A euphemisim for suicide. While phrases such as "six feet under" and "pushing up daisies" apply to other forms of death, this is specifically for suicide. It was inspired by the Heaven's Gate cult, who committed suicide in the belief that their souls would ride a "spaceship" hidden near the Hale-Bopp comet when it passed by Earth in 1997.
Did you hear what happened to Joe?
Yeah. After he lost his job and his wife left him, he couldn't take it anymore. He decided to ride the comet.