brdavis

No, not that kind of dating… http://www.jpl.nasa.gov/news/news.php?release=2013-356 From the standpoint of KSP, note that this is the first in situ use of radiometric dating techniques (potential science instruments anyone?), as well as a deeper analysis of both the history and environment of the surface of Mars. Interesting.

What stars? I see… nothing, even in HD, at those points. Odd. Yeah, the whole "I must let go of the rope" thing bugged me. It (a) wasn't needed, ( didn't make sense (if that was the solution, he doesn't sit there talking to her before releasing… he lets go), (3) only use was to make her alone and have sort of surprise later. Gravity was good. The number of mistakes, gaffs, and fudges in it was huge. The amount it got right(ish) compared to any other similar movie was astounding… but I'm not sure if that says volumes for the quality of Gravity, or volumes for the poor competition. Probably both. If they were rotating… haul him in. Even just a little. Tether goes slack, secure to parachute cords, repeat. No, pulling him towards the center of rotation would not speed up (in any realistic sense) any rotation of the station (compare the moment of inertia for the station to that of George Clooney. His a big star.. but not that big).

A known event, releasing known contaminants, at a known time and location? That's called a "tracer"… it was nice of the Chinese to do it for LADEE . Seriously, it's had a chance to characterize the system, and now it has a chance to characterize the system when it is perturbed out of equilibrium. This isn't a problem as much as it's a golden opportunity handed to them.

While on the subject of hard SF… Hal Clement worked with Asimov a lot in some of his work (often more chemistry, but an awful lot of physics too… Mission of Gravity is very good, but he has a lot of others. Rama I loved (worked out the shape of the waterfall and how the atmosphere would change with height in grad school), and Robert Forward's "Flight of the Dragonfly" (or "Rocheworld") are also good in terms of good hard science (the last having some of the most bizarre orbital mechanics ever… including how to get what is effectively a boat into orbit by riding a waterfall between two planets… and, yeah, the physics works). Allen Steele has also done some good stuff in terms of near-term "orbital construction jocks" and what's entailed.

What he/she just said. Seriously, that hit all the main points. I went with a good pair of 10x50 binoculars with a tripod mount and a tripod for about 8 months - saw all the planets except Pluto, more than half a dozen asteroids, lots of lunar features, geosync satellites, some of the brighter galaxies, double stars, etc. Then I stepped up to an XT-8 (the 8" version of the first telescope recommended, a Dobsonian reflector) with a couple of eyepieces. To start, this is all you need. To continue… well, this is still pretty much all you need.

I was thinking that it would be option two, but the planet/moon would be just far enough away that tides n both would be comparable to earth. From other comments, I think you mean something like option 3: the planet rotates, and so has tides induced by the moon, but the moon has become tidally locked to the planet, and so has not tidal variation, just "fossil" locked-in tidal bulges. The equilibrium tidal height scales as: (m_source / m_target) (r_target / a_orbit)^3 where "source" is the object inducing the tides, and "target" is the object experiencing tides. You can use this to compare a system to Earth. For example, here "Earth" and "Moon" pertain to the normal astronomic bodies, where "planet" and "moon" correspond to the objects in the story: { (m_moon/m_Moon) / (m_planet/m_Earth) } * { (r_planet/r_earth) / (a_moon/a_Moon) }^3 { ( 28 / 1 ) / ( 1.34 / 1 ) } * { ( 1.1 / 1 ) / (70,606 / 384,400) }^3 { 20 } * { 1.1 / 0.1837 }^3 4,294 Very roughly, tides on the planet induced by the (much much more) massive moon that is located (very very much) close to the planet will be huge… roughly 4,000 times higher than lunar tides on Earth, which are on the order of 11 cm. So we're talking tides on the order of half a kilometer, more or less. Surfing is going to be AWESOME More likely, tides like these lead to planets scoured flat, or tidal locking occurring in very short periods of time. You can do a rough estimate of the tidal locking timescale (& if you want I can give you a formula), but now all go really well out on a limb and say yeah… these will be very very VERY tidally locked. Mutually locked. So no tides. Back to hawkinator's Option 2.

