If a dinokoller asteroid was spotted on collision course for Earth with impact in the next ten years, it would be a race to divert it. I happen to disagree with many of my friends here that everyone would pull together to solve the problem. I happen to believe that all too many of the would be working their own agenda at the expense of us all. I also believe that many would oppose any effort to avert disaster with the belief that we would only make things worse, or possibly even that we planned on taking a near miss and turning it into a bulls eye strike on an enemy. Enemy can be anyone in the world if you go deep enough into the paranoia that tends to exist in many places.
In my opinion, what we need is a series of methods of mitigating the potential disaster of a real strike. This post is a thought experiment for using almost entirely ISRU resources with equipment that could be in space within the next decade without bankrupting the organization that funds it. By using assetts that are already in space, and under the control of people with real expertese in operating those resources, the possible political and hysteria roadblocks can be mitigated to a considerable degree.
The first set of assetts that absolutely must be developed is a survey of every reasonable threat to this world. Earth and space based telescopes and databases have to be developed in such a way that a comprehensive knowledge of every Earth crossing body of above a thousand tons or so is known and mapped for the foreseeable future. Second is to find and map orbits on the smaller bodies and bodies that intersect the potential dangerous Earth crossers. It would be cold comfort knowing that Humanitykiller One  was caused by two ‘safe’ rocks that collided in such a way as to send either or both of them our way. Only after a thorough catalogue is available can we say with reasonable certainty that no danger exists within some given time frame.
The second set of assetts is an affordable and robust transportation industry to at least LEO and hopefully cislunar space as well. Developing asteroids is dependant on the first and would benefit hugely by the second. A fledgling asteroid development industry would vastly improve the Dinokiller diversion effort I am going to suggest, and a mature one would make protection trivial.
My suggestion this time is that a fully mapped NEO inventory could use one asteroid to bump another off course. If a gigaton rock were going to hit Earth in a few years, then a diversion of a single meter per second would make it miss by a substantial amount. A meter per second is 3.6 km/hr, which is 86 km/day, which is over 30,000 km/year. While theoretically a few months might be enough to save the Earth, I would prefer a lot more margin.
Say this gigaton rock named Eight Ball is scheduled to land in the Earth pocket in October 2025. A second asteroid named Cue Ball orbits inside of Earths orbit and has a near miss with Eight Ball in January 2024. A mining operation is diverted to Cue Ball in 2022 to fully characterize its’ orbit, mass and composition. In the meantime a comprehensive survey expedition is sent to Eight Ball. Both teams follow transponder units that have been sent ahead to tag both bodies in order to nail their orbits down to within a few meters at any given time for the next decade. By the time both missions arrive, it has been firmly established that Eight Ball will strike Earth on October 11, 2025 at 9:35 PM Grenwich time with the center somewhere in Iowa. The daylight strike will obliterate the US instantly with the rest of the world to follow in short order.
Cue Ball is determined to mass a megaton itself and has a rubble pile composition. A large boulder or bag of pebbles is lifted off the surface of Cue Ball and suspended a short distance above the surface, The mining ship reels out enough tether to be well clear of both the bag of material and the asteroid itself. Using solar sails or ion engines or both, the mining ship uses the ISRU material as a gravity tractor to change the orbit of Cue Ball. With a constant track of orbits of Cue Ball and Eight Ball, the adjusted orbit is changed enough that Cue Ball will strike Eight Ball dead center on January 15 2024.
The mining ship stays to Shepperd Cue Ball all the way to impact. With a megaton of rubble impacting a gigaton of asteroid at 10 km/sec, the orbit of gigaton Eight Ball, or its’ resulting rubble, should change by an average of 10 m/sec. If everything went according to plan, Eight Ball, or its’ debris field, should miss Earth outside the orbit of the moon.Â
The mining ship for Cue Ball is now in the wrong orbit to follow the results, but the  survey expedition to Eight Ball can stay on site for a while after the impact to assess the result. If Eight Ball has been determined to now be in a safe orbit, the survey expedition does a comprehensive after strike analysis of the results. Any useful and readily available materials are collected for return to whichever facility seems appropriate for research or exploitation.
If Eight Ball has not been safed, then the survey expedition must go to diversion plan two. Diversion plan two is the one you suggest.
johnhare
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Projected energy weapon to target the threat, http://www.ia.ucsb.edu/pa/display.aspx?pkey=2943
Total vaporization shouldn’t be needed. Ablating a surface layer should provide some kick.
Spin it ’til it explodes? It can’t have that much gravity, can it? You wanted imagination, didn’t ya?
