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Vultur
2008-Oct-04, 12:36 AM
I can't believe I can't figure this out, it ought to be obvious, but:

Since the same face of the Moon always faces the Earth, why does it have craters? Why didn't Earth block those meteors? Did they just come in at an angle?

Neverfly
2008-Oct-04, 02:01 AM
I can't believe I can't figure this out, it ought to be obvious, but:

Since the same face of the Moon always faces the Earth, why does it have craters? Why didn't Earth block those meteors? Did they just come in at an angle?

Bear in mind that the Moon did not Always keep the same face toward Earth.
The far side of the Moon is more heavily cratered now though.

Also, yes, impactors can come at many angles. Consider how far the Earth is from the Moon and how large the Earth is, it's not a very effective shield anymore than the Moon is for the Earth. A little bit- but not much.
Most of the shielding is from gravity slinging objects, not from the Earth literally jumping out in front of asteroids and physically stopping them from hitting the Moon.

01101001
2008-Oct-04, 03:51 AM
Looking at the scale, do you still think the Earth would provide much protection?

From Wikipedia: Illustration of Earth and Moon at average distance from each other (http://en.wikipedia.org/wiki/Image:Earth-Moon2.jpg)

http://upload.wikimedia.org/wikipedia/commons/thumb/c/c0/Earth-Moon2.jpg/800px-Earth-Moon2.jpg (http://en.wikipedia.org/wiki/Image:Earth-Moon2.jpg)

Ozzy
2008-Oct-04, 04:01 AM
I love that image!

PraedSt
2008-Oct-04, 05:18 AM
I may be wrong...

1. The vast majority of impacts took place a long time ago.

2. When those occurred, the same side did not always face us (the moon rotated faster). As Neverfly points out.

novaderrik
2008-Oct-04, 05:51 AM
Looking at the scale, do you still think the Earth would provide much protection?

From Wikipedia: Illustration of Earth and Moon at average distance from each other (http://en.wikipedia.org/wiki/Image:Earth-Moon2.jpg)

http://upload.wikimedia.org/wikipedia/commons/thumb/c/c0/Earth-Moon2.jpg/800px-Earth-Moon2.jpg (http://en.wikipedia.org/wiki/Image:Earth-Moon2.jpg)
there is also an image like that out there that puts the solar system in a similar scale.. there is a LOT of side scrolling going on in that pic..

astromark
2008-Oct-05, 08:34 AM
Not all the objects that are orbiting the sun are in the planetary plain of this solar system. The gas giants can and do remove some objects that might be coming this way. Just as the object nearing the moon might be pulled around it as it spirals into it. Its a complex subject with many variables. No definite yes, no maybe and because can be stated without much more information. Trajectory and velocity and mass all play important rolls in the track of any object near a larger mass object.

mugaliens
2008-Oct-05, 03:13 PM
Did they just come in at an angle?

Yes. That and the fact that Earth, when seen from the Moon, doesn't occupy much space in the sky.

Only about 2 degrees.

cjameshuff
2008-Oct-05, 03:49 PM
Yes. That and the fact that Earth, when seen from the Moon, doesn't occupy much space in the sky.

Only about 2 degrees.

Earth is 3.7 times as big as the moon, and the distance is obviously the same whether you're standing on the Earth or the moon...so it would appear 3.7 times as large in the sky as the moon does on Earth. An impressive sight, but not much of a shield.

It could tidally disrupt a significant fraction of impactors, biasing the size distribution a little more toward the lower end for recent (post tidal lock) impacts. This would also increase the odds of an impact, since the result would be a spread of smaller debris that would have higher probability of intersecting the moon.

The presence of Earth has likely decreased impact events overall, due to it destabilizing the orbits of likely impactors and forcing them into other orbits or ejecting them from the system entirely. I wouldn't expect that to lead to any bias in effects on one side of the moon, though.

Eroica
2008-Oct-05, 04:31 PM
Looking at the scale, do you still think the Earth would provide much protection?

From Wikipedia: Illustration of Earth and Moon at average distance from each other (http://en.wikipedia.org/wiki/Image:Earth-Moon2.jpg)

http://upload.wikimedia.org/wikipedia/commons/thumb/c/c0/Earth-Moon2.jpg/800px-Earth-Moon2.jpg (http://en.wikipedia.org/wiki/Image:Earth-Moon2.jpg)
Is the relative size of the Earth really significant? I would have thought that the key factor is the Earth's gravitational focusing of potential impactors, which would tend to increase the likelihood of the Moon being hit than would be the case if the Moon was a lone planet in its own right?

It's still intriguing that the Moon displays this hemispherical dichotomy. A number of satellites in the outer Solar System (Rhea, Dione, Iapetus) have different leading and trailing hemispheres, but I don't know if any display the same sort of dichotomy as the Moon.

a1call
2008-Oct-05, 04:40 PM
The presence of Earth has likely decreased impact events overall,

It certainly does look that way:

See the far side. (http://apod.nasa.gov/apod/ap981008.html)

PraedSt
2008-Oct-05, 05:22 PM
I wouldn't expect that to lead to any bias in effects on one side of the moon, though.

