Asteroid Deflection

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    Sixty-five million years ago an order of animals that had dominated life on Earth for over one-hundred million years, Order Dinosauria, got almost completely wiped out: only a small group of theropods, which became the birds, survived. The Great Extinction came from an asteroid hitting Earth just north of the Yucatan Peninsula, both from the immediate phenomena attending the impact and from the flood vulcanism that the shock wave instigated in India, thereby creating the Deccan Plateau and the Deccan Traps. In many ways we live in conditions more precarious than those occupied by the dinosaurs, conditions that make us more vulnerable to the effects of an asteroid strike. So how do we prevent such a disaster from happening?

    That is not an idle question. In 1178 (Jun 25) five British monks witnessed the asteroid strike on the moon that created the crater Giordano Bruno and brought on The Little Ice Age by way of dust blasted in the retrograde direction along the moonís orbit. In 1908 something came out of the sky and blasted the Tunguska region of Eastern Siberia. In February 2013 another something exploded in the sky ten miles above the Russian city of Chelyabinsk and blew out most of the windows in the city. We live in a cosmic shooting gallery, so the sooner we develop a means of deflecting incoming asteroids, the better we will make our chances of not going extinct.

    Note that we want to deflect any disasteroid headed toward Earth, not smash it. Simply bombing the body converts a cannonball into a shotgun blast, which doesnít do Humanity any favors. We would better shove the asteroid off its trajectory by an amount that will make it miss Earth.

    The main problem in shoving an asteroid comes from the fact that asteroids rotate. Unless we build them on one of the asteroidís poles of rotation, we do ourselves little good by putting giant rocket motors on the asteroid. And even in that limited case we may run into an unfortunate fact: the asteroid may be nothing more than a loosely packed pile of rubble. In that case the rocket motor might punch through the asteroid, scattering its components as would the above mentioned bomb.

    Astronautical engineers have come up with an elegant solution of the problem Ė a gravitational tractor. Certainly nothing that John Deere would ever recognize, the tractor consists of a huge, massive ball (presumably made of solid iron) connected to an array of electric rocket motors through a long shaft. The motors angle away from the shaft in order that their exhaust plumes do not strike the asteroid and push it away from the tractor.

    In operation the tractor approaches the target asteroid and comes to a halt with its ball as close to the asteroid as possible without risking a collision. As gravitational attraction acts to pull the ball and the asteroid closer together, the tractor fires its rockets with just enough thrust to counter the attraction and thereby keep the ball and the asteroid at a constant distance from one another. With its engines firing continuously, the tractor gently pulls the asteroid off course and away from a collision with Earth.

    To gain some idea of what the tractor involves, letís consider a spherical asteroid with a radius of one kilometer, assume that it heads straight at Earthís center and that we begin the deflection one year before the anticipated collision. We thus have 31,557,600 seconds to deflect the disasteroid 6356.9 kilometers off course. That deflection requires a continuous acceleration of 1.2766x10-8 meter per second per second acting at the asteroidís center. If we can bring the tractorís ball to a position in which its center lies two kilometers from the center of the asteroid, then the ball must ponder 765,600 tonnes in order to create the necessary acceleration. We thus need 139,200 cubic meters of iron in the ball. We thus need a ball a little over 32 meters in radius.

    This project obliges us to assemble nearly one million tonnes of iron in space, to equip it with giant rocket motors, and to drive it around the solar system at high enough speeds to intercept asteroids headed for Earth. In order to acquire the cue stick with which we play interplanetary billiards we need to carry out a major construction project in deep space, something analogous to asking Henry Hudsonís Holland to build a modern skyscraper in Manhattan shortly after Hudson discovered the place.

    The hardest problem requires us to move the tractor around the solar system to intercept the disasteroid, to rendezvous with it, and to deflect it. Solving that problem requires building rocket motors that generate giganewtons of thrust and use megatonnes of propellant. We have no hope of meeting the power requirements of such a vehicle with current technology. Nor do we have any hope of building and deploying the fleet of refueling vehicles that the project would require.

    We have an easier way available to us. Instead of building one giant spacecraft, we build many small ones.

    Imagine floating next to a disasteroid heading for Earth. As a workable example letís again consider a spherical asteroid with a radius of one kilometer, assume that it heads straight at Earthís center and that we begin the deflection one year before the anticipated collision. Multiplying the mass of the asteroid by the acceleration that we wish to impose upon it tells us the amount of thrust that we must exert. In our example we need a force less than the weight of a 25-tonne body on Earth, roughly half the weight of a modern diesel-electric freight locomotive.

