After spending not a few words talking about Armageddon and fake space rocks last Wednesday I was slightly surprised when an actual real-life asteroid tore through the skies above Russia and disintegrated/detonated in midair somewhere above Chelyabinsk. Thanks to the asteroid’s passage over populated areas and the modern ubiquity of smartphones with some kind of video capture capability – not to mention the uniquely Russian preponderance of car dashboard cameras to provide some protection against the now-famous driving standards in the country, as well as the notoriously corrupt traffic police – this has been by far the most well-documented asteroid “strike” in history, so I thought I’d take a little while to talk about it, and the reaction to it.

First, there’s the question of nomenclature. I’m going to be calling the Russian object an asteroid in this piece, but you should know that the term “asteroid” is rather ill-defined (in fact all the terminology used to refer to things smaller than dwarf planets is rather ill-defined), and the best I can come up given currently-existing IAU guidelines is “a lump of rock that is smaller than a dwarf planet but larger than a meteoroid”. Meteoroids are currently defined as “a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom”, but that definition dates from 1968 and we’ve since determined that meteoroids and asteroids are basically the same population of bodies, with the term “meteoroid” being used to refer to any small-scale asteroid (10m or below) that manages to enter Earth’s atmosphere. Most meteoroids burn up in the atmosphere; the ones that make it through to hit the ground are called meteorites, which is what the majority of the press is calling the Russian asteroid.

Are they correct? Well, yes and no. There are two problems with calling the Russian asteroid a meteorite:

1)      It was actually quite large – NASA says around 17 metres – which puts it out of the generally accepted size range for a meteorite.

2)      It didn’t actually hit the ground. While it didn’t burn up completely, the Russian asteroid broke up just above ground level thanks to the immense heat and pressure of entering the atmosphere.This will have scattered chunks of asteroid everywhere and there’s already been several fragments and craters reported found. Astronomers have a specific term for asteroids that create fireballs and break up in the atmosphere like this: we call them bolides.

However, if the press went around referring to the “Russian bolide” in their news reports nobody would have any idea what the hell they were talking about, so in the absence of any better term calling it a meteorite is fine. This is the IAU’s fault for not ever bothering to classify small Solar System objects properly and relying on fuzzy definitions that are decades-old, not bad reporting.

Now, let’s take a moment to watch a video of the asteroid’s passage.

(I have desperately tried to avoid lifting the video links straight from Bad Astronomy guy’s excellent rundown of what was going on at the time, but the originals I saw have since been buried by recycled clips from news agencies and I didn’t save them for future reference.)

That gets across just how bright the asteroid’s entry into the atmosphere was; it’s a large object travelling at around 18 kilometres per second (i.e. far, far faster than a supersonic jet aircraft, which would be lucky to manage a kilometre and a half per second) so it’s generating an awful lot of ram pressure, which heats up both the asteroid and the air flowing around it. If you direct that amount of force and energy against a lump of rock and metal it is rather understandably going to start falling to pieces, which is exactly what the Russian asteroid did; it lasted barely thirty seconds in the Earth’s atmosphere and broke into bits over the Urals near Chelyabinsk. But did it explode?

This video shows that there was a very large bang about thirty seconds after the asteroid’s passage through the atmosphere. This is caused by a sonic shockwave, which is what broke all the windows, smashed up that zinc factory and injured a thousand people, but the shockwave is not the product of the asteroid exploding or hitting the ground. Instead it’s been caused by the simple passage of the asteroid through the air at such a ridiculous speed. You’ve heard aircraft make sonic booms as they break the sound barrier? This is that, except magnified about a hundred times. The shockwave of a sonic boom is created by air piling up in front of the aircraft, and as the aircraft goes faster and faster the air simply cannot get out of the way fast enough and becomes compressed into a single shock front moving at the speed of sound. Since the aircraft is moving faster than the speed of sound you’ll see a supersonic jet a few seconds before you hear its sonic boom. Scale up from an aircraft to a large chunk of rock 17 metres on a side, and make the time lag thirty seconds (because it was around 30-50 kilometres up, an altitude much higher than aircraft fly at) and you get the shockwave caused by the Russian asteroid.

