Killer Asteroids.

Factual Questions
41

An object from another star could be moving faster. We haven’t seen any objects that were moving fast enough to definitely come from another star. That means they’re much less common than comets and asteroids in orbit around the sun (since we see plenty of those).

You can’t definitively say any object moving faster than solar escape velocity is from outside the solar system. It could be a solar system object that was slingshotted around another solar system object. The Voyager probes are human-made examples of this.

We think an object from another solar system would probably look like one from our own (but probably with a subtly different chemical composition). We could see it as well as we can see a regular solar system object. We might not see it until too late if it were headed right at us at high speed, but that could happen with a solar system object, too.

Asteroids in our solar system tend to be closer to the sun than comets. We assume this would be true of other solar systems as well, so any interstellar object is likely to be a comet. Comets from our solar system can come from any direction and at high speeds, though, so it might not be easy to tell where a comet originally came from.

42

The Yarkovsky effect is a curious phenomenon which messes up the orbits of small asteroids, making their exact courses difficult to predict; it is caused by sunlight and the rotation of the asteroid.

Attempting to use Bill Nye’s shiny wrap method of deflection would certainly affect the orbit of a small asteroid, but it seems likely to be an unpredictable process, and might even make things worse. If you redirect the course of an asteroid towards a city or populated territory you would be liable, even if it was an accident.

I am quite fond of the idea of a gravity tug - a spaceship that pulls the asteroid off course without touching it, simply by hovering nearby.

You can increase the mass of the gravity tractor by scooping up material from the asteroid itself, which should make the process more efficient.

I expect it would take a few decades to build and test a gravity tractor, so we probably won’t have one ready for Apophis in 2029.

43

Thanks Anne, for your answer. I didn’t think about slingshot approaches potentially boosting a body to greater than solar escape velocity. Isn’t the ‘window’ for such an approach awfully small though? I mean, I thought that Jupiter acted like a giant vacuum cleaner for comets and other bodies, either capturing them outright, or retarding their velocity enough to direct them into the Sun. What’s the likelihood of a body, orbiting the Sun, picking up enough velocity through slingshots to exit the Solar System? It seems like it’d be insanely rare, but I don’t know.

Anyway, here’s a relevant, recent Ars Technica article on researching ablating cosmic bodies through detonating nuclear devices near them. They don’t mention Stranger on A Train’s idea of detonating the device next to a block of low-Z material, and letting that vaporized block do the pushing, but I thought it interesting all the same. Seems like a great use for a directed radiation nuke. Short, interesting 1984 paper on feasibility of same.

44

A 1995 LANL paper (.pdf) on modelling the most effective means for utilizing nuclear devices to deflect asteroids.

If the paper is accurate, I think our means for deflecting a potential Earth-impacting body are a lot more robust than feared. You’d have to break a few treaties relating to nuclear explosions in outer space, and building the deflecting spacecraft is probably fraught with potential error, but it’s not like we don’t have heavy lift vehicles or spare thermonuclear weapons lying around.

45

Yeah, you’d probably want something that would give you a little more fine-grained control than that, if possible. Of course, your choices are going to be limited by how long you have to change the orbit of an incoming object. You could have anything from a couple of years to thousands of years.

The liability and international issues if a deflection method failed, or if you couldn’t deflect it from hitting the Earth but had some control over where it hit, might be interesting.

You probably wouldn’t want to crash it into the ocean if you can help it. That would be likely to generate a megatsunami that would affect everybody living on the shores of the ocean that it hit. There are estimates that the K-T impactor (a 10 km object) created megatsunamis 5 km in height. If Apophis would also generate one that was half the height of the impactor, that would be a tsunami on the order of 100 m high. Earthquake tsunamis, by contrast, don’t get much higher than 10 m (they can rise up much higher when they approach land, but that depends so much on local conditions it’s hard to predict). It’s hard to predict exactly what a 100 m tsunami would do, but I’ll guess it would not be good. Unfortunately, with Earth’s surface 70% covered by oceans, this is the most likely scenario.

I looked at what would happen if Apophis crashed into the earth from this simulation program (created by someone I know, and a lot of fun). If this can be believed, it would cause the equivalent of a 7.8 earthquake when it hit. That might not be so bad if it hit an unpopulated area. Though it might get interesting from an international diplomacy standpoint if the best choice were to steer it into Siberia. Vladimir Putin would probably have something to say about that, if he were in power when it happened.

