# Avoiding Armageddon!

Let’s say we’re having a nice day here on Earth; the Sun is shining, the clouds are sparse, and everything is just looking like a peach:

And then Lucas goes and tells me,

Oh my God, Ethan! It’s Armageddon! An asteroid is coming straight for us! You’ve got to stop it!

Really? Me? Well, how would I do it? Let’s say we’ve got some reasonably good asteroid tracking going on, and we’ve got about 2 months before the asteroid is actually going to hit us. We’d like to do something with the situation on the left, to avoid the situation on the right:

Well, what we really have to do is change the asteroid’s trajectory by a little bit, so that instead of hitting us, it flies-by Earth without ever colliding with us.

This means making a teeny-tiny change in the velocity of the asteroid; we just have to give it a little push. How little? If we can get to the asteroid with a month left, just five miles per hour. If we can get there immediately, with two months left, only 2.5 mph. That’s a pretty small number! But how can we do it?

Asteroids are huge, massive things. The one that came close to us recently, 433 Eros, has a mass of 6 x 1015 kg, or about 100,000 times as massive as Mount Everest! Sure, we might only have to change its speed by a small amount, but how do you do it for something that massive???

Well, we’ve got to smack something into it with enough momentum to change its course by a large enough amount. That means we need something moving with enough speed and enough mass to smack into that asteroid hard enough to make it change course.

Well, we can build rockets with masses up to about 1,000 tonnes, and they typically have payloads of up to about 20 tonnes. (The payload is the part that can get launched into deep space.) We would have to launch that rocket at nearly the speed of light to change the course of that asteroid! NOT COOL!

If we really wanted to stop the asteroid, we’ve got two realistic, and very expensive options:

1. Make a rocket massive enough to do the job. How massive would this be? About 100,000 times as massive as the Atlas V rocket shown above. And realistically, that’s how much it would take. This assumes that we can make it fly at typical rocket speeds with perfect control over its trajectory. This will also require about 100,000 times more rocket fuel than we’ve ever used before.
2. Load a regular rocket with the largest multi-phase Thermonuclear Bomb ever made, just for this purpose. If we can take a 20-tonne payload and make the majority of it out of fusionable Hydrogen (which can be fused into Helium) that’s 10% efficient, we can get about 5 x 1017 Joules of energy out.

If we blow up the bomb on impact with the asteroid, half of this energy will go into changing its velocity. If we hit it at the right angle and in the right spot, we can change its velocity by up to about 20 miles-per-hour, or just enough to change its course and have it miss Earth.

And that’s my solution to this problem, with the second one being far more feasible and less expensive. Are we going to be serious about protecting the Earth from catastrophic asteroids? If we are, this is the way to do it. And just like all the greatest sci-fi movies ever, this is the scientist cautioning you:

If this device should fall into the wrong hands…

1. #1 brian
May 8, 2008

Ethan brings up two possible ways to deflect an asteroid: ram it, or explode it. These were prevalent solutions to the problem a few years ago, but now most space scientists tend to discount these ideas as impractical. We can’t make an object massive enough for the ramming idea to work, and even if we could it would be way too expensive. If we were able to make a nuclear bomb big enough to do the job, the problem could get much worse. It could blow up the asteroid into chunks, each of which would still be on a collision course with Earth. If anything, the damage would be worse because it would be spread over more area.

A third alternative that has been around a long time is to move the asteroid’s orbit more slowly (assuming we have more than 2 months). One way to do this is by attaching a rocket to the asteroid and having it burn in a direction to push the asteroid where you want it to go. Another idea is to somehow paint the asteroid to change its albedo so that solar radiation has a greater effect on it and could push it.

