The eighth moon sounds exciting... "it travels in the same region of space as the retrograde moons, but in the opposite direction... That means it could easily smash into the retrograde moons, pulverising itself into oblivion."
Setting up science nerds for thousands of years of suspense!
Wouldn't it be a spooky experience to stand on one moon, and have another pass close enough for you to touch it? If they were of similar mass my intuition is that you'd go weightless while it was happening. Might be best to be harnessed during something like that.
While I'm thinking about spooky space stories. Jupiter creeps me right the hell out. Thinking about being on one of its moons where Jupiter takes up a HUGE percentage of your visible sky and is just screaming radiation at you. Personify the gas giant a little and you've got a decent proxy for hell. https://solarsystem.nasa.gov/resources/184/simulated-view-fr...
> If they were of similar mass my intuition is that you'd go weightless while it was happening.
Remember that they would be attracted to each other, so the orbit would have to be absolutely perfect for this to work.
Imagine an X - each moon traveling on one of the lines, and just barely touching at the center of the X. Their speed would be incredibly high - don't try to touch the other moon, in fact it would be so fast you might not even be able to see it. (In reality X would be very curved.)
That would be amazing, it looks like Europa travels at 14 km/s or 31,000 mph. With no atmosphere there would be no sound or pressure wave. And the bodies are probably small enough to not shatter because of the gravity interaction. Escape velocity is about 36,000 mph. So you could see a huge Manhattan size chunk of rock flying at you at rocket speeds. Would be amazing to see it pass by 100 feet above you.
> Would be amazing to see it pass by 100 feet above you.
At those speeds, and the necessary angle, I'm not convinced you would be able to see it at all.
The actual encounter would last for too short of a time to see (like trying to see a bullet in flight), and their motion would take them out of your field of view, unless the planet you were on was rotating very fast.
The two moons are going in opposite directions, so the speed guess is really 28 km/s.
For a full second it would be 2km across and 14km or less away (.5 seconds before and .5 seconds after it's overhead). That's 16 degrees of visual angle. The moon is .5 degrees of visual angle, so you get a full second of something 32 times bigger than our moon.
It would be moon sized about 230 km away, which is over 8 seconds away. So you'd get to see a moon grow quite large over 8 seconds.
So you would see it, but would it be awesome?
Lets say our eyes can see things in the 100 ms range (really a bit better). For that 100 ms, the object would be < 1.4km from you and subtend fully 71 degrees of your vision. That's about 2 basketballs width at arms length.
Which sounds cool though maybe not awesome. But hey you are looking at a moon rushing at your face like a basketball, I would totally go.
You have to check the angles. Think of the X I posted earlier. Imagine your goal is to stand at the point looking "up" (because that's were they will meet), both moons were traveling bottom to top, and you were on the moon that started bottom left. (So your gaze is at an angle to direction of travel.)
For most of the time the moon will be behind you to your right out of sight. After the interaction, if the other moon was larger than yours, you might be able to see it.
But if it was smaller, then your own moon's horizon would block your view of it.
But it's worse than that - even if you could see it, you would have to turn your head to the left to see it, but it's moving so fast you just don't have enough time to physically move your head before it's far far away.
i.e. It would whiz by across your field of view, not toward it. Which makes it really hard to see.
Edit: I'm tying to visualize how it would look, but I may have missed some detail. It would be cool if someone could program a visualization of this.
In the spirit of "could this be awesome" rather than is it likely...
So pretend we stand on the side of the moon facing toward Jupiter so that Jupiter is straight overhead. And pretend the other moon will pass just inside our orbit 100 ft above our head.
Imagine 2 quarters overlapping, representing the orbits. Pushing the top one up a bit keeps the inner moon completely above the horizon for the whole approach. You can also push the top quarter to the left a smidge so that the inner moon orbit is coming from high and to the right. This would be a moon sized object popping up high and right in the sky and then disappearing in a blink below the horizon to your left.
Speed isn't an issue because for the 8 full seconds before it's right overhead it will be our moon sized and bigger. Up to a 2 "arm's length basketballs" when right overhead. You could definitely track the moon across the half the sky over 8 seconds.
I was thinking a bit more on this today. Specifically If a moon did this past earth and assuming it was made out of something that wouldn't just break apart in our atmosphere.
The amount compression on the air around you might set you on fire.
This article is a bit incomplete. A planet can not just capture a moon from something that just happened to come near it.
In order for a moon to enter orbit around a planet it must lose energy, there are two ways for it to do this: Crash into another object, or complicated 3-body interactions, which fling another object away.
> It may be the leftovers of a bigger cosmic collision in the past, Sheppard says.
That is the mostly likely scenario: Two objects collided, one shattered into 7 moons traveling in one direction, and the other ended up as this moon traveling backwards.
Setting up science nerds for thousands of years of suspense!