When talking to my son about the Geminid meteor shower tonight, we read up on 3200 Phaethon, and were curious: why is there only one meteor shower from 3200 Phaethon, and not two? Given its orbit, the Earth crosses the sunward orbit of 3200 Phaethon in December, and the after-sun orbit the next September or October (see this orbital diagram on wikipedia for example.)

Why does only the one side of the orbit have a major meteor shower, and not the other (as far as I know, anyway)? This seems true for any of the major showers - Swift-Tuttle also crosses our orbit twice as well, of course, once in (northern hemisphere) spring and once in August, as must any other comet (assuming none are perfect tangents to our orbit, which seems unlikely). Why does only one side of the orbit cause a meteor shower?

And, why it is it not always the same side of its orbit - 3200 Phaethon causes the shower heading towards the sun, while Swift-Tuttle causes the Perseid shower after it's past the Sun and is headed back out? (I'd thought perhaps the shower was caused by debris from the sun approach at first, but of course that mustn't be the case if I read the orbital diagram properly; and then later I thought that maybe the Sun vacuumed up the debris, but then Swift-Tuttle has the opposite result.)

Is it simply orbits in three dimensions (and the comet/asteroid coincidentally crosses Earth's orbit once on Earth's orbital plane, but the other crossing is on a different plane)? Would both sides of the orbit produce similar effects, if the comet was in precisely the same plane as Earth? Or are there "dusty" and "clean" portions of the asteroid's/comet's orbits?


Note the short straight lines that hang down (or stick up) from the orbital track of 3200 Phaethon you reference.

These indicate the distance above or below the plane of earth's orbit (the ecliptic) 3200 Phaethon is at that point.

So 3200 Phaethon passes well above Mars' orbital track and passes through the Earth's orbital track (just about)

Then it passes under Venus, under Mercury, makes a U-turn around the Sun, and passes back over Mercury, Venus Earth, and Mars.

So your supposition in the last paragraph is correct.

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    $\begingroup$ That's what the image indicates, but I'm not sure that's the actual reason because (and I hate to just use common sense as an answer), but it doesn't make much sense that at it's 2 points, 1 AU from Earth, one would be in line with Earth's orbit and the 2nd would be distinctly above it. I want to say that the reason there's only one by most accounts is because the other is a sun-side approach and we don't see meteors during the day very well, but I haven't been able to verify that. $\endgroup$ – userLTK Dec 14 '20 at 8:14
  • $\begingroup$ @userLTK : Put a narrow parabola with focus at the Sun and axis if symmetry perpendicular to the plane of the planets. Now tilt the axis of the parabola in the plane of the parabola until the "left" half of the parabola meets the orbit of Earth at the point P. Make a new axis of revolution of the parabola along the line through P and the Sun. Rotate the parabola around this new axis -- notice that the parabola meets Earth's orbit a second time only twice and at all other orientations misses Earth above or below. Meeting twice requires that the parabola lie in the plane of Earth's orbit. $\endgroup$ – Eric Towers Dec 14 '20 at 16:29
  • $\begingroup$ Thanks, this makes sense - I guess it's easy to forget that non-planetary objects don't have much of a tendency to hang out in the ecliptic. I'll accept in a day or so if there's nothing else! $\endgroup$ – Joe Dec 14 '20 at 16:41
  • $\begingroup$ Also - I really like the links from uhoh in the comment on the question, that seem to support this - would it be okay to have those edited into your answer, in particular this one? $\endgroup$ – Joe Dec 14 '20 at 16:42
  • $\begingroup$ @EricTowers OK, that makes sense. I was having some trouble imaging it. Thanks. $\endgroup$ – userLTK Dec 14 '20 at 17:16

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