Testing of SLUSH ice penetrating probe in Europa, Enceladus, and Mars environments

1 Introduction

The Icy Worlds of Europa and Enceladus, and the subsurface of Mars are primary targets in the search for past and present life. Europa and Enceladus possess global liquid-water oceans, a key ingredient for life, beneath ice shells estimated to be a few kilometers to tens of kilometers thick (Howell et al., 2021). On Mars, polar ice deposits more than 3 km thick (Plaut et al., 2007) may cover localized layers of liquid water or brine, though this interpretation remains debated (Orosei et al., 2025). Analogous sub-ice habitats on Earth host microbes in small melt pockets and ice pores (Malaska et al., 2020).

Reaching these subsurface bodies of water requires a probe that must penetrate the ice while moving the excavated material behind it. This operation can be achieved via two methods: thermal (melting) or mechanical (cutting) (Bar-Cohen and Zacny, 2020). Mechanical drilling systems are efficient at breaking ice formations but inefficient at transporting the broken cuttings (Zacny et al., 2013; 2016). Thermal drilling is effective at transporting the melted formation behind the probe but is inefficient because of the need to melt an entire body of ice ahead of the probe (Dachwald et al., 2023). Thermal probes are used terrestrially and trace back to the first freeze-in probe by Karl Philberth, which stored a tether for power transmission that passively unspooled as the probe descended (Aamot, 1967). Evolutions of this concept continue to be developed and field-tested for glaciological applications in Earth’s polar regions, offering the potential for clean, low-power access to subsurface environments (Winebrenner et al., 2013; Espe et al., 2024; Sun et al., 2024). These designs use resistive heaters fed by electrical power sent through the tether. Alternative thermal architectures have been explored to improve speed and efficiency. Stone Aerospace has developed two cryobot technologies: VALKYRIE, which uses Direct Laser Penetration (DLP) to transmit laser power through a fiber-optic tether and melt ice without resistive heaters (Stone et al., 2018), and the Closed Cycle Hot Water Drill (CCHWD) systems THOR and PROMETHEUS, in which onboard heaters warm a recirculating water loop that jets pressurized hot water from the probe nose to melt the ice ahead of it; this approach has demonstrated high penetration rates but requires a pressurized liquid water environment to operate (Stone et al., 2023; Guerrero et al., 2022). To improve efficiencies of thermal approaches, some designs have augmented melting with rotary mechanical action. IceMole, developed at FH Aachen University of Applied Sciences, combines a differentially heated melting head with a hollow ice screw at the tip for forward propulsion; it was successfully field-tested in Antarctica (Dachwald et al., 2014; Baader et al., 2024). The Vertical Entry Robot for Navigating Europa (VERNE), developed at Georgia Tech, similarly employs a rotating auger to improve penetration through the range of ice conditions expected on Icy Worlds (Bryson et al., 2020). A comprehensive review of thermal ice drilling technology across terrestrial and extraterrestrial applications is provided by Talalay (Talalay, 2020) and Dachwald (Dachwald et al., 2023).

Probes for extraterrestrial ice must be designed to perform in extreme environments. On Mars, the temperature on the coldest portions of the south pole surface is around 140 K, with the planet averaging an atmospheric pressure of about 4.5 torr (Kieffer, 1979). The environments of Europa and Enceladus are more extreme: surface ice temperatures closer to 100 K are expected, and atmospheric pressures are orders of magnitude below the sublimation point of water ice (Ashkenazy, 2019; Bland et al., 2012). The ice temperature on these bodies is expected to increase to 273 K as the ice transitions to liquid water at depths of kilometers or tens of kilometers. Mechanical properties vary in this range; ice becomes more brittle at temperatures below 160 K (Arakawa and Maeno, 1997). In addition, non-icy material such as salt lenses and rocks are expected to exist throughout the ice layer. Mechanical drilling would be required to penetrate these obstacles, as a melt-only approach would be unsuccessful (Ulamec et al., 2007).

The Search for Life Using Submersible Heated drill (SLUSH) probe is a hybrid, thermo-mechanical probe that combines the most efficient aspects of both mechanical and thermal drilling (Zacny et al., 2018). The design consists of a heated drill bit in front, a power and anti-torque section in the middle, and a tether bay for communication in the back (Figure 1). SLUSH utilizes mechanical drilling to break the ice into chips (the fragmented ice cuttings produced by the drill) and heat to partially melt these chips into a flowable mixture of ice and meltwater, referred to as a slurry or slush, which enables the efficient transport of material behind the probe. The slurry behaves like a fluid despite being partially frozen, enabling a significant reduction of the power required for melting the full volume of ice. This drilling approach, dubbed ’slushing’, also results in higher penetration rates compared to pure melting, allowing the probe to reach a subsurface body of water in a much shorter time.

The full-scale SLUSH flight concept is approximately 5 m in length and 50 cm in diameter. The diameter is mainly driven by the size of the nuclear reactor. As such, if the reactor can be reduced in diameter, the entire probe will get smaller too. It is worth noting that even a small reduction in diameter has a significant effect on probe speed and energy efficiency; the area of ice that needs to be penetrated is proportional to the square of the diameter. Concepts for Icy Worlds require larger, more powerful probe designs with onboard nuclear sources for both electrical and thermal power. Leading options include concepts that explore using a General Purpose Heat Source Radioisotope Thermal Generator (GPHS-RTG) (Hockman, 2023; Bryson et al., 2020) or a miniaturized fission reactor (Stone et al., 2023; Oleson et al., 2019). The SLUSH flight concept uses NASA’s Kilopower nuclear reactor for both thermal and electrical needs (McClure et al., 2020). The fission reactor can be turned on/off and is self-moderating, allowing the probe to control the amount of heat generation to simplify thermal management. In contrast, a radioisotope-based thermal system, such as an RTG that employs Pu238 as its heat source, would require more complex thermal management and the ability to absorb or dissipate a significant amount of heat. This poses a challenge should the probe encounter a material with low thermal conductivity, such as a layer of salt. Calculations show that a 10 kW RTG on a 1-ton, mostly metallic probe would heat it up by 100 °C in an hour if there is no heat sink available. It is counterintuitive, but the major issue with planetary nuclear probes will be keeping them cold as opposed to keeping them warm. In fact, high-temperature technologies developed for Venus applications would be directly applicable to planetary ice probes.

