Earth may once have had two moons | Hacker News Reader

Earth may once have had two moons

bbc.co.uk

75 pointsespeed18 comments

Nature's summary, with a bit more detail: http://www.nature.com/news/2011/110803/full/news.2011.456.ht...

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.

  > 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?

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.

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

That's kind of strange, "speed of sound" in a mainstream publication almost certainly means the speed of sound through air at sea level (~.3 km/sec)

But if the scientist or paper talked about the speed of sound while referring to sound in rock, would the science reporter have realized that this wasn't the speed of sound in air? In my experience of science reporting in mainstream publications: no.

That might very well be the source of the strangeness :-)

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."

"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.

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

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.

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

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

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

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'.

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.

From the article:

  > Quasi-satellites orbit the Sun...

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

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..."

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.

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