How could a specific asteroid be diverted to impact the earth?

Question

Imagine someone wanted to divert a specific asteroid to impact the earth. The Current Impact Risks page at nasa.gov shows a list of known impact risks based on observations. Most of these asteroids are very small; this person would not be interested in them.

Some of these, however, look promising for widespread destruction if they were to impact the earth. Fortunately for us, the probability is very, very small that they would actually do so.

Take a sample one: 2011 SR52. According to the linked page^{(page no longer exists)}, this object has a rough mass of 2.4e+13 kg and a rough diameter somewhere around 2.6 km. The page estimates the impact probability is about 2.5e-10 that it will impact the earth on 2034-03-30. And with future observations, the probability is certainly going to drop to zero.

Let's assume someone wanted to divert this particular asteroid onto a path that will guarantee a collision.

What would it take to move this object's orbital path (2011 SR52) to collide with the earth?

The page estimates the magnitude of the impact at 8.6e+05 MT. Assume it hits the Pacific Ocean. Could humanity survive this impact?

Answer 1

Here's a quick guide to changing the orbit of an asteroid:

1. Figure out where it is. This is the crucial bit. You need to know its position, mass, velocity, orbit, size, density, and composition, and perhaps one or two other parameters. These are crucial, especially the first three. You'll need all this data to model its orbit and what will happen when you deal with it. Now, you might not have time to launch a probe to it (or divert another probe near it), so you'll have to make some educated guesses. If you can get good images of it, you should be able to figure out its size, position, and velocity.

2. Predict its future. You'll need a lot more than a crystal ball for this. Create a computer model (multiple ones for different scenarios, if possible) that predicts its orbit, both past and future. Create a model depicting the solar system, and put in the asteroid. Then you can figure out its closest approach to Earth in the near future. Take some measurements from this model, especially concerning its position and velocity at a certain time.

3. Start building. Unless you're a Jedi, you won't be able to move this thing with just the power of your mind. You'll have to send something up there to do something about the asteroid. Either retrofit an existing rocket or build one from the ground up (though again, you may not have time to start from scratch) and make sure it is capable of delivering objects beyond Earth orbit. Also, create a payload. This is going to be the thing that will interact with the asteroid.

4. Launch. At the proper time, launch the rocket and its payload. Make sure the payload isn't injected into Earth orbit; if this happens, it will be useless. The craft must escape from Earth's gravity if it wants to do anything. This is the only way to get it near the asteroid.

5. Interact with the asteroid. This is a bit of a broad title, but there are few different approaches you could take. I would ram the asteroid with the craft to transfer momentum and change the asteroid's orbit. This would have to be done with extreme precision, though. Ram it at just the wrong velocity and it could miss Earth by thousands of miles.

6. Continue taking data. I almost forgot - make sure you have a backup craft (and rocket). Lots of things could go wrong along the way, and if you're out to destroy civilization, you need a contingency plan. Even if you manage to hit the asteroid, you could have hit it too hard or too softly. Be prepared to launch again to correct the orbit.

   Anyway, keep collecting data from the asteroid. You'll get better information as it nears Earth, which could help you if you need a second launch. If you have a lander on the asteroid, that could be a great source. Alternatively, have part of the spacecraft continue to do a flyby with the asteroid while the other half impacts.

7. Wait. There isn't a lot you can do from here. Just sit back, relax, and get your affairs in order. If something has gone wrong, though, make sure you have redundancy and go back to step 4. Or step 3, if you have the time and completely forgot.

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I don't know how much force it would take to move the asteroid in question. The NASA page says it has a mass of $2.4 \times 10^{13} \text { kg}$, which is quite sizable. The craft would have to be either very massive (a downside because you have to get it off Earth) or be going very fast to make a difference. The choice is yours. It also depends on just how much the asteroid's orbit must be changed in order for it to hit Earth.

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As you asked,

The page estimates the magnitude of the impact at 8.6e+05 MT. Assume it hits the Pacific Ocean. Could humanity survive this impact?

I think we'd have some trouble. The largest nuclear bomb ever detonated, the Soviet Tsar Bomba, had an energy output of an astonishing $50 \text { megatons}$. This asteroid's impact (as NASA's estimates have it) would produce many times that amount - even if it didn't directly impact Earth.. Now, if we changed the orbit of the asteroid just enough so that there would be a direct hit, a lot more energy could be released. But it would have to be timed just right. Precision here is everything.

What would be the effects of this? Let me name just a few:

• Tidal waves. If you chuck a large rock into the middle of a pond, it makes quite the splash, right? Okay, now make that rock much bigger, and going at a much faster speed. I'd think you're going to have some issues. If it hits in the Pacific Ocean, many island nations could have severe flooding, and some minor atolls could sustain severe damage. The damage to continents would depend on just where the asteroid hit.
• Impact winter. We're going to have some chilly weather for a while. Clouds of gas and dust could be stirred up, although I've only heard about impact winters when asteroids (or other rocky objects) hit land. Hitting the ocean could reduce this issue. Still, I'd prepare for a long winter. You could get some serious sledding in.

