Castalia was proposed in 2015 and 2016 to the European Space Agency within the Cosmic Vision programme missions M4 and M5, but it was not selected.
[1] Castalia would rendezvous and orbit comet 7968 Elst–Pizarro and measure isotope ratios, plasma and dust properties, while also mapping its interior structure with a radar.
[1] The mission architecture and spacecraft design were initially developed in 2013 by the German Aerospace Center (DLR) in Bremen and OHB SE.
[1] Castalia was proposed, in slightly different versions, to the European Space Agency M4 (2015), and M5 (2016) missions within the Cosmic Vision programme.
The mission concept is named after Castalia, a nymph in Greek mythology that was turned into a fountain by a god, and she could inspire the genius of poetry to those who drank her waters or listened to their quiet sound.