So pulling some numbers together from this and other posts, and making some stuff up as I go… let's assume the density of both planet & moon is the same as Earths (5520 kg/m^3), just to ballpark things. If the surface gravity is 1.1 G's, then the radius of the planet is 1.1 R_earth and for a moon surface gravity of 0.7 G's the radius there would be about 0.7 R_earth. Using those radii and the assumed density of 5520, you can calcuate their masses: M_planet = 7.985e+24 kg (1.34 M_earth) m_moon = 2.058e+24 kg (0.34 M_earth, or 28 M_Moon, Earth's moon… this is a Big Moon ) Well, I'm going to spitball an estimate of the moon being in an orbit with a period of 40 hours… certainly above synchronous orbit. In this system the 40 hours orbit would have a semi-major axis of 70,607 km (18% of the distance between Earth and the Moon). That's pretty close. The SOI for the moon would be around 41,000 km (I say "around", because really a SOI approximation is not going to hold here - the SOI is a significant fraction of the orbital distance), which means that there won't be any stable planet-synchronous orbits… at all. In fact there aren't going to be many long-term stable orbits more than around 25,000 km up around the planet… probably a lot lower. So… that may be a problem. With a higher gravity the scale height should be smaller, so I'm guessing either you have a higher temperature planet here or one with a significantly different gas mix? For an Earth-normal atmosphere, the slightly increased gravity means the scale height should drop from around 8.4 km for 1 G's to 7.6 km for 1.1 G's. A "sea level" pressure higher than the terrestrial value should be fine… even make sense (larger planet, more to outgas, so perhaps more atmosphere/hydrosphere). Mountain climbing will be a little tougher due to thinner air I guess. Hmm, here I'm confused. With a surface gravity of 0.7 G's, the scale height should be, all other things being equal, much greater than on the planet. H_scale = R T / g, and if R (determined by the gas composition) and T (the atmospheric temperature) are the same, H_planet / H_moon = g_moon / g_planet = 0.7 / 1.1 = 0.636. So if the scale height on the planet was real 9 km, the moon should have a scale height around 14 km. for the 7.6 km I got for an "Earth normal" atmosphere, the moon would end up with a scale height of 11.9 km. Lower surface gravity implies taller scale height, other things being equal. So… are you taking into account some temperatures and gas mixes I'm not, or is something else up? Note that with the reduced gravity, the lapse rate will also change - about 6.9 K/km on the moon. That may present a problem in that the atmosphere will not get terribly cold terribly fast as you go up, so it may lack a good cold trap… which in turn means over geologic time, this moon is likely to start loosing the oceans due to UV photolysis and Jean's escape. Atmospheric loss in general is going to be a potential issue: v_RMS for O at T=1000 K (about the temperature of Earth's exosphere) would be 1,248 m/s, and with an escape velocity of just 7.83 km/sec, there's going to be a significant fraction of free oxygen radicals that are at or above escape velocity. It can probably hold onto an oxygen-rich atmosphere for billions of years, but probably not tens of billions - there might be significant long-term atmospheric loss here. That's going to be an outrageous magnetospheric environment - when they are both aligned, it will be a strong magnetosphere, when anti-aligned I'm not sure what you will get (but, again, atmospheric loss might come up, like Mars in our own system). Probably even the smaller body will have enough internal heat to do this however, so that's not a big problem. More than a billion years between the formation of the star and the formation of the planet? That seems… more than a little long, to me. But probably not an important story detail. star larger than our own and as old or older than our own implies it;s going to be brighter, so the planet (to have Earth-normal insolation) will orbit further away. The details on that will depend on the stars mass (fairly sensitive to it). If the moon is in tidal lock, it's going to have essentially zero obliquity relative to its orbit… and furthermore unless the situation is really unusual (look up Cassini states), the moon spin axis will align with the moons orbit will align with the planets spin axis (Earth's Moon being a very unusual exception). So if the planet has an obliquity of 9°, so will the moon. The Hill sphere and stability criterion for this planet, with a moon like that anywhere near synchronous orbit, isn't a problem - it's well within the planet's stability criterion. Oh yeah, tides. Next post.

