the intelligence of the group regarding the science of this issue, but your steering into the mysterious conspiracy behind it is extremely lacking in common sense.
The Bush-hater mentality that drives these conspiracy theories fails the common sense test. You spout rhetoric as to the incompetence and failure of the current US Administration, then assign a hyper-competent successful conspiracy theory to it. Well, which is it?
The difference between the, so called, “Bush Administration spin” and your “paranoid conspiracy theory spin” is that your accusations are based on assumptions and do not require any proof whatsoever - Just an active imagination.
Allow me to poke some holes in your hatred based theories.
The US does not need an excuse to test its missile systems. Why can’t your mind grasp the possibility that this was a convenient coincidence for an opportunity to use a weapons system for good?
The threat of hydrazine surviving re-enty is nominal, but contingencies are measured against a worst-case scenerios. Imagine if someone did get sick or died from a minor exposure to hydrazine. The argument that the US figured there would not be any danger would be argued as unacceptable, most likely, from the very people voicing their negative opinions right here. Do not forget that traces of hydrazine were discovered on the Columbia debris.
The worst case scenerio also applies to national security. Sorry professor, simply “HOPING” that any classified technology does not survive re-entry does not factor into the equation.
Any debris sent into higher orbit will just take a bit longer for its orbit to decay and return to Earth. But one of the goals of this mission was to ensure that the debris would be much smaller.
And this “pissing contest” as you refer to it, may just save your life one day. Your children are more likely to be slaughtered by terrorists as they are to be hit from falling debris. But let’s just “HOPE” they’re not.
Meanwhile, relax and enjoy your freedom. Thanks for the support.
Astroprof on March 3, 2008 at 2:19 pm: 18
KSoldier,
I never said that there was anything wrong with shooting down the satellite. Rather, I indicated that the stated reason was probably not the sole motivation. It is not unusual for there to be more than one motive behind anything that the military does. And, while national security may be a motivating factor, it is almost never the reason given. After all, that would be just advertising to the rest of the world that if anything of the satellite survives and falls onto a hostile nation that they should go over it with a fine tooth comb to figure out what we didn’t want them to find.
If you read my later postings, you will see that I also indicated that this was likely a convenient target to test a missile system that was bound to be tested again anyway. Given that the missile system would be tested again soon, anyway, then destroying the satellite is a no brainer. It reduces the small chance of injury of someone on the ground, and it makes sure that classified technology does not fall into unfriendly hands. The way that this was done minimizes orbital debris from the satellite’s destruction.
KSoldier on March 5, 2008 at 9:30 am: 19
Astroprof - My responses were not directed towards your comments as much as they were for the other posts painting this task as an evil government conspiracy. In fact, your comments reflect the common sense lacking in the the other theories. It is obvious that there were multiple reasons behind the reasons to shoot the satellite out of the sky; simplicity, availability, and efficiency being the top three. The ability to test our defense systems was just a convenient bonus.