Right. For all the reasons given above, I don't think it has.

Both sides were impacted equally. The far-side appears more cratered because it has less maria.
Near-side maria have filled in most of the craters there, giving it a less cratered appearance.

I note from wiki that there is no consensus as to why the near-side has more maria.
http://en.wikipedia.org/wiki/Lunar_mare

This is because they haven't yet considered the possibility of ancient lunar civilisations, monkey colliders, and Demigrog's astute analysis.
http://www.bautforum.com/off-topic-babbling/79613-super-monkey-collider-loses-funding.html

a1call
2008-Oct-05, 05:43 PM
Geological Survey in Flagstaff, Ariz., found that the far side has about twice as many rayed craters under 10 kilometers in diameter as predicted by the crater counts from the near side. They reported these results last month at the Lunar and Planetary Science Conference in Houston.Source (http://findarticles.com/p/articles/mi_m1200/is_/ai_18205260)

PraedSt
2008-Oct-05, 06:00 PM
a1call

Good link.

BUT, I notice that they are applying the results of the 'new' count towards a) estimating the population of asteroids and comets, and b) towards refining their estimate of the age of planetary surfaces.

Despite it being the obvious thing to do, they are going nowhere near a theory of a differential rate of impacts. Not even mentioned once!

Probably because they're satisfied that both sides were hit equally (due to all the reasons given above, by most posters), but that the near-side record is partly obliterated; by maria.

Indeed, that's why they re-counted the far-side in the first place, a 'near-perfect' record.

No?

a1call
2008-Oct-05, 06:10 PM
Like any wisely written scientific article it does avoid stepping on any mainstream toes but my quoted text does, IMHO clearly imply a bias towards excess of young and small craters on the far side.

PraedSt
2008-Oct-06, 05:30 AM
a bias towards excess of young and small craters on the far side

Ok! Any ideas why?

djellison
2008-Oct-06, 10:18 AM
Because the near side is younger - because volcanic activity lasted longer on the near side than the far side due to the tidal influences of the Earth.? I don't think this is a symptom of impact, rather a symptom of what's happened since impacts. Crater counts are used to age things. Fewer craters - younger surface.

hhEb09'1
2008-Oct-06, 10:56 AM
Is the relative size of the Earth really significant?I think the illustration shows that the size is not significant, in shielding the moon.
I would have thought that the key factor is the Earth's gravitational focusing of potential impactors, which would tend to increase the likelihood of the Moon being hit than would be the case if the Moon was a lone planet in its own right?I'm not sure how that gravitational focusing would work to make one side have more impacts than the other, though. Wouldn't it just draw more impactors to the vicinity, from all sides? I mean, if it were effective at all.


It's still intriguing that the Moon displays this hemispherical dichotomy. A number of satellites in the outer Solar System (Rhea, Dione, Iapetus) have different leading and trailing hemispheres, but I don't know if any display the same sort of dichotomy as the Moon. Of course, the moon's sister planet does display a dichotomy :)

Eroica
2008-Oct-07, 11:56 AM
I'm not sure how that gravitational focusing would work to make one side have more impacts than the other, though. Wouldn't it just draw more impactors to the vicinity, from all sides? I mean, if it were effective at all.
That was the point I was making.

JustAFriend
2008-Oct-08, 01:46 PM
http://upload.wikimedia.org/wikipedia/commons/thumb/c/c0/Earth-Moon2.jpg/800px-Earth-Moon2.jpg (http://en.wikipedia.org/wiki/Image:Earth-Moon2.jpg)


I liked seeing the photo to scale.

The biggest problem I have with sci-fi the last few decades is they have taken away ALL the scale.

Starships fight battles right off each other's noses at light-speed like old pirate movies.
Planets look like they're only a few miles from each other.
Solar systems look like they're only a few hundred miles across.
Everything is RIGHT IN YOUR FACE.

I'm a writer and have done CGI so I know its all about storyline, but there's a LOT of the (ahem) simpler (ahem) people out there who really don't have a clue. And the schools aren't helping.......

cjameshuff
2008-Oct-08, 02:28 PM
Starships fight battles right off each other's noses at light-speed like old pirate movies.
Planets look like they're only a few miles from each other.
Solar systems look like they're only a few hundred miles across.
Everything is RIGHT IN YOUR FACE.

The new BSG miniseries did a good job using camera zoom to give a relatively realistic sense of distance. Don't know if they kept it up in later seasons. Early Andromeda episodes also did pretty well, at least making an effort...though it went way beyond unrealistic and outright absurd to just plain bad by the time things got to the "worldship".

Then there's Firefly...not so much a poor concept of scale as an almost entirely absent sense of scale. All moons appear to be approximately Earth-sized and about the same distance from the sun, great for terraforming, though some of them are far from civilization while others are right in it. The movie committed the worst sins as far as inter-spacecraft distances, though.

mugaliens
2008-Oct-10, 09:33 PM
Everything is RIGHT IN YOUR FACE.

:lol:

From an entertainment value perspective, Star Trek got it right with their huge, wonderous planets beneath the Enterprise. Dot's don't sell commercials.