|Fig. 1: Illustration of 4 types of thermal drill probes: (a) a single tethered probe with nuclear power source in the surface lander, (b) a single tethered probe with nuclear power source in the subsurface probe, (c) a tetherless multi-probe network with small nuclear power source in each unit, and (d) a tetherless single probe with "breadcrumb" RF devices.|
It is difficult to imagine such a continued widespread interest and investment in far-space exploration if not for the sometimes fantastical hope of the existence of extraterrestrial life. Based on Earth's example, these searches typically focus on locations with known or suspected sources of water, such as the planet Mars and moons Enceladus, Titan, and Europa of our solar system. However, due to the distance from the sun any water at the surface exists in solid ice form. In environments such as Europa, where liquid water may exist deep below an icy outer shell, small probes with the capability to actively melt a pathway into the ice in order to reach encased liquid water have been proposed. Even when no liquid water is thought to exist, as on Mars, such probing techniques would allow for valuable observation of geophysical layers spanning millions of years of planetary history.
Most proposed thermal probes have been based on the established radioisotope generator technology, which acts as a thermal energy source through a steady process of particle decay. These generators typically make use of a core of plutonium oxide and have been well- demonstrated in combination with thermoelectric generators for United States space missions since the 1960s.  Radioisotope Power Systems (RPSs) are good options when electric power requirements are relatively low, on the order of 100 We. However, above 1 kWe RPSs are expected to become prohibitively massive and expensive, and fission power systems have been proposed for more demanding missions. 
Initial designs by NASA's Jet Propulsion Laboratory (JPL) planned for a fission- powered surface lander that would transmit electric power (~3 kWe) by cables to electric heaters in the small subsurface probe (1 m long/0.1 m diameter), but subsequent ideas included the incorporation of a fission reactor directly into a larger probe (3 m long/0.5 m diameter) to take advantage of a much larger thermal heat source for direct melting (~15 kWt).  Both designs require transmission of electric power and signals between the probe and lander via a tethering system.
An alternative approach was proposed by Powell et al. in which a network of several nuclear cryoprobes are released into the ice layer to simultaneously gather data over a wider area.  In this design, each probe would produce on the order of 500 kW of thermal power and several kW of electrical power. Powerful RF transmitters would allow communication through the ice layer, and the network of cryoprobes would be able to relay data to the lander, extending the operational range of the furthest probe far beyond the transmission distance feasible for a single probe. The concept of tetherless RF relay has been considered in other designs, as there is concern that motion within ice layers would shear a tether or cause significant noise and attenuation of acoustic signal over the several kilometers of ice likely to be encountered. One alternative tetherless design involves RF relay "breadcrumbs" dropped by the probe periodically during its descent.  A general illustration of each technology discussed here is presented in Fig. 1.
Thermal drilling approaches may be particularly important for extraterrestrial applications. Since payloads must remain small and thermal-to-electric conversion efficiencies by standard thermoelectric elements are often low, bulky and power-intensive mechanical drilling approaches may be prohibitive. The large polar ice caps of Mars are expected to provide excellent geological records dating back to times when liquid water likely existed on the surface. 
Europa has become an increasingly-interesting option for surface and subsurface exploration as deep space missions return more information about its makeup. It is now suspected that a salty ocean rich in minerals exists under the vast ice crust at the surface (~3-30 km thick), which presents a remarkable opportunity for the existence of past or present lifeforms. [4,5]
In some cases this approach is even relevant to Earth to access isolated subglacier lakes, and "Cryobot" probes have been tested in terrestrial glaciers recording penetration speeds of near 1 m/h.  Lake Vostok in the Antarctic, which has been sealed under 3 km of pack ice for a million years or more, is one example of targeted terrestrial use. External contamination of the prehistoric lake could be limited because the water melted in front of the Cryobot would be refrozen behind it as it moves. 
In particular for extraterrestrial applications where the ambient temperatures are significantly below the melting temperature of water ice, there is potential concern that proposed radioisotope devices may not even be able to melt the ice and would stall on or shortly below the surface of the ice. A thermal analysis was provided by Lorenz approximating such a device as a point or sphere that determined three typical radioisotope sources would be unable to function as a thermal drill in the conditions on Titan, and one of the three would even be unlikely to work on Mars.  However, these assumptions are relevant for current radioisotope generator technology, and does not consider fission-based generator concepts which may provide 1+ kW of electric or 10+ kW of thermal power. 
Environmental contamination is a very legitimate concern for such a device, especially in a sealed environment with the potential to support life. It is important to not only eliminate the accidental introduction of organic compounds and microorganisms which could falsely indicate the presence of extraterrestrial life, but also to limit any potential hazardous contamination of the environment. This is particularly relevant when radioactive materials are involved. It is difficult to predict the effects of such contamination in an environment such as Europa, which we know relatively little about, and most discussion of thermal probe devices has been limited to technological innovation and payload limitations. As a practical mission comes closer to realization, more detailed attention to environmental concerns is likely to follow.
© Marc Dunham. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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