Hmm. Tricky. First I think you can have life in tidally locked worlds (even once-faced worlds tidally locked to their stars it seem are OK if they have even an Earth-normal style atmosphere for heat transfer). I'm mostly pointing out that for a tidally locked satellite the solar day can be fairly long, which is something to consider… and something you can change (for story purposes). Move it into a lower orbit, and it orbits faster, and even if tidally locked has a shorter solar day. The limit there is probably that you shouldn't have a moon (especially a large one) below synchronous orbit (moons above synchronous orbit tidally evolve outward, which is probably OK... below synchronous orbit they tidally evolve inwards, which can result in serious consequences for anyone on the planet below ). But even there you have a lot of flexibility… if you need a fast moon orbit, put it just outside synchronous orbit, and make synchronous orbit low by having a rapidly spinning primary*. Secondly, you can work out, roughly, the tidal locking limits at a given time… but before I want to do all that, there are a bunch of things to consider. Masses and radii of both bodies, how far apart they are, and the "Q factor" (how good they are at dissipating tidal energy). If you have advanced life here, the system needs to be… well, we're not sure how old. Life seems to kick off almost as soon as a planet has liquid oceans (it seems to have taken at most a 100 million years or so for Earth - which is a very short time, geologically speaking). Complex life, multicellular life, seems to have taken a good deal longer, possibly waiting until you started having some oxygen in the atmosphere… billions of years. So I'd guess the system in question might be at least 3 billion years old, based on Earth and "optimistically" shifting things around… but could be at least 5-6 billion (more than that… the biosphere is likely to run into other problems, like carbon starvation due to the carbonate-silicate cycle, etc., etc.). While we're at it, what type of star is this planet orbiting? There's a maximum distance you can have a moon at (related to the SOI, but a little bit different - research Hill Radius). *Yes, there are limits there - spin it too fast and it comes apart. I can work the numbers, when there are firmer numbers to work with, but for discussion let's assume a day length no shorter then 3-4 hours or so.

True. I was drawing the line at "interfere with the water-based chemistry of life". And given the context of the discussion… I thought that was a reasonable line to draw. Especially when you consider how much of "biochemistry" has to deal with things like conformal changes in protein structure (not a chemical change at all).

There's no covalent bonding going on… but there are plenty of chemical effects. After all, the lipid membranes of the cell aren't kept together by covalent bonding (but weaker VanDerWalls forces), and even isotopic difference, which are *very* much "non-chemical", can kill you (don't drink the heavy water, it's a great way to screw up relative reaction rates… and *that's* certainly chemistry).

