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\begin{document}

\title{PROMETHEUS: Progress Toward an Integrated Cryobot for Ocean World Access}



\author[1]{Kristof Richmond}%
\author[1]{Bartholomew Hogan}%
\author[1]{Alberto Lopez}%
\author[1]{Harman John}%
\author[2]{Krista Myers}%
\author[1]{Veronica Guerrero}%
\author[1]{Ephraim Lanford}%
\author[1]{James Ralston}%
\author[1]{Neal Tanner}%
\author[1]{Victoria Siegel}%
\author[1]{William Stone}%
\affil[1]{Stone Aerospace}%
\affil[2]{Scale Free Systems}%


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  \date{June 13, 2024}


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\begin{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.%
\end{abstract}%



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