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?