This repository provides the necessary components to build and install a Flatpak of Celestia. This is not in any way associated with the Celestia project. I just made this for personal use.
$ flatpak-builder build --user --install --force-clean space.celestia.celestia_qt6.yml
This may complain about missing dependencies. If it does, try running the following first:
$ flatpak-builder build --install-deps-only --install-deps-from=flathub space.celestia.celestia_qt6.yml
It also may give a cryptic error about "rofiles". If so, simply add the --disable-rofiles-fuse
option.