Abstract
Ocean Worlds in our Solar System are attractive candidates in the search
for extra-terrestrial life. The best chances for detecting biosignatures
and biology on these bodies lie in in situ investigations of sub-ice
oceans in contact with rocky interiors. The actual conditions that will
confront an ice-penetrating vehicle (“cryobot”) performing such
investigations are largely unknown. However, any Ocean World cryobot
must be able to, at a minimum, successfully negotiate five different
operating regimes to have a chance of reaching a subsurface ocean:
starting at the surface in vacuum at cryogenic temperatures;
brittle/cold ice transit; ductile/warm ice transit; negotiating or
penetrating salt or sediment layers, and other obstacles; and detecting
and transiting ice-water transitions such as voids and the final ocean
entry. PROMETHEUS (nuclear-Powered RObotic MEchanism Technology for
Hot-water Exploration of Under-ice Space) represents a full cryobot
concept and set of key technology demonstrations that advance the
capability to perform such investigations. The PROMETHEUS concept is
targeted for deployment on Europa, and consists of a fully-instrumented
science vehicle able to actively control descent through the ice shell
and into the subsurface ocean. The concept employs closed-cycle hot
water drilling (CCHWD) technology as the primary means of penetrating
ice, and making forward and turning progress. A “passive” (purely
conductive) heat transfer system enables penetration starting on the
surface where liquid water cannot exist until hole closure is achieved
and the system proceeds inside a melt water “bubble”. PROMETHEUS is
compatible with a small fission reactor (the NASA Kilopower design) and
employs a vertical motion control system using a trailing tether frozen
into the ice to guard against falling through voids and enabling
controlled entry into the sub-ice ocean. The design is capable of
achieving a 20 km descent through a Europan ice profile in under a year
and under 500 kg vehicle mass, including reactor mass.