Expert: Courtney Seligman - 11/3/2010

Question
How is it that  meteor craters are perfectly formed with regular sides .  It stands to reason impacts would have to be from directly perpendicular from above to get this. Dont space objects and meteors arrive thru the atmosphere at an  angle across the sky, not like being dropped from directly above. There should be a lot od debris at ground zero pushed up at the end of the crater an virtually nothing but an entry gap at the front that should be nearly level os a actually deeper. I have dropped marbles and golf balls in the sand as an experiment and the only way to acheive a near perfect crater is to drop the object from a perpendicular height.  How do you explain this please. I think we are being conned. Regards  Paul

Answer
(Note added afterward, in response to a followup comment about the effects of a lack of atmosphere: You are correct in that having or not having an atmosphere makes a difference. Since the Moon has no atmosphere, even microscopic objects make craters much larger than their own size when they are brought to a sudden stop at the lunar surface. But on the Earth and other bodies with an atmosphere, small objects are vaporized or slowed down by the atmosphere, so they either do not reach the surface or land with relatively small speeds (this is where meteorites come from). Objects much larger than a house can reach the surface of the Earth with substantial velocity, but things much smaller than that almost never do. (End of added note. Original answer follows.))

You have a good point. In fact, for a long time astronomers were divided over whether lunar craters were due to impacts or volcanic activity, because of exactly your line of reasoning. It was presumed that impacts at various angles would leave craters of different shapes, and ejecta blankets in different directions.

However, it turns out that in most cases this is wrong. The key factor is that the impact velocity is very high (tens of thousands of miles per hour), so the craters are not "scooped out". Instead, the impacting object is going so fast that it buries itself it in the ground before there is time for either it or the ground to react. Then, as its energy of motion is turned into heat it explodes, melting and vaporizing and generally pulverizing a volume of rock as much as ten thousand times larger than its own size. That explosion produces a crater that is almost always more round than not, and gives little if any indication of the direction it came from. Only craters formed by very low angle impacts show irregularities due to the angle of impact (such as Halley, which is a double crater with a noticeably directional ejecta pattern, and non-round shape).

Eventually, laboratory experiments with very high velocity impactors showed that non-round craters would be the exception, rather than the rule, and the impact theory of crater formation soon eclipsed the volcanism theory; but if memory serves, it wasn't until the 1950's or 60's that we realized that. So although the impact theory is now firmly established, astronomers who are now in their sixties and beyond can remember when views were very different.