You're right, we've gotten off-topic… but it was a fun direction to take it Well… work the numbers. If it has the same composition as Earth (5520 [kg/m^3]) and was 10% larger, the mass would be 7.98e+24 [kg], or 1.33 [Me]. But surface gravity scales nicely with the mean density: g = (4 pi / 3) G rho R. So if it's the same density, the best you've got is about 1.1 [G's] for a surface gravity. To push that up to 1.5 [G's] on a planet with a radius of 1.1 [Re] would require a density 36% larger than Earth's bulk density, or about 7,527 [kg/m^3]. That's a problem. Self-compression means things are denser than you would normally think, but from what we know that's a really high density for a terrestrial planet. You're inventing a big moon here, and maybe you could explain the high density for the planet as due to a Moon-forming impact leaving the planet with a larger than average core… but Earth already had that going for it. Could you get there, yes… but it's going to take a really really large core of metal. Well, shoot… I did just say you could figure this out, right? OK… assume the mantle averages 5000 [kg/m^3], and the core 11000 [kg/m^3]. To get the bulk density around 7,530 [kg/m^3], the core will have to stretch 75% of the way to the surface (as near as I can tell) Possible? Well, perhaps… but the moment I saw "1.5 G and 1.1 Radius" my suspension of disbelief radar went off on high. I may have a much lower tolerance than your readership, but I suspect that's the critical thing here. If I was reading it, I'd start questioning it… if a Star Wars fan was reading it, they would likely happily continue and not realize anything was amiss. So… who are you writing for? Note I'd also say this object would likely be more volcanically active than Earth (given the same age). If they are in a mutual tidal lock there's no additional tidal input of energy, but if they are not yet in such a lock, tidal dissipation might be significant (it still is with Earth, for example). And in terms of internal radioactivity, an object 1.1 [Re] would have a heat flow about 1.1x the Earth, per square meter (the square-cube law is your friend). So the planet will have potentially plate tectonics, crustal recycling, and something called the carbonate-silicate feedback. It's in good shape. The moon is a harder case. Smaller, almost certainly tidally locked, it's not going to have much in the way of internal heat… so unlikely to have plate tectonics even with extensive oceans (yeah, Venus is telling us you apparently need those). So no CO2 feedback, no (known) way to adjust for the star getting brighter, far less likely to remain habitable though out the life history of the star. Without crustal recycling, you aren't going to have mountains… especially not with an active hydrosphere. So, this is going to look a lot like Epona I suspect… not great for the inhabitants, aquatic or otherwise. Have you figured out the atmospheric pressure and composition of these two worlds? Scale hight should be interesting… and atmospheric loss is something you'll need to be concerned on for at least the moon. Not to mention day length… Yeah, I'm a geek. But I'm a geek with a PhD in physics who assigns world building projects to his college classes, so… yeah, this is my thing

Well… yeah, you'd think that. Gaseous nitrogen is pretty close to a nobel gas, but humans have problems with nitrogen narcosis. Xenon is an excellent anesthetic. Helium has effects on the nervous system at high pressures. I don't think any of that is a problem is you evolved there… but for visiting humans, well, we have a rather narrow tolerance on a whole host of gases (oxygen is toxic, nitrogen is toxic, etc.) True, but depends on the climate and metabolism. If they have a slightly lower metabolism, or a slightly higher internal temperature, they will have greater heat loss… and even with terrestrial life there's a wide range (tigers and lions are roughly human-sized, and live in hot climate… and certainly aren't naked). Humans are rather uniquely good at heat dissipation, and so might not be the ideal model here. But I couldn't agree more with this

I wouldn't, and I agree… but it would be interesting to have a way to patch in limited N-body code ("hard wired" binaries for example). Really interesting. Just… not possible, given the limitations. I'm frankly still in awe with how well you can approximate a patched-conic and get fun gameplay at the same time. And I'll note that in fiction, it's been done, and done very very well - Rocheworld. The question isn't what wonderful settings you can come up with in this regard, but what wonderful settings you can come up with and still stay within a patched-conic approximation.

Although to be fair to your point, for a small (enough) object, spread enough, it might be better (if a significant portion of the mass misses the disk of the Earth entirely… or if the air is heated by the impact but only to the point of raising it a couple hundred degrees or so - uncomfortable, but no global firestorm). Or hit the side of it with a really big bag of chalk dust wrapped around a firecracker, and let Yarkovsky do the driving. You don't even have to worry much about direction - if it's on a collision course, " any orbital change is for the better (speeding it up or slowing it down likely being the best… don't even think about kicking it sideways). Going to try to pin me down to an answer? sigh, let me guess… - below 100 [m] scale, evacuate (and film… YT will be saturated) - 100 [m] to 1000 [m], maybe… maybe… try to fragment it, if it is really weak (gravel pile style) - 1 [km] to 5 [km], probable let it hit intact and consider having Lloyds of London write a policy for the continent - 5 [km] and up, ignore it - get a possibly self-sustaining colony to Mars if you have a decade or two, and use the rest of the world economy on the biggest end-of-the-world party you've ever seen. Or ever will see.

