Starship Asterisk*

by **FieryIce** » Wed Aug 08, 2007 2:40 am

An interesting comment on LPOD July 31, 2007 about our Moon.
“the Moon is credited with protecting the Earth from large impact events that are hazardous to life”
Comment…. “Or has the Moon shielded us from bombardment through some completely different mechanism?”
THE MOON AND LIFE

by **makc** » Mon Aug 06, 2007 1:55 pm

13 Rabbit wrote:Most of the craters seem to have six sides.

Right, left, top, bottom, above and below. Generally, it's 2 x number of space dimensions (6 = 2 x 3).

by **Qev** » Mon Aug 06, 2007 7:52 am

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

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.

A good demonstration of how weak gravity is would be a small rare-earth magnet suspending a significant mass against the entire gravitational pull of the Earth.

by **Tom of Dreams** » Mon Aug 06, 2007 7:33 am

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

by **craterchains** » Sun Aug 05, 2007 10:45 pm

Then there is earth's moon that has it's outboard edge all cratered, and the inboard (facing earth) mostly free of craters?!?!

by **bystander** » Fri Aug 03, 2007 12:52 pm

inertnet wrote:...In Dione's region, there must be a significant number orbiting in the opposite direction...

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.

by **inertnet** » Fri Aug 03, 2007 6:24 am

I've been trying to come up with a reason why the impacts are mostly on the trailing side.

You have to consider Saturn and its moons on its own. Although the system is orbiting the Sun, I think this will have only a minor 'impact' on where most of the impacts will end up. Along with its moons, Saturn is also dragging a large number of asteroids along, all of them in some kind of orbit around the planet. In Dione's region, there must be a significant number orbiting in the opposite direction. I think that's the most logical explanation why the majority of impacts are on the trailing side of Dione.

by **Qev** » Fri Aug 03, 2007 5:32 am

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.

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.

by **13 Rabbit** » Fri Aug 03, 2007 1:13 am

Most of the craters seem to have six sides.

by **Keldor314** » Thu Aug 02, 2007 11:12 pm

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. The typical velocity would be around Saturn's orbit velocity + Dione's orbit velocity + Dione's escape velocity.

Finally, the .1% of asteroids that happen to hit at a low velocity will also leave much smaller craters than similar sized asteroids would at more typical speeds. Hence, in order to find any greatly elongated craters, you'll have to bring out your magnifying glass and look around for quite a while.

by **Pete** » Thu Aug 02, 2007 9:26 pm

craterchains wrote:
Pete wrote:
craterchains wrote:How nieve it is to assume that ALL objects are traveling at hyper velocities.

Think about it.
And no, a planet or large moon can't "sneak up" on asteroids traveling at almost the same velocity and gently sweep them up.
Guess then our solar system just couldn't have been formed by that very method? *coughs*

Planets and large moons weren't around before the formation of, well, planets and large moons

My intent was to explain why tossing rocks into sand, a process which easily produces noncircular "craters," is not analogous to the process behind the circular craters of moons and rocky planets. I was referring to the impossibility of the scenario in which a massive body (say, a large moon) traveling at 1 km/s approaches a little asteroid traveling in the same direction at 1.001 km/s and gently scoops it out of space at a closing speed of 1 m/s. No, impact speed will at least equal the moon's escape speed.

Over some range of escape speeds, impact energy is low enough to allow oblique crater formation; considering the noncircular craters that Chris pointed out, Dione is around the upper limit of that range.

by **Chris Peterson** » Thu Aug 02, 2007 4:13 am

quixote218 wrote:Seems to me that the more heavily cratered side of Dione should be the trailing side.Right?

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.

You can see this effect on Earth by looking at the frequency of meteors. There are many more meteors towards dawn, when the zenith is above our own leading edge, than in the early evening when it is above our trailing edge.

by **Chris Peterson** » Thu Aug 02, 2007 4:06 am

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.

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.

by **quixote218** » Thu Aug 02, 2007 3:04 am

Seems to me that the more heavily cratered side of Dione should be the trailing side.

An entity in a nearly circular orbit around something like Saturn is likely traveling at a slower speed than anything else at its orbital altitude. Anything slower and still at that altitude would be in a degrading elliptical orbit that causes it to get pulled into Saturn.

So if slower is almost ruled out, then faster in the orbit means that Dione is being attacked from behind.

Right?

by **craterchains** » Thu Aug 02, 2007 2:10 am

Pete wrote:
craterchains wrote:How nieve it is to assume that ALL objects are traveling at hyper velocities.

Think about it.
And no, a planet or large moon can't "sneak up" on asteroids traveling at almost the same velocity and gently sweep them up.

Guess then our solar system just couldn't have been formed by that very method? *coughs*

by **Pete** » Wed Aug 01, 2007 10:56 pm

craterchains wrote:How nieve it is to assume that ALL objects are traveling at hyper velocities.

Think about it.

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.

by **jimmysnyder** » Wed Aug 01, 2007 9:35 pm

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?

Here are a couple of sites with information.

http://zebu.uoregon.edu/~imamura/121/nov10/crater.html
http://zebu.uoregon.edu/~imamura/121/le ... rater.html

by **craterchains** » Wed Aug 01, 2007 8:54 pm

How nieve it is to assume that ALL objects are traveling at hyper velocities.

Think about it.

by **Pete** » Wed Aug 01, 2007 6:56 pm

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.

I wondered about that too after reading about it in Lucifer's Hammer (comet impact disaster novel).

Astronomical impactors deliver explosive amounts of kinetic energy to tiny spots on their targets, excavating circular holes much larger than the original infalling bodies. Crater formation is governed by the impactor's energy rather than its shape, dimensions, or trajectory. a stone tossed into sand at a shallow angle produces an elongated "crater" because it doesn't reflect the dynamics of asteroid impacts, unless you're tossing stones at the Moon from an orbital railgun or something

(you listening, NASA?)

search the web for better info; I'd look for and paste some sources, but I need to get back to work too!

by **tbranch** » Wed Aug 01, 2007 3:56 pm

Perhaps this is a naive thought, but one explanation for more craters on the trailing side of Dione, is that Dione is orbiting more slowly than other debris, and therefore the debris hits it from behind rather than Dione hitting the debris.

by **craterchains** » Wed Aug 01, 2007 3:01 pm

Looking at this picture it is obvious that there are many "odd" craters on Dion, , ,

http://antwrp.gsfc.nasa.gov/apod/ap070801.html

by **jimmysnyder** » Wed Aug 01, 2007 12:14 pm

Dione always has one side that faces Saturn, and always has one side that faces away.

As reassuring as this is from a logical point of view, I don't think it is what the annotator meant to say.

by **zeilouz** » Wed Aug 01, 2007 8:13 am

Obviously,our moon have more craters compared to Dione.. 8)

by **craterchains** » Wed Aug 01, 2007 7:44 am

Go back to work, there is nothing odd about Dione, , ,

by **Alan327** » Wed Aug 01, 2007 7:40 am

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.

Starship Asterisk*

Cratering on Dione question (APOD 01 August 2007)

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Cratered from behind

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Dione moving slower than other bodies?

Cratering on Dione question (APOD 01 August 2007)