So the bang wasn’t caused by any explosion in the conventional sense; it’s not like the asteroid reached the end of its trajectory and abruptly exploded like a nuclear weapon. Instead, as bits and pieces sheared off of the asteroid it would have released energy with the same effect as a trail of smaller explosions. NASA puts the total energy released during the asteroid’s passage through the atmosphere at around 500 kilotons, which is roughly equivalent to a mid-sized nuclear bomb, but this energy was released gradually instead of all in one go which is why the property damage was mostly limited to a lot of smashed glass.

That’s what we think happened, but sadly there’s a couple of myths that have already grown up around the Russian asteroid:

• That it was something to do with the asteroid 2012 DA14, which was scheduled to make a very close approach to the Earth on the same day. This is stupid because the two objects were on completely different trajectories as demonstrated by this video; the Chelyabinsk asteroid grazed the Earth’s atmosphere on an east-west trajectory, while DA14 passed it by north-south. They are completely different bodies that just happened to get noticed by the human race on the same day; thousands of meteoroids/meteorites enter the Earth’s atmosphere every year and the vast majority of them are completely unknown to us because they burn up too high, or they come in during the day, or they descend over the ocean. The only things that make the Russian one special are its size and that it happened to come down over a populated area.

• That it was intercepted by Russian air defence forces and destroyed. This is insane for a number of reasons but I’m going to pick the most salient one: the best air defence technology on earth currently cannot make a reliably successful interception on an ICBM moving 7-8 kilometres per second. It would have no chance of hitting an asteroid moving at 18 kilometres per second.

Finally there’s the media reaction to it, which if you look past the usual idiotic misreporting is basically one of total ignorance. Despite reporting on 2012 DA14 for a week or two the fact that an asteroid could actually make it through (most of) the atmosphere and cause some damage to a populated area seemed to take them by surprise. I lost count of the number of hastily-commissioned opinion pieces I saw which basically said “Whoa, maybe we should be a bit worried about this!” like they previously thought it was something that only happened in movies. The good news is that even if 2012 DA14 had hit it would have caused only localised damage (roughly equivalent to one city, and that’s only if it had managed to score a direct hit), and that really threatening impacts only occur over geologic timescales. The bad news is that there is literally nothing we could do to stop either kind of impact without a decade of lead time to prepare – and as the Russian asteroid showed, often we don’t even see them coming at all.

(Just as an aside, every news article under the sun mentioned the Tunguska event as another meteorite event which happened in Russia, but nobody brought up the Sikhote-Alin meteorite which came down over what was then the Soviet Union in 1947. Unlike Tunguska this was witnessed by people who understood what they were looking at, including an artist who immediately drew this. It’s strikingly similar.)

The post Asteroids Again. appeared first on The Scientific Gamer.

gnomishlich asks

Armageddon, terrible movie about oil drillers on an asteroid how would a space shuttle or other space traveling vehicle fare for persons odds of arrival, survival, and departure from an asteroid?

Oh god, what have you done. WHAT HAVE YOU DONE.

You may think Armageddon is a terrible movie, and you would hardly be wrong to do so. Michael Bay’s job is to make blockbuster movies that make extensive use of special effects and loud action scenes to cover up the fact that they have no logical plot or consistent characterisation to speak of, and are designed to cater to people whose brains are operating on the mental level of a twelve year-old boy. Literally every single one of his films has been offensively bad. Like, not just the regular kind of bad, where you sit through it and it’s bad and at the end of it you think “Well, that was bad, I won’t be watching that again.” No, that would be too easy for Bay, and in fact if he ever made a film that was simply bad I’d think he was slipping somehow. What sets Bay’s films apart from the rest of the lowing, braying herd of CGI summer blockbusters is that they seem to be designed to piss the audience off. I honestly don’t know how you can watch a Transformers film and be anything other than utterly and completely ashamed and insulted – ashamed because you spent two-plus hours of your life watching this crap and you don’t want anyone else to know, and insulted because Michael Bay thinks you are stupid enough to enjoy something pitched at the level of a Transformers movie. It’s only a Michael Bay film that can make me feel soiled somehow after I’m done watching it, like I need a day-long shower to wash off all of the dumb that has built up on my skin. It’s hard to accurately recall just how horrible I felt after watching Transformers 3, but thanks to the internet my immediate reaction has been preserved for posterity:

Transformers 3 should be submitted to a peer-reviewed scientific journal because it conclusively proves many things that otherwise would have taken decades if not centuries of study and observation. The intellectual bankruptcy of Western civilization and culture. The non-existence of God. The ultimate futility of living in a world where the laws of physics allow such crimes to be perpetrated against the very nature of the universe. Like some sort of Lovecraftian horror that has slithered forth from an unholy nether dimension, Transformers 3 is so utterly and fundamentally at odds with everything good and sane that the mind atavistically recoils and refuses to comprehend it.  And even if I manage to purge the experience of having watched it from my memory it will still leave signs of its passage; a broad swath of tainted brain matter that cannot be reused for any other purpose but which instead grows and expands, subverting healthy tissue and corrupting my mind to its vile purpose until I am a twisted ur-man no longer capable of rational thought. Ia! Ia! Baythulu fhtagn!

Right. Yes. Anyway. Michael Bay films. Very bad. Very bad. Armageddon is a bad film, and it is no less bad for having been made during Bay’s early period (it was his third film or something), but by the standards of Michael Bay it is unremarkably bad. There’s nothing to make it stand out in terms of badness from the likes of Pearl Harbor or The Island – yes, it’s stupid, and shot with Bay’s trademark oversaturation of American flags fluttering in the breeze, and the scene with the minigun makes no logical or rational sense, but neither do any of his other films so there’s nothing here to mark it out as different in any way.

That is, unless you happen to have a PhD in solar system impact physics, at which point Armageddon becomes tied with Mission to Mars for the Worst Cinematic Atrocity Perpetrated Against Science award. You’ve got an unrealistically sized asteroid heading towards Earth which is stated to be “the size of Texas”, so about 700 miles across, except there’s only one asteroid (now dwarf planet) in the belt anywhere near that big and we’d notice if anything happened to it. The asteroid has been knocked out of its orbit by a comet; this is a chunk of ice which is typically Not Very Big and has sod all mass thanks to being made of spongey ice, and would have little impact on the orbital trajectory of a an asteroid 700 miles on a side¹. The asteroid itself looks nothing like an asteroid that’s just been walloped by a comet; there’s a few chunks of rock and clouds of debris orbiting with it like you’d expect, but I don’t think any asteroid would look quite as spiky as the one in Armageddon does. It should look just like a big rock, or like a big pile of agglomerated rubble compressed down into a sphere by its own self-gravity. Instead it looks all gothic and nightmarish and there’s lots of weird green and blue hues used in the lighting (again, trademarks of Bay) because that shit looks cooler, I guess.

Then you’ve got the actual asteroid impacts on Earth, which are also awful. The one at the start creates a moving wall of fire that eventually consumes the entire planet, and I don’t even know how this would work. Asteroid impacts tend not to be about fire so much as they are massive earthquakes and huge blast waves. Oh, anything caught nearby is going to be a crispy critter (read the first statement from a witness of the Tunguska event) thanks to the immense heat of the asteroid’s entry into the atmosphere and sheer quantity of energy liberated when it either hits the ground or explodes in an airburst, but we’re talking something about the size of a big nuclear fireball here, and not some all-encompassing global catastrophe. While the shock waves would devastate an area hundreds or thousands or miles around the impact site, truly global damage would be done through dust clouds/climate change for the smaller asteroids, and the physical disruption of the Earth’s crust for the larger ones.

The second thing this – and all other Hollywood films about asteroids, including Deep Impact – gets wrong is the speed of the asteroid. Asteroids in films move slowly, almost leisurely, like an out-of-control jet airliner rather than a piece of space rock moving at 20-30 kilometres per second. People on the ground have plenty of time to see them coming and make futile attempts to escape. In reality, unless the asteroid comes in at an incredibly oblique angle the time lag between an asteroid entering the Earth’s atmosphere and hitting the ground is about one second. Try snapping your fingers once; that’s how fast an asteroid impact would seem to a watching human. If they’re standing close enough to see it happen they’re screwed, and even people who are safely over the horizon will be killed by a blast wave pushing a wall of moving rock and debris towards them faster than the speed of sound. Even smaller asteroids (and we’re talking the metres-scale stuff that makes it to the ground here) would pack enough punch to explode with the force of a moderately-sized stack of TNT. You see the Barringer Crater? The thing in North America that’s over a kilometre wide and 200 metres deep? We think the rock that created it was about fifty metres in diameter. Fifty. Metres. Just one of the “tiny” rocks featured at the start of Armageddon would devastate a significant area of New York if it managed to score a direct hit, rather than just chopping the top off of the Chrysler building. Movies dramatically understate how lethal and destructive asteroid impacts are. It’s one of the many cases where the reality is actually far more terrifying than what’s on film, but it’s deliberately understated so as not to scare the living daylights out of the audience.