When you are in a car and the car hits a wall, do you just happily stay in your seat? No. Inertia. It's reality. Even if you don't think it is You do realize if the Earth stops rotating, it would still have days and nights? Admittedly rather long ones… I'm not even sure exactly what you'd get on a tidally locked Earth, with one side in perpetual sunlight. Some simulations have been done, and for even very moderate atmospheres you can redistribute heat surprisingly well. Yeah, not 'comfortably'… but at least the Larry Niven "the atmosphere freezes out on the backside" seems to have been fairly discredited. No. Not really.

Agreed; ‘shotgunning’ results in a higher percentage of the energy deposited in the atmosphere. There is, actually, a difference in terms of things like changing the planets rotation, because that depends on where (not just with what momentum) the objects hits. A ‘shotgun’ blast would result in less torque (center of impact being equal, a ‘shotgun’ blast may have a portion of the debris miss the disk of the planet)... but honestly it makes no difference even for a ‘dinosaur killer’ - they are just way way too small to deliver much angular momentum on these scales. There’s another thing to consider. That ‘massive thermal energy increase’ in the atmosphere is actually pretty dang small, but the intensity matters (and it is intense). Take that hypothetical 1 [km] radius asteroid, massing 8.4e+12 [kg] (and let’s say hitting at 15 [km/s]). In terms of energy, that’s popping in with 9.45e+20 [J]... but just one day worth of sunlight on one hemisphere of the Earth is around 7.6e+21 [J]. So as drastic as this is... it’s the equivalent of having two Sun’s in the sky for a total of an hour and a half. It’s not going to effect the weather, or the global circulation, or the jet stream... at least not for more than a few days to a week at best. The intensity might be the issue however. If the incoming objects are dropping enough energy to briefly heat the air to incandescence, then you have the potential for this glowing oven-temperature sky to ignite fires... over an entire hemisphere. That can put out far far FAR more particulates than the original impact would. That’s why in certain size ranges... you’d really like the thing to hit intact, on land. Because the “dump the energy into the atmosphere†solution is far worse. Nuclear weapons do far more damage in air bursts than in ground detonations. There’s a lesson there, and it has to do with shock physics (dumping that energy into the ground really doesn’t result in much destruction... there’s a LOT of ground) and thermal physics (the heat pulse in the air is deadly and a distance, and couples energy into an air shock... not so much for ground impacts, er, I mean detonations). Perhaps the biggest problem in this discussion is what scale of impact we're talking about. For really large ones (10 [km] scale), the shotgun effect results in a global firestorm that is far more damaging than the "localized" impact, but those pack so much energy that re-entering ejecta from the main impact are enough to start their own global firestorms. At the very small end, it doesn't matter. At the very large end, it also doesn't matter. In between… it probably depends on the size in a rather complex way. I don't think global earthquakes are a significant problem however… at least not until you start hitting Chicxulub-style impacts or Caloris-basin-forming events (that one did leave geologic traces on the far side of the planet).

Hmm… addendum, dealing with how "real" SAS is done… failure modes of space hardware on the ISS (with lots of details about why certain positionings were done on the ISS, how the system functions, etc): http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100021932_2010023824.pdf

That's a really good question, and I hadn't realized that - got to watch the video I guess. In principle, rotating the ISS doesn't "cost" even any electricity - yes, you might need to "spin up" a reaction wheel, but you can recover that energy by "spinning down" (electromagnetically braking it) when you approach your desired attitude. I'm not sure if the ISS CMG (control moment gyroscopes… similar to reaction wheels, but used in a slightly different way) systems do that or not, but they could in principle. And it's not like you need to turn the ISS fast (nor could you… i'm actually surprised they turn it at all for reasons of microgravity experiments). Interesting.

