Feasibility of an Ice Penetrating Robot for Europa

Bart Romgens, 1173197, b.a.romgens@student.tudelft.nl

November 24, 2008

Abstract

Previous missions into the outer solar system have found a natural satellite around Jupiter,Europa, that may contain a liquid ocean under its icy surface. A liquid ocean would be the firstplace to look for life as we know it. A small, 12 cm diameter, 2 m long, Ice Penetrating Robot(IPR) could bring a miniaturised science submarine through 10 km of ice, to the liquid ocean.It can use a heat element and hot water jet to melt through the ice. This journey will take 2-4years and requires wireless transponders, radioactive isotope power sources and autonomousattitude and position control. The main difficulties with designing such an IPR are the longrequired lifetime, the heat transport during flight to Europa and the communication through10 km of ice. A explorative mission to get more information about the ice layer on Europa isneeded before a detailed design of an IPR can be made.

Contents

1 Introduction 1

2 Supporting mission and assumptions 32.1 Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Supporting mission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 Functional requirements 3

4 General design (options) overview 5

5 IPR Segments 65.1 Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65.2 Propulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.3 Guidance and Navigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105.4 Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135.5 Thermal control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145.6 Computer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6 Conclusion and recommendations 156.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156.2 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1 Introduction

Europa is the fourth largest moon of Jupiter and is almost as large as our own moon. The Voyagermission provided the first close-up images of Europa when it flew by at an altitude of 130,000km. These pictures showed a surface of water ice. The next mission to get close to Europa, wasthe Galileo spacecraft in 1997. This craft flew by Europa several times, at one time skimming itssurface at 200 km altitude. Analysis of the data gathered by this mission strongly suggests therecould be a large portion in permanent liquid state underneath the frozen surface.[1][2]

What if there is an ocean? What would this mean? Fact is, where there is a large amountof liquid water, an energy supply, and protection against most high-energy cosmic events, there

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Figure 1: Optical image of Europa surface. [2]

is potential for life. A liquid ocean underneath the ice and possible volcanic activity would makeEuropa the most likely place for life outside Earth. When it is certain there is an ocean underneaththe ice, exploring this ocean will be on top of the wish list of many scientists and space agenciesaround the world.

A scout or pathfinding mission will be needed to find out if there is an ocean and characterisethis ocean before a mission will be send to explore a possible ocean. NASA and ESA both hadplans for such an exploration mission, but NASA ran out of funds for its robotic exploration pro-gram and cancelled a Europa mission [3]. The only mission currently planned by ESA to go toEuropa is the Jovian Europa Orbiter (JEO), however, a launch date has not yet been determinedand development came to a hold [4].

This essay will ask questions, and tries to answer them, about a mission to actually explorea possible ocean. In particularly the part about penetrating the ice to get to this ocean. It isthus assumed there has already been a mission to Europa to determine whether there actuallyis an ocean and how deep the ocean and ice are. Because such a preparatory missions is not indevelopment, it could take several decades before an actual ocean exploring mission will be sendto Europa. Although it may therefore seem a bit premature to examine an Ice Penetrating Robot(IPR), it is useful for the following four reasons.

First of all, before sending a preparatory missions to Europa, it is useful to know whether it isfeasible to explore a possible subsurface ocean. An expensive preparatory mission would be lessuseful if we know it is impossible to reach the ocean, may we find one. Funds may be directedtowards other missions if this is the case.

Second, there is another satellite in our solar system that has a large water ice layer with apossible liquid ocean beneath it; Enceladus, a natural satellite of Jupiter. [5]

Third, there probably is a large layer of water ice in the polar regions of Mars. A similar icepenetration mission could be send, and may have a higher priority, to Mars to explore the ice onMars. [6]

And last, there are unexplored glacial lakes deep under ice on Earth. Lake Vostok is probablythe most well known, but there are many more unexplored subsurface lakes on Earth. Lake Vostokis located 4 km under the surface of the central Antarctic ice sheet. This lake has not yet beenreached because normal drilling methods may contaminate the lake that has been sealed off forthousands of years. There are great similarities between lake Vostok and a possible ocean underEuropa and finding life in lake Vostok would strengthen the prospect for the possible presence oflife on Europa. [7] Exploring lake Vostok would be a great scientific mission in itself while it alsoprovides a test environment for a Europa lander mission.