This paper presents the design, development, and initial proof-of-concept testing of a scaled SLUSH prototype, focusing on the prototype rather than the full flight system described above. The objective is to demonstrate the viability of the thermo-mechanical slushing approach and identify performance trends, rather than to establish optimized operating parameters, which will require dedicated parametric studies. The remainder of the paper is organized as follows. The Analytical Framework introduces a power-balance model that decomposes the slushing power requirement into mechanical and thermal contributions relative to a conventional melt-probe baseline. Materials and Methods describes the prototype and the test setups across the three ice environments. Testing Results presents the drilling performance of each mode, and the Discussion interprets these results and their implications for the slushing approach and flight design. The Conclusion summarizes the findings and identifies priorities for future work.

2 Analytical framework

Table 1 lists the nomenclature and units used throughout this work. Several studies have addressed thermal modeling of ice probes descending through ice (Aamot, 1967; Ulamec et al., 2007; Schüller and Kowalski, 2019; Li et al., 2020; Durka et al., 2022; Do Vale Pereira et al., 2023). Modeling has been focused on the traditional thermal approach, where the probe uses only heat to penetrate the ice. Analytical models are conceptually simple; thermal power into the probe is used in one of four ways when operating in steady state: heat to melt the ice beneath the probe Q ̇ 0, heat to warm up the ice beneath the probe Q ̇ 1, heat that is lost to the side wall around the probe as it descends Q ̇ 2, and heat that is lost by the flow of warmed up water above the probe Q ̇ 3. This thermal model is visualized in Figure 2. Aamot was the first to tackle this problem, developing an analytical model to estimate Q ̇ 0, Q ̇ 1, and Q ̇ 2 (Aamot, 1967). Roughly 40 years later, Ulamec et al. (2007) extended the approach to extraterrestrial environments, demonstrating feasibility in cold, vacuum conditions and estimating the energy demand for a space mission. Schüller and Kowalski (2019) further developed the model to include the advective loss Q ̇ 3, and Li et al. (2020) studied how various parameters affect power consumption and proposed an approach to the thermal design of ice probes. The most developed model to date is the Aamot++ numerical model of Durka et al. (2022), which adds (1) the temperature dependence of ice properties, which is important for cryogenic ice, and (2) the melt jacket, or the lateral distance between the probe side wall and the water-ice interface. This model was recently validated in controlled laboratory testing by Do Vale Pereira et al. (2023). Gaps remain between predicted and measured performance, however: Baader et al. (2024) found that models overestimate melting velocities and efficiencies by 15%–50%, with the deviation increasing as ice temperature decreases. Reliable modeling for a spaceflight application will require continued comparison and refinement. The full derivation and coefficient values for Q ̇ 0 through Q ̇ 3 are given in Durka et al. (2022); this work applies their result directly. The total thermal power from the probe into the environment is the sum of these four terms:

Q ̇ total = Q ̇ 0 + Q ̇ 1 + Q ̇ 2 + Q ̇ 3 (1)

TABLE 1. Nomenclature and corresponding units.

Equation 1 provides a baseline for understanding the power requirements of a conventional thermal probe and serves as a useful benchmark for comparing the performance of SLUSH to a theoretical thermal probe.

SLUSH modifies this baseline in two ways: it adds mechanical power to break the ice rather than melting all of it, and it changes how the remaining thermal power is distributed (Figure 3). We capture both effects by adding a mechanical drilling term, W m, and four non-dimensional correction factors, σ 0 through σ 3, that scale the conventional thermal terms relative to the melt-probe baseline. Each σ is a power modifier on its corresponding term in Equation 1: σ 0, σ 1, and σ 3 attenuate their terms ( 0 ≤ σ ≤ 1 ), while σ 2 amplifies its term ( σ 2 > 1 ). Their definitions are summarized in Table 2.

TABLE 2. Definitions of the mechanical power term and the slushing correction factors. Each σ modifies the corresponding conventional thermal term from Equation 1 relative to a melt-probe baseline.

The total required power for a slushing probe is then:

Q ̇ slushing = W m + σ 0 Q ̇ 0 + σ 1 Q ̇ 1 + σ 2 Q ̇ 2 + σ 3 Q ̇ 3 (2)

Equation 2 is proposed as a conceptual framework; the correction factors σ 0–σ 3 are not independently measured in this work and cannot be uniquely determined from the available single-run data. The side-wall factor σ 2 exceeds unity because the ice cuttings suspended in the annulus draw heat from the meltwater in addition to the heat lost through the borehole wall; this additional draw grows with the quantity, temperature, and total surface area of the cuttings, so finer cuttings increase σ 2. This surface-area dependence is consistent with measurements of ice-slurry heat transfer, in which the heat-transfer rate rises with the total surface area of the suspended ice particles (Suzuki et al., 2024). The contents of the annulus, the cuttings, partial slurry, refrozen material, and any vapor voids, form a layer whose effective thermal and transport properties differ from those of either solid ice or liquid water. This layer may aid heat transfer by adding contact area and mixing, or hinder it where porous refrozen material or vapor gaps insulate the bit; we cannot tell which dominates from a single run, which adds to the uncertainty in the correction factors. Even a pure melt probe shows this sensitivity, with the conductance of the thin meltwater film at the head setting the penetration rate (Li et al., 2021).

The mechanical power W m and the melt fraction σ 0 are kept as separate terms, but they trade off against each other: ice melted below the probe (captured by σ 0) does not need to be broken mechanically, and ice broken mechanically does not need to be melted. The limit σ 0 = 1 with W m = 0 recovers a pure melt probe, while σ 0 = 0 with W m at its maximum represents purely mechanical drilling. Not all of W m breaks ice, however; some is lost to vibration, and much of the rest is converted to heat by shear deformation and friction at the bit, warming the cuttings. This heating has been studied for rotary ice-core cutting: Cao et al. (2015) measured cutter temperature rises of only a few degrees Celsius during dry cutting, and Azuma et al. (2007) and Chen et al. (2024) obtained comparable results from cutting and heat-transfer models. This contribution is therefore minor for cryogenic ice, where the cuttings must be warmed substantially to reach the melting point, but becomes significant for warm ice already near its melting point, where a few degrees can be enough to initiate melting. SLUSH additionally applies percussive impact, whose heat contribution we do not characterize here. Because W m, σ 0, and the heat dissipated by drilling are not quantified by the present single-run data, we retain W m and σ 0 as independent terms.

The transition from mechanical drilling to slushing, and the transport of cuttings that follows, are both governed by the liquid fraction of the cuttings, ϕ. Below a critical fraction ϕ crit, the cuttings are near-dry and cohesive and resist flow through a finite yield stress, so they must be cleared mechanically by the auger; above it, enough ice has melted that the mixture mobilizes and flows around the probe, consistent with ice-slurry rheology measurements (Ayel et al., 2003). This ϕ is a local property of the cuttings at the point of mobilization, distinct from σ 3, which describes the slurry advected above the probe; we do not measure ϕ crit in this work and identify it as a key parameter for future characterization. The cuttings produced by drilling are rough and irregular, which hinders mobilization: engineered ice slurries show that rough, non-globular particles entangle and resist flow at low ice loadings, whereas only smoothed, globular particles flow freely at high loadings (Kauffeld et al., 2010). SLUSH therefore cannot rely on the favorable flow of engineered refrigeration slurries, and its cuttings sit toward the harder-to-mobilize end of this range. Once mobile, the slurry is transported by the rotating auger and by the buoyancy of unmelted ice in the surrounding meltwater; the effectiveness of this transport depends on solids loading, cutting size, and the annular clearance between the probe and borehole wall (Suzuki et al., 2024). Because buoyancy contributes, transport would be less effective in the lower gravity of Europa (1.3 m/s 2) or Enceladus (0.11 m/s 2), where auger-driven transport would be expected to dominate; quantifying this gravity dependence is an important direction for future work.

3 Materials and methods

3.1 SLUSH prototype

A scaled down prototype of the drilling subsystem has been developed to study the efficacy of thermo-mechanical drilling (Figure 4A). The prototype probe body has a diameter of 2 inches (5 cm) and a bit diameter of about 2.5 inches (6.4 cm). This results in a 0.7 cm radial clearance between the probe body and ice–sufficient for ice chips to accumulate and be transported. The design uses two independently controlled brushless DC motors to power the rotary-percussive drilling assembly. The gear train section of the probe transmits the torque of the motors to the auger and percussion mechanisms. The slip ring section passes power and temperature sensor signals to the rotating heated auger and bit. Four ceramic heaters inside the heated auger and bit section conduct heat out through the auger and bit to partially melt the ice cuttings. Each heater contains an embedded thermocouple, and a single thermocouple was used to control the temperature of all four heaters to prevent overheating and burnout. A single custom external wall heater, with a thermocouple on its underside to keep it within its temperature limit, wraps around the probe body to prevent the melted cuttings from freezing the probe inside the borehole.

To be able to conduct testing with a fully submerged probe, a standalone prototype was designed and built in-house at Honeybee Robotics (Figure 4B). Relative to the drilling subsystem, the integrated probe adds three subsystems: an anti-torque assembly to cut into the borehole wall and keep the entire probe from rotating, a tether bay and tether to transmit power and data to the probe’s motors, heaters, and temperature sensors, and an electronics bay. It also expands the side-wall heating from the single heater on the drilling subsystem to six identical wall heaters distributed around the probe body. The anti-torque assembly is designed to maximize reaction torque and minimize axial friction along the probe wall. This allows the probe to use more torque to drill and maximizes preload against the bit, which improves drilling performance. The design uses spring-loaded, heated skates to glide and melt along the borehole wall. These wall heaters and their thermocouples line the outside of the probe to measure the sidewall temperature and prevent freeze-in. The tether bay also includes an encoder reel that accurately measures probe depth as it unwinds. Unlike conventional tethered systems in which the cable is paid out from a surface winch as the probe descends, the SLUSH tether is stored on a spool aboard the probe itself and unspools passively behind the vehicle as it penetrates downward, leaving the tether frozen into the ice above rather than managed from the surface. For the prototype design, a large flex cable was used for the tether, to allow for most of the electronics to live outside of the probe body. Future iterations will have the electronics embedded, to allow for a smaller tether design.

3.2 Test design

Tests were designed to characterize slushing performance across the ice conditions expected during a planetary mission and to compare it against mechanical-only and thermal-only drilling. A fourth configuration tested the integrated prototype probe in a warm ice tower to demonstrate end-to-end system functionality. Each condition was tested once; statistical repeatability and a parametric study are identified as priorities for future work.