I'd tend to agree with Steven Mading. The question, as stated, seems to be trying to find a physically reasonable answer to a situation that is not only physically impossible, but so poorly defined as to be meaningless. If some magical super-force were to slow the Earth rapidly to a (rotational) halt… why isn't that magical super force also acting on the crust, atmosphere, oceans, and poodles? first, you'd need to define in what way the Earth was "stopping", before you could even begin to talk (scientifically) about how other things (like poodles) would respond to "non-stopping". Heck, Earth is slowing down now - it will never be tidally locked to the Sun (timescales much longer than 10 billion years required for that), although we might* end up locked one-faced to the Moon eventually (the "day" would equal the "month" at about 50 current days long… now that would make for a long work day). Even then, we'd still spin… and tidal forces are about the only thing I can think of that slows down the rotation of large bodies in space. *I say "might" because when you figure in solar tides, the Moon starts having its orbit shrunk sometime after mutual tidal lock occurs, and the Moon is dropped into a lower orbit until it enter's Earth Roche limit… except the Sun would go all red giantish first, and anyway by the time the mutual tidal lock occurs the Moon is out nearly to the point of instability due to solar perturbations, so we might loose it after all.

On the whole, you probably want an impactor hitting intact, not as a "shotgun blast". As others have noted, the amount of energy dumped into the biosphere is identical in either case - in the case of a single impact, you have an amazingly hot impact zone. That means (if it is on land), it can radiate at least some of that energy away efficiently… and a lot more energy is partitioned into throwing around lots of rock. Not a lot of fun in the immediate vicinity (and, admittedly, for a big enough rock the immediate vicinity starts being measured in hemispheres), but it does allow the thermal radiation a chance to radiate away from a high-intensity source. Now picture all that as gravel. Pea gravel. Falling at hypervelocity into the atmosphere. Same energy… but now spread over the entire visible sky (if the debris cloud had spread that far). The result is a lower temperature, so heat is not radiated away as efficiently as in the single impactor case, but the total energy load is the same. Above a certain size, the result is a global* firestorm, with the entire sky horizon to horizon going very very rapidly to the radiative equivalent of an oven. Poof, there goes the biosphere on that side of the planet**. For similar reasons you probably would prefer the impact hits land, not water - a water impact is going to be extremely good at capturing the bulk of the energy (either thermally… look up "hyper canes"… or in the form of really largish tsunamis), while a land impact just really drastically overheats a very small area that can radiate the heat back to space more effectively than most alternatives. Yeah… don't send Bruce Willis. If it's big enough, deflect, don't break, and if it's on the borderline you'd almost certainly want it to hit intact anyway (and evacuate that continent… or that entire oceanic coastline). *OK, it wouldn't immediately be a global firestorm, just the facing hemisphere. Still, a pretty poor day to suntan. **And given the amount of dust and soot generated by this, the biosphere on the far side of the planet is unlikely to be happy either.

No; blowing it up should work just fine. A 1 [km] asteroid would have a mass of 8.4e+12 [kg] and a surface escape velocity of 1.1 [m/s]. If you "blow it up", plenty of stuff will be moving faster than that, and a lot of stuff slower than that. Assume you over-estimate, and accelerate every bit of gravel to 10x faster than needed to disassemble the asteroid - at 10 [m/s], with an asteroid approaching Earth at 15 [km/s], the pieces will travel 1,500 [m] forward for each 1 [m] sideways perpendicularly-blown bits will spread - the expanding cloud will have an opening angle of about 0.04°. I think a shotgun blast may have a wider spread than that. If the asteroid had been aimed "dead center" for the disk of the Earth, it would take more than two months for those debris to just start clearing the edge of the Earth.

