How do the Moon and Jupiter protect the Earth from meteorites?

April 30, 2024 9:05 AM

I've read that the Moon and Jupiter protect the Earth by slinging space junk away from us. But naively, for every space rock that was heading towards us that gets slung away, isn't it equally likely there was a space rock that wasn't heading for us that now gets slung at us?
posted by musofire to Science & Nature (10 answers total)

Birth of the Solar System
Here is an exquitsitely beautiful explantation of the current theory on planetary development that includes how Jupiter and the moon shield the earth. About twenty five minutes long.
posted by effluvia at 9:19 AM on April 30 [1 favorite]

Best answer: This is actually an incredibly good question -- and one that doesn't seem to have been entirely resolved. There have been studies suggesting that perhaps Jupiter isn't the shield it has long been assumed to be; see for example this study or this study, both attempts to replicate the hypothesized "Jupiter shield" effect via N-body simulation. The latter paper also has a section that explains the historical origin of the idea that Jupiter acts as a shield, and why people once assumed that to be true.
posted by etealuear_crushue at 9:28 AM on April 30 [4 favorites]

My dumb response (because I don't really know anything about this) but, like, it wouldn't be equally likely, would it, because slung away from us is a huge space (any direction that is not us) and slung at us is a very tiny space (us). The proverbial needle in a haystack.
posted by kbanas at 10:21 AM on April 30

I'm pretty sure kbanas has got it. If things are being flung in random directions then the possibility of hitting Earth is pretty low.
posted by Tell Me No Lies at 10:52 AM on April 30

My layman's understanding is that it's largely to do with gravity. To understand gravity, the classic thought experiment is to imagine space as a rubber sheet. The heavier something is, the deeper the "dip" it makes in the rubber sheet. The Sun is by far the biggest, heaviest thing in the solar system, so it makes by far the biggest dip. Jupiter is much, much smaller than the Sun but still many times bigger than the Earth. So the dip created by Jupiter is much smaller than the dip created by the Sun, but still much bigger than the dip created by the Earth.

Now imagine rolling a ball towards one of those dips. If you roll the ball directly towards the centre of the dip it will most likely roll straight down the dip and hit the middle of the dip. This is what happens when a meteorite hits a planet - the meteorite is on a direct collision path with the planet and goes straight into it. (This happens every day on Earth but most meteorites are so small they burn up in the atmosphere so we don't notice them.)

But if you roll the ball close by to the dip then if the ball is going fast enough it will enter the dip, change direction a bit, and then escape the dip and keep going, just on a slightly different path. This also happens every day on Earth (and every planet) but again the meteorites are either so small we don't notice them or large enough that we do, but can calculate if they're going to hit us or just change direction a bit.

The Earth is the 3rd planet out from the Sun. Jupiter is the 5th planet out from the Sun. Any asteroids or comets that might cross Jupiter's path around the Sun and come into the inner Solar system have a good chance of being perturbed by Jupiter's large gravitational field and either hitting Jupiter or being flung off in a different direction. That direction is much more likely to be away from us than towards us, partly because we're a small target, but mainly because if something is coming from outside of Jupiter it will most likely hit Jupiter or be flung outwards away from Jupiter, rather than somehow go past Jupiter into the the inner Solar system.

So as Jupiter revolves around the Sun it "picks up" rocks that might otherwise make their way unhindered towards Earth's orbit and either gets hit by them or flings them out away from us thanks to gravitational mechanics. Because Jupiter is so big, with such a correspondingly large gravitational dip in space, it picks up a lot of rocks.

(That's a very simplified explanation of a complex dance of forces and I'd be happy for any non-layman to correct it or even say it's flat out wrong. Plus I recognise what others above have said about this explanation no longer being as confidently proclaimed as it used to be. But it's basically how most popular astronomy books explain it.)
posted by underclocked at 12:09 PM on April 30 [3 favorites]

I'm pretty sure kbanas has got it. If things are being flung in random directions then the possibility of hitting Earth is pretty low.

Sure, but if things are flying randomly through space then the possibility of them hitting the Earth is also pretty low. The question is if Jupiter's involvement makes it even lower.
posted by It's Never Lurgi at 2:04 PM on April 30 [3 favorites]

Gravity is an attractive force so rather than thinking about Jupiter slinging rocks all over the place, just think of the rocks as being drawn towards Jupiter and therefor away from Earth. The flinging and slinging isn't really the point.
posted by grog at 3:35 PM on April 30 [1 favorite]

Your intuition is correct, that the energy of the system is conserved whenever an object is perturbed by Jupiter. In fact, astronomers use this fact to decide whether a new object is likely to be the same one they've seen before.

But Earth cares mainly about whether, on the average, Jupiter tweaks objects in such a way that they cross Earth's orbit. Does it often lower the objects' periapsis (closest point to the Sun) to below 1 AU? This is usually bad.

But if the encounter also raises the eccentricity of the orbit to near-parabolic or hyperbolic, it might be ejected from our solar system forever. That'd be good, but it only applies to long-period comets, like those from the Oort cloud. These are going so fast that a little push from Jupiter is all they need to attain escape velocity. This is where the "Jupiter vacuum cleaner" theory comes from.

However, there are lots of other nasty rocks that bounce between Earth and Jupiter's orbit, and Jupiter's shepherding of these objects is not as easy to determine. They're also going slower, and not likely to be ejected from the solar system. Is Jupiter making their orbits better for us, or worse? And are they more dangerous overall than the sexy Oort cloud comets? These are the open questions.
posted by credulous at 5:11 PM on April 30 [2 favorites]

Best answer: It is not really proven. I think people believe its true because the Moon for example has so many craters. But we all know thats only because erosion hasn't made them fade away like on Earth.
Were those object that would have hit Earth? Its not clear.

There's a good N-body simulator out there called Rebound and I suppose you could try this yourself. Certainly for a "rain" of asteroids from all directions. But most asteroids that are dangerous to us are repeat visitors that are roughly sharing the same orbit around the Sun as us so not "random" in a true sense.
posted by vacapinta at 6:30 AM on May 1

Sure, but if things are flying randomly through space then the possibility of them hitting the Earth is also pretty low. The question is if Jupiter's involvement makes it even lower.

But comets aren’t randomly flying through space. Due to their highly eccentric orbits they spend half of their lives headed directly for the inner system.

In any case, I am really over my head here so I will leave it to the experts.
posted by Tell Me No Lies at 7:38 AM on May 1