The best way to prevent an asteroid from hitting the earth is not to blow it up but to deflect it using a solar powered gravity tractor.

https://en.wikipedia.org/wiki/Gravity_tractor

https://b612foundation.org

Or a solar mirror, from afar, to divert it using light. Gravity tractors arent easy. They need fuel and there are exhaust issues that limit performance (exhaust has to miss the rock). A bunch of big mirrors can impart energy/motion without having to stay in a paticular position.

Do you have a reference? Has someone actually done an analysis? I've never heard of this idea before, and offhand it seems impractical. I think you'd have all the same problems that you have with a tractor, plus a few extras.

You're right that gravity tractors aren't easy. Nothing is easy in space.

As a simple approximation, debris of an explosion would need to reach the escape velocity of that asteroids gravity field to permanentely be separated from the asteroid. To "blow up" an asteroid, one would need a sufficiently strong explosion to reach the escape velocity for at least a large part of the debris. The recombination of the asteroid would take quite a lot of time, considering the weak gravity. So if it is blown up at the right moment, recombination does not have enough time. Just reducing the density of the asteroid or rather its cloud of debris beyond a certain point might be sufficient to prevent a large disaster on earth. It then doesn't matter if the asteroid would recombine after crossing the earths path.

The idea of earth ghosting through a giant rock certainly does sound pretty cool.

A very naive question, but could we rotate an asteroid apart? Suppose we continuously hit it with a laser at an angle, so the resulting heat would cause a bunch of small explosions and impart angular momentum, until it rotated so quickly that it simply tore itself apart.

To be able to hit a specific part of the asteroid, you either have to wait until it is very close, or you would need very good aim.

Also, there’s beam divergence (https://en.wikipedia.org/wiki/Beam_divergence). That has a theoretical lower limit (http://www.aml.engineering.columbia.edu/ntm/level1/ch02/html...). Reading https://www.reddit.com/r/askscience/comments/1ihca9/how_much..., that limit is about 1:3000, so a laser beam is at least 100 km wide at the earth-moon distance.

I guess you would want to start rotating that asteroid way earlier, and that you would want to hit asteroids smaller than that. That makes it practically infeasible (Given infinitely precise aim, you theoretically still would be able to hit it only at one side, by aiming off-target, but that would wasting most of your beam energy)

Also, I am not convinced evaporating the surface by heating it from an angle would significantly impart angular momentum.

One of the asteroid redirection ideas is to have a probe with a laser evaporate part of the asteroid, the exhaust would then impart a force to nudge it off course. If caught earlier enough it's sufficient to prevent a collision with Earth.

Probably not - but it is easy enough to deflect an asteroid's trajectory to avoid earth. Over astronomical distances simply heating a portion of the asteroid can provide enough thrust to sufficiently avoid a collision.

I believe you need enough advance warning and it's actually quite _simple_ as far as these things go.

Always surprised how little science there is in this stuff, both detection and diversion it is a real existential threat to humanity given enough time and yet militaries around the world are out there tilting at windmills spending all their cash on outdated bloated acquisition projects to kill each other better in the most peaceful time on Earth.

The reason is probably same as the reason toddlers aren't afraid of guns.

Is that easy? To hit the right spot on a 1km target from a n x 10^9 km (presumably from space and lifting the necessary equipment, otherwise include correcting for atmospheric distortion?). Also, wouldn't power / beam dispersion be a big problem?

It should be significantly easier in space thanks to the lack of an atmosphere that would mess with your beam.

I tried to figure out just how accurately we are able to point stuff in space and found out the hubble telescope is able to "lock onto a target without deviating more than 7/1000th of an arcsecond, or about the width of a human hair seen at a distance of 1 mile." [1]

That's pretty amazing to me. Though I'm not sure if that would be enough, or whether we can be even more accurate.

With a laser beam it might also not be a problem if you're just slightly inaccurate, say only hitting your mark 70% of the time as long as you still manage to heat your target spot up sufficiently.

[1]: https://www.nasa.gov/mission_pages/hubble/story/index.html

I was more interested in the dispersion aspect of it. I'm not a specialist in this area, but I think a good dispersion rates for a high powered laser is in the area of 0.3 mrad regardless of the medium of transmission. And with that dispersion, targeting an object as far as Jupiter (600M km) would mean the beam would be 180,000km wide when it arrived.

