Apr 17 2009
Saving the Earth from an Asteroid
In Death From The Skies, Phil Plait discusses all the ways in which the universe might wipe out life on Earth. Perhaps the most obvious is by asteroid or comet impact. This one comes readily to mind probably because the current majority opinion is that the dinosaurs met their end at the hands of a large asteroid striking the earth near modern-day Mexico. This has also been the plot of two recent movies – Deep Impact and Armageddon, with nice visual effects of the impact.
Most readers are probably aware of the fact that the Earth is threatened by asteroids which cross our orbit. There are none right now on a known collision course with the Earth, but the threat remains. Apophis will come close in 2029 and 2036, but probably won’t hit. But eventually a large asteroid’s path will intersect the Earth again – it’s just a matter of time.
Therefore astronomers have been thinking about ways to protect the earth from such an inevitability. One feature of any scheme to protect the Earth is the tracking of all near-earth objects. We cannot protect ourselves from a threat if we don’t know it exists. Also, the more lead time we have, the easier it will be to deflect an incoming rock.
But what do we do if we detect an impending impact? There are numerous suggestions, and recently aerospace engineer, David French, has suggested a new one. He proposes attaching a heavy object by a long tether to the asteroid. This, he argues, will shift its center of gravity and thereby alter its trajectory. Given enough time, and a long enough tether (from 1,000 to 100,000 kilometers) even a large asteroid could be made to miss the Earth.
This sounds like a plausible method. I leave it to the astronomers to do all the tricky calculations, but I don’t see anything wrong with the basic concept. The engineering, however, seems non-trivial. It would be difficult to manufacture such a long tether (probably only doable with carbon nanotubes, which we cannot yet make indefinitely long). Also, many asteroids are tumbling and it seems to me this would make it difficult to attach the tether and keep it extended, rather than just wrapping around the asteroid as it spins. Tricky – but not impossible.
What other methods have been proposed? Probably the first one people think of, and the one used in the movies, is to just blow up the asteroid with nukes. This makes for good cinema, but is not very practical. It would take quit a bit of power to blow up a solid rock or hunk of metal. Further, the explosion is unlikely to pulverize the asteroid. Rather, it would more likely just break it up into many pieces, most of which would still hit the Earth, perhaps doing more total damage than the single impact.
Attaching a rocket to the asteroid and blasting it to a different trajectory makes sense, but would be technically difficult to impossible for most asteroids. As I said, asteroids tend to tumble, which would make the timeing of firing the rockets tricky and limit their total time for firing.
Perhaps the most intersting suggestion I have read is to use a gravitational tug – to have a rocket not attached to the asteroid, but next to it. The rocket would fire with just enough power to suspend it above the asteroid, counteracting its gravitational attraction. The rocket’s gravity would then provide a slow steady tug on the asteroid. If we have a few years warning, this technique could work even for large asteroids.
Yet another method is to paint the asteroid so that it reflect more sunlight, and therefore gets a little push from the sun which might nudge it far enough to miss the Earth.
Perhaps my favorite suggestion is to just mine the asteroid. Land one or more mining ships on the asteroid, break it up into ore, and cart it away to the Moon or somewhere else where the metals can be used to construct a station or something else. The asteroid will then just cease to exist, and we get the minerals to boot.
These are all technically challenging methods, and it may be years before we could pull any of them off. Efforts are already underway to track all near Earth objects, and this is good, because lead time will be critical no matter what method we use. If we had enough advance warning, I am confident that we have the technology currently to develop one of these methods to thwart the incoming asteroid.
One thing is for sure – any of these methods, based in science and technology, are more likely to work than having the world’s “psychics” use their telekinetic powers to push the asteroid away, or by having practitioners of “The Secret” wish it away.
19 Responses to “Saving the Earth from an Asteroid”
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There is an interesting (and entertaining) talk by Neil DeGrasse Tyson available at http://fora.tv/2008/02/19/Neil_DeGrasse_Tyson_Death_by_Black_Hole
Chapters 4 and onward are about Apophis and avoiding asteroids from hitting the earth.
Another one I’ve heard is attaching solar sails to the asteroid. Definitely one of the cooler methods in my book.
Here’s an entertaining ,if not frightening description of this by Neil deGrasse Tyson : http://digg.com/d1hold
That seems like an awfully big job! And it reminds me of something Steven Wright said:
“It’s a small world…
…but I wouldn’t want to paint it.”
