Nessus is an unstable body known as a centaur, between being classified as a comet and a planetoid. It’s incredibly irregular orbit has been adjusted by the Vex, who have converted Nessus into a Machine World. The colony ship Exodus Black crash-landed on Nessus after its launch in the Golden Age.
There are reports of a colossal entity approaching Nessus, but the Vanguard has yet to verify these claims.