Why are all Moon's craters round? A round crater is made by an object falling perpendicular on the surface; an object hitting the surface at an angle different than 90 deg. will made an approx. elliptical crater. This mean that either all objects hitting the Moon were of low kinetic energy (either slow or small) and deflected by Moon gravity, or they were pointing toward Moon.
What do you think?
Cornelius.

My understanding is that in most cases, the moon's gravity will start tugging on the meteoroid on it's way down and the object will eventually fall straight down. If you go outside and throw a softball, it will travel a distance but when it begins to fall, it will gradually loose it's foward momentum while it begins to drop. Eventually, it looses all foward momentum, but continues to be pulled twards Earth by gravity. End result: hits the ground straight down.

If you were to throw the same softball off a building say a few stories tall, you would get the same result but the result would be far more obvious. No matter where you threw that softball, it would hit the ground straight down.

Now, imagine an object (no matter what size) comming twards the moon with several thousand miles to fall?

Great question though :)

Originally posted by cornelius
Why are all Moon's craters round? A round crater is made by an object falling perpendicular on the surface; an object hitting the surface at an angle different than 90 deg. will made an approx. elliptical crater. This mean that either all objects hitting the Moon were of low kinetic energy (either slow or small) and deflected by Moon gravity, or they were pointing toward Moon.
What do you think?
Cornelius.

Actually, unless an object strikes at a very shallow angle, it will always produce a round crater. The reason is that as the object strikes the surface, it is largely vaporized, which releases a great amount of energy (like a bomb). It is this explosion that creates the crater. This "bomb crater" effect overshadows any elliptical effect that may be caused by the impact angle.

It would seem that the moon should be covered by all kinds of skid marks caused by glansing blows. Also here on the Earth one side of a meteor crater is higher than the other showing that the object came from a certain direction. I imagine if a close-up analysis is done on the moon's craters one side of the crater will be higher than the other.

Originally posted by Xevious
My understanding is that in most cases, the moon's gravity will start tugging on the meteoroid on it's way down and the object will eventually fall straight down. If you go outside and throw a softball, it will travel a distance but when it begins to fall, it will gradually loose it's foward momentum while it begins to drop. Eventually, it looses all foward momentum, but continues to be pulled twards Earth by gravity. End result: hits the ground straight down.


Well, softballs and meteorites don't compare that well, as softballs travel at tens of metres per second, and meteorites tens of kilometers per second, so a factor of a thousand will have a significant effect.

Also, the earth and the moon differ significanly, in that the moon has no atmosphere to cause drag on an object, and having a fraction (1/6) of the gravity of earth, that will cause even less of a deviation to it's flight path.

So many, many meteorite impact at angles.

In fact, the 'dinosaur killer' at Chixulub impacted on earth at an angle (so even atmosphere and higher gravity have little effect on large objects), which is why it sprayed so much matter into the atmosphere.

So, your softball analogy sucks.

The purpose of the anology was to demonstrate a principal about how gravity works - and in that context it was an entirely valid and correct statement. The fine points you listed such as air drag and gravitational differences between the Moon and Earth have no relevance to the basic premice I stated.

Yes, glancing impacts do happen, but unless the object is traveling at an incredibly high rate of speed, the gravity of the Moon or for that matter any other object, is going always going to decrease the foward momentum of an object that does not have sufficient velocity to achieve orbit, or escape. This is exactly what "falling" means! Unless you believe that all impacts are caused by head-on collisions (and not capture / decelerate) then my example is perfectly correct.

Originally posted by Xevious
The purpose of the anology was to demonstrate a principal about how gravity works - and in that context it was an entirely valid and correct statement. The fine points you listed such as air drag and gravitational differences between the Moon and Earth have no relevance to the basic premice I stated.

Yes, glancing impacts do happen, but unless the object is traveling at an incredibly high rate of speed, the gravity of the Moon or for that matter any other object, is going always going to decrease the foward momentum of an object that does not have sufficient velocity to achieve orbit, or escape. This is exactly what "falling" means! Unless you believe that all impacts are caused by head-on collisions (and not capture / decelerate) then my example is perfectly correct.

You're in a hole, stop digging!

Your analogy was flawed, because on earth, the softball loses forward momentum _solely_ due to air drag. Gravity only affects the vertical component of the movement. So, a softball launched with _any_ forward motion, would _always_ impact at a glancing angle if there were _no_ air drag.

"unless the object is traveling at an incredibly high rate of speed",

what, like, 10km/s? That's pretty fast.

"gravity ..., is ... always going to decrease the foward momentum of an object".

NO! Gravity only affects the _vertical_ component, and _increases_ the vertical speed of the incoming object! It's forward (horizontal, perpendicular to gravity) component, however, remains the same.

I think what you are getting confused over, is that for a slow moving object, it's trajectory will bend more and more towards the perpendicular, as the vertical component increases due to gravity, and becomes more significant wrt the horizontal component.

But for a fast meteorite, approaching at 10km/s, for arguments sake, aiming for a perfect glancing tangential hit, gravity would have to accelerate that object to 10km/s to make it a 45degree hit, which means, at a constant acceleration of 10ms^-2 (earth), the meteorite would have to experience a gravitational pull for 1000 seconds. 1000 seconds away though, the meteorite is 10,000km away, now plug that into your inverse square law for gravitational attraction, and you get gravity only exerting 1.49N on the object at that distance. Oh, that's not enough sustained gravitational attraction to cause a 45degree impact is it? So to work it out, you'll have to integrate the function, and re-work the numbers. Can you do that?

OK, I do see where you are comming from. Yes it's true, an object will not change one direction without an opposite reaction.

This mean that all meteorites which hit the Moon, were not pointed toward the Moon, but their kinetic energy was not enough to counteract the Moon's gravity, and in the last seconds before impact, their trajectory become almost perpendicular to the surface; this make sense, except the fact that this happened with all, none being lesser diverted in order to have an impact at a shallow angle (which will make an elliptical shaped impression).
Thanks,
Cornelius.

Why are all Moon's craters round? A round crater is made by an object falling perpendicular on the surface; an object hitting the surface at an angle different than 90 deg. will made an approx. elliptical crater. This mean that either all objects hitting the Moon were of low kinetic energy (either slow or small) and deflected by Moon gravity, or they were pointing toward Moon.

Actually, they aren't. I did a little digging and found a few places saying that about 5% of craters are elliptical.

As for why most are round, Janus is right. Unless something hits at an extreme angle, the blast from the impact wipes out any slight elliptical tendecies in the crater.

Heres a link to a paper I found on elliptical craters that might be of interest...

http://www.lpi.usra.edu/meetings/LPSC99/pdf/1441.pdf

My understanding is that in most cases, the moon's gravity will start tugging on the meteoroid on it's way down and the object will eventually fall straight down. If you go outside and throw a softball, it will travel a distance but when it begins to fall, it will gradually loose it's foward momentum while it begins to drop. Eventually, it looses all foward momentum, but continues to be pulled twards Earth by gravity. End result: hits the ground straight down.

The softball falls straight down because friction (air resistance). This of course, requires an atmosphere. The moon has very little atmosphere. Also as was previously mentioned, the softball is moving very slowly, meteorites move quickly.

In a friction free or low friction environment such as the surface of the moon, the path of the softball would by a symmetrical arc.

You guys might find this link interesting

http://www.etsimo.uniovi.es/solar/eng/tercrate.htm