And then we get to your question, which is: could we fly a spacecraft up to an asteroid and land on it? The answer is, yes we can. In fact we have, and they even managed to get the spacecraft back to Earth orbit afterwards. Armageddon’s rendition of military space shuttles that handle like jet fighters in outer space is completely ludicrous, of course, but we’ve managed it with a robot probe so there’s absolutely no reason why we couldn’t do with an appropriately-designed manned spacecraft. Before the Orion program was gutted by the US administration one of the proposed missions was a trip out to a Near Earth Object – in other words an asteroid orbiting relatively close-by to the Earth (out past the orbit of the Moon, but not too far out) – so it’s something that is seriously considered by space agencies today, and it’d actually be considerably safer than a trip to Mars thanks to the shorter mission duration and the fact that “landing” on an asteroid doesn’t mean marooning yourself at the bottom of a gravity well.

This is what happens when somebody mentions Armageddon to me; I spend 1500 words ranting and then finally get around to actually answering their question in a single paragraph. The movie’s science was deconstructed years ago by somebody with far more patience than me², and it used to be shown to new management hires at NASA as a training exercise to see how many inaccuracies they could spot. It is quite literally the textbook case of How Not To Do It. Please, nobody ever bring it up on here again.

(Incidentally if you want to know if we could use Armageddon as a blueprint for saving the world from a real rogue asteroid, I already covered it in a previous post.)

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Having explained the basic concept of gravitational barycentres, I can now get to the meat of what I wanted to talk about: Lagrange (or Lagrangian) points.

If we can find the gravitational barycentre between two objects, we should in theory be able to predict how they move based on their current position, velocity and their mutual gravitational position. Most of them time it’s not that simple, however. For instance, if we really want to plot the orbit of the Moon around the Earth, and do it accurately, we need to take other significant gravitational influences like the Sun into account. Predicting the orbital motion of the Earth-Moon-Sun system as they undergo mutual gravitational interactions is an example of what’s called the three-body problem, and it’s one that physicists and mathematicians have been trying to solve since the 17^th century. While solutions for special cases of the three-body problem have been found a general solution for all cases is still very much beyond us, and increasing the number of gravitational influences still further (up to n bodies, which is known as the n-body problem) makes things even worse. We can simulate the interactions of n-body systems using very powerful supercomputers but this is only an approximation, and it’s possible that the n-body form of the problem has no solution since the approximation involves an infinite series of expansions that has to be truncated at some point in order to get a finite answer; in other words, solving the n-body problem is kind of like trying to find the exact value of pi.

Lagrange points rise as part of the three-body problem, but happily it’s one of the special cases we have an exact solution for known as the restricted three-body problem. In the restricted version of the problem the third body has negligible mass in comparison to the other two – it’s a probe or an asteroid rather than a moon or planet – and so exerts practically no gravitational force on the other two bodies in the system. The problem therefore becomes one of figuring out how the two bodies that do have mass affect the movement of the third massless body, and since the solution to the two-body problem is known this is something we can do.

One quirk of the solution to the restricted three-body problem is that it indicates there are certain places where you can put the massless object that will cause it to – given the appropriate velocity – orbit the common barycentre of the system at the same rate as the two objects with mass. This is a fancy way of saying that it will be stationary with respect to the two massive objects and will be located at the same position relative to them at any point in their orbital system. These stationary points are called the Lagrange points, and there are five of them for every two-body system. For examples I’ll use the Earth and the Sun as my two-body system since they’re easy to understand, but Lagrange points also exist for the Earth and the Moon, the Sun and Jupiter and so on.

L1: Situated directly in between the Earth and the Sun, very close to the Earth. This is the point where the gravitational forces of the Earth and the Sun “balance”. Stuff orbiting closer to the Sun will do so at a faster velocity and over a shorter period, while stuff orbiting further away will orbit at a slower velocity and over a longer period. Ordinarily anything placed at the L1 point would orbit the Sun more quickly than the Earth and would thus be asynchronous with it; however, L1 is in just the right spot for the Earth’s gravity to provide a slowing force on any hypothetical body placed there that causes it to orbit the Sun with the same orbital period as the Earth.