You wouldn’t apply “paint” with a brush, you would vaporize a reflective material and in the vacuum of space the mean free path of the molecules is long enough that they would travel line-of-sight and coat what ever they hit.
You could supply heat to vaporize metal via a chemical reaction. A hundred kg of vaporized aluminum could cover 370 square meters in a layer 100 nM thick. Magnesium might be a better choice because it has a somewhat lower vapor pressure and MgO is a good reflector of visible light too.
A mixture of elemental Mg and MgO2 would react to form MgO and generate a lot of heat. Excess Mg would be vaporized and would deposit on what ever surface it hit. If you set off such a device 50 meters off the surface of an asteroid, half the reaction products would hit the surface and coat it. Half would be lost into space. Doing it with an explosive device like this also ensures that only a portion of the asteroid is covered. The device wouldn’t even need to land.
daedalus2u-” You wouldn’t apply “paint” with a brush, you would vaporize a reflective material and in the vacuum of space the mean free path of the molecules is long enough that they would travel line-of-sight and coat what ever they hit.”
Oh my God! Space paintball, how cool is that! But am I missing something here? If that asteroid is tumbling, wouldn’t that either cancel out the directional forces of the “push”, or at least make them unpredictable?
Maybe the brains at NASA have sufficient calculating skills to take that into account, but I hope they don’t mix up their English and metric systems again.
@tmac57: Actually, it might be better to paint both sides, since the direction of movement would always be away from the sun.
Seems to me you’d want to paint as much of it as possible, especially if it’s tumbling. Only the sunny side is going to recieve the push from the sun, so painting the entire thing would give you more total exposure to the paint as it tumbles.
Btw, do nukes produce shock waves in space? Probably not right?
“Daddy?”
Yes, honey.
“They were talking about jobs today in school.”
Were they? And?
“What’s your job, Daddy?”
Well, I’m an asteroid painter, honey…
The initial expansion of the fireball of a nuclear explosion is via radiation, even in air. Air is heated by the radiation, it gets so hot that it starts to emit black body radiation as x-rays which have a short path length in air before they are absorbed and heat that air hot enough to emit black body radiation as x-rays.
Nukes do not produce shocks in space, but what they do produce is a very high flux of gamma which is absorbed in the surface of the asteroid. That material is very rapidly heated. Depending on the energy flux, the temperature rise is a few degrees or a few million degrees.
If the outer few inches of an asteroid is vaporized to plasma, it will expand and exert a reaction force against the remaining unheated part of the asteroid. It will also generate a shock wave in the asteroid which may eject material by spallation (if the asteroid is solid), or by lesser effects if the asteroid is a loose rubble pile.
The problem with that approach is that now there are zillions of smaller asteroids on now different trajectories. Maybe they are not big enough to cause damage once they go through the atmosphere, but they could completely screw up any object in Earth orbit that they hit. That could make Earth orbit unusable for a long time (centuries or longer) due to debris in orbit.
I think that using nuclear explosives is simply too risky due to the potential for putting debris in places where it would do bad things. Keeping the asteroid intact and moving it is the much safer course of action. If you landed instrumentation on the asteroid to track its position, you would know with great precision exactly the course it is taking and how it is changing over time.
I think the greatest danger is trying to do something like this on the cheap without enough redundancy. If an asteroid is to be diverted, the solution has to be multiply redundant even in case of multiple failures of multiple systems multiple times. If the first 10 missions fail to prevent a collision that will kill a billion people, you don’t have the option of not doing missions until you are successful and until you know you are successful. You don’t get 99.9999999% reliability with a single approach until after you know it has been successful.
That may be a good next step for tracking the near earth orbiters. Just a small little thing that would bury itself in the asteroid and emit some type of ID.
“Yet another method is to paint the asteroid so that it reflect more sunlight, and therefore gets a little push from the sun which might nudge it far enough to miss the Earth.”
Wouldn’t increasing the reflectivity of an asteroid make it absorb less energy from the sun? It seems like this would be more likely to make it undershoot the earth. To get a push from the sun wouldn’t you want to paint it darker?
I believe you want the sun’s photons to bounce off (reflect), as that way you get twice their momentum as a reaction force, whereas if they hit and stick, you get one times their momentum. Think of throwing tennis balls at an oncoming object to slow it down.
