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In multiple different movies and fiction, there's the threat of a burning space comet heading for Earth, threatening to kill everyone and destroy the planet.

One example is the animated Comet in Moominland (1992).

When they look at it in a telescope, and later it's seen on the sky, it's depicted as a burning ball with a trail, seemingly going sideways, to the right, past Earth. If it were heading toward Earth, wouldn't it have no visible "tail", but simply be a big round burning ball which gets increasingly bigger?

Am I fundamentally missing something here? Why do they so often depict it appearing "sideways" rather than "frontways"? Would a comet heading straight to Earth ever appear to be going past Earth, or for some reason have a trail/tail to the left of the big burning sphere, even though it's coming for us?

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    $\begingroup$ This is an interesting question and the details of actual comet tail behavior is well covered in Astronomy SE already so if asked there it would probably be quickly closed as duplicate. However a question about comets in movies would be in my opinion a great question in Movies SE or Science Fiction and Fantasy SE! Have a look around those sites and see what you think. $\endgroup$ – uhoh Nov 2 '20 at 2:02
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    $\begingroup$ Any video of a ship with smoke stacks chugging along on a windy day? Or someone walking around with a flag on a windy day? $\endgroup$ – Nat Nov 4 '20 at 8:15
  • $\begingroup$ This would seem not to be a question of space exploration, but astronomy. $\endgroup$ – James K Nov 4 '20 at 21:56
  • $\begingroup$ Opinions of "better suited to Astronomy" do not make something off-topic here. There are currently 55 questions tagged with comet including 22 also including "tail" the newest of which is Have spacecraft ever passed through the tails of comets? Voting to reopen because this is not off-topic. $\endgroup$ – uhoh Nov 5 '20 at 22:01
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    $\begingroup$ @uhoh - while questions about comets do exist, they generally relate to space exploration through the interaction between spacecraft and comets, or how a comet might be explored, etc. This is just a question about the general behavior of comets when viewed from Earth. It's on the edge, but if you want to give a reason, then it would fall under "heliophysics and planetary science", not "we have some already". $\endgroup$ – IronEagle Nov 6 '20 at 16:30
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The comet's tail always points away from the Sun. Yes, even when the comet is heading back into the outer solar system.

This is because the tail isn't a 'trail' of where the comet has been, like a rocket exhaust or contrail, but instead it's gas, ice and other debris blown off by the stellar wind.

(There's actually two tails, one made of charged particles, and one of neutral dust, but that's not especially relevant to the question)

You can see the tail behaviour clearly in the image below. comet tail always points away from sun image

So yes, while movies about comets are often notoriously inaccurate, you can indeed have a comet coming toward earth with a tail pointed sideways. But, I suspect accuracy is not why they depict the comet with a sideways tail in movies. A front-end comet will not be nearly as recognisable to viewers as a sideways one.

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    $\begingroup$ "This is because the tail isn't a 'trail' of where the comet has been, like a rocket exhaust or contrail, but instead it's gas, ice and other debris blown off by the stellar wind." Even if it was, it'd be entirely possible for it to be positioned "sideways" compared to the Earth as it came in for a collision, if its trajectory was perpendicular to the Earth's. $\endgroup$ – nick012000 Nov 3 '20 at 3:57
  • $\begingroup$ @nick012000 As Ingolifs has pointed out, the comet's trajectory doesn't matter, only the position relative to earth and sun. If it came from the direction of the sun, the tail could even (nearly) point towards earth. $\endgroup$ – Zac67 Nov 3 '20 at 11:10
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    $\begingroup$ “isn't a 'trail' of where the comet has been, like a rocket exhaust” – rocket exhaust also only forms such a trail whilst in the atmosphere. In vacuum, the exhaust tends to disperse in all direction, but even if you avoided that (which would be possible with an extremely large nozzle extension) then it wouldn't point “backwards” in the intuitive sense either. Away from the rocket's back, yes, but depending on what maneuver you're performing the rocket could point in any direction, not necessarily along its orbit (though that is usually most efficient). $\endgroup$ – leftaroundabout Nov 3 '20 at 13:55
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    $\begingroup$ @Zac67 Yes, I realise that. My point was that even if that wasn’t true, you could still have a comet moving sideways and hitting the Earth. $\endgroup$ – nick012000 Nov 3 '20 at 22:32
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    $\begingroup$ weird. This has now become my highest rated answer on this SE and I almost posted it as a comment. I don't ever think I'll get used to the weird Pareto-law stuff that happens to posts here. $\endgroup$ – Ingolifs Nov 5 '20 at 2:54
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While those movies probably do it because that's how comets are commonly depicted, it might not be that inaccurate. Remember that the Earth itself is moving around the Sun, so if a comet is heading for Earth, that means it's heading for a point where Earth will eventually be, not where it currently is. It would therefore be possible to see such a comet be "sideways".

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    $\begingroup$ @uhoh I'd go a bit further... It's half the answer. The other half is in the answer by Ingolfis. $\endgroup$ – Oscar Bravo Nov 2 '20 at 10:51
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    $\begingroup$ This doesn't really have anything to do with it - the trajectory of a comet makes no difference whatsoever in where the tail points. Yes, you can see a comet tail side-on from the earth, but whether the comet is heading toward earth, or toward where the earth will, or in any other direction won't change a thing about where the tail points. How a comet's tail looks only depends on where it is, not where it's going. $\endgroup$ – Nuclear Hoagie Nov 2 '20 at 17:48
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    $\begingroup$ @NuclearHoagie I think the point though is that even if the tail points directly behind the comet, that doesn't necessarily mean you won't see it sideways. The question could be construed as "if a comet was on a collision course with Earth, wouldn't its trajectory be directly toward an observer on Earth?" and I would say the answer to that is no. $\endgroup$ – DarthFennec Nov 2 '20 at 18:27
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    $\begingroup$ @bdsl By "directly behind" I of course mean "opposite to the direction of motion". I thought that was obvious from context but sure, I could have been more clear. $\endgroup$ – DarthFennec Nov 2 '20 at 19:49
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    $\begingroup$ @NuclearHoagie I read the question in two parts. One, making the incorrect assumption that the tail is "behind" the comet. Two, making the incorrect assumption that the trajectory of a collision course would be directly toward an observer. Showing that either assumption is false is enough to answer the question, but I would argue that you need both for a complete answer. So, sure, the question is technically asking about the tail, so focusing on the tail is valid, but I think it ignores the other (more implicit) half of the question. $\endgroup$ – DarthFennec Nov 2 '20 at 19:59
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If you're the comet, the way to hit Earth is not to head directly for it. That's because Earth is orbiting the sun: you need to aim at where Earth will be, not where it is right now.

For example:

Animation of InSight trajectory.gif

By Phoenix7777 - Own work Data source: HORIZONS System, JPL, NASA, CC BY-SA 4.0, Link

This shows a transfer orbit from Earth (blue) to Mars (green), but the basics of orbital mechanics are the same for a comet. Just imagine the probe (purple) is the comet, on a trajectory to collide with Earth (green).

As you can see, the comet isn't headed "right at" Earth. Rather it's mostly headed in the same direction as Earth, which is a counter-clockwise rotation in this image. If you could somehow see the path the comet is traveling from Earth, it would not be behind the comet, but to the side.

But also, the trail of a comet does not leave debris in its path. Burning/smoking/vaporizing things do this on Earth because as the gas/small particulates/whatever is being blown off by the air as the object flies through Earth's atmosphere. Since the apparent direction of this wind is behind the direction of travel of the object, the trail traces out a path of where the object has been.

But in space there is no atmosphere, and so the "wind" blowing stuff off the comet is not related to the comet's direction of travel. Instead, the wind is always coming from the sun. Consequently, the tail is predominately away from the sun, regardless of which way the comet is traveling.

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  • $\begingroup$ What a well-written answer. $\endgroup$ – Fattie Nov 5 '20 at 2:09

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