MadSci Network: Astronomy
Query:

Re: If astroids pass close how will tides be affected or how will the orbit of

Date: Fri Oct 29 12:42:46 1999
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Astronomy
ID: 933646581.As
Message:

If asteroids pass close how will tides be affected or how will the orbit of the moon be affected at various points in its orbit? Or with a direct hit on the moon what will happen to its orbit at various points?


The answer depends on two things: how big the asteroid is, and how fast it is traveling. Because this is rather vague, I will try to give some qualitative answers.
  1. Tides. A passing asteroid will have no tidal effects worth mentioning unless it is very, very large and passes very, very close. How close depends on how large, but your average (or even above-average) chunk of space junk that gets the name of "asteroid" isn't going to do anything worth talking about unless it strikes either the moon or the earth. To have serious tidal effects, the object also needs to be around for an appreciable length of time, and since most asteroids are moving pretty darned fast...

  2. The Moon's orbit. Nothing short of a Moon-sized body passing very close to the Moon (again, how close depends on how large; an Earth-sized body would not have to be closer than a million miles or so, but a Moon-sized body would have to be as close - or closer - to the Moon as the Earth) will have any serious effect on the Moon's orbit unless it strikes the Moon. Remember that no known asteroid is even half as large as the Moon.

  3. A direct hit on the Moon. As anyone can see by looking at the Moon, with or without a telescope, the Moon has taken quite a number of direct hits. They don't seem to have done much to affect its orbit, though. The reason for this can be understood by a few simple calculations.

    The Moon masses about 7.35 x 1022 kg. Its average distance is 3.844 x 105 km from the Earth, and it orbits the Earth in about 700 hours, or 2.52 x 106 sec, for an average orbital velocity (around the Earth!) of about 1 km/sec. The Moon's kinetic energy relative to the Earth, then, is about 3.5 x 1022 joules, equivalent to 8 million megatons of TNT or at least several thousand times the total destructive force of the entire world's nuclear arsenal.

    To substantially effect the orbit of the Moon around the Earth, an asteroid would have to strike the Moon with a substantial fraction of that energy, let's say 1 million megatons. Since objects at the Earth's distance from the Sun are moving at about 30-50 km/sec relative to the Sun, and kinetic energy is given by E = ½mv2, we can quickly calculate that we would need an asteroid massing more than 1012 kg, or about a billion tons. This is large, but within the range of masses of known near-Earth asteroids such as 253 Mathilde or 243 Ida.

    The effect of such a huge strike would depend on the exact details, including the direction of impact. But the odds are low, so low that nobody really knows them exactly (compare the likelihood of a strike by an object large enough - from one to five tons - to reproduce the 1908 Tunguska explosion, estimated as 0.25 to 2 per century).

Dan Berger
Bluffton College
http://cs.bluffton.edu/~berger



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