OK, I've just got to say as a physics professor who teaches this stuff for a living, and is writing next weeks lectures on rotation motion, torques, and angular momentum now… This is fun. I should go get some popcorn. -- Brian Davis

Well, yeah. It makes sense. Do a detailed search of a small area with a known number of stars, and a known observation bias (you only see transiting planets, and some stars are poor targets due to stellar phenomenon), and generalize from their. Not too different than estimating the number of blades of grass in my yard by counting the blades of grass in a single square meter and generalizing. I don't think it's really that dubious. Peer-review sort of takes care of a lot of that. I think (still) that the "Rare Earth Hypothesis" was poor when it was proposed… and the more we learn, the poorer it seems to be. Upshot - there's a very very large number of terrestrial worlds located in the habitable zone of their stars (with a preponderance of them being tide-locked in all likelihood). That's… potentially a lot of real-estate. -- Brian Davis

Well, not exactly . There are going to be dang few 250 [km] objects coming in from anywhere. So, the premise itself isn't very realistic (and the discussion it generated is interesting). In the last 3 billion years, there's little evidence for impactors an order of magnitude less than this. The reality is this isn't a realistic situation. Actually, let's run the numbers. Assume the 250 [km] diameter object has a density of 3000 [kg/m^3] (high, but not bad). The mass would then be 2.45e+19 [kg]… the Moon is 7.34e+22 [kg] - about 3,000 times more massive. If the interstellar interloper was on a head-on collision course with the Moon, the impact velocity would be at best (highest) around 74 [km/sec] (42.2 for Vesc at 1 [AU] + 30 for Earth's orbital velocity + 1 for the Moon's orbital velocity relative to Earth). So, let's see, delta-v for the Moon post-impact you can get through momentum conservation: it would be around 24.7 [m/s]. So you would reduce the Moon's orbital velocity, at best, by about 2%. You're not de-orbiting it. You're barely changing it's orbit at all. If it was in a circular orbit 384,400 [km] in radius before, the new semi-major axis is 366,829 [km], and the new period would be just 93% of the old one… if it took 27.3 days to orbit before, it would take 25.4 after the impact. It wouldn't affect our orbit at all. Nor our seasons. It would throw up one heck of an ejecta cloud, and it wouldn't even remain in the Moon's orbit around the Earth (ejecta velocities would be more than sufficient to put it on Earth-crossing trajectories). But considering the tiny tiny size of the Earth relative to the volume the debris would be spread through, it might not be fatal (but it would make for a very interesting next few of million years as the ejecta is re-accreted on the Earth and Moon).

