From Discovery Wiki

Location E3, Omicron Epsilon
Edge Worlds
Technical information
Docking No
Terrain Barren
Diameter 3,509 km
Mass 2.31 x 10e24 kg
Temperature -240°C to -14°C
Escape velocity 2.20 km/sec

Callista is a small moon of methane ice and rock that lacks sufficient mass to sustain any kind of atmosphere. The surface of the planetoid reaches extreme temperatures far below freezing, rendering any attempts to further explore the desolate rock incredibly dangerous.