Castalia is a proposed mission concept for a robotic spacecraft to explore the main-belt comet 7968 Elst-Pizarro and make the first in situ measurements of water in the asteroid belt, and thus, help solve the mystery of the origin of Earth's water. The lead is Colin Snodgrass, from The Open University in UK.

Castalia was proposed in 2015 and 2016 to the European Space Agency