The goal of this essay is to get an overview of the feasibility and mayor technological challengesof a small Ice Penetrating Robot (IPR) that can bring a scientific payload (sensors) to a possiblesubsurface ocean on Europa. This essay will focus on the electronic, mechanic and computer parts

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and their interconnections to create a system that fulfils all mission requirements stated in Section3. It will try to give a general system overview that pinpoints important relations, problems anddifficulties that need attention. It is not a study into the scientific requirements and how to fulfilthem. It’s about the mechatronics that has to serve a science mission.

Two ice penetrating robots prototypes have already been build and tested; Cryobot[8] andSIPR[9]. They are shown in Figure 2.

Figure 2: left: SIPR prototype[9]. right: Cryobot prototype[8].

To achieve the goal mentioned above, a few assumption are made and the supporting missionis described in Section 2. An overview of all the functional (non-scientific) requirements of theIPR is given in Section 3. After the requirements a basic design overview is presented in Section4. In Section 5 all major system segments are discussed in some detail; problems are analysedand possible solutions are presented. This essay closes with a conclusion and recommendation inSection 6.

2 Supporting mission and assumptions

2.1 Assumptions

The thickness of the ice layer on Europa is currently unknown. All estimates of the thickness arebased on Galileo’s observations and thermal models. Estimates range from 1 to 200 km, althoughmost estimates range from 1-20 km[1][10][11][12][13]. It is therefore assumed that a preparatorymission found a liquid water ocean on Europa, under an ice layer of 10 km.

2.2 Supporting mission

It is a big undertaking to softly land an object on the surface of Europa. A launch vehicle willhave to launch a spacecraft into orbit towards Jupiter. At Jupiter it will have to send a landerto the surface of Europa. Since Europa has a virtually non existent atmosphere[1], parachutes areno option and all braking has to be done with rocket engines. The lander has to put the IPR invertical position such that it can enter the ice. The lander also has to serve as a communicationhub between the IPR and a relay satellite, that on its turn will serve as a communication hubbetween Earth and the lander. A Ground Station has to monitor and control the whole mission,including the IPR.

All these supporting mission segments are taken into account, but are not treated in this essay.

3 Functional requirements

This section presents the functional requirements of the IPR. What functions should it perform andhow well should it perform these functions? The functions are categorised in payload, autonomy,

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guidance and navigation, communication and power.

PayloadThe IPR has to bring a payload to the liquid-ice boundary. One obvious payload to explore the

ocean is a miniature submarine. There already are two conceptual designs for such a submarine,the MASE[14] and the MEMS[15]. Both have a 5 cm diameter and are 20 cm long. The mostadvanced concept is the MEMS, shown in Figure 3. The IPR should be able to bring a MEMSsized submarine to the ocean. When the ocean is reached, the IPR should still function as a powerand communication relay station for the submarine.

Figure 3: An illustration of the MEMS enhanced miniature submersible vehicle. The internaldistribution and dimensions of the vehicle is shown. [15]

Guidance and NavigationThe IPR should move downwards through a thick layer of ice. It should be able to detect and

navigate around possible objects obstructing its way. It should also determine and control itsposition and attitude in the ice.

CommunicationThe IPR should have a data link with Earth, to receive commands from Earth and send house-

keeping and science data back to Earth. The communication link should be able to transmitimages, although no realtime streaming is required. 10 Mbit/day is assumed to be enough.

PowerThe IPR should provide electric power to the science payload (miniature submarine). This

science payload power is estimated to be 8 W in case of the MEMS submarine [15]. The IPRshould also power all its own subsystems.

AutonomyA signal from Earth to Jupiter and back travels for ∼40 minutes. There are also moments when

no communication is possible for hours due to obstruction by Jupiter[1]. The IPR therefore cannot be directly controlled and it should be able to move, navigate, communicate and secure itselfautonomously for hours.

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4 General design (options) overview

There are two basic ways to make it through the ice: by finding a natural hole or by making a hole.Scientists think there may be large cracks in the ice due to tidal movements [1]. Those are thelines that can be seen in Figure 1. These are, most likely, not straight static cracks to the oceanthat can be used to get to the ocean. This should, however, be investigated by the preparatorymission to Europa. It is, for now, assumed that the IPR has to make its own hole trough the ice.

The methods that are mainly used on Earth to drill through ice require a large base station ontop of the ice. This would require too much mass and is therefore not an option for Europa. Theonly option is a small probe that melts or drills through the ice, while the hole behind the probecloses again. The easiest way to move the ice from the front of the probe to the back is by meltingit. A probe that melts through the ice is therefore initially chosen as best option.

