The Sciences

Crowdsourcing Astronomy: DISCOVER Readers Fight Asteroid Threat

Out There iconOut ThereBy Corey S PowellOct 16, 2013 7:47 AM


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Four months ago, NASA issued what the agency—in all its acronym-loving glory—called an “RFI for the Asteroid Grand Challenge.” Translated into English, that means the agency was opening its doors to outside ideas about how to locate, study, and deflect potential Earth-threatening asteroids. (RFI stands for “request for information.”) More than 400 organizations and individuals responded.

NASA's planned mission to capture and tow a small asteroid for scientific study could be modified to deflect a potential threat. (Credit: NASA) On September 4, NASA announced that it had identified 96 submissions that merited further study. Because of the insane government shutdown there has been little progress in that direction, but that hasn’t prevented the general public from continuing to think through the challenge. Over the past few weeks, DISCOVER readers have sent in a number of provocative ideas, written in response to my September and October Out There columns about asteroid hazards. The readers’ solutions may not be practical, exactly, but even the most outlandish ideas contain hints of practical ways to control our destiny. There’s also an intriguing theme running through these suggestions: turning the threat around and using asteroids to our advantage, an astronomical version of natural pest control.[For related news and information, follow me on Twitter: @coreyspowell] It appears nobody has considered using one asteroid to deflect another. We could deflect a "small" asteroid such that it would collide with and deflect a bigger asteroid. This could be cascaded, as needed, to deflect even bigger asteroids for as many times as needed. --Dexter S This is one of those ideas that makes great sense until you begin to parse the realities of what it would take to do it, and what you would achieve when you were done. First, you have to find an appropriate target asteroid. You have to rendezvous with it and find a controlled way to change its path--all the same things you'd have to do to deflect an asteroid in the first place. Then there are extra things you have to do. You need significant additional lead time, because you are deflecting two different asteroids in sequence. And you have to deflect the first asteroid in a specific manner--not just away so it misses the Earth, but on an extremely precise path so it hits another asteroid. That makes the job exponentially harder. Next there is the matter of the impact on the second, larger asteroid--the one you are really worried about. The incoming asteroid is a dumb missile, hitting the big asteroid in an unpredictable way. It has some spin, it has an irregular shape, and it has internal structure. All the same are true for the second asteroid as well. There are so many variables that it is effectively impossible to predict what the debris field will look like after the impact. Will some of it still be headed toward Earth? Do you now have a whole family of mid-size asteroids incoming instead of a single big one? You see what I mean. This isn't like billiards where every ball stays intact and rebounds in a controllable way. Asteroids are sloppy, complicated objects. They may be rubble piles or they may be single, cohesive forms. Dealing with one is hard enough. Dealing with two, and the interaction between them, is basically a leap of faith.

One spacecraft makes a targeted crash into an asteroid while another watches, in the "Don Quijote" concept from the European Space Agency. (Credit: ESA/AOES Medialab) Fortunately, this kind of bootstrapping is probably not necessary anyway, because nature is stacked in our favor. Larger asteroids are easier to detect, and their orbits are more predictable than those of smaller asteroids. The bigger and more dangerous the asteroid, therefore, the longer your advance warning. And the earlier your advance warning, the less effort you need to deflect the object. For a really big asteroid—dinosaur-killer size—you might have decades, even centuries of time to prepare for a projected impact. That means you could use a relatively gentle, inexpensive means of deflecting the asteroid (such as hitching it to a solar sail or pulling it into a new orbit using the gravity of a massive spacecraft) that takes a long time to do the work. That way you have control, you can correct for errors, and you don't have to play a dangerous game of celestial billiards. Could nudging a cosmic body out of a threatening path invoke the old flapping butterfly wing scenario? Everything is space is tied to one another through gravity that rules over a very delicate dance of many partners. Would moving a cosmic body alter the dance motion of other bodies? --Dan T Indeed it would. Decades ago, scientists realized that the motions of many smaller objects in the solar system are chaotic, in the sense that their long-term locations are fundamentally impossible to predict [PDF link]. Small, random motions lead to secondary gravitational effects, and so on, so that the movements become increasingly uncertain as you project further and further into the future. The downside of that fuzziness is that there is a serious limit on how far ahead of time you can predict Earth-threatening asteroids. Fortunately, as I noted above, the uncertainty is greater for small objects than for small ones. The asteroids that are really confounding are the small ones--ones that might level a city or a neighborhood, say, comparable to the 1908 Tunguska explosion--which are much harder to spot and much harder to forecast. (A major, quite surprising reason why small asteroids are so unpredictable: Radiation pressure from sunlight. The sun’s rays heat the surface of the asteroid, which then emits thermal radiation that provides a slight push on the asteroid. The phenomenon, called the Yarkovsky effect, depends on the color, reflectivity, shape, composition, and rotational velocity of the asteroid. Not only are these attributes hard to measure for a small asteroid, but the intensity of the Yarkovsky effect is greater for smaller asteroids because they have more surface area relative to their volumes.) There is an upside to this celestial butterfly effect. If you give an asteroid a little nudge now (like the beating of a butterfly's wings), you can have a big effect on its location at a much later time. In particular, there are locations in space called gravitational keyholes, which you can think of as delicate balance points in an asteroid's path. If it passes just the right way through the keyhole, it will hit the Earth. If it does not, it misses. And if you can figure out where those keyholes are and get to the right asteroid at the right time, you can prevent a future collision with remarkably little effort.

