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If we discovered a sizeable (Tunguska-like mass or larger) near-earth object tomorrow that was on a definite collision course with Earth in the near future, could we deal with it with our current levels of technology?

Theoretical asteroid avoidance or deflection protocols are interesting to analyze, but I'm curious about a situation where we get caught unawares, Chelyanbinsk-style, and suddenly discover a previously-unknown large object that's aimed right for us. What, if anything, could we do with the technology and space programs available today?

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    $\begingroup$ Chelyabinsk bolide isn't really the best example since it caught us completely unprepared. We were all busy observing an altogether different asteroid on a close proximity flyby that day (367943 Duende). So we obviously wouldn't be able to do anything about it, if even Russian air defense didn't have the time to scramble anything into the air. Beyond that, it depends on many parameters, from size, mass, impact velocity, impact angle, composition / compactness, its spin rate, and most importantly time before impact to do anything about it and determine its trajectory with any precision. $\endgroup$ – TildalWave Jan 28 '15 at 5:34
  • $\begingroup$ Practical asteroidal defense is entirely about early detection. Early detection allows time for action as well as effective action with minimal energy. Our biggest blind spot are those objects (like Chelyanbinsk) that approach with the sun at the backs (so to speak). $\endgroup$ – Erik Jan 28 '15 at 5:44
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    $\begingroup$ You might also find this Purdue University's Earth Impact Effects Program interesting. I can't vouch for its accuracy but playing with various parameters (example) and observing what impact a hypothetical meteor would have is kinda cool. $\endgroup$ – TildalWave Jan 28 '15 at 12:31
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At this point in time it seems doubtful that we could intercept and deflect an asteroid large enough to justify being deflected. This is simply a matter of momentum, a large asteroid has a great deal of momentum and the puny little spaceships we can presently intercept an asteroid with can impart only very little momentum. The only way we can substantially alter this formula is by using nuclear weapons.

The wikipedia article states:

If the object is very large but is still a loosely held together rubble pile, a solution is to detonate a series of nuclear explosive devices alongside the asteroid, far enough away as not to fracture the potentially loosely held together object. Providing this stand-off strategy was done far enough in advance, the force from any number of nuclear blasts would be enough to alter the object's trajectory to avoid an impact. By the 2020s NASA has concluded that 1 mission utilizing nuclear stand off, can deflect NEOs of 100–500-metre (330–1,640 ft) diameters two years before the estimated Earth impact, and larger NEOs with a five year warning.[38]

A NASA analysis of deflection alternatives, conducted in 2007, stated:[39]

Nuclear standoff explosions are assessed to be 10-100 times more effective than the non-nuclear alternatives analyzed in this study. Other techniques involving the surface or subsurface use of nuclear explosives may be more efficient, but they run an increased risk of fracturing the target NEO. They also carry higher development and operations risks.

Nuclear standoff (detonating a nuclear warhead near the asteroid) would seem to be feasible, but would require at least 2 years warning.

There is one way to deal with an incoming asteroid, not actually precluded by the question, that we definitely have the technology for - evacuate the area and let it hit us. These asteroids, while destructive, are not that destructive. The Tunguska event, for all it's boom, was only like a large nuclear bomb. Humans have detonated larger nuclear bombs on the surface of Earth, most famously the Castle Bravo nuclear test, and the Tzar Bomb. We can survive being hit by an asteroid. Most of Earth's surface is sparsely or unpopulated and an asteroid impact would carry little danger to humans.

Even much, much larger asteroid impacts leave no evidence in the fossil record of species extinction. For example the Manson Crater in USA:

Most researchers agree that the 35-km-diameter Manson Crater of Iowa, a possible source of shocked quartz in the K-T boundary sections, was too small and insignificant to have caused mass extinctions. source

The impactor in that case is thought to have been a 2km diameter stony meteorite, much larger than the Tunguska event impactor, and it is still small and insignificant when it comes to causing extinction. Now this is not to say that such a large impact would not be incredibly disruptive, but it would certainly be survivable, it is not like there is an imperative to avoid it.

This makes evacuation a legitimate strategy, the impact is only going to be extremely lethal at ground zero, or where it raises tsunamis if it lands in the ocean.

With forewarning - even only weeks or days - there would be time to relocate people who would be at ground zero. Those areas which would be at risk from secondary effects such as ejecta, tsunamis, earthquakes and volcanic eruption could also be evacuated, or at least emergency services could be on heightened alert.

Note that if the asteroid is broken up rather than deflected while still somewhat distant from Earth, it would cause much, much more damage. This is the MIRV warhead principle, a lot of smaller explosions spread over a large area causes much more damage than one large explosion. Thus intercept is not without risk and it might be safer to take the hit than to risk breaking up a 'rubble pile' asteroid with a nuclear explosion. Nevertheless, if the asteroid is aimed for a major city, a mission to deflect it might well be cheaper than the costs of dealing with a natural disaster of that magnitude. For example cleanup/reconstruction costs for large earthquakes which level a major city measure in 10's of billions, while space missions are often only in the 100's of millions or a few billion. In the event of a major impactor, it would come down to a cost analysis like this, how much would it cost to deflect it, versus the costs of dealing with the damage an impact would cause?