The most logic shape for the IPR is a cylinder with the smallest possible diameter. A smallerdiameter means that less ice has to be melted. The nose of the IPR will have the shape of anellipsoid to create enough contact with the ice and guide the melt-water along the cylindrical bodyof the IPR. The smallest possible diameter and related length of the IPR will be estimated in thenext sections.

Melting of the ice can be done by an active and passive heating system. The passive heatingsystem is a static heat element and the active system could come in the form of a hot water jet.Both options and their combination will be considered in the next section.

Figure 4: A conceptual design overview of the IPR.

The science payload will be located after the melting system and water tank in the nose, ascan be seen in Figure 4. This payload, together with the nose, will be dropped in the ocean. IPR’scomputer and communication system should, however, still work when the science payload (minisubmarine) is dropped in the ocean. It is therefore located behind the science payload and willhave to fix itself in the ice before it reaches the ocean.

The IPR electric power options are an onboard radioisotope thermoelectric generator (RTG), anuclear reactor, fuel cells or batteries. The electric power source will be located between the maincomputer and the communication segment, this to decrease large temperature differences withinthe IPR.

Communication with the Europa lander can be done in multiple ways. A long tether couldcreate a connection between the lander and the IPR and could even provide the IPR with powerfrom the lander. One of the disadvantages is that a tether could break when the ice starts tomove and shear. A second option is to drop wireless transponders in the ice while moving downthrough the ice. These transponders could be used as hubs for communication between the IPRand lander. A third option would be to have a very strong transmitter that could send a wirelesssignal through the whole ice column. A last option would be to make the hole, which refreezes,very conductive by adding some material to the meltwater. The frozen hole could then be used asa wire to communicate electric signals to the lander. This option has the same main disadvantageas the tether, but may be done with less mass.

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5 IPR Segments

This section goes into more detail about the different segments of the IPR. The segments treatedhere are communication, propulsion, guidance and navigation, power, thermal control and thecentral computer.

5.1 Communication

Communicating through 10 km of ice is one of the biggest challenges for the IPR. There are threeways to do this; by tether (wire), wireless transponders or by using the ice as medium (electric oracoustic).

TetherTethers are reliable for slow-moving ice, can also carry electrical power, and have a very large

bandwidth. They, however, have strict length limits, are risky in moving ice (they can break) andmay involve a lot of mass. The winch and the mass of the tether has to be on board of the IPRbecause the ice behind it refreezes and fixes the tether in the ice. Using a tether on Europa isconsidered too risky because the ice layer is very dynamic, it may flex 10’s of meters per 3.5 Earthdays (one Europa revolution). [1] [16]

Ice mediumThe acoustic properties and electric conductivity of ice could be used, or even created, to transmit

a signal through the ice. Cracks and cavities in the ice will greatly reduce the penetration depthand bandwidth. These cavities and cracks may already exist or may be created while meltingthrough the ice. Europa’s is very dynamic and therefor too risky to rely on the electric or acousticconductivity of 10 km ice.

Wireless transpondersWireless transponders could be deployed from the back of the IPR. These transponders could

transmit through the ice at optical and radio frequencies, as can be seen in Figure 5. As thetemperature of the ice decreases, it acts less as a conductor and the signal can penetrate a greaterdistance[8]. The surface of the ice on Europa is -170 C, at which a 1GHz signal penetrates 30-50times deeper than through ice close to the freezing point [16][8]. Electro-magnetic penetrationdepth favours longer wavelengths, but antenna size and data rates favour shorter wavelengths.

An estimate, by NASA[8], of the wireless transponder deployment depth is shown in Figure 6.This graph is based on 120 mW transmission power, for which a total of 1.3 W electrical power isneeded. This figure shows that at least 14 transponder units are needed.

The transponder units will consist of a power source, antenna and electronics. Extra sensors,i.e. pressure and temperature, could be added if there is enough space and power available.

The only power source that is small and can produce power for years at a place where there is nosun, is a miniature radioisotope power unit. Since only 400 mW electrical power can be producedin the small (height = 2-4 cm, diameter = 10-15 cm) transponder, it will need a capacitor and usedata burst[16]. The power source of the transponders is discussed in more detail in Section 5.4.

The outher shell of the transponder will function as a 1 GHz patch antenna. The 1 GHz patchantenna designed for the MUSES-C could be used [16].

