Maybe I just need to go back to work.
![Wink :wink:](./images/smilies/icon_wink.gif)
I wondered about that too after reading about it in Lucifer's Hammer (comet impact disaster novel).Alan327 wrote:Maybe this is a stupid question, but why are all of the craters round? Why did none of the meteors impact at an angle and make an oblong or oval crater?
Maybe I just need to go back to work.
Here are a couple of sites with information.Alan327 wrote:why are all of the craters round? Why did none of the meteors impact at an angle and make an oblong or oval crater?
Thinking about it, a small gravitating body (an asteroid) "at infinity" (very, very far away) from a much larger body (a moon) will have zero gravitational potential energy by definition, and will accelerate toward the planet, infinitesimally at first, but faster as it approaches. Equate potential energy with kinetic energy at the planet's surface, solve for speed, and you get a figure of the order of km/s, which is a lower bound on impact speed (for airless worlds). And no, a planet or large moon can't "sneak up" on asteroids traveling at almost the same velocity and gently sweep them up.craterchains wrote:How nieve it is to assume that ALL objects are traveling at hyper velocities.![]()
Think about it.
Guess then our solar system just couldn't have been formed by that very method? *coughs*Pete wrote:And no, a planet or large moon can't "sneak up" on asteroids traveling at almost the same velocity and gently sweep them up.craterchains wrote:How nieve it is to assume that ALL objects are traveling at hyper velocities.![]()
Think about it.
Generally true, of course. But Dione has an escape velocity of only 500 m/s, which sets the lower limit for a collision. That's low enough that a moderately oblique hit will create an out-of-round crater. And indeed, looking at the images such craters exist. But most impacts will be at higher speeds, making round craters likely.Pete wrote:Equate potential energy with kinetic energy at the planet's surface, solve for speed, and you get a figure of the order of km/s, which is a lower bound on impact speed (for airless worlds). And no, a planet or large moon can't "sneak up" on asteroids traveling at almost the same velocity and gently sweep them up.
Dione has about an equal chance of hitting a prograde bit of debris on the leading or trailing edge. But there's bound to be lots of retrograde debris as well, which will selectively hit the leading edge. Also, retrograde debris impacts at higher velocity, meaning that smaller stuff will produce craters, and that craters will generally be larger. All in all, it makes sense that the leading edge should have more craters.quixote218 wrote:Seems to me that the more heavily cratered side of Dione should be the trailing side.Right?
Planets and large moons weren't around before the formation of, well, planets and large moonscraterchains wrote:Guess then our solar system just couldn't have been formed by that very method? *coughs*Pete wrote:And no, a planet or large moon can't "sneak up" on asteroids traveling at almost the same velocity and gently sweep them up.craterchains wrote:How nieve it is to assume that ALL objects are traveling at hyper velocities.![]()
Think about it.
Wouldn't that be pretty much a case of the object winding up in an horseshoe orbit with Dione? Which means it could never actually impact with the moon? I guess that makes it doubly unlikely.Keldor314 wrote:Also, you have Saturn's gravity to worry about for any asteroid that drifts into Dione. It's rather unlikely that a random asteroid, traveling in a more or less random direction at a very high speed will just happen to match Dione's orbit around Saturn, AND Saturn's orbit around the sun so that it can close at a velocity near that of Dione's escape velocity.
With asteroids traveling in the opposite direction of Dione, there would be a "head on" collision and the resulting impact would be on the leading side, not the trailing side.inertnet wrote:...In Dione's region, there must be a significant number orbiting in the opposite direction...
Gravity is actually the weakest of the forces, but it seems dominant in our everyday lives thanks to a couple factors. First, the enormous bulk of the Earth under our feet. Second, unlike the other forces, gravity is always attractive; the other forces tend to cancel themselves out over relatively short distances. It tends to be the dominant force in the evolution of the universe thanks to it's infinite range and lack of a canceling countercharge.Tom of Dreams wrote:oh I guess it would be at least the escape speed. hmm
gravity always seemed a little strange to me. It is described as a large force where as the nuclear forces and electro magnetism are small. however gravity is minuscule in most cases. I know I may be a little rambling so I will put it a different way.
I am affected by gravity most by closer and larger massed objects, but overall, I am acted upon more by the small things like a particle of dust halfway across the galaxy.
If the force of dust acting on me from halfway across the galaxy isn't considered small I will eat my hat
Right, left, top, bottom, above and below. Generally, it's 2 x number of space dimensions (6 = 2 x 3).13 Rabbit wrote:Most of the craters seem to have six sides.