Testing was conducted in a 5 m tall walk-in freezer and in a 3.5 m tall thermal vacuum (TVAC) chamber, using warm 260 K ice and cryogenic 130 K ice. In both cases, homogeneous crystalline ice blocks made from distilled water were used to produce ice free of any contaminants that could affect drilling conditions. Figure 5 illustrates the three conditions relevant to Europa simulated by the test setups. Cryogenic vacuum testing is analogous to the probe entering the top few meters of the ice shell, when the borehole is exposed to vacuum. Cryogenic testing at ambient atmospheric pressure is analogous to the moment the probe is fully submerged in the ice shell, as the sublimated ice will form a pressurized environment inside the cavity once the borehole closes above the probe. The pressurized environment affects the phase change of the ice, as it will now melt rather than sublimate. The warm ice ambient pressure testing is analogous to the probe approaching the body of water, where the ice temperature is considerably warmer. Recent modeling predicts Europa’s ice thickness to be between 23 and 47 km (Howell, 2021), meaning pressures up to the ice-ocean interface may be as high as 26–57 MPa, estimated from P = ρ g h using ice density ρ = 917 kg/m 3, g = 1.315 m/s 2, and the predicted ice thickness range. While these pressures are extreme and must be accounted for in the probe design, the solid–liquid transition remains near 273 K across this pressure range.

3.3 Drill subsystem test setup

The drill subsystem was mounted to a linear stage with an in-line load cell for testing (Figure 6A). The linear stage was used to apply a consistent drilling preload and monitor rate of penetration (ROP). A weight-on-bit (WOB) of 125 N was chosen for all tests, as it is representative of the weight of the integrated prototype probe at this scale. In a true flight environment, WOB will be governed mainly by the mass of the probe minus any buoyant force when submerged in water, and minus friction from the anti-torque assembly.

Cryogenic testing was conducted in the 3.5-m tall Honeybee TVAC chamber, capable of vacuum levels of less than 1 Torr (Figure 6B). Electrical feedthroughs connected SLUSH and the linear stage to the EGSE (Electrical Ground Support Equipment) outside of the chamber. The EGSE and software setup allowed for control and power measurements of the heaters and motors. Chamber instrumentation included a pressure transducer to monitor chamber pressure and type-K thermocouples frozen into the ice to monitor ice temperature.

A custom cooling system was developed to cool the ice to cryogenic temperatures. The cooling system consisted of a double-walled barrel with a channel to cool crystalline ice blocks using liquid nitrogen (LN2). Approximately 350 L of LN2 were required to cool the ice to around 130 K. Vacuum was maintained around 0.5 Torr, below the triple point of water (4.5 Torr). Below this pressure, water has no stable liquid phase, so heating the ice drives it directly from solid to vapor (sublimation) rather than melting.

3.4 Drill subsystem test procedure

Using the drilling section of the prototype probe, slushing (thermo-mechanical drilling), mechanical drilling (no heat applied), and melting (heating only) tests were conducted in warm ice at ambient pressure and in cryogenic ice in vacuum. Slushing tests were also conducted in cryogenic ice at ambient pressure to isolate the effect of vacuum, by direct comparison with the cryogenic-vacuum slushing run. Mechanical-only and thermal-only modes were not repeated in this environment, as they were already characterized in the other two environments and each test is resource-intensive; this case was included specifically to isolate the pressure variable for slushing. The following mechanical and thermal parameters were held constant across all tests to isolate the effect of drilling mode and environmental condition.

For all motor-on tests, the auger was rotated at 120 rpm. Percussion was applied at 2.5 J/blow and 8.1 blows/rev for all percussion-on tests. The percussion rate was chosen to expose fresh ice to the bit cutters on each auger rotation, and the impact energy was carried over from previous Honeybee drills (Zacny et al., 2025), where it was sufficient to fracture rock and therefore expected to be more than sufficient for ice. The 125 N WOB described above was applied via the linear stage. For slushing and thermal tests, the auger heater was temperature-controlled to a setpoint of 353 K. The resulting heater power ranged from 60 to 930 W depending on drilling mode and environment, reflecting how effectively heat conducted from the drill bit and auger into the environment under each condition.

Prior to each test, ice temperature and ambient pressure were recorded before initiating drilling. The melting test ran until a constant penetration speed was observed for more than 20 minutes. The mechanical test ran until the ROP plateaued. The slushing test was stopped at a maximum depth of 250–300 mm to prevent water ingress into the top of the assembly. Throughout each test, EGSE recorded temperature, heater current, motor current, and motor position data at 40 Hz via a LabJack data acquisition system (DAQ). Motor position was measured by rotary encoders on the motors: the linear-stage encoder provided rate of penetration, and the auger and percussion motor encoders were used to control and monitor mechanical drilling. Heater current was used to calculate heater power ( W t ) and motor current was used to determine mechanical power ( W m ). A camera recorded video of each test.

3.5 Integrated prototype probe test setup

To test the integrated prototype probe, a tower of ice was assembled in the walk-in freezer. A warm plate was used to melt a layer of ice between discrete 50 × 50 × 25 cm ice blocks before stacking, fusing together a continuous 1.5 m column of ice (Figure 7). A deployment structure kept the probe upright and provided anti-torque until the probe was fully submerged in the ice.

3.6 Integrated prototype probe test procedure

A single slushing test was conducted with the integrated prototype probe in a warm, 260 K ice tower at ambient pressure. The same motor parameters used in the drill subsystem tests were applied. The probe drilled under its own weight, providing an approximate preload of 125 N. The auger heater was temperature-controlled to 353 K, operating nominally at 200 W. Wall heaters (50 W each) were activated when the sidewall temperature dropped below 273 K to prevent freeze-in, and skate heaters (60 W total) were activated on ice contact to allow the anti-torque assembly to smoothly melt into the borehole wall. Drill depth was recorded via an onboard encoder reel that unwound as the probe descended. EGSE recorded telemetry via a LabJack, while cameras recorded video of the test.

3.7 Theoretical melt probe benchmark

In addition to the experimental thermal-only tests, a theoretical thermal probe benchmark was used to compare slushing against an idealized conventional melt-probe approach. This benchmark is not a physical test article; rather, it is an analytical reference based on Equation 1, using the thermal probe model from Durka et al. (2022) described in Section 2. The benchmark was included because the prototype drill bit and auger were designed primarily for mechanical and thermo-mechanical drilling, not for efficient heat transfer in a melting-only mode. As a result, the experimental melting tests reflect limitations of the prototype hardware as well as the melting method itself.

For each test condition, the theoretical benchmark was calculated for a perfectly efficient thermal probe of the same diameter and power budget, operating in the same ice temperature as the corresponding slushing case. The predicted ROP was determined by matching the total applied power to the thermal power required for penetration (Q ̇ 0 + Q ̇ 1). For the integrated probe benchmark comparison, Q ̇ 2 is calculated as well. This benchmark provides an idealized upper-bound reference for the melting approach, not a directly achievable hardware prediction or a model validation.

4 Testing results

Figure 8 shows depth versus time with the drilling subsection of the probe in each test environment: warm ice at ambient pressure (A), cryogenic ice in a vacuum (B), cryogenic ice in ambient pressure (C). Each panel also includes the calculated steady-state ROP of the theoretical melt probe benchmark, computed at the same total power as the slushing test in that environment and shown for comparison with slushing rather than with the thermal-only test.

Table 3 summarizes the measured ROP, specific energy, heater power, and mechanical power for all tests, including theoretical melt probe benchmarks at equivalent power budgets as described in Section 3.7. Reported ROP values are taken from the steady-state portion of each test, excluding the initially faster penetration during hole start. These measurements are used to calculate specific energy (SE), defined as the total drilling power normalized per unit volume of ice excavated directly beneath the probe.

TABLE 3. Summary of all drilling tests. Environmental conditions are given in group headers. Power is broken out into thermal ( W t ) and mechanical ( W m ) inputs. Reported ROP is the steady-state penetration rate, computed after the auger is fully submerged and therefore excluding the initially faster hole-start penetration observed in the drilling subsystem tests. Each value is from a single test. Theoretical benchmark rows are calculated values for a perfectly efficient melt probe described in Section 3.7.

Calculated value assumes a 2.5-inch diameter melt probe: with ROP set so that Q ̇ 0 + Q ̇ 1 equals total slushing power.

As note (a), but uses latent heat of vaporization for calculation (vacuum environment; no liquid water phase).

As note (a), but also includes the effect of heat loss to the side wall, Q ̇ 2 using a probe length 1.38 m to match the integrated prototype length.

Figure 9 shows an in-progress image of each drilling subsection test and the resulting drill chips. In the warm ice testing in an ambient environment, mechanical-only drilling created very dry cuttings that accumulated in the borehole and were eventually unable to be cleared, causing the probe to stall (Figure 9A). A similar result was seen in the mechanical-only drilling test in cryogenic ice in vacuum (Figure 9E). The drill chips when slushing in warm ice (Figure 9C) and in cryogenic ice (Figure 9D) in an ambient environment show wet clumps of frozen together ice chips, reflective of the slurry created by the combined mechanical and thermal drilling.

The thermal test in warm ice ambient pressure (Figure 9B) was power-limited by poor heat conduction from the drill tip into the ice; the heater was restricted to 60 W to prevent overheating. While this limitation can be significantly improved with design changes to the drill bit geometry and heater placement, it highlights the importance of thermal design and the need to modulate power to prevent overheating.

The thermal test in cryogenic ice in vacuum (Figure 9F) suffered the same power-limitation as in the warm ice test: the heater was restricted by poor heat conduction from the drill tip into the ice, resulting in the lowest ROP and highest specific energy of the three modes. Unlike the warm ice test, the thermal drilling mode in vacuum created a borehole of the same size as the auger rather than a water well. Since there is no fluid to effectively transfer heat in the vacuum environment, only the ice in direct contact with the auger was penetrated.

In vacuum, cuttings from slushing are dry, as liquid water cannot exist below the triple point pressure (Figure 9G). Sublimation was indirectly observed during the slushing test, as pressure from the gaseous H₂O was sufficient to blow cuttings out of the borehole (Figure 9H). Clearing ice chips via water-ice sublimation has a major advantage of increasing penetration rate and lowering drilling power (Zacny et al., 2004). This effect, however, will only occur during initial penetration into the ice shell, when the borehole is open to vacuum.

The preceding drill-subsystem tests isolate the drilling physics at the bit and establish that slushing is viable across all three environments. A full probe is expected to drill more slowly and less efficiently for several reasons: more heat is required to offset thermal losses along the longer borehole wall, and the anti-torque subsystem draws additional power to melt its skates into the ice. Friction from the skates and reaction effects from the tether also reduce the WOB, which lowers ROP. The integrated prototype test described next is a proof of concept for slushing as a standalone system; its expected performance penalty is examined in Section 5.1.

4.1 Integrated prototype probe testing

Figure 10 shows the results of the integrated prototype probe tested in the 1.5-m, 260 K ice tower. Despite several technical issues during the test, an uninterrupted period of drilling near the probe full submersion provides a useful result of thermal power, mechanical power, and speed data recorded, which is used for comparison in Table 3.

The probe operated in slushing mode throughout the test, forming an ice slurry, as seen in (Figure 11A). Wall heaters were mostly unused. Cracking in the ice tower was observed during the test, either due to thermal or mechanical shocks from slushing (Figure 11B). The heaters on the skates in the anti-torque assembly were activated as they entered the ice, allowing them to smoothly melt into the borehole wall (Figure 11C).

5 Discussion

5.1 Slushing vs. thermal

Several caveats apply when comparing slushing and melting test results. First, the auger and bit were designed for mechanical drilling, not heat conduction, which hampered the thermal-only tests by limiting how effectively heat could be transferred from the drill tip into the ice. The heaters are placed farther from the ice interface than they would be in a dedicated melt probe, and the bit was made from aluminum 6061 rather than from a higher-conductivity copper alloy. These choices trade thermal performance for mechanical robustness. Second, power levels were not matched across the tested modes, because the heaters were temperature-controlled to prevent burnout. The melting test was restricted to 60 W in warm ice and 360 W in cryogenic ice, while slushing operated at total powers of 250 W and 1000 W, respectively. As a result, direct mode-to-mode comparison is misleading. With these caveats in mind, the raw comparison between the tested modes in warm ice shows slushing at 5.2x the efficiency and 22x the speed of the tested melting configuration, where efficiency is interpreted as the inverse of specific energy. This difference is largely a consequence of the power mismatch and bit design rather than an intrinsic property of the two approaches.

A more meaningful comparison is against the theoretical melt probe benchmark, where slushing achieved 1.1x the efficiency and 1.3x the speed. The theoretical melt probe benchmark assumes idealized heat transfer from the probe to the ice and does not account for practical engineering losses; it represents an idealized upper-bound reference, not a directly achievable hardware performance target. Practical performance of an actual thermal probe would depend strongly on the engineering design and would likely fall between the theoretical benchmark and the tested thermal-only configuration. For example, modeling shows that a theoretical probe can become more energy efficient at higher speeds because side-wall losses are reduced, but this requires higher total power. At the same time, actual melt probe performance is less efficient than idealized modeling and depends on design considerations beyond speed and specific energy (Talalay, 2020).

For the full integrated prototype test in warm ice, slushing achieved 0.16 Whr/cc and 0.90 m/h, compared with 0.08 Whr/cc and 1.10 m/h for the drilling subsystem alone. Thus, integration increased the measured specific energy by about 2x and reduced the speed to about 0.8x the drilling-subsystem value. As anticipated above, these reductions have distinct causes: the lower speed reflects reduced weight-on-bit from skate friction and tether reaction loads, while the lower efficiency reflects the added power lost into the borehole wall along the probe’s full heated length, rather than at the bit alone. Even with these added system-level loads, the integrated probe exceeded the speed and efficiency of a theoretical melt probe at the same power budget, achieving 1.19x the efficiency and 1.6x the speed.

In cryogenic ice at ambient pressure, slushing achieved about 0.43x the efficiency and 0.43x the speed of the theoretical melt probe benchmark. While the theoretical performance is idealized, this discrepancy is more significant than in the other environments. One possible explanation is that side-wall heat transfer becomes more significant as the temperature difference between the melting point of ice ( T 0 ) and the environmental ice temperature ( T ∞ ) increases. Even a short length of borehole wall could have conducted a significant amount of heat out of the slurry created by the drill, effectively contributing a partial Q ̇ 2 term not accounted for in the theoretical comparison in Table 3. Using the side-wall heat loss expression from Durka et al. (2022) to estimate Q ̇ 2, approximately 640 W of thermal power could be attributed to side-wall heat loss along the length of the heated auger during the cryogenic ice ambient-pressure test. This is an approximate, first-order estimate based on the model rather than a direct measurement; we did not measure the borehole-wall temperature. Still, the size of the observed performance gap together with this estimate indicates that side-wall heat transfer is a major term in the thermal power budget in cryogenic ice.

The vacuum cryogenic ice environment is where the slushing approach showed the greatest advantage. Slushing was over 7x faster and about 1.5x more efficient than the thermal-only test. The thermal-only test struggled to conduct heat into the ice because the vacuum environment prevented formation of a stable liquid melt pool that could transfer heat away from the drill. Input power therefore had to be limited to prevent overheating. This limitation is relevant to melt-probe operation during early penetration, before borehole closure and pressurization allow a stable liquid phase to form. During slushing, the auger and percussion mechanisms mixed small cuttings and increased thermal contact with the heated drill surfaces, allowing heat to be transferred into the ice more effectively. In the upper portion of an icy-world ice shell, where cryogenic ice and vacuum conditions are expected before borehole closure, the slushing approach is therefore expected to be most beneficial. Compared with the theoretical melt probe benchmark, slushing was about 2.3x faster and about 1.3x more efficient.

5.2 Drilling in vacuum

The effect of vacuum during slushing is significant. Ice sublimation energy (2.6 kJ/cc) is 8.4x greater than the energy to melt ice (0.31 kJ/cc). In testing, slushing in cryogenic ice in vacuum required only about 1.4x more specific energy than slushing in cryogenic ice at ambient pressure (0.48 Whr/cc vs. 0.35 Whr/cc) at a similar ROP (1.14 m/h vs. 0.91 m/h). The partially liquid slurry that forms at ambient pressure pulls heat away from the drill much more effectively than dry cuttings in vacuum, likely due to convection and larger contact area. The effectiveness of slushing in vacuum was due to sublimation of the ice in contact with the heated auger and bit. As seen in video Figure 9H, the resulting gas created local pressure differentials large enough to blow cuttings out of the borehole. Since cuttings buildup below the drill is the primary cause of slow penetration and stalling, this clearing mechanism was extremely effective at maintaining a good penetration rate. This ejection was strongest early, near the surface, when the borehole was open to vacuum; as the probe drilled deeper the effect became more sustained but weaker, and cuttings were pushed up and away from the drill bit rather than ejected completely. The transition is seen in the slushing depth versus time curve in Figure 8B, as the ROP transitions to a slower, more constant rate. In vacuum, where no stable liquid pool can persist, heat transfer at the bit shifts from convection through meltwater toward solid–solid contact, sublimation, and vapor transport, so this vapor-driven clearing is also the dominant means of moving material behind the probe.

This behavior is consistent with recent low-pressure ice-penetration work. Koßagk et al. (2025) drilled ice in a vacuum chamber below the triple point and observed vapor ejecting solid particles, both ice grains and dust, from the borehole. Because their laser drill ablates rather than mechanically cuts the ice, the vapor-driven ejection of mechanically generated cuttings we observe appears uncharacterized elsewhere. Numerical models of heated drilling in icy regolith similarly show sublimation, vapor migration, and refreezing governing low-pressure heat and mass transport (Wang et al., 2023), though for water extraction from porous regolith rather than forward penetration of a continuous ice shell.

Partial borehole closure was also observed, likely due to the buildup of cuttings (Figure 9H). Borehole closure is vital for penetrating an ice shell as it allows a pressurized environment to form around the probe before it reaches the subsurface body of water. Recent studies observed borehole closure and pressurization occur at about 4 times the length of a thermal probe descending through pure, cryogenic ice at vacuum conditions (Stone et al., 2023). On a planetary body, this also depends on the porosity and fractures that may be present in the ice shell and prevent borehole closure at greater depth.

5.3 Mechanical drilling

Mechanical drilling proved useful in starting the hole and has the lowest specific energy in each environment, but consistently stalled once the auger was fully submerged and chips piled up in the borehole. Hence, this mode is viable when the probe first enters ice, but once fully encased within the ice shell, the probe will need to transition into slushing or melting. Removing cuttings and re-compacting behind the probe is the toughest problem that mechanical-only systems have to tackle. There have been commercial companies, such as Badger Explorer, that attempted to develop such a system for terrestrial applications with mixed success (Stokka, 2006). Honeybee Robotics has developed alternative solutions: cable-suspended drills such as WATSON (Malaska et al., 2020), Planetary Deep Drill (Zacny et al., 2016), and Autogopher (Bar-Cohen et al., 2018). This approach, also used predominantly in Antarctic core drilling, solves the chip transport problem by lifting the drill out of the hole, emptying cuttings or a core container, and lowering the drill back in. This only works in stable, dry boreholes—which will not be the case on Europa and Enceladus. In testing, mechanical drilling was more effective in cryogenic ice (0.83 m/h, 0.027 Whr/cc) than in warm ice (0.42 m/h, 0.031 Whr/cc). Cryogenic ice is significantly more brittle than warm ice, making rotary-percussive drilling more effective at fracturing (Arakawa and Maeno, 1997).

5.4 Slushing analytical model implications

This subsection presents a preliminary, first-order estimate of the slushing correction factor σ 2, derived from a single test under simplifying assumptions stated below.

Decoupling the heat distribution and slushing coefficients in Equation 2 from the integrated probe testing data is non-trivial, as the heaters in the auger contribute to Q ̇ 0, Q ̇ 1, and Q ̇ 2. Heat generated from friction during mechanical drilling also contributes to the total heat output. Several assumptions are made to obtain an initial estimate: (1) penetration is assumed to be purely mechanical, (2) all thermal power is applied to the sides of the probe, and (3) advective heat loss Q ̇ 3 is assumed to be negligible. Applying this to the results of the integrated prototype probe slushing test from Table 3 means all 295 W of thermal power are going into counteracting the thermal losses out of the side wall ( Q ̇ 2 ). This value can be compared to the theoretical melt probe benchmark, where Q ̇ 2 is calculated at 191 W using the side wall heat loss expression from Durka et al. (2022). This warm-ice integrated-probe value differs from the ∼640 W estimated earlier for the cryogenic ambient-pressure case, where the larger ice-to-melt temperature difference drives a much higher side-wall loss. This gives an initial estimate of σ 2 ≈ 1.54, meaning the SLUSH prototype probe required about 1.54x the power to keep the water in the borehole liquid compared to a theoretical melt probe. The efficiency gain for SLUSH comes from penetration, as it takes 0.15x the power to penetrate ice compared to Q ̇ 0+Q ̇ 1 from the theoretical probe. These two factors are where the slushing model (Equation 2) departs from the conventional melt-probe model (Equation 1): slushing trades a large reduction in penetration power for a moderate increase in side-wall power, and this trade is what the framework is intended to capture. The estimate for σ 2 is sensitive to design parameters and conditions. For example, faster speeds and finer cuttings may increase σ 2, as the cuttings are colder and have a larger total surface area to pull heat away from the liquid water faster. Dedicated testing is recommended to study this more directly and reduce sensitivity to the assumptions above.

5.5 Design lessons learned

Testing of the prototype SLUSH probe highlighted several areas of improvement for future designs. The heated drill bit had difficulty conducting heat out into the ice. Improved heater placement, a drill bit geometry designed for heat conduction, and intermediate materials such as tungsten-copper that combine high strength with good thermal conductivity would improve performance for both slushing and melting-only modes. Figure 12 shows a comparison of the SLUSH bit versus a more conventional melt probe designed for heat conduction. Melt probe thermal head optimization testing and optimization is explored in other studies (Sysoev et al., 2025; Palmowski et al., 2022).

An actuated anti-torque subsystem is also recommended, as the passive, spring-loaded design used is difficult to tune to maximize reaction torque while minimizing axial friction, which is vital for probe performance. As drilling and ice parameters change, so does the borehole size, and the anti-torque must adjust accordingly. The tether was also a source of inefficiency and was not representative of a flight-forward design. Future iterations should integrate electronics into the probe and reduce the tether from 50+ conductors to 2 conductors for power and a fiber optic for data.

Weight-on-bit (WOB) is one of the most important factors for slushing performance. A higher WOB improves the speed and efficiency of rotary-percussive drilling (Mellerowicz et al., 2022). For thermal and thermo-mechanical drilling, WOB primarily ensures contact between the heated tip and the ice. ROP is governed more strongly by available thermal power than by axial load (Schüller and Kowalski, 2019; Sysoev et al., 2025). The integrated prototype was designed for the sole purpose of testing drilling methods and was therefore lower mass than a more developed, fully instrumented probe would be. During testing, variations in the audible noise from the percussion indicated moments of lower WOB where the anvil would not retract quickly enough to be hit by the hammer, leading to lost energy. There is likely an optimal combination of WOB and percussion rate (BPM) that would improve probe performance. Additionally, a heavier and more rigid probe reduces side-to-side wobble, which was often observed during drilling subsystem testing. This caused the drill to create a slightly larger borehole than its outer diameter, reducing performance; though it is worth noting that a small borehole clearance also reduces the risk of the probe becoming stuck due to refreezing on the side wall.

5.6 Limitations and future work

This study is intended as a proof-of-concept evaluation of a thermo-mechanical drilling method. The results presented in this paper are subject to several limitations that should be considered when interpreting the findings.

5.6.1 Single-run results

Each test condition was run once; no duplicate runs were performed, as noted in Section 3.2. The reported ROP and SE values therefore reflect individual test outcomes and do not carry statistical uncertainty estimates. Future work should conduct multiple runs per condition to quantify variability and establish confidence bounds on the reported metrics.

5.6.2 Pure distilled-water ice only

All tests used homogeneous crystalline ice blocks made from distilled water. The ice on Europa, Enceladus, and Mars is expected to contain inclusions such as salts, dust, and gas bubbles, as well as fractures and porosity. These features could affect heat transfer, phase change behavior, chip transport, and borehole closure. Testing in inclusion-bearing and fractured ice is a priority for future work.

5.6.3 Slurry state not characterized

The chip size, liquid fraction, and whether the meltwater forms a continuous phase were not measured. Ice chips and a flowable slurry were observed qualitatively (Figures 9, 11A), but these properties govern slurry mobility and the correction factors in Section 2, so we identify them as key measurements for future testing.

5.6.4 Fixed RPM, BPM, and WOB

Auger rotation speed (120 RPM), percussion rate (972 BPM) and weight-on-bit (125 N) were held constant across all tests. The interaction between these parameters, ice temperature, and drilling mode is not characterized. A parametric or test design varying RPM, BPM, and WOB is recommended to identify optimal operating points for each environment. WOB is especially relevant for planetary drilling, as the lower gravity environments of Europa, Enceladus, and Mars will significantly affect the available WOB of the drill system.

6 Conclusion

To improve drilling speed and minimize energy usage, the SLUSH probe combines thermal melting with mechanical cutting: a rotary-percussive drill fractures the ice while applied heat partially melts the cuttings, creating a slush that transports chips behind the probe. This slushing approach is designed to allow the SLUSH probe to penetrate through the ice on Europa, Enceladus, and Mars. A scaled prototype drilling subsection of the probe has been developed and used for testing to quantify the differences between the slushing, thermal-only, and mechanical-only drilling methods. The design was tested in icy environments relevant to what is expected on planetary bodies: cryogenic ice (130 K) at vacuum pressure (to simulate the ice surface and hole start), cryogenic ice at ambient pressure (to simulate the upper layers of the ice), and warm ice (260 K) at ambient pressure (to simulate the layers of ice near the body of water). A scaled, integrated prototype probe was developed to further test the slushing approach in a warm ice tower. Testing demonstrated that the slushing approach is viable across all tested environments. It uniquely avoids the chip-transport stalling of mechanical-only drilling and the thermal-conduction bottleneck of melt-only drilling. While the single-run, fixed-parameter test campaign does not permit definitive performance ranking, the results are sufficiently encouraging to warrant parametric optimization and repeat testing.

Since the prototype design was not optimized for melting, test results were compared using the most recent analytical model for a melt probe going through extraterrestrial water ice. Using this modeled, perfectly efficient melt probe as a benchmark, slushing was comparable in speed and efficiency in a warm ice, ambient pressure environment, was slower and less efficient in a cryogenic ice, ambient pressure environment, and was significantly faster and more efficient in a cryogenic ice, vacuum environment. A conceptual power-balance framework was introduced that decomposes the slushing power requirement into its mechanical and thermal contributions, providing a structure for interpreting the slushing performance rather than a predictive performance model.

These results suggest that slushing may be especially beneficial in the upper meters of Europa’s surface ice and in the warmer ice near the subsurface ocean. Because the method incorporates mechanical cutting, it has the potential to penetrate impurity-rich ice (e.g., salts, dust, rock inclusions), though this remains to be experimentally verified. The findings support slushing as a promising method to improve the drilling performance of a planetary ice probe, warranting further investigation toward access to subsurface bodies of water on Icy Worlds.

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Footnotes

Edited by: Antonio Mattia Grande, Polytechnic University of Milan, Italy

Reviewed by: Shuangyu Wang, Harbin Institute of Technology, China

Stephanie Cazaux, Delft University of Technology, Netherlands