The electronics have to receive, amplify and transmit a signal using the patch antenna men-tioned above. An example of a simple commercial miniature receiver and transmitter is shown inFigure 8. The operating temperature of the electronics is between -20 C and 90 C. The ice tem-perature surrounding the transponders is between -170 C and 0 C. Small pellets of plutonium-238may be used for extra thermal control when the 400 mW and isolation is not enough to keep thetemperature within the electronic operating range.

The transponders will be deployed from the back of the IPR. Four ice-hooks on the outside ofthe transponder (see Figure 7 ) are pushed outwards by a coil spring when it leaves the IPR tube.The ice-hooks will grab the ice and fix the transponder in the ice. The transponder is released bya wire pulling mechanism that is pulled by a winch. This releases one transponder and moves therack with remaining transponders one step upwards.

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Figure 5: Penetration depth vs wavelength for ice at different temperatures. [16]

Figure 6: Transponder deployment depth graph. [16]

A possible temperature sensor that can be embedded in the shell of the transponder is the S900Silicon diode miniature cryogenic temperature sensor as shown in Figure 9

An conceptual overview of the transponder, described in this section, can bee seen in Figure 7.

5.2 Propulsion

The propulsion system of the IPR has the sole purpose of bringing the IPR through the ice. Theonly feasible way to accomplish this, is to melt the ice in front of the probe and let the melt waterflow around or through the probe so it can refreeze behind the probe. Heat is needed to melt thewater. This heat can be directly transferred from the shell/nose of the IPR to the ice, or water

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Figure 7: Concept version of the IPR transponders.

Figure 8: left: Transmitter example [17]. right: Receiver example [17].

Figure 9: The S900 Silicon diode miniature cryogenic temperature sensor [18].

can be heated and jetted out of the nose.A passive heating system may not be sufficient when small dust/particulates, in the range of 10

to 750 microns, are present in the ice. The 750 micron particulates are dense enough to sink to thebottom of the borehole and form a thermal insulation boundary which caused a JPL test probe tostop after 10 cm. [19] This means that an active water system is needed to remove accumulateddust in the borehole that may bring the IPR to a hold.

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A passive heating system alone is not enough to remove dust while an active water systemalone is not efficient without a passive heating system [20][19]. A design with a passive heatingsystem and active water jetting system is therefore chosen as the optimal propulsion system.

Passive nose heatersModels and experiments conducted by NASA/JPL [19] have determined that between 800 W

and 20 kW thermal energy is needed for probes of different sizes to keep a positive melting speed.They also determined that a thermal power source of 1 kW should be most efficient for a 12 cmdiameter probe, in terms of W per cm. Figure 10 shows the drill speed and thermal power availableat different ice temperatures. From different models and experiments[8][20][19] a melt rate of 0.3-0.7 m/hour can be expected. This means that 10 km ice would take the IPR 1.5-4 years to reachthe ocean. This means that the IPR should be very reliable for long periods. This certainly willbe one of the major problems in designing and testing the IPR.

Figure 10: Actual and predicted melt rates for the cryobot as a function of ice temperature/pressurefor a 1 kW thermal energy source [8].

To fulfil these requirements, the nose of the IPR can be fitted with Plutonium-238 that generatesheat (390 thermal Watts per kg) by radioactive alpha decay. To generate 1000 W of thermal power,2.5 kg and 130 cm3 Plutonium-238 is needed. This Plutonium will be placed in the nose of theprobe while heat will be transported, through the shell, to the back of the IPR to make sure waterdoesn’t refreeze around the back of the probe. The 130 cm3 easily fits into a nose with a diameterof 12 cm and a height of 10-25 cm while leaving more than enough room for structures, insulation,water jet system and sensors.

Active hot water jetAn active hot water jet in front of the probe will melt the ice and create enough fluid movement

to transport small dust particles in the ice to the back of the probe. The water that is used,is gathered by an inlet at the sides of the probe, just behind the nose. This is also where thetemporally water reservoir is located. This means that the heat from the Plutonium-238 can heatthe water while the water protects the rest of the probe against the heat from the Plutonium-238.

