Pedagoguery

Take a look at nearly any solid body in the solar system and one commonality becomes clear: craters. Nearly all of them are covered by craters. Most of those craters are millions or billions of years old, but there are still many random chunks of ice and rock out there that can strike larger bodies. What can we do if we discover that one is headed our way? Given enough warning, the answer is that there are actually many different ways of avoiding a collision.

Asteroid or cometary deflection mechanisms can be broadly categorized into quick jolt and slow push options. The quick jolt options are pretty obvious. Even Hollywood has explored them, although it has gotten at least one of them wrong. In the movie Armageddon, the astronauts were tasked with taking a nuclear bomb to the asteroid and burying it, then setting it off. In reality, that would probably be counter productive. Such an explosion would likely fragment the asteroid without materially affecting the trajectory of most of the fragments. The result would be multiple impacts, and those pieces that missed could well be sent on a trajectory that would lead to an impact several years down the line. In reality, the nuclear device would be detonated 15 to 25 feet off the surface of the object. The resulting radiation would boil off some surface matter, resulting in a kick in the opposite direction. For a kilometer-sized asteroid, this would impart enough of a kick to turn an impact 7 to 10 years in the future into a harmless miss.

The second quick jolt method is one which we have actually already done – slam a spacecraft into the object. This was done by the Deep Impact mission, which slammed a copper projectile into the comet Temple 1. The primary purpose of the mission was to see what sort of material the comet gave off, but a side benefit was to see how the comet's trajectory was altered by the event. It turns out that the majority of the change in trajectory was delivered by the material blasted off of the surface of the comet, not by the impact itself. This method is considerably less efficient than the nuclear bomb method, but if the potential impact is far enough down the line, then one or more impacts properly timed could be enough to do the job.

There are far more slow push options than quick jolt options. In every case, the principle is that even a very low acceleration applied over enough time is enough to move something in a significant way. Most of these methods also have the advantage is that they provide a much greater degree of control.

The first slow push method is the mass driver. This is essentially a fancy way of throwing rocks. A typical mass driver is a magnetic cannon. The rock would either be coated in iron or have some ferrous content so that it would respond to the magnetic field. In theory, the rocks could be thrown with a very high velocity, so that even though their mass is microscopic in comparison with the asteroid, each one could provide a small kick that over time would allow precise steering of the asteroid. Unfortunately, such cannon are not within our technical capabilities right now.

Another option is a solar sail. A sail could be used in a couple of ways. First of all, it could be attached directly to the asteroid and used to steer it directly. The question there becomes how to moor the sail to the asteroid. If the asteroid were a solid body, then the trick would be to have a cable and anchorage strong enough to allow the asteroid to be moved. If the asteroid were a loose collection of smaller bodies, then this method would probably not be feasible at all. Another alternative would be simply to concentrate sunlight on the right point on the asteroid and boil off surface material to form a kind of rocket. This would work with a number of types of asteroids, but you have to be wary of the boiled off material re-condensing on the surface of your mirror.

A laser or particle beam could be used in much the same way as a mirror. They offer some advantages. First of all, they are not as subject to the re-condensation effect. Secondly, they could be used further from the sun, since they are not dependent on ambient sunlight. Lasers might be technically ready for such a task, but deploying one and keeping it in the right place over the time scales involved is still not within our technical capabilities. As for particle beams, they are still well beyond our capabilities at this time.

Another method takes advantage of something called the Yarkovsky Effect. This is the effect of uneven heating caused by the rotation of an object. If the object rotates in the same direction as its orbit around the sun, then the “afternoon” side of the asteroid is warmer than the “morning” side. This causes an excess in radiation (mostly infrared) to be emitted from the warmer side, giving the asteroid a kick in the direction of its orbit, causing it to spiral outward. If the object rotates in the opposite direction, then the effect is the opposite and the object spirals inward. By altering the reflectivity of the asteroid, we can manipulate this effect, causing the asteroid to change its orbit at our direction.

The final mechanism is in some ways the most feasible in the short term. It is the gravity tug. Any self-powered spacecraft keeping station with an asteroid will affect the asteroid's orbit by the spacecraft's gravity. The effect is not large, but, once again, over time it does add up. Low thrust, high efficiency engines like ion engines or solar sails would be ideal for such a spacecraft.

We have a number of possible ways to deflect an incoming asteroid or comet, provided we learn of the potential impact far enough in advance. This makes the search an even more important endeavor.

Next time, what Cassini has revealed about Titan.