The most recent suggestion that has rapidly gained favor in the community is the so-called “gravity tractor” approach, which was first described in a 2005 Nature paper by astronauts Ed Lu and Stanley Love:

http://www.nature.com/nature/journal/v438/n7065/full/438177a.html

This approach involves positioning a small spacecraft in a strategic location near the asteroid and then slowly fire the spacecraft’s thrusters to move it in the direction you want to deflect the asteroid. The gravitational attraction between the asteroid and craft then pulls the asteroid along with the spacecraft. It’s an elegant approach that is much easier than all previous ideas. To quote the paper, “The mean change in velocity required to deflect an asteroid from an Earth impact trajectory is about 3.510-2/t m s-1, where t is the lead time in years. So a 20-tonne gravitational tractor hovering for one year can deflect a typical asteroid of about 200 m diameter given a lead time of roughly 20 years. Deflecting a larger asteroid would require a heavier spacecraft, more time spent hovering, or more lead time. However, in the special case in which an asteroid has a close Earth approach, followed by a later return and impact, the change in velocity needed to prevent the impact can be many orders of magnitude smaller if applied before the close approach5. For example, the asteroid 99942 Apophis (2004 MN4), a 320-m asteroid that will swing by the Earth at a distance of about 30,000 km in 2029, has a small probability (10-4) of returning to strike the Earth in 2035 or 2036. If it is indeed on a return impact trajectory, a deflection of only about 10-6 m s-1 a few years before the close approach in 2029 would prevent a later impact. In this case, a 1-tonne gravitational tractor with conventional chemical thrusters could accomplish this deflection mission as only 0.1 newtons of thrust would be required for a duration of about a month.”

2. #2 ethan
May 8, 2008

Brian, with the exceptions of ramming or nuclear weapons, none of these other options has a shot with a lead time on the order of months. And for most asteroids that would get put on a collision course with us, months is really the most realistic scenario. A nuclear explosion could fragment the asteroid, but it could be our only shot at survival, too.

Of course, given years of lead time, the options you suggest are much safer and saner. But do you have a better suggestion than the nuclear strike one on a 2-month timescale?

3. #3 brian
May 8, 2008

That’s why the first point you raised in this post is the most crucial. We must have highly accurate detection and tracking of near earth objects (NEOs). Although that program is underfunded, it has enjoyed tremendous success over the past decade. Assuming the program of getting orbits for potential NEOs continues and improves such that we find all big objects (including hard to detect high inclination objects like comets), I see no good reason why we would be surprised by an object on a collision course with only 2 months left. Yes, there are gotchas, like what if the object comes from near the Sun, but I’m confident we’ll be able to overcome some of those observational problems in the near future.

The gravity tractor paper points out with the Apophis example that we could deflect it with a 1-tonne craft with thrusters of only 0.1 N force to impart a delta-V of 10-6 m/s over a period of only 1 month. This is assuming we know the asteroid’s orbit reasonably well and can plan the deflection to occur at the optimal time during its first close approach.

There are still many in the military space community who think nuclear bombs are the best option, but from what I’ve seen on the civil side of things, almost no one thinks it is a reasonable response anymore. A large object hitting in one spot on Earth might be preferable to two or three large (but slightly smaller) ones striking different parts of the Earth. Add to that the fact that the rock might become radioactive due to the nuclear explosion, and you have just created a radioactive shower of rocks on the Earth. Not good.

4. #4 V. H. Hammontree
May 8, 2008

I think your idea is a good one. It reminds me of Kary Mullis idea on “Dancing Naked in the Mind Field.” Can’t go too far wrong with a Nobel Laureate.

5. #5 Lucas
May 8, 2008

If I alerted you to the impending doom do I get to hook up with Liv Tyler at the end?

6. #6 ethan
May 8, 2008

Only if you can convince Bruce Willis to kill himself in the nuclear explosion.

7. #7 ethan
May 8, 2008

Brian, the energy required to do the deflection you’re talking about is actually MORE than either the ramming or nuclear explosion strategy. You need a tremendous amount of time (like decades) to make it work. Since we believe that most “Earth-threatening” asteroids come from unpredictable gravitational encounters that change the trajectories of asteroids, we’re unlikely to have more than a couple of months notice. Maybe many months, but that’s about it.
And there’s no danger of making things radioactive with a fusion bomb; that’s purely a fission phenomenon.

May 8, 2008

Wow I love you guys!

9. #9 brian
May 9, 2008

Ethan, I’m not sure about the energy requirements of each option. I’d had to do some back-of-the-envelope calculations to see. I disagree with your statement that we’ll only have a couple of months notice of impending NEO impacts. Yes, orbits can change in ways we don’t predict, and that can cause us some surprises, but I think on the whole the timescales are years, not months, that we’d have to react. Again, this assumes we have a robust and effective detection and tracking system in place. The radioactivity comment is well taken; I guess I assumed a fission bomb for some reason.

