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NEO Deflection: All the Myriad Ways

The asteroid Apophis is extremely unlikely to hit the Earth any time soon, but we do know that it’s slated to make two close passes, closing to a distance of 36,000 kilometers or so in 2029 and again in 2036. These events should give us pause — this is an object some 335 meters in diameter weighing an estimated 25 million tons. It’s 90 stories tall, if you like to think in skyscraper terms, which is what Greg Matloff probably likes to do, given that the physicist and asteroid deflection expert works at New York City College of Technology (City Tech).

Of Apophis, Matloff says, “We don’t always know this far ahead of time that they’re coming, but an Apophis impact is very unlikely.” A good thing, too, for a strike by an object of this size would be catastrophic. This City Tech news release offers a look at Matloff’s ideas on what to do if we find a Near-Earth Object on a collision course. He’s a proponent of diverting rather than destroying such objects because of the potential for debris striking the Earth after an explosion.

Asteroid Deflection the Slow Way

Wouldn’t we need a huge nuclear explosion to divert an asteroid’s trajectory in the first place? Not necessarily. Matloff worked with a team from Marshall Space Flight Center (Huntsville, AL) in 2007 to study methods for deflecting NEOs, finding that heating the surface could alter an object’s trajectory. That heating project is another potential use for the solar sail technologies Matloff has been investigating for the past thirty years, going back to the days of a seminal paper in interstellar studies (written with Eugene Mallove) called “Solar Sail Starships: Clipper Ships of the Galaxy,” which appeared back in 1981 in the Journal of the British Interplanetary Society.

But when it comes to asteroids, solar sails play a different role than leading humanity’s push to the stars. The idea is to configure twin solar sails to act as concentrators of sunlight. Imagine a highly reflective sail that faces the Sun, focusing solar photons on a smaller thruster sail. Both sails would be stationed alongside a Near-Earth Object, with the thruster sail focusing sunlight on its surface. In his book Paradise Regained: The Regreening of Earth (with Les Johnson and C. Bangs), Matloff notes the result:

[The] thruster directs a concentrated sunbeam on the NEO’s surface. If the NEO is coated with layers of dust, soil, or ice, a jet of superheated material (like a comet’s tail) may be raised in the direction of the thruster sail. The reaction force to this jet pushed the NEO in the opposite direction.

The potential is to create a jet stream of sufficient strength that, over time, it would nudge the NEO into a different trajectory. Creating a steerable jet involves penetrating the object’s surface with photons, but by just the right amount to create the deflection. According to Matloff, it could be as little as a tenth of a millimeter.

Probing an Asteroidal Surface

Here the need for missions to one or more NEOs again comes into focus, but while we wait for the development of the necessary tools and funding, Matloff and colleagues at City Tech are working with red and green lasers to study how deeply they penetrate asteroidal rock, using meteorite samples from the Allende meteorite that fell in Mexico in 1969. Their first results were presented at the recent gathering of the Meteoritical Society, which met in New York last July. “To my knowledge,” says Matloff, “this is the first experimental measurement of the optical transmission of asteroid samples.”

And given the significance of the work, we can assume it won’t be the last. We won’t know whether creating a jet stream by long and slow application of light reflected off a solar sail will work on an actual object without analyzing a wide variety of Near-Earth Objects. And that raises the question of how to proceed. The City Tech story quotes Matloff on the matter:

“At present, a debate is underway between American and Russian space agencies regarding Apophis. The Russians believe that we should schedule a mission to this object probably before the first bypass because Earth-produced gravitational effects during that initial pass could conceivably alter the trajectory and properties of the object. On the other hand, Americans generally believe that while an Apophis impact is very unlikely on either pass, we should conduct experiments on an asteroid that runs no risk of ever threatening our home planet.”