L2: Like L1, but backwards. An object at L2 would ordinarily orbit the Sun more slowly than the Earth due to being further away; this time, though, the gravitational pull of the Earth works in the opposite direction, giving the L2 body a boost in speed and (again) causing it to orbit the Sun with the same orbital period as the Earth.

L3: Directly on the opposite side of the Sun to the Earth, L3 does not lie on the Earth’s orbital path as the diagram suggests; instead it’s just inside it as a result of the Earth-Sun barycentre being shifted slightly towards the Earth. L3 is the point where the combined gravitational pull of the Sun and the Earth act along the same line and provide a single unidirectional force that is equivalent to the gravitational force the Sun exerts on the Earth, causing an object placed there to orbit the barycentre just like the Earth does.

L4 + L5: The trickiest Lagrange points to explain, L4 and L5 lie at the apexes of a pair of equilateral triangles which have the Earth and the Sun as two of their corners. Objects at L4 and L5 are the same distance away from the Earth as they are from the Sun, forming an equilateral triangle! I was going to explain exactly how this happens but it involves trigonometry which I’m sure everyone is doing their damndest to forget, so I’ll just steal the following picture from Wikipedia.

Right, so, here the examples are the Earth and the Moon; the gravitational force of each combines into a single resultant force which is deflected to a point slightly away from the centre of mass of the Earth which just happens to be the barycentre of the Earth-Moon system, causing objects placed at L4 and L5 to – again – orbit the barycentre along with the Earth and the Moon.

So that’s what Lagrange points are and why they are. Why are they important? Well, the examples I used of our massless third body being an asteroid or a probe weren’t just for fun since that’s exactly the sort of thing that tends to be present at a Lagrange point.

L1: In the Earth-Sun system a probe placed at L1 will never be out of sight of the Sun (i.e. no eclipses by the Earth), making it very useful for solar observatories. In the Earth-Moon system you can stick a space station up there and use it as a halfway house for lunar exploration. I mean, if you really wanted to.

L2: Opposite of L1 (again); a probe at L2 will have almost all of the solar radiation from the Sun blocked by the Earth, so this is where we want to put space telescopes for the best result since solar radiation is the space equivalent of light pollution from cities. The successor to the Hubble, the James Webb space telescope, is going to be put at the Earth-Sun L2 when they eventually get around to finishing the damn thing.

L3: Kind of boring! It does kind of amuse me though because this is where some people thought you might be able to hide a secret planet; since it’s on the other side of the Sun it’d always be out of sight of the Earth. These people did not take into account the massive gravitational perturbations such a secret planet would cause, nor the fact that L3 is inherently unstable and no object can persist there over solar system timescales.

L4 + L5: You know how Jupiter gives asteroids, comets etc. that have the temerity to venture inwards towards the Sun a massive gravitational slap that sends them careening out of the Solar System towards parts unknown? The L4 and L5 points in the Jupiter-Sun system is where the lucky survivors of this process tend to congregrate; each of them is populated by a cluster of asteroids (well, I say cluster, there’s like a million of them) which are collectively referred to as the Trojans, and which make the asteroid distribution in the Solar System look like this.

The reason the Trojans are spread out over a fairly large area is that the regions around the L4 + L5 points where you can get a stable orbit are actually pretty large. This image maps gravitational potential as contour lines; while the L1, L2 and L3 points are all tight gravitational troughs, the L4 and L5 points are broad gravitational peaks that an asteroid can, with a bit of luck, perch on top of over long timescales (millions of years) without too much difficulty.

The post Home On The Lagrange. appeared first on The Scientific Gamer.

Empress Josh asks

This wacky Lets Mine All the Asteroids scheme. Viable y/n.

Ah, a question where my answer isn’t immediately “That’s stupid” or “We couldn’t ever do that.” Mining asteroids isn’t stupid, and we probably will be able to do it at some point. The real question here is, can we do it today or (as I suspect Sergey Brin et al. damn well know) even fifty years in the future? And is it going to be worthwhile?