Since people smarter than me have floated this as a way of deflecting asteroids, I assume there's some kind of way to focus it over those distances? It seems nontrivial though.

Hit it with a laser or whatever force far away and 1mm movement over there means that earth is saved and then some.

I actually took a class on this stuff in college. Basically, the forces involved are so tremendous that there's probably nothing we can do to prevent an asteroid impact. Also, odds are we wouldn't even detect it until maybe a few days before.

Maybe if we had 20 year's notice, we might be able to do something. But even then, probably not.

Wouldn’t transforming the asteroid from a solid rock to a ball of sand already be an improvement? 1) it looses density 2) it loses cohesion

It depends. The kinetic energy of the impact wouldn't be changed. The nature of the impact would be changed though. A pile of sand would probably transfer its kinetic energy into the atmosphere instead of impacting the ground. This could be good. But then, setting the atmosphere on fire might not be that great of an idea.

If the sand cloud has expanded enough before impact, then a large portion of it may miss earth entirely and therefore reduce the kinetic energy earth's atmosphere has to absorb.

As the post I was answering to just talked about converting the asterioid to sand, I assumed, that all of it would hit the atmosphere. But indeed, if in this way part of the asteroid misses the earth entirely, it would be a win. I was also a bit simplifiying/exaggerating. When talking about heating up the atmosphere, the consequences entirely depend on how high the thermal impact is going to be. If spread far enough, there could be a "global warming" which is below the limit for an instant disaster. (And when caused by a single event, would decay quickly)

Something else to potentially consider is the spread in the 'z axis', which would spread out the heating across time, not just space. Although I think that only helps if there is enough time for Earth to radiate a lot of that heat.

At large scales, asteroids beyond a few 100m, cohesion doesnt matter. It will travel through the atmosphere like a bullet through metal foil. It wont break up, instead punch a relatively clean hole.

> Using computer models, scientists simulated a 4,000-foot asteroid smashing into a 15.5-mile asteroid at 11,200 miles per hour.

The biggest known potentially hazardous asteroid is only 7 km in diameter. (https://en.m.wikipedia.org/wiki/(53319)_1999_JM8).

> The study has limitations. Both asteroids are modeled as simple, nonrotating chunks of rock, whereas real asteroids are far more variable. In addition, the larger asteroid, despite featuring a starting collection of cracks, did not have a history of multiple impacts as true asteroids would.

Sounds like this model was designed to generate sensationalized headlines like this one.

Models are not reality, you exclude some details from the model, can you expand on your criticism and explain why the missing details would have different impact on the result?

Based on this I am guessing they didn’t simulate the gravitational effect of any surrounding bodies. If the asteroid core is the only object giving off a gravitational force of course all of the fragments would reform around it. This simulation just feels very incomplete and provides no real actionable results.

My guess is that without rotation ... Why the model doesn't have rotation????? It doesn't sound too complicated to simulate.

My guess is that without rotation, the tidal forces of the other bodies that are far away are small enough to be ignored. In a perfectly even gravitational field, all the parts of the comment has the same acceleration from the other bodies and it can be canceled. (Like the "zero gravity" effect in the ISS.) So only the forces between the fragments of the asteroid are relevant.

Anyway, the Shoemaker–Levy 9 comet suffered some disruption due to gravitational tidal forces because it passed too close to Jupiter, and the fragments formed a line instead of a ball. https://en.wikipedia.org/wiki/Comet_Shoemaker–Levy_9

Seems like two explosions would be much better. One to break up the rock, then one to disperse the cloud as widely as possible. If you don't have the payload to impart escape velocity onto the debris field with the second bomb, offset both explosions such that the combined delta-v changes the path enough to reduce or eliminate the likelihood of impact. We probably don't want irradiated sand burning up in the atmosphere at scale...

Land a mass-driver on the asteroid. Escape velocity is small, perhaps a few meters per second. Scoop up bits of rock, throw them away. Throwing them in a consistent direction changes the orbit of the asteroid very slightly. With enough lead time, throwing enough mass away may reduce the size of the asteroid sufficiently or cause it to miss the earth.

Makes sense. I think this is a great way to slam Ceres into Mars and cut this whole terraforming thing to the chase.

Wow, who would have guessed it, Newton was right!

sounds like devops work.