In any case, further City Tech work by physicist Lufeng Leng has shown through scans of the Allende sample that the composition of the surface material through which the light passes governs the depth of the light’s reach. The results show that lasers from a space vehicle placed near an NEO can help us understand its composition, allowing subsequent sail missions to focus solar photons with the precision needed to create the trajectory-bending jet stream. It’s an ingenious use for solar sails, but we’d better be sure we understand the objects we’re heating well enough to ensure a successful result.

Consider: We have much to learn about the mechanics of keeping a twin solar sail mission deployed on station near an NEO for long periods of time. Moreover, a deflection option like this one (or a similar idea Matloff discusses, in which astronauts land on an asteroid and set up a highly reflective thin film sail on the surface to exert a small but constant force on the NEO), are optimised only for certain kinds of NEOs. Would they work on a ‘rubble pile’ asteroid that’s barely held together by its own gravity? Other options, like the so-called ‘gravity tractor,’ seem more useful in that context, a reminder that NEO deflection may have many potential solutions.

Related: Ray Villard on asteroid deflection. The next Sputnik moment?

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Comments on this entry are closed.

  • Ron S February 1, 2011, 12:01

    This comment from Matloff intrigued me: “…On the other hand, Americans generally believe that while an Apophis impact is very unlikely on either pass, we should conduct experiments on an asteroid that runs no risk of ever threatening our home planet.”

    Why would it matter? If its trajectory is disturbed by any experimentation, all that means is that it would still miss the Earth, though it will do so on a different path. It’s not as if Apophis is a malevolent spirit that will somehow take direct aim at the Earth if we interfere with it. That is, a random perturbation changes the trajectory but does not shift the impact probability in an unsatisfactory direction.

    If Apophis is a convenient target we should at least go and check it out.

  • The Cosmist February 1, 2011, 16:21

    On the subject of asteroid deflection, I’ve been trying to think of some way their kinetic energy could be harnessed. I have a vague idea that deflecting asteroids into some kind of orbiting generators could be a way to supply energy to future colonies in the outer solar system. This idea is probably unrealistic, but can someone who is a better physicist than me explain why?

  • Eniac February 1, 2011, 23:46

    For sheer energy and speed, the nuclear approach is hard to beat. The partitioning of the asteroid into “payload” and reaction mass can be fine-tuned by detonating a variable distance beneath the surface. The projectile could be an off-the-shelf bunker-buster type nuclear warhead, delivered by a standard space launcher. You could probably put together a crash program in weeks, and provide a large enough push for less than a year’s warning. Best to launch several, though, to be safe…

  • Doug M. February 2, 2011, 5:45

    Okay, let’s try some math.

    Apophis masses ~25 million tons. To deflect it, we’d need some delta-V. How much depends on how early we get started. Let’s say, for argument’s sake, just a single meter/second will do it; after all, the “keyhole” is less than a kilometer across.

    An acceleration of 0.00001 m/s^2 — that’s one-one millionth of a gee — maintained for just over a day, would do it.

    How much force is required to accelerate Apophis at a millionth of a gee? F= MA = 2.5 x 10^10 kg x 1 x 10^-5 m/s^2 = 2.5 x 10^5 Newtons.

    Now, doing this with a laser… I dunno, man. You’d need a pretty bigass laser. 2.5 x 10^5 Newtons is about one/sixth of a Space Shuttle main engine. And ablating the surface of an asteroid doesn’t seem like a very efficient way to generate thrust.

    It gets easier if you have more time, of course. If you can match orbits with the asteroid and spend months zapping it, the force required drops by a couple of orders of magnitude. But this doesn’t seem like anything we’d be able to do for at least a couple of decades.

    OTOH, nothing’s getting close to us for a couple of decades. So perhaps.

    Doug M.

  • Eric SECT February 2, 2011, 8:55

    Using a solar sail to heat up a spot to cause thrust; what do you do if the object is rotating or tumbling? Dont you have to stop that first? Otherwise the jet will be spraying out all over the place as the object moves and will cancel out thrust or make it chaotic.