In sum, early detection may well be the best form of preparation we have, as it gives time for orderly evacuation of the danger zone, and the relevant emergency services can be ready to respond.

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  • $\begingroup$ Why 2 years? The Dnepr launches is only one example of ICBM's which already launch payload to orbit. And couldn't the military within weeks improvise a nuclear weapon payload on any launcher which is already prepared and scheduled for some other purpose? I'm not sure it would be effective, but I'm sure it would be attempted. $\endgroup$ – LocalFluff Jan 28 '15 at 10:18
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    $\begingroup$ There is a big difference between launching a warhead into space and launching it on a trajectory to deflect an incoming asteroid. Much of that two years will be travel time - you want to hit the asteroid probably at least a year before it would arrive at Earth as even a nuke wont deflect it by much, so you'd probably need to launch the warhead at least 2 years in advance. You may also need to wait for a launch window (Earth travels in it's orbit at 30km/s, so you'd want Earth to be travelling 'towards' the asteroid at time of launch). That's lots of time even if you have a rocket handy. $\endgroup$ – Blake Walsh Jan 28 '15 at 10:46
  • $\begingroup$ +1. I think also that Deep Impact mission revealed that deflecting a comet maybe more complex than anticipated. Some trajectory change was expected, but didn't occur. Could you talk about this too? $\endgroup$ – mins Jan 30 '15 at 7:39
  • $\begingroup$ I disagree with the evac plan. It is difficult to predict how a man made sat will aerobrake. But a natural astroid...small deviations would mean it could land minutes early or late. Given that it is likely traveling at km/s... That could potentially mean being hundreds of km off... $\endgroup$ – Aron Jan 30 '15 at 12:22
  • $\begingroup$ @Aron I think that depends. Man made sats have to come in shallow, because that requires the least delta-v from LEO, it would take a great deal of fuel to make a direct descent. It also minimizes gee-forces on humans. Natural meteors are under no such constraints, and could come in directly, passing through much less atmosphere, and with a much higher velocity (more momentum). So I think that at least some meteors would be quite predictable. Still, “Could we predict where a meteor would strike” would make an interesting question in it's own right. $\endgroup$ – Blake Walsh Jan 30 '15 at 13:11
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It is possible we could deal with an asteroid large enough to warrant action, but only if we had many years of warning.

We cannot destroy a large asteroid or comet, that's science fiction now, the best we can do is deflect it. The technology we have now is only capable of causing very small course changes to large objects, however a small change is still enough if it happens soon enough as even small changes will add up.

The key here would be very early detection which is the major problem. Technologically we can do much better than we are at detecting dangerous space objects, however IMO there is a lack of investment which means we don't use what we have to best advantage. The technology could also be substantially improved with investment.

There's also no deflection system ready to go, if we did discover an asteroid which poses a threat it would take months or years of precious time to design, build, and launch one.

What this all adds up to is that it is possible that we could detect and deflect an asteroid with today's technology, but unlikely it would actually happen. We'd have to get lucky. With investment in developing the technology and having something ready to do quickly we could substantially improve our chances and luck would have less to do with it.

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Your question doesn't limit it nearly well enough.

We almost certainly could intercept a Tunguska-sized rock on a notice of days. Such a rock is small enough that brute force at the last instant will do the job, fit a standoff detonator to an ICBM and launch it into the path of the rock--you need just enough standoff distance that the warhead goes off before it's crushed by the impact. The rock is blown apart, the fragments burn, the biggest threat is the EMP from the warhead. (Which might actually make this not a good idea.) Note that it merely needs to get in the path of the rock, it doesn't need to match velocities--hence the need of a standoff detonator as it's going to be smacked at several miles per second. Sure, the fragments will still come down pretty close to each other but remember Chelyabinsk didn't do anything beyond break glass--a bunch of Chelyabinsks in close proximity would likewise not be a big issue.

Blowing the rock apart is only an answer for small stuff where the fragments will burn without causing devastation. Blowing apart a bigger rock is worse than doing nothing at all--blowing a large rock into 8 equal pieces approximately doubles the damage it causes.

Also, there's a political factor here. What we could accomplish and what would be accomplished aren't necessarily the same thing. Politicians love to cut corners. Suppose we are going to nudge it aside with nukes. The blasters say we need 30 bombs on target. The rocketeers say there's an 80% chance a booster functions correctly and fires it's warhead at exactly the right spot. How many boosters will fly? I would be surprised if more than 40 rockets fly despite the fact that that gives nothing like certainty of deflection.

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  • $\begingroup$ rocketeers... that word deserves to be used much more often. So much sexier than 'rocket scientist'. $\endgroup$ – kim holder Jan 31 '15 at 19:48
  • $\begingroup$ ICBMs won't help here - they are ballistic missiles while you need something that can leave earth orbit. $\endgroup$ – Martin Schröder Feb 3 '15 at 14:32
  • $\begingroup$ @MartinSchröder Note that the low end of the question was a Tunguska-level event. Apogee of a ballistic missile is enough distance to do the job in this case. Speed doesn't matter other than for accuracy purposes--the fact that the warhead would fall back if it didn't take out the rock has no bearing on whether it can take out the rock. $\endgroup$ – Loren Pechtel Feb 4 '15 at 1:02

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