Speaking of “The Secret”, wasn’t the peak of its popularity just before the current recession and financial crisis? I wonder if Oprah has considered that and will do an update retracting her recommendation of it? I can’t think of a more dramatic repudiation of its claim to bring people financial success. (I assume millions of people weren’t wishing for what we got.)
I think ya gotta approach this problem from the other end. Instead of thinking of ways to affect the asteroid, just move the Earth! I propose planting a series of land based rocket boosters pointing straight up along all lines of longitude at 5 degree intervals. As each line of longitude faces the direction of Earth’s movement around the Sun, fire all the rockets on that line. Then in sequence fire each line in turn. This will slow the Earth down, allowing the asteroid to pass harmlessly in front of us. Afterwords, fire the rockets on the opposite side, speeding the Earth back up to it’s original velocity.
OK engineers, there ya go.
Great brainstorming exercise. It’s interesting to see the creative ideas that people can come up with.
The best measure at present is to monitor an asteroid orbit. Many factors alter its trajectory through space. Adophis will advance towards earth near the end of my statistically predicted life expectancy. I have alot more socioeconomic concerns about the U.S. prior to proposed impact. If we had to act with regards to an asteroid, we would have to follow emergency evacuation procedures, assuming the public would even be informed prior to the actual event. We have the capacity now for nuclear pulse propulsion, but we most likely need a world treaty for joint cooperation. I like the concepts involving solar energy sails, but I will leave that to the stimulus package researchers involving energy technology.
I don’t get it.
Changing an object’s center of mass doesn’t change its orbit. The center of mass of the asteroid + ballast system will continue to move on the same path after the tethering as it had before the tethering. That’s just Physics 101.
The article was pretty light on details, so there may be more to the plan than just connecting masses.
Tethers work in principle, but as Novella pointed out, actually building them is trickier.
In fact, tethers *have* been tested, albeit much smaller, and attached to things in near-Earth orbit. However, their effect has more been from drag in the upper atmosphere than anything else, and the main focus has been on electricity generation from dragging past all of those free ions. The main drawback is that it decelerates the spacecraft.
Believe it or not, though, changing the center of gravity is significant, and this is because of the gravity gradient operating on the object. (WARNING: non-expert explanation! May contain errors! Get out your salt shaker and check with actual experts or at least Wikipedia before calculating how to deflect Apophis!)
Gravitational attraction doesn’t actually act on an entire body but on the particles that make it up. And since gravitational attraction is inversely proportional to the square of the distance between any two objects, it’s easy to see that the near side of an asteroid is experiencing a greater attraction towards the Sun than the far side.
For an object the size/distance from Sun of Apophis, this is negligible. For a larger object, it can be pretty significant. The most obvious example is our own Moon. Because of the gravity gradient, and the fact that neither Earth nor Moon are totally uniform bodies, there are tidal interactions as they rotate and as the Moon orbits the Earth (or, more properly, as they orbit the Earth-Moon barycenter). Over time, these interactions transfer momentum, accelerating the Moon’s orbit, slowing its rotation, and also slowing the Earth’s rotation. Today, the Moon orbits synchronously — that is, it rotates such that it always faces the same side towards the Earth. The system is not yet stable, because the Earth is so uneven and still rotating out of synch. Our day slowly gets longer and the Moon’s orbital period keeps lengthening. Theoretically, Earth and Moon will eventually be mutally synchronous.
This has already happened for the Pluto-Charon system. Pluto’s “day” is the same length as Charon’s “month”, and the same length as Charon’s “day”. Thus, each is fixed perpetually in the other’s sky, while the stars (and Sun and planets, and Pluto’s two smaller moons, Nix and Hydra) wheel endlessly behind. One can only imagine what sorts of mythology might develop on such a world!
Synchronous rotation turns out to be pretty common. Most of the large moons rotate synchronously, always pointing the same face towards their primaries.
The same basic physics could allow manipulation of an object’s orbit simply by dropping a tether. But it would have to be very long and very strong (else the tidal forces will simply snap it), and you’d need to do your calculations very carefully.
Note: I’m told that the gravity gradient even affects objects as small as the Space Shuttle, though it’s worth noting that it is extremely close to its primary (Earth), which greatly amplifies the effect.