  • Joe February 2, 2011, 12:11

    Logical arguments against using nuclear weapons for deflection purposes has been contaminated somewhat with the political ant-nuclear movement.

    It is proven technology and the amount of force scales very well with what we need to do.

    I agree a test run is needed that is safe. Pushing a small asteroid into Mercury that allows for a very large margin of error in all potential trajectories being safe would be a good experiment.

  • Rob Henry February 2, 2011, 15:27

    It is hard to read this article and not get blown away by the casual acceptance of being able to calculate orbits around our neighborhood, whose typical scale factor should be about 1AU (150 million km) to within 36,000 km over a couple of decades. It is thus easy to forget how deficient astronomy is in some other respects. I recall in particular a lack of certainty over how frequently moderately sized asteroids and comets hit the Earth. When I last followed the debate I discovered that there was about two orders magnitude difference between the highest and lowest estimates of this flux. It has been pointed out that that dearth of such impacts over the last ten thousand years could not be used as evidence of our safety, since the trappings of civilised society could only be maintained if we were not hit by anything ‘too large’, and since primitive troupes of hominids would not debate such matters. I can not help wondering if this debate has now been settled, and how lucky have we been so far.

    Also, Ron S, part of scientist’s job should be to protect the image of science. To allow such preliminary work on an object that could, in principle, be deflected to hit this Earth, would invite rumor if not subterfuge itself. Just imagine the (hopefully irrational) panic caused if a bug was found in the software directing the deflection of such a mission.

  • Ron S February 2, 2011, 17:29

    Rob, I do understand your point and considered it myself. What I’d really like to know if this is official policy for this reason, or another that is based on something other than public reaction.

  • ant6n February 2, 2011, 18:17

    @Doug
    To accelerate a 25 million ton object to 1 m/s you need 360 watt years of energy. So if you have a year, and your laser ‘propulsion system’ has an efficiency of 1%, you ‘only’ need a 36.000 watt laser.

    If one uses a laser anyway, can’t it just be located on earth? One could probably spread a bunch of lasers throughout the planet to have continuous coverage and collective power.

  • Wedge February 2, 2011, 21:42

    @Eniac, sub-surface detonations are an inefficient and risky method of diverting asteroids. The preferred method would be a standoff explosion which ablates part of the surface of the asteroid through x-ray radiation, effectively creating a rocket engine. If you do the math this turns out to be one of the best ways to divert an asteroid given our current technology.

    Long term, looking at robust impact diversion infrastructure there are some clear trends. First, it pays to invest in detection capability and to use low delta-V methods of diversion as much as possible (a small nudge tens or hundreds of years in advance saves having to use more dramatic means with less time left). Second, there’s no getting around needing a fast-response very high delta-V infrastructure. It’s not technologically feasible to be able to detect all of the members of the kuiper belt and oort cloud out to several hundreds of AU, which means that there will always be the possibility of a very large cometary impactor detected only a few years or perhaps a decade in advance of the impact.

    That means that we’ll need the technology to a: intercept the impactor as much in advance of the time of impact as possible and b: apply huge amounts of delta V to it rapidly. A likely candidate for providing such a capability would be a nuclear Orion craft, which could accelerate quickly to an intercept course and then launch a series of its “propulsion units” (bombs) on trajectories to create a series of stand-off explosions which apply the necessary nudge to move the impactor off course.

  • Eniac February 3, 2011, 0:20

    @Wedge,

    I don’t see how subsurface would be inefficient. It seems that most of the energy of the detonation would be captured this way, and transformed into kinetic energy of the ejecta, which would give you the maximum delta-V. Stand-off explosions would be wasteful, for two reasons: First, more than half of the energy would go unused into into empty space. Second, X-ray ablation sounds like high Isp and low reaction mass, thus low delta-V. Given a fixed amount of energy, the highest delta-V is achieved with the lowest Isp, the extreme case of which is splitting the asteroid in half. Splitting in half might be difficult without risk of splitting into three or more pieces, in which case some may still hit. A subsurface crater, on the other hand, could be expected to safely throw the ejecta out to one side, and give the rest a good push to the other side, with nothing substantial left in the middle.