A solar sail--basically a giant, extremely thin reflective kite--could create the gentle nudge needed to redirect a hazardous asteroid. (Credit: Bong Wie/Iowa State U) Instead of trying to blow an asteroid to smithereens just as it is barreling down on the planet--fantastically difficult--you can prevent yourself from ever reaching that sorry state of affairs by making uncertainty work in your favor. In essence, there are many paths an asteroid can take but only one that leads to an impact. Kick the asteroid a little at the right time--much easier--and chaos will send it on one of the other, safe paths. That is why early warning is so important. It is also why low-key deflection techniques like solar sails and gravity tractors could be sufficient to do the job. If you had a very wide catcher's mitt, could a comet's energy be harnessed to pull vessels by stealing its momentum? You'd need a net located in the path of a comet, attached to several very long bungee cords. The cord's length would be designed to give an acceleration less than bungee cord breaking point. As the bungee cords inevitably retract the attached vessels could be traveling at nearly twice the comet's velocity. -- Roger & Sandy I love this question. It’s utterly whimsical but has a serious heart. The short answer is: a definite no, but also a qualified yes. Don't you love the way science works? The no part: Stealing the momentum of a comet (or an asteroid—same argument) doesn't make sense because of the logistical problem of how you would do it. To catch a comet in a net, you'd need to get the net into space ahead of the comet, waiting. How do you do that? Well, you need to launch a rocket with the net as the payload. You have to navigate to the exact location where the comet will pass. You have to match your speed roughly to the speed of the comet--if you are too far off, your net will snap. You need to get the position and orientation of the net exactly right to intercept the comet. Then you probably have to cancel out all the rotational motion of the comet, too. By this point, you've done more work and spent more energy than if you just sent your rocket where you wanted to go in the first place. Moreover, you are now stuck going wherever the comet is going. Yes, you could try to steer the whole comet, but that requires far more energy than just steering your own (much smaller) rockets. I get the appeal of this idea. The comet is moving much more quickly than the Earth, so why not use some of that extra speed? But no conceivable net material would be strong enough to borrow a meaningful amount of the comet's momentum. Think about it this way. If you sent up a net that exactly matches Earth's orbit (so you are just getting it off the ground and expending as little rocket power as possible), its velocity relative to a comet like Comet ISON would be roughly 30 miles per second--about 100,000 mph. That is not merely fast enough to break the net. That is fast enough to vaporize the net, instantly, no matter what it is made of. That is the speed of an impact that blasts a giant crater in a planet. It would be like trying to throw a net over a nuclear bomb. But wait, there is also a yes part. The underlying idea of stealing momentum is a really good one, so good that space engineers use it all the time. It's called a gravitational slingshot or gravity assist, and it's a lot like the idea you describe, except that it uses gravity (not a net) to steal momentum, and it uses massive objects (not small comets) to steal from.

The Juno probe just flew past Earth, stealing some of our planet's momentum to speed its journey to Jupiter. (Credit: NASA/JPL) It works like this: If you send a spacecraft past a planet or other massive body in the right way, the planet's gravity slings the spacecraft off onto a new path at a higher velocity. In the process the planet loses an equivalent amount of momentum--but because it is so much more massive, the effect is unmeasurably small. This is the process the Voyager 1 and Voyager 2 spacecraft used to tour from planet to planet and then fly out of the solar system. NASA's Juno spacecraft just did a slingshot maneuver past Earth to pick up speed on its way to Jupiter. Not only does the gravitational slingshot require no bungee cords, it also doesn't force you to go whichever way the comet happens to be headed. Depending on how you steer a spacecraft toward a planet, you can control your aim, how much momentum you gain, or even lose a specific amount of momentum if that is your goal instead.

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