Saturday, December 30, 2017

Asteroid Defense

We live in a fairly benign solar system. Earth has not had a major asteroid impact for 60 million years, however, that one was an extinction event. Large asteroids hitting an inhabited planet can create large shock waves that sweep through the atmosphere, megatsunamis that can cover almost all of the land surface, dust to fill the atmosphere and block photosynthesis long enough for most plant life to die, vulcanism at the impact site or at the antipodal point, or simply pumping so much heat energy into the atmosphere that non-aquatic life mostly dies out, as well as near-surface dwelling sea life.

In a solar system with more frequent extinction events, with the events happening more frequently than the recovery time needed by life on the planet, asteroid impacts might simply result in almost uninhabited planets, with only single-celled organisms or perhaps small creatures remaining. Is it possible that these are the norm? If so, it could prevent any alien species from ever arising and taking on the job of traveling to other solar systems beyond their own. Earth, as an exception to this norm, would be alone in the galaxy.

Asteroid impacts consume asteroids. Each time an asteroid hits a planet or a satellite, it is destroyed and there is then one less asteroid to impact a planet later. Asteroids do not typically form after the formation time of the planets, so any period of heavy consumption will free up later periods to have periods long enough for intelligent life to form. Astronomers like to talk about the Early Heavy Bombardment, when there were many more asteroids than now, and very many of them were hitting planets and satellites. One might assume it was a sort of random process, and try to figure out the decay time of the rate. If it was short enough, an alien planet might become free of asteroid impacts early on, and easily proceed to evolve all manner of life. If it was long, meaning the cleaning process to remove asteroids did not work very quickly, asteroid collisions would be spaced out so that impacts, not being common initially, did their work for a long time.

Where the asteroids are, and what type of orbits they occupy, makes a great deal of difference in this cleaning process. If there were only a few large asteroids, and they occupied stable orbits in reference to a couple of shepherding gas giants, there would be no collisions, just like we do not expect Mars to hit Earth any time soon. The only problem is that this doesn’t work as well with tiny bodies like asteroids, as there is likely to have to be some dissipative forces to move planets into stable orbits, relative to the gas giants. Asteroids are too small to have enough of these, as they would go as a second power or higher of the mass of the asteroid. Thus asteroids just keep the orbits they were born with, unless they do a gravitational slingshot with a giant planet. They may be in the vicinity of a stable orbit relative to the shepherding gas giants, but not as close as a rocky planet or something larger.

The sweeping clean of orbital areas should be proportional to the cross section of impact, and a gas giant has hundreds of times the cross section for collision that a small planet would. Thus, those orbits which intersect the gas giant’s orbits are likely to get swept clean early on, but not the orbits which intersect the small planet’s orbit. The orbits from which a gravitational slingshot can happen are those which will be swept clean early, so the later periods of a solar system would be when asteroids were concentrated in the areas where there were minor planets, which are the ones where life can form.

Let’s do some numbers on asteroid collisions. First, suppose there was an asteroid in an orbit similar to Earth’s, fairly circular, in the same stable notch caused by the gas giants. That notch might be of the order of a quarter of the distance from the mean orbital radius of Earth to that of Mars. The cross section of Earth compared to the cross-section of the notch, assuming it is circular is of the order of ten to the seventh. If the difference in periods is about ten percent, this implies the cleaning of Earth’s co-orbital vicinity is short compared to the age of the solar system. But for asteroids not in co-orbit with the Earth, another factor, comparable to the radius of the Earth and the radius of its orbit, about a million, comes into play. This makes the cleaning time much longer than the age of the solar system, and implies that for some solar system like our own, with an Earth-sized alien planet at something like the radius of Earth from its star, asteroids will be coming for the whole lifetime of the solar system.

As soon as an alien civilization became expert at astronomy, it would realize this peril existed. Probably there would be geological evidence of the same thing. Thus, asteroid defense would be considered. Is it feasible that an alien civilization could prevent an asteroid from hitting their planet?

Destroying a larger asteroid is probably impossible, but fracturing one into two pieces might work for some of them. However, the destruction operation is likely to involve large amounts of explosive, which means that the resulting course of the pieces is uncertain. This would therefore have to be done far away from the planet to allow the pieces to go far wide of the alien planet. Other asteroids, larger ones, might be impossible to fracture. Diverting them would be possible, if their orbits were sufficiently precisely determined.

Before any destruction or diversion can happen, the asteroids would all have to be located and tracked. Those with the possibility of crossing the orbital radius of the alien planet would have to be tracked more precisely. Could the alien planet find the resources to do this? If the number of asteroids was in the millions and their orbits ranged over the whole solar system, they might not.

The other defense, a backstop to anything else, is to make self-sufficient outposts on other planets or satellites so that if there was a large collision with an asteroid, it would not spell complete extinction of the alien species. This raises the question of whether in a typical solar system, if a very competent and dedicated alien species could figure a way to sustain life on at least one other planet or moon in the absence of support from the home planet. Would doing that leave any signatures that could be detected from Earth with a sufficiently large telescope or other observing devices?

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