Actually, spray painting it is probably the best (or second best) solution. I've not read all of this (sorry), but thought I'd throw my two cents in - I'm teaching this stuff in my 100-level astro class on Wednesday, so… I've got some background. First, the damage is done by the kinetic energy deposited upon impact. That doesn't matter if it comes in as one chunk, or one billion - the energy deposited is the same. So "gravelizing" it doesn't help - even if you vaporized it and the gas was what was hitting the Earth, you haven't made the mass go away… so breaking it up doesn't help. Worse, a single impact superheats one point on the Earth's surface, but a lot of that energy is radiated back into space (because the impact zone is so hot, this outward radiation is efficient. But if you are hit be the same mass spread out, the impact temperatures are lower (same energy, larger area), so it's less efficient at radiating. Net result? Turning it to gravel makes the problem far worse, not better. Think "atmosphere on half the planet glowing horizon to horizon for several minutes to tens of minutes at the same temperature as the Sun". Bar-b-que anyone? Worse still, the asteroids we have good data on have in general low density; less than rock. So most of them (larger ones, anyway, 100's of meters to kilometers and up) are probably more like gravitational bound gravel piles. If you don't want them to fragment, you probably need to be very gently with your pushes. Surprisingly, a stand-off nuke can do this: the blast in space is essentially a very high-intensity radiation front (in the gamma region without an atmosphere to interact with). When that radiation hits the surface, it flash heats and spalls the surface on the side facing the detonation. Those spalling debris push uniformly on the asteroid as a whole, so it's like a giant catchers mitt. If you are careful (not too close, not too far), and the asteroid isn't too weak, this might work… barely. Note that using a nuke to deflect a smaller object (smaller asteroid) unto a collision course does get you more bang for the buck - you are using a small delta-v to direct an already large momentum onto a collision course, taking advantage of that pre-existing momentum. But… you are likely hitting a gravel pile with a bullet (or another gravel pile) - see hemispheric bar-b-que above. Second… and really as KSP players we should know this… if you want to miss a rendezvous, you don't thrust sideways, you thrust prograde or retrograde… either one… to change the semi-major axis, and the orbital period. All you have to do is delay the impactor by 4 minutes. That's it. At an orbital velocity of 30 [km/sec], Earth moves its own radius in about 4 minutes. So if the impactor is headed straight at us, all we have to do is slow it down slightly or speed it up (yep, adding to its kinetic energy is just as effective here). By a tiny bit. Let's say you change the orbital period as a near-Earth asteroid by 10 seconds (one part in 3.1 million… I don't think the maneuver nodes will let you do tuning this fine ). That means it will be perhaps 10 seconds "late" for it's Earth-orbit-crossing on the first orbit, but 20 seconds late on the next one, and 30 seconds late on the third… if you had a lead time of 30 years, that's more than sufficient. As to actual mechanisms… look 'em up. Asteroid deflection strategies have a long history in the technical literature… and yes, one of the better ways (as in low-mass, low tech, quick to implement) is the Yarkovsky effect. It's not direct "light pressure" on an asteroid (look it up on Wiki, it has to do with the rotation of the asteroid as well as the albedo and surface heating), but it is a significant effect for asteroids in certain size ranges. And it doesn't take a lot of "paint" - try chalk dust wrapped around a stick of dynamite. Approach trajectory on the surface, expand the dust curtain before impact so it coats a large spot on the surface, and let Yarkovsky do the driving. Ironically you probably don't know how the orbit will change… but if it's on an impact trajectory, you don't really care. Any change in the orbital parameters will result in an impact becoming a miss… in, out, faster, slower. So for a validated impactor, just make any change to the orbit possible… essentially, it can't get worse. A "gravity tug" (spaceship that hovers near an asteroid, very very slowly changing the asteroids orbit due to gravitational interactions) is another possibility… if you can keep a 'tug' working for years or decades. A solar sail (or, better yet, a mirror gravitational bound to the asteroid, but reflecting light onto the surface and strongly heating one point) is another one, but requires control over long periods of time. Nukes you need to detonate at the right standoff, and hope the asteroid is strong enough to take the "push". Mass drivers sitting on the surface are great, year-after-year throwing stuff away in a single direction… but are working mechanical nightmares that have to mine the surface ("Dear Jeb, welcome to your decade-long maintenance job… PS, if you fail everybody dies, but we have a great retirement package"). Chalk dust may not be as sexy as Bruce Willis… but it will get the job done Incidentally all this depends on size and lead time - the longer the lead time, the better off (more options) you are. If all you have is a single orbital period, we're probably in very very deep trouble. But if you find things a long time in advance, very simple solutions start becoming practical. For small objects (on the order of 10 meters to perhaps 100 meters) moving everyone out of the way on the ground below is probably the best bet (again, assuming you know they are coming). Above 100 meters into the 1-10 kilometer range, it's time to do some very serious mitigation. Above 10 kilometers, roughly speaking, start planning end of the world parties, maybe to watch the select 100-400 land on Mars (Phobos would be better, but probably not long-term). Above 10 kilometer… well, I'm not going to worry about it As for "moving asteroids is too hard, let's move the Earth out of the way"… um, what are you thinking? No. No no no no NO. Sorry for the tl;dr -- Brian Davis