Hard to imagine a collision between two planets? Well, don't feel too sorry for yourself. It's difficult for us to imagine, too, and we in the planetary-science community have been studying impact and its effects intensively since the early 1960's. It's not a simple thing to comprehend the energy released in even a small nuclear weapon, but the energy represented by the impact of even a little comet is so stupendously greater that a comparison between the two is basically pretty silly. Being silly never stopped us, so we'll make one anyway, because it might make you think of things a little differently. Consider a 1-megaton hydrogen bomb for a moment. If it detonated near a city, the amount of destruction would be so great that I would imagine few people -- if any -- would be able to comprehend it. I doubt that anybody needs convincing that a 1-megaton explosion is incomprehensibly violent. I was going to use a graphic here. I can't, though, because computer screens are too small, so I'll just have to use numbers. Take the energy released by this hydrogen bomb and represent it by the area of a circle a centimeter in diameter. That's the size of a typical button on a shirt. Now consider the impact of a comet. Comets appear to come in all shapes and sizes, so let's take a small one that's only a kilometer across, and assume that it has a density of 1 g cm-3, about that of ice. Calculations indicate that a typical encounter velocity between a comet and the Earth would be around 40 km s-1 (Yes, that's about 24 miles per second, and no, that's not a typo.), so we'll use that as the impact velocity. Some will be faster, some will be a little slower. The equation for the kinetic energy E of a moving object is simple: where m is the mass of the object (the comet, in this case) and v is its velocity. Plug in the mass of a 1-km comet (at 1 g cm-3) and the impact velocity of 40 km s-1, and you'll get a kinetic energy of 4.2x1027 ergs. A megaton of TNT releases 4.2x1022 ergs when it's detonated. The kinetic energy of this impact would be represented by a circle 3.56 m in diameter --that's more than 99,700 times greater than the energy contained in that hydrogen bomb!! All this for a small comet, too, with a density only equal to that of ice. If it were an asteroid instead, with a density of about 3 g cm-3, we'd require 300,000 hydrogen bombs to equal the impact's energy, and the circle would be almost 6.2 m across. Planetary-scale impacts are inherently very violent events. The next time you hear on the news that an asteroid half a mile across is going to come within a few hundred thousand miles of Earth, and that astronomers are excited about it, please (1) think 300,000 hydrogen bombs going off all at once and (2) be very thankful that it's a few hundred thousand miles away. And now maybe you'll have some idea why those astronomers will be so excited over a speck of light so far away...