I expected somebody to ask how? Since that didn’t happen I’ll suggest something. You put an electric motor and solar panels on one of it’s poles (assuming it already has some kind of spin.) That motor feeds out a cable with a mass taken from the asteroid itself. It wouldn’t take a very big motor, although it might take a long cable. The mass spins one way, the asteroid the other until… kabloowee.
Ken,
I will let it perk awhile and then run a few numbers. If it works and is done that way I think it will be a byproduct of mining delivery techniques.
Peterh,
At some point in the future those energy methods will work on that scale. I don’t think that handling a gigaton of rock like that is going to happen in the near future.
I expect you would be letting out cable while bringing the spin up. Once you have enough rotation, you pull the cable in for the final ballerina.
Destruction by spinning depends on what material the meteor is made off. Sandstone will go quickly but iron will need a fast spin.
Why the desire to smack or hit or blow something up near an object before it strikes the earth. If you have a decade or two of warning then why not change the mass of the NEO slightly by removing a small part of the asteroid. If its a rubble pile you could attach rockets to selected bits of the rubble and move them farther away from the center of mass. If it’s a single, solid object you could drill out the center of the object.
Or you could add mass to the object. Over a period of years, you could attach tanks of water to any potential impactor until the mass has changed enough to move it off the collision course.
Jardinero1,
If the asteroid is in a normal solar orbit, I doubt you could change its mass enough for non-linear effects (like light pressure from the sun) to alter its course. And in normal Newtonian terms, changing the mass of the asteroid won’t alter its trajectory. (Unless you use them as propellant in an engine to drive the asteroid in the other direction.)
Jon Hare,
If you want a real challenging scenario, have a look at the trajectories of the first-timer comets that we’re getting over the next couple of years. From detection to perihelion inside a year, even with robotic scanners; coming in on trajectories that can’t be matched from Earth even if you launched on the day of discovery.
While steering a handy asteroid into the path of the incoming comet might be an option, it’s hard to imagine that we’d have enough time to use your scenario, even if we happened, by amazing coincidence, to have a mining operation at the nearest-to-perfect cue-ball already stocked with the equipment needed to “tow” it into an intercept path.
Paul451,
For the comets it’s much harder. The only reasonable answer I am aware of is the H-Bomb push for moving them in time. I’m thinking about it though since you brought it up. Any economical way of altering such a mass with so little warning should have some valuable commercial uses.
Andrew Swallow,
most are not solid metal, but if they are you don’t explode them, you simply cut the cable.
Ken,
The numbers don’t seem to work for me. Spinning up a gigaton asteroid with anything under a megaton of material doesn’t seem to make sense. Spinning a megaton of material at tether speeds of 3,000 m/s would require most of a megaton of tether alone and would only take the gigaton dinokiller to about 3 m/s.
What did I miss? How do you get a gigaton of mass spinning fast enough to self destruct with an equipment mass in the sub kiloton range? Are you thinking of slinging off reaction mass material as in the asteroid beanstalk post?
From a google…
A 100 kilometer diameter asteroid with a density of 3000 kilograms per cubic meter would have a surface gravity of 0.042 meters per second.
If you can get that baby anywhere near 3 m/s parts are gonna fly!
Earth has a surface acceleration of 9.81 meters per second, and is rotating at over 400 meters per second, but escape velocity is 11,200 meters per second.
What is escape velocity on that particular asteroid? What is the mass of tether to spin it up to escape velocity instead of surface acceleration? What is the cohesion of the non-rubble bits that tend to hold it all together?
You’re right. Escape Velocity =√2 R* g (R)
It could only work for smaller asteroids.
25143 Itokawa
Mass 3.5E10kg, Radius 200 to 300 m
Escape Velocity 0.2 m/s
I don’t know how to factor in cohesion although I expect lumps would fly off rather than sand or dust.
I doubt we could rotate this mass over 0.2 m/s but if we could, I would expect chunks to fly?
Ken,
While I am not convinced this would be a good method of destroying the asteroid, or even if destruction is the best option*, there does seem to be a way of doing some of what you suggest with far less tether mass than the initial pass. Use a sling launcher tether with a hub as close to the surface as possible. Send material out the tethers for random dispersal off the ends. The torque of spinning up the chunks translates into equal but opposite torque and therefore spin on the asteroid. This cuts way down on required tether mass and tip velocity. See comments on Moving the Asteroid from 6-Sep-09.
*Henry Vanderbilt made a good case against turning a rifle bullet impactor into a shotgun blast impaction group.
See, that’s why they call it brainstorming. I also agree that destruction may not be the best option.