I feel like the answer can only be “ping pong” with another gravitational body, but that seems far too coincidental.
In: Planetary Science
We have to distinguish “the universe” with local space. The *universe* is, as far as we know, expanding, but it’s not expanding at a rate that makes an appreciable difference to local systems (yet. One theory posits that the speed of expansion will continue to get faster and faster until eventually not even individual atoms will maintain their composition, the theory is sometimes called the Big Rip)
Comets like Halley’s Comet are still gravity bound to our sun and orbit it just like Earth does.
The fact that *space* is expanding doesn’t mean it’s expanding *fast enough* to impact orbits of objects in a local system. It might. Eventually. But not yet.
Those asteroids and comets reappear because they’re orbiting the sun, just like Earth and everything else in the solar system. Their orbits are often eccentric (highly elliptical, and/or tilted with respect to the planets), but they’re still gravitationally bound to the sun. The effect of expansion is only seen in the distances between galaxies.
Asteroids and comets come back because they follow a path, like a race track in space. The Sun and planets act like magnets, pulling them around in circles, so they keep coming back the same way every time. It’s not random, they’re just following their track!
Same reason the planets do.
They’re basically all going around the Sun with us, just at different speeds and distances.
Asteroids are basically mini-planets; small lumps of rocks (anywhere up to a thousand kilometres across) orbiting the Sun, minding their own business, but not big enough to collapse into a full planet and “clear” the space around them.
Comets tend to have fairly squished/elliptical orbits, which means they can kind of [swing by](https://en.wikipedia.org/wiki/File:Comet_Kohoutek_orbit_p391.svg) and then zoom off into the outer Solar System for a long time before coming back again.
It’s actually really hard for things to “escape” the Solar System; they have to be going very fast – if not they’ll end up looping back in again eventually. Kind of like throwing something up in the air – unless you throw it very hard it will fall back down again.
There are also an awful lot of these things. [This little animation](https://en.wikipedia.org/wiki/File:PIA22419-Neowise-1stFourYearsDataFromDec2013-20180420.gif) shows the things detected by Nasa’s “Near-Earth Object Wide-field Survey Explorer” programme, from 2014-2018. The blue rings are the orbits of the inner planets (the paler blue one is us, Earth). Each dot represents an asteroid or comet – the green ones are asteroids that pass near the Earth at some point in their orbit, and the yellow dots are comets that tend to be more far-flung and just zoom by (but will come back eventually).
While space is ever expanding, it is only expanding on truly huge scales – at scales between clusters of galaxies. Asteroids and comets are things within the Solar system, so much, much more local.
Asteroids and comets which re-appear are just orbiting the sun, the same as all the planets are.
They simply have *really big* orbits, so they’re only near us every so often.
things like comets and asteroids aren’t just floating around in space randomly, they are following an orbit that takes them around big celestial bodies, like a race track. Think of it like a nascar going around a nascar track.
Comets are in orbit around the Sun, like the planets, but they are on a weird arc where some of the time they are close to the Sun and some of the time are near the outer planets.
They have an orbit. Halley’s comet, for example, orbits our sun. It has a highly elliptical orbit meaning that its distance from both the sun and Earth and so it’s only visible from Earth every 70 years or so.
All* asteroids and comets we’ve ever seen are in orbit around the sun.
*: except that one time
Interestingly the comets don’t fly that far away from sun. Orbital period of Halley’s comet is like 75 years and the furthest it goes is just around Neptune.
Or the Hale-Bop comet (seen in 1995) will return in around year 4400 and max distance will be just 10x the distance of Neptune (2 light days). Voyager 1 is already half that much away from us.
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