    Now, risky is a different question. You want a crater, not a fissure, so that would require some research, and perhaps experimentation.

    Orion type craft would be cool if you had one, until then I think existing rockets and warheads will do and can be cobbled together much more easily.

  • ljk June 16, 2012, 1:10

    http://www.technologyreview.com/view/428165/solar-powered-laser-spacecraft-could-prevent/?ref=rss

    Solar-Powered Laser Spacecraft Could Prevent Apophis Hitting Earth in 2036

    Spacecraft equipped with lasers powered by light from the Sun are our best defence against incoming asteroids, say aerospace engineers

    KFC

    Friday, June 8, 2012

    In 2004, the Earth crossing asteroid Apophis generated much interest when astronomers announced that there was a 2.7 per cent chance that it would hit the Earth in 2029.

    The excitement died down when more detailed observations showed that Apophis would actually miss Earth in 2029. And yet, Apophis might still hit in 2036 or 2037–we simply cannot know until nearer the time.

    An important question, then, is what to do if astronomers spot Apophis coming our way–what can we do to push this 46 million tonne object away?

    Last year, we looked at a Chinese plan to deflect the asteroid by smashing a spacecraft into it using a solar sail.

    Today, Massimiliano Vasile and Christie Maddock at the University of Strathclyde in Scotland reveal a plan to blast the asteroid with solar powered lasers, ablating its surface and steering it away from us. “[Our] paper demonstrates how significant deflections can be obtained with relatively small sized, easy-to-control spacecraft,” they say.

    Laser ablation is not a new idea. The basic idea is that the material vapourised from the asteroid’s surface, pushes it like rocket exhaust, generating thrust. Until now, space scientists had always thought that a job this size required a megawatt class laser, which would need to be powered by a nuclear reactor.

    That introduces a host of challenges, not least of which is launching such a device safely and then dealing with the huge amount of heat it produces.

    But Vasile and Maddock say that instead of a single large laser, a better option is to use lots of small ones–kilowatt-class lasers, which could each be powered by the Sun.

    The advantages are many, they say. First, the problem of dissipating heat in space is a serious one and does not scale linearly with mass. Small spacecraft are easier and cheaper to cool because a smaller percentage of their mass needs to be devoted to radiators and related equipment.

    Next, solar powered lasers have the obvious advantage of requiring no fuel and being far simpler and safer to launch than nuclear devices.

    And finally, having many small spacecraft ablating the asteroid gives greater scope for redundancy. If one goes wrong, there are several others to plug the gap.

    That’s not to say that such a mission would be easy to mount. A significant problem for all ablation schemes is that the vaporised rock from the asteroid can end up coating the spacecraft optics and ruining their efficiency.

    That’s particularly acute for spacecraft that must orbit close the asteroid, such as those that might use mirrors to focus the Sun’s rays onto the surface, as some astronomers have suggested.

    But laser beams can be collimated and so aimed from much further away. That vastly reduces the risk from ablated material.

    Then there is the problem of asteroids with highly eccentric orbits, which are too far from the Sun for much of their orbit for solar power to be much use. In this case, Vasile and Maddock say that solar power spacecraft could still deliver a large enough kick to steer such an asteroid away from us, given enough lead time.

    Vasile and Maddock make no attempt to calculate the cost of such a mission or compare it to the cost of other plans. However, there’s no question that the price of preventing an Apophis-sized asteroid hitting Earth pales into insignificance compared to the cost of dealing with the consequences of the impact itself.

    These ideas might sound like science fiction today but the only question is not whether we will ever need to put such a plan into action but when. If Apophis turns out to be heading our way in 2036, it will turn out to be extremely useful to have sketched out the details already.

    Ref: http://arxiv.org/abs/1206.1336: Design of a Formation of Solar Pumped Lasers for Asteroid Deflection