A complication is that, in general, if you deflect an asteroid from hitting the Earth, you most likely haven’t gotten rid of the threat forever. It’s probably still in a similar orbit that will bring it close to Earth periodically. You will still have a potential threat every time it comes near the Earth. You could have to go through the same process again a few years later. If you can make it hit the Moon or something else, you could get rid of the threat, but that’s much harder than just deflecting it away from Earth. Throwing it into the Sun or out of the solar system (a la the Futurama episode A Big Piece of Garbage) is likely to require a lot more energy than would be feasible. You’d need to slow it down by about 30 km/s to crash it into the Sun. To escape the solar system, you’d have to speed it up by about 13 km/s. Neither of those would be easy.

A gravity tractor might be a good way to deal with a tumbling asteroid, since you don’t have to land anything on the asteroid or worry about the asteroid’s rotation making it point in the wrong direction.

Actually, it’s not that unlikely for an object to get ejected from orbit. Jupiter may have ejected a giant planet from the solar system early in the solar system’s history. I have heard, but can’t find a cite right now, that, in a system where you have three bodies interacting gravitationally and one of them is much smaller than the other two (this would be the case for an asteroid orbiting the Sun and interacting with, say, Jupiter), the most likely scenario is that the smallest object eventually gets ejected. Comets and asteroids obviously have collided with Jupiter in the past, a famous recent example being Comet Shoemaker-Levy 9’s collision with Jupiter in 1994.

What is much harder is to use a gravitational slingshot to send an object exactly where you want it to go. That’s what we were doing with the Voyager probes. We didn’t just want to slingshot them out of the solar system, we wanted them to make close approaches to other planets on the way.

Well, that depends on how much warning you have, and how big the incoming object is.

I’d like to think that there wouldn’t be much trouble with setting aside treaties about nuclear weapons in space in the face of a threat from an asteroid or comet. But things like the government shutdown make me a bit less optimistic on this…

46

I have sat in on presentations from astronauts and an astronomer who work at NASA on this very topic. He’s a buddy of mine, who was the guy talking to the White House about Chelyabinsk. Unfortunately, I don’t recall details of numbers and sizes. However, it is a topic of interest.

The gravity tractor approach has the problem that first you have to build a fairly large vehicle (like multiples of the ISS in size/mass), then you have to move that vehicle into the path of the asteroid in question somewhere out there in space way away from Earth, and then you have to have time for the gravity tractor to work. The larger the object, the longer it takes and the more mass you need in your tractor. Using some of the asteroid itself is an idea to increase the mass “on site”, but that has complications of its own. Suffice it to say we wouldn’t be able to do this anytime soon.

The stand off nuke approach actually is the best option for most cases. It doesn’t rely on the object having structural integrity so you can push against one side of it and the force will transfer through. Many of these objects are “rubble piles”, aggregates of rock. The gravitational force holding them together is tiny. Astronaut Stanley Love built a table top demonstrator - he suspended rocks from string in a framework so they hang next to each other. Push on a rock, and the others just slide out of the way. And he says that is something like 10 times stronger than the gravity in question.

The stand off nuke also works for objects of a larger size than anything we could reasonably try to affect with a gravity tractor, and the time scale to cause the effect goes down. Of course, for best effect, you want a series of stand off nuke blasts.

There are basically two options for changing the path. You don’t want to try nudging sideways. That takes far more energy, and on rotating bodies is going to have even more complications. Rather, you want to either slow it down or speed it up. That means either blasting in front of the path or behind the path. The idea is to put the asteroid through the Earth orbit window in a different time than when Earth is hitting that spot.

Then you get the tricky politics of do you speed up or slow down, what does that do to the impact point, does it move it into another country? The sensible answer is to move it the least to the most desolate area, but nobody is going to want you to move the asteroid impact into their country in case the result is not completed or off target. They will want it moved away from them. If everyone wants it moved away from them, there’s nowhere to move it.

Right now, the emphasis is on (a) identifying as many NEOs as possible down to some “reasonable” size, and (b) characterizing the asteroids, their structures, and what happens when you try to do things with them.

(a) is complicated by the very few number of people actually working on it. It was described as currently there are about as many professional astronomers working on this as the staff of a moderate-sized McDonalds. That may have doubled to 2 McDonalds by now. It’s also complicated by defining “reasonable size”. The smaller they are, the harder to find. But even 20 m objects are dangerous in the sense they can cause damage and loss of life - witness Chelyabinsk. Right now, I think “reasonable size” has been stated at 50 m.

(b) is where robotic missions to comets and asteroids like Hayabusa come in. It’s also where the proposed trip to an asteroid would fit. The modification to use a robot to drag a small asteroid to Earth orbit is less useful from that standpoint - the method of trying to grab it with a bag will do messy things to the structure. And it’s not particularly useful for testing methods for moving larger asteroids.

I would estimate that even if we had no political or financial hurdles, an we had an existing launch vehicle and space delivery vehicle to get the warheads to wherever, it would take a minimum of 2 years to be able to do something useful against an asteroid that would be a significant climate change event. YMM definitely vary.