    To exert a thrust equivalent to the weight of one tonne on Earth, an engine with an exhaust velocity of 100 kilometers per second must spew 98 grams per second of propellant into space. To gain that kind of performance we need to use an electromagnetic rocket. For propellant the rocket will use dust obtained from the asteroid itself.

    In the worst-case scenario the density of the asteroid is too low for it to be made from solid rock. This would mean that the typical small asteroid is not a monolith but rather a "rubble pile" formed from fragments that have accumulated and cohered over time. A certain amount of material may have cold welded over geological time in the vacuum of space. Still, it may possess a certain fragility and we will need to take care that our effort to alter its trajectory does not make it break up. That care involves sending multiple spacecraft that will spread the total thrust applied to the asteroid over a wide area.

    Thus we send out a team consisting of dozens of robot cockroaches drawing power from a flock of robot butterflies flying in formation with the disasteroid. Each robot constitutes a complete spacecraft in its own right, but they work together as a single flexible unit. Built in Earth orbit or on the moon, the robots fly themselves out to the disasteroid, take up their positions on and around it, and push it off course.

    Like the insect for which they are named, the cockroaches are simple creatures. Each roach has a head that contains the sensors and computers needed to guide the robot to its target, land upon it, and then walk around on it. The body contains a device that will scoop up material from the asteroid and pulverize it for use as propellant. On the roachís back we find an electric rocket that can spew the dust into space at speeds as high as 100 kilometers per second. The legs, which enable the roach to walk around a rotating asteroid, end in broad pads that spread the roachís thrust over a wide area on what we assume is a soft surface. And the wings are rectifying antennae that take in microwave power beamed to the roach from a separate power source and convert it into electricity.

    Like the cockroaches, the butterflies are simple creatures that, from a distance, resemble their namesake insects. Their wings are giant solar panels. Inside their bodies resides the apparatus (likely klystrons, which work well in the vacuum of space) that converts the electricity from the panels into microwaves. The robotís head contains the computers and sensors needed to guide it through space and to locate its targets. Special antennae, where the butterflyís legs would be, beam the microwaves at the targets. Each butterfly also has a dust-spewing electric rocket for propulsion and a tank for storing pulverized rock as propellant.

    Carrying out their rendezvous with the asteroid, the cockroaches survey it. Then the flock lands upon the body and each roach takes up a position from which it begins to exert thrust. Because the asteroid rotates, the cockroaches must walk around it in order to keep their thrust vectors properly aligned to push the body onto the trajectory that will miss Earth. As the cockroaches walk around the body, they might walk slightly ahead of the ideal thrust point, so that their thrust also exerts a torque upon the asteroid and thereby controls its rotation.

    We would not see a great plume of dust, like a cometís tail, coming off the asteroid, but the dust is there nonetheless. Since even a grain of dust can leave a significant mark on a spacecraft, we might think that our robot cockroaches have simply replaced one problem with another. But they will use the finest dust available to them, because it makes the best propellant in this case and they will blast it into space at a speed that well exceeds solar escape velocity. Further, in this case they send the dust north, outside the Ecliptic plane and, thus, far from any foreseeable spacecraft trajectories.

    Approaching Earth at a speed as high as 44 kilometers per second, the cockroaches apply their thrust right up to the last minute available to them. Crossing the moonís orbit, less than three hours from perigee, they shut down their effort to move the asteroid and begin securing themselves: thereís nothing more they can do. Throughout near-Earth space satellites and other spacecraft have moved away from the asteroidís trajectory. All anyone can do now is wait and watch.

    They almost didnít succeed. Radar tracking, continuously updating the computer projection of the asteroidís trajectory, shows that the asteroid will make a shallow dip into Earthís atmosphere at its perigee. To avoid getting blown off the asteroid and destroyed by the glowing-hot, hypersonic wind that will blast the asteroid, the cockroaches trek to the aftmost part of the body. There they will wait while the asteroid bounces off the upper atmosphere as a stone skips off a pond.

    Standing on the aft side of the asteroid, the robot cockroaches look up and stare at the sheet of thin flame rising above them. Far below them the icy desolation of Antarctica slides past as the asteroid reaches its perigee, coming almost too close to Earth. Will they feel anything? Will they possess enough sentience to feel a sense of satisfaction in what they have accomplished? Will they understand the meaning of what they have achieved? Or will they, like the stereotypical robots they resemble, merely go on to the next task assigned to them?

    The answer to those questions will say more about our moral development than about our technical capability.


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