

Earth may once have had two moons - espeed
http://www.bbc.co.uk/news/science-environment-14391929

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jerf
Nature's summary, with a bit more detail:
[http://www.nature.com/news/2011/110803/full/news.2011.456.ht...](http://www.nature.com/news/2011/110803/full/news.2011.456.html)

Since I occasionally pound on this point, I'd observe this is a _good_ use of
computer models; they take their hypothesis, explore its possibility, and
generate real-world predictions that we can test against to determine if it
accurate. (Albeit perhaps decades later, but that's not their fault.) And no
claims about how it has now been proved or anything.

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biot

      > It was a rather gentle collision at around 2.4km per
      > second; lower than the speed of sound
    

Is that the speed of sound through solid rock, or should that be m/s instead?

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ominous_prime
No, that's about right. Seismic waves propagate between 2-8 km/s depending on
rock density. 2.4 m/s is an easy run for most people.

~~~
biot
I suppose that's the problem with speeds in space... 2.4 km/s would be
anything but "a gentle collision" on Earth.

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eru
Interesting quote: "The researchers involved hope that data from two US space
agency (Nasa) lunar missions will substantiate or challenge their theory
within the next year."

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astine
"For decades, scientists have been trying to understand why the near side of
the Moon - the one visible from Earth - is flat and cratered while the rarely-
seen far side is heavily cratered and has mountain ranges higher than 3,000m."

Is it possible that the Earth has shielded the moon from asteroid collisions?
This seems to me to be a far simpler explanation as to why the side facing
Earth has fewer collision marks.

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AlecSchueler
It's the opposite way round. The side that's not facing the Earth has fewer
craters.

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sesqu
I'm under the impression that's not true. Looking at
<http://www.lpi.usra.edu/lunar/missions/clementine/images/> I see a smoother
surface on the earth side. I also found on the Internet some paired images
from different cameras, which of course can be chosen to emphasize either
side.

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mturmon
You are correct. There is a nice browser with a good DEM at

<http://pub.lmmp.nasa.gov/lunarmapper/lm.html?host=pub>

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singingwolfboy
Earth currently has two moons, or even more, depending on how you define
"moon": <http://en.wikipedia.org/wiki/Other_moons_of_Earth>

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joejohnson
How are you proposing a moon be defined? From the Wikipedia article: "There
have been large generic searches for small moons, actual proposals or claimed
sightings of specific objects in orbit, and finally, analysis and searches for
those proposed objects. All three of these have failed to confirm a permanent
natural satellite."

Also from Wikipedia: "A natural satellite or moon is a celestial body that
orbits a planet".

There have been no other confirmed moons of Earth besides 'The Moon'.

~~~
singingwolfboy
They're not officially categorized as "moons", but the article lists several
"quasi-satellites" that orbit the Earth. There is a distinction, but
personally, I would consider them similar enough to count as other moons.

~~~
biot
From the article:

    
    
      > Quasi-satellites orbit the Sun...
    

By definition, that makes them not a moon of Earth.

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hugh3
I'd just like to congratulate everybody on the fact that there are seven
comments so far and not one of them says "That's no moon..."

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vorg
Why the congratulations? If people are no longer saying "That's no moon..."
then perhaps the deception is complete.

> Researchers suggest the collision may explain the mysterious mountains on
> the far side of our Moon.

The best way intelligent life, such as humans, can travel between stars is
embedded deep inside a large pre-existing object, rather than a flimsy
arficially-made spaceship. That object could be modified so it can navigate
around by putting large booster rockets ON ONE SIDE. If humans wanted to
observe a planet for terraforming and colonization, they would navigate the
object into orbit as an apparent Moon around that planet. If their habitat in
that moon was on the opposite side as the booster rockets, then by leaving
those rockets, which would look like mysterious mountains, on the permanent
far side from the Earth, they could keep the Earth permanently within
observation for terraforming, etc.

