Re: Various Moon Questions

The moon has zero atmosphere, and no magnetic field. Without these protections, the radiation from the solar wind burns down on the surface 328 hours a day. This radiation is not dangerous for short visits, but poses a real risk of cancer to people on a permanent lunar colony. This problem can be solved by building a moon base underground, or covering it with a few meters of lunar soil, called regolith.

The moon's interior is much less interesting than Earth's. It appears to have a semi-solid mantle beneath the crust, and perhaps a very small iron core. The absence of a liquid iron core and the Moon's slow rotation rate are probably the reason it has no magnetic field.

If an object the size of the Tunguska impactor hit the moon, it's very likely that we on Earth would notice. With no atmosphere, an airburst cannot occur, and everything that hits the moon will make a crater. You can consult the impact table in this page on meteorites, treating all impacts as land impacts of iron meteorites. A Tunguska-sized event would be as powerful as a very, very big hydrogen bomb, and leave a new crater easily visible by lunar-orbiting space probes like Clementine, and perhaps barely visible by Earth-based telescopes. If the impact occurred on the near side when the moon was visible from Earth's night side, the explosion would probably be visible to the naked eye. In fact, I remember an old report from a European monastery in the middle ages which claimed to see an arc of light rising over the limb of the moon; many feel this may have been the ejecta plume from a meteor impact just on the far side of the moon. Unfortunately, I can't remember where I heard about that. But I doubt such an impact would cause serious effects on Earth: the rock thrown off the surface of the Moon might eventually hit our atmosphere, causing some very pretty meteor showers but no damage.

The moon is getting slowly farther away from the Earth. If you know some physics, you'll realize that it must be gaining energy and angular momentum to do so. It gets these from the Earth's rotation: the length of Earth's day is growing by 1.48 milliseconds/century, and the Moon is getting farther away by 3.8 centimeters per year. It's interesting you mentioned the effect on the Earth's tides: it's the tides that are causing this effect. The moon raises bulges of water on the Earth: the Earth's rapid spin pulls these bulges ahead of the point on Earth directly beneath the moon. The gravitational attraction between the near bulge and the moon tends to accelerate the moon and retard the Earth's spin. The moon probably spun rapidly too, long ago, but the opposite effect (the Earth raising tides in the rock of the Moon) caused the Moon's rotation rate to slow much faster, eventually "locking" the moon with one face always towards the Earth.

Here's something to puzzle over: How can scientists measure the rate at which the Moon is getting farther away? How could they measure the rate at which the Earth's spin is slowing down?

More info on the moon can be found here.