I would think that's it. The water is going to absorb a lot of the energy of the asteroid. Also, it could break up upon re-entry, creating a lot of smaller rocks (yikes!). This would further dissipate the energy, although a much bigger are could be effected, and the resulting damage could be substantial.

Answer 2

Rockets. And math.

The way you make an asteroid meet up with (read: smash into) another body is exactly the same way you make a spacecraft meet up with (read: dock with) another craft: You adjust its trajectory.

Course corrections take exponentially less energy the further out you make them: 1m/s ΔV at time T is worth far more than 1m/s ΔV at time 2T. This page doesn't appear to list the orbital parameters of the asteroid, but it's likely far away (especially given that it won't approach a possible-though-unlikely impact for another 20 years) and thus easy to change its path to hit us. How easy? Well, to know that, you need to know its mass (listed on that page), its current orbit, and where (relative to Earth) it will cross Earth's orbit; after that, it's a simple matter of calculating the transfer burn to get an intercept. Given that it's still so far out and yet (on an astronomical scale) going to pass quite close, even though it's quite massive the amount ΔV needed is almost certain to be very small.

Most likely, your antagonists would make the initial burn as far out as possible (the asteroid's apoapsis). This would most likely consist simply of attaching a rocket to the asteroid and executing a controlled burn, although a shaped charged (i.e. explosion) might do it as well (just be careful you don't blow apart your asteroid in the process). Using a separate spacecraft to tow it into its new orbit would also work. Again, the key is simply the ΔV, how exactly you get it isn't that critical. After that, they'd monitor its progress towards Earth, and make correction burns as necessary (again, ideally doing them as far away as possible).

The downside (for them) to this is that we here on Earth would see its approach from a long ways off. Fortunately (again, for them), it's not likely we'd be able to do a whole lot about it, really.

Would we survive? Yes, probably -- but not all of us, not by a long shot. The page estimates the impact energy of this sucker at 850,000 MT of TNT. That's going to hurt, no doubt. But it's several orders of magnitude smaller than the estimates of the Chixulub impact that is supposed to have wiped out the dinosaurs; mammals far less intelligent than we are today survived that, so I think it's a given that we'd survive a smaller one. You'd still probably have a pretty severe Impact Winter to contend with, but less than what wiped out the dinosaurs.

Answer 3

The answer to this is that we have the technology right now to modify this object's orbit to guarantee its collision with earth.

What it would require is a major, probably unconcealable, launch of one or more multi-tonne unmanned spacecraft which would rendezvous with this object and attach itself (or more likely several sub-packages) to the object. The sub-packages would then fire their own engines to subtly alter the orbit of this object over a period of days or weeks to ensure its capture by earth's gravity well and its inevitable collision with earth. With continued operation, it would be possible to hit pretty much any desired target on Earth. It might even be possible to cause a glancing impact where the object would enter the atmosphere, scrape a city off the map, then bounce back out into space, doing far less than the maximum possible amount of damage.

This would take a lot of planning, money, labour and computer time to achieve, and would not be a one-man job, though it is possible that a handful of people (educated lunatics?) with enough money and the right skills could achieve this.

While the launch(es) necessary to divert the object would be unconcealable, since undoubtedly the US and Russia amongst others are still looking out for potential ICBM launches, there is nothing preventing a commercially funded overt launch from being a cover for the launch of this project. The commercial launch could then be declared a failure if necessary. The modification of the body's orbit is fairly likely to go unnoticed long enough for an operation to readjust its path to be unfeasible in the remaining time after discovery of its new path and destination.

Yes, humanity would probably survive an 860 GT impact, but not much of it at all. This would probably be an extinction-level event, causing major ecological damage similar to the event which finished off the dinosaurs 65 million years ago. A direct impact in an ocean would cause giant tsunamis (as opposed to the normal kind of tsunami), and throw up uncountable tons of dust and rock that would fall to earth as meteorite impacts right around the globe, raising the air temperature significantly for days, before an impact winter set in.

An impact in the pacific would wipe out the American western coast cities, as well as Japan, SE Asia and Australia's east coast from the tsunami effects alone, which would likely be a hundred meters or so high when it hit land. Collateral projectiles re-entering the atmosphere would cause global damage. It would actually be better for Earth if such an impact occurred on land.

Answer 4

For diverting the asteroid: if you have sufficient time and the asteroid is not huge kinetic impactor or ion beam shepherd, depending on the type of asteroid (e.g. material porosity) and its orbit. For very large asteroid (> 400 - 500 m in diameter) or last-minute impactors a nuclear bomb is the only available option.