First, the why: why bother mining asteroids in the first place? Yes, the juicier ones will contain a vast, vast quantity of valuable resources, but asteroids are all the way up there and there’s still plenty of unexploited resources down here. Making a trip to an asteroid is currently a hugely expensive undertaking, and returning material from an asteroid to Earth is even more so (the Great Oracle tells me that it’s going to cost NASA $1 billion to get sixty grams of asteroid). While getting at Earth-based resources is going to become more difficult and more expensive over time as easily-accessible supplies run out, it should still be far cheaper to mine them from a source that’s already at the bottom of Earth’s gravity well instead of expending the huge amount of energy required to transport raw resources to and from a nearby asteroid.

That’s going to be the case for the short- to mid-term future. However, fairly soon now – and we’re talking within a period of time not more than one century – we’re going to start running out of the rarer resources our decadent Western civilisation requires for its technology and industry. Crucial elements used in electronics like gold, iridium and copper are going to become extremely thin on the ground, and consequently far more expensive. At that point we’re going to be faced with two alternatives: we either learn to live without our smart phones and our Google glasses and go back to a basic agricultural existence (which incidentally would require the die-off of about four-fifths of the human race) or we start getting our resources from further afield. Nobody is suggesting we start mining asteroids tomorrow, or even in the next couple of decades. In a hundred years, though, we’re pretty much going to have to, and so it’s good that somebody is starting to seriously think about it now.

As for the how, that’s a bit more complex. I’ve expounded before on this blog about what a bastard it is to get enough delta-V to get out of Earth’s gravity well, and at the moment this requirement makes any potential asteroid mining endeavour ridiculously expensive to the point where it would consume far more resources than it ever produced. You’ve got the following considerations to deal with:

• Lifting the equipment required to extract useful resources from an asteroid out of Earth’s gravity well and shooting it off to a nearby asteroid. This will not be a small amount of equipment; using today’s technology it would likely exceed the lifting capacity of all the major space players for the next dozen years.

• Developing technology which will autonomously harvest asteroid resources without human intervention. This is going to be absolutely necessary if we ever want to do it properly; we have a hard enough time getting minerals out of the ground on Earth where humans can do it, and sending humans to an asteroid would be a complete waste of time and effort since there would be little they could effectively do that robots couldn’t.

• Getting the mined resources back to Earth. This is somewhat less acute than the gravity well problem, but you still have to build transport vehicles in situ on the asteroid and then fuel them somehow.

In order to overcome these problems we are going to have to make some very significant advances in robotics and/or nanotech-scale devices, but the good news is that if we manage it its going to be kind of a catch-all solution. If we can somehow build functioning self-replicating machines then most of the problems facing the concept of asteroid mining are greatly alleviated. It would still be a pain to get the initial payload out of Earth’s gravity well, but you’d only need one self-replicating machine to start the ball rolling making that initial payload comparatively small. Any machine with the ability to autonomously build a perfect copy of itself would have absolutely no problem strip mining an asteroid. Ditto for returning the raw/processed materials to Earth; assuming sufficient volatiles could be found within the asteroid to fuel a transport spacecraft (for “volatiles” read “hydrogen and oxygen” or “water”) then the machines could build that as well. That’s the nice thing about asteroids: they’re so big that all the raw material to do this stuff is already on-site waiting for us to do something with it.

The bad news is that this is the only realistic way I can see of making asteroid mining work on any kind of practical level. Any other solution runs smack-bang into one or more of the three problems outlined above which spell doom for any putative asteroid mining effort: you have to either take the equipment required to mine the asteroid out to the asteroid (nigh-impossible) or else physically move the asteroid to Earth orbit where you have an easier job of getting at it (actually impossible*). What can I say? Physics is a bit of a sod when it comes to space travel.

Still, Planetary Resources Inc. is one of those things that sounds completely ludicrous but which makes perfect sense if you think about it for a bit. Half the trouble with national spaceflight programs is that they’re projects requiring planning horizons of several decades or more which are completely at the mercy of ADD-riddled governments who are only concerned with the outcome of the next election cycle. Asteroid mining is a necessary technology that could easily take 50-100 years to come to fruition and so it’s probably a good idea for this company to begin working on it now, because I guarantee you nobody else will until it’s far too late.

*On any kind of useful timescale, anyway. I said in the nukes post that you could deflect an asteroid with long-burn rocket motors, but parking an asteroid in a stable Earth orbit is a different kettle of fish entirely.

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