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Asteroids, Comets and Earthlings

© Wesley Colley

Well, with two movies about celestial bodies impacting the earth, I suppose it's appropriate to give some lip service to the possibility. I won't delve into the several egregious scientific errors in these movies, but will provide some scientific facts about such things.

Asteroids and comets are basically any objects in the solar system not directly associated with one of the nine major planets. In fact, some would call Pluto an asteroid, since it's barely larger than Ceres in the main asteroid belt.

There's really not a big distinction between asteroids and comets, except that comets tend to come from orbits beyond Pluto, and up to 10,000 AU (astronomical units, the Sun-Earth distance), while asteroids generally reside inside of Pluto's orbit. There they receive enough solar flux that their icy components have long since fizzled off and what's left is just a rocky chunk. Comets still have their ice, which gets fizzled off as they zip around the sun, and that's what makes the beautiful tail.

The main asteroid belt the sits between the orbits of Mars and Jupiter - This is likely a would-be planet that just never quite collected enough mass to form into one object. In fact, the total mass in the belt is about that of our moon. These asteroids, for the most part, happily orbit the sun for eons and eons. Occasionally, though, one enters a resonant orbit, in which it finds itself being tugged on Jupiter and/or Mars in a way that makes its orbit unstable. For instance, it might orbit five times for every three Jupiter orbits, and hence every fifteen orbits, Jupiter is pulling on it in exactly the same way. Over time, this repeated tugging in the same direction can pull the asteroid out of its orbit, and deflect it inward or outward. Mars has collected two of these wayward asteroids as small moons, and Jupiter has clumps of asteroids that lead and follow in its orbit (the Trojan asteroids).

Eventually, some of these unstable orbits wind up on orbits that cross the Earth's, and hence, pose a threat of hitting the Earth.

Now the largest asteroid is Ceres, 933 km in diameter (smaller than Texas, Mr. President). But even much smaller asteroids, only 10 km in diameter, can end civilization on the Earth. The reason is that they are travelling typically 50,000 miles per hour relative to the Earth. That means they pack a tremendous kinetic energy that gets deposited into the earth upon impact.

mass = 4 * pi * (radius)^3 / 3 * 10 gm/cm^3 ~ 4 * (1 million cm)^3 * 10 = 40 million gm = 4 * 10¹⁶ kg

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