10. #10 ethan
May 9, 2008

Brian,
The gravitational tractor option requires the same amount of total energy as putting a rocket on the asteroid and pushing it. It doesn’t matter whether you use the force of gravity or the force of a thruster; the energy is the same. The whole point is that if you change an object’s velocity instantaneously, through a collision or explosion, it has more time to move with that new velocity than if you accelerate it gradually. That’s why the ramming and explosion options save you energy!
Of course you’re right that if we know an asteroid is going to hit us 20 years from now, that’s much better than knowing it’s going to be 2 months from now. But if we knew that, we’d only need to change its speed by 0.025 miles-per-hour, or a much smaller nuclear explosion. A nuclear explosion would have the added bonus that first off, you could use a much smaller nuclear device. Second, if it didn’t work right away, you can send up another one and try again!

11. #11 brian
May 9, 2008

There have been whole international conferences and UN sessions devoted to this topic. I wish I were more keyed into their findings, as this is an issue of key importance to the future of humanity.

12. #12 Mike
May 9, 2008

How about an inflaton beam? (perhaps better yet, a giant solar sail)…

13. #13 Mike
May 9, 2008

My back of the envelope calculation says that if you’re allowed a 20 metric ton payload, and 10% of that is allowed to be sail, you get around 120km^2 of sail (assuming common aluminum foil, but it would probably be thinner and lighter material) at up to, maybe, 2kW/m^2… So if my algebra is right that’s a respectable tug of 0.24GJ/s or so.

14. #14 ethan
May 10, 2008

Mike, if your algebra is right, that means you need about a decade of advance notice with the solar sail deployed properly, docked on and attached to the asteroid. But managing a solar sail that large is unfathomable at this point; the largest one that NASA has even concept-designed is only about 0.25 km2, and no successful deployment of even a smaller solar sail has even been made to date. It’s a nice concept, though, and could be useful, again, if we had decades to plan for it.

15. #15 Fran
May 17, 2008

Maybe we could ‘park’ a bunch of iron rocks into elliptic orbits around the Earth and ready them to be slingshotted into incoming asteroids. Fight fire with fire!

16. #16 ethan
May 17, 2008

Fran, planned collisions are very dangerous because they run the risk of fragmenting the incoming asteroid. As difficult as it is to deflect one large incoming asteroid, it is enormously more difficult to deflect hundreds of smaller ones!

Of course, this is a danger with a nuclear explosion as well, although marginally less so, so long as the bomb is detonated a sufficient distance from the asteroid.

17. #17 John Hunt
July 20, 2008

It would be good to have some information about how fast progress is being made on finding all or nearly all asteroids over a particular size. I believe that we are close to detecting 90% of all of the civilization-destroying asteroids. As we continue we should be able to keep reducing the size of the unfound asteroids so that the risk keeps dropping. At some point we will get down to Tunguska size which would be bad if it hit a city but at least humanity would survive.

Along the way we’ll calculate a trajectory of one that will hit us. But it will likely be years away from doing so and so it will need a smaller nudge and we’ll have more time to develop solutions and even experiment. And then there’s also the strategy of giving people two months time to catch a bus to a thousand miles away from the calculated impact site.

I for one think that we are being concerned about a pretty small near-term risk. I think that there are greater risks than an asteroid such as the likelihood that someone within this century will develop self-replicating nano, bio, chem, or AI.

18. #18 ethan
July 25, 2008

John, I don’t think we’re that close. Anything over a kilometer in size has the potential to be devastating. We’ve got maybe 90% of the ones over 50 km, but we’ve got a long way to go. But yes, the risk is small, near-term. We’re looking on things that have about a 0.01% likelihood every millenium. Still, if you even had a 1 in 10,000 chance of crossing the street unsuccessfully, you might think twice. Your call.

19. #19 barry
October 8, 2008

would be be possible to capture an asteroid and keep it in orbit around the moon, to then drag off and hurl it at any other asteroids which are on a collision course?