Comets pose problems for flat earthers.
Comets are objects made of ice that usually go on trekking tours around the solar system, like the famous Halley's Comet which has an orbit of about 70 or more years around our host star, Sol.
It doesn't make any sense for comets to go around the sun if the sun goes around the earth when the earth is only 3,000 miles away from the sun so the comet could just orbit that instead, and it also doesn't make any sense for Halley's Comet to have the ability to travel at over 250,000 km/h if the diameter of the flat earth from either walls of its dome is only 40,000 kilometers distant from each other. If Halley's comet was trapped inside and going at it’s speed, it would be able to bounce off one of the inside sides of the dome once every 10 minutes. This never happens.
Even if Halley's comet wasn’t going at this speed, being enclosed inside a dome this tiny would mean Halley's comet would ALWAYS be visible in the sky somewhere on the flat earth… But it only appears in the sky every 70 or so years.
Of course, flat earthers will probably say that God is just making Halley's comet appear every 70 or so years inside the dome and then takes it away not long after to make the world a prettier place to live as it shines in the sky.
This is delusional nonsense.
Now, it only gets worse from there.
As we go around the sun, we pass through two meteor showers every year left over from the debris trails of Halley's comet; The Eta Aquariids in Early May, and the Orionids in late october. These showers happen EVERY year during these times and is completely consistent with the earth going around the sun, rather than this rubbish with the earth being stationary and not moving or passing through ANYTHING.
(CoolHardLogic has a pretty good video explaining why it won't work in his Testing Geocentrism series. The earth is spherical in geocentrism, but at the end of the day the earth MUST be moving in order for meteor showers to make sense.)