After patching (if necessary), the Launcher closes when 'Launch' is clicked but Descent: Underground does not launch.
On Linux machines which don't have the Simple DirectMedia Layer library (LibSDL) installed, the Game is unable to run. LibSDL is meant to be installed automatically but has been known not to install.
You will have to install LibSDL from your distribution's package manager. Here are the commands for some common distributions:
Debian, Ubuntu, Mint
sudo apt-get install libsdl2-2.0
Red Hat, CentOS, Fedora
sudo yum install SDL2
sudo emerge libsdl2
You should not need to install SDL on a SteamOS box as it is included by default with the SteamOS install.
For other distributions, please consult your distribution's website or package manager. Alternatively, you can attempt to do a source install from SDL Wiki.
Once LibSDL is installed, please attempt re-launching and you should be able to enjoy the game.