The heated water from the reservoir can be accelerated by an electric pump. This, however,means that precious electrical power has to be used while there is more than enough thermalpower. It would therefore be best to not use a electric/mechanic pump, but a steam driven pumpthat is powered by the heat elements in the nose. The temperature in a smaller pressure reservoircould be raised above boiling point and released from the nose while mixed with normal heated

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water from the major reservoir. This avoids the need for very inefficient thermal to electric powerconversion that requires precious mass and space. If this may not be feasible, a small centrifugalelectric water pump could be used.

Since there is no control over the thermal output of the Plutonium-238 and RTGs, the waterreservoir is the only thermal control mechanisms possible. The water inlet, outlet, temperature,pressure and water volume in the reservoir should therefor be measured and controlled. Miniaturepressure and temperature sensors are connected to the main IPR computer that in its turn activatessmall valves.

A CMOS piezoresistive micro pressure sensor chip is small ( 5x5x2 mm) and light (<1 g)and would be suitable for use in the IPR [21]. Such a pressure sensor is shown in Figure 11.The S900 Silicon diode miniature cryogenic temperature sensor, proposed for the communicationtransponders, can also be used in the IPR itself.

Figure 11: CMOS piezoresistive pressure sensor chip. [21].

5.3 Guidance and Navigation

The IPR should be able to determine its (vertical) position in the ice to know when to deploy thecommunication transponder modules. It also needs to determine the local gravity vector (i.e. theinclination of the IPR) to make sure it is going straight down towards the sea. The IPR shouldnot only determine, but also control its position, to avoid obstacles in the ice and make sure theshortest path to the ocean is followed. This means that obstacles have to be noticed and analysedin time and that a control program has to be created to manoeuvre the probe around the obstacle.The next three paragraphs will explain how obstacles are found and its position and inclinationare determined and controlled.

Position and inclination determinationTo determine the vertical position of the IPR, three concept are considered; a small wire con-

nected to the Europa lander rolling out from the IPR, a small wheel rolling against the ice in thebore hole, and a wireless GPS style position determination based on signal travel time.

The small wire/thread rolling out from the IPR, while the revolutions of the winch are countedand provide the distance travelled, may break due to ice movement as long as at least one part isstill frozen in the ice and the wire is under tension. Bringing 10 km of wire is possible if the wireis very thin (1-10 micrometer) and strong. This is possible with carbon fibre.

A small rough wheel on the outside of the IPR, pressed against the ice by a spring, wouldmeasure the distance travelled by the number of rotations of the wheel. This is a simple solutionthat is accurate enough for the transponder deployment. The major disadvantage is that this unitis placed outside the shell of the probe and therefore not protected against temperature, water(flow)and small obstacles in the ice. It may also not touch the ice when the bore hole becomes too wideand water flow may rotate the wheel meanwhile. This should be tested experimentally. The massand power impact of such a distance sensor is very small and it’s easy to place more than one forredundancy.

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A GPS like wireless position (distance) determination system would work like the Earth basedGPS system, but with only one transmitter (on the Europa lander) and provides only the distancebetween the lander and the IPR. The transmitter has to be very powerful to get through 10 km ofice. The ice and different layers within the ice will also cause significant time delays that make theposition determination less accurate. It probably is the most reliable method of the three, withthe main disadvantage the mass and power budget impact on the Europa lander.

All three method are feasible and could work. More detailed analysis is needed to pick the best.This choice has very few implications for the rest of the IPR design.

Inclination, with respect to the local gravity vector, can be determined with an inclinometer/tiltsensor. Tilt sensors are used in many Earth applications and come in many different forms. A fewminiaturised tilt sensors that could be used are shown in Figure 12. The smallest is 42 g and ismore than accurate enough for the IPR.

Figure 12: A variety of miniature tilt sensors made by Applied GeoMechanics [22].

Position and inclination controlThe IPR can only go downwards. It can thus only reach positions below its current position.

Since the ice mainly melts at the nose, the IPR always moves with the nose in front; it can notmove sideways or backwards. This means that the only way to change the position of the IPR isto change its inclination. Changing the inclination is difficult in a borehole that is not much widerthan the probe itself. This means that the inclination change is very slow (in terms of degreesper meter descend) or that, locally, a wider borehole has to be created. When obstacles can bedetected a few hundred meters up front, there is no need for a quick inclination/position change.

The slow change in direction (inclination) can be accomplished by aiming the active water jetsuch that it melts and removes a little more ice at one side of the borehole. This means that theborehole becomes slightly inclined and the whole probe will follow this tilt. The same thing couldbe accomplished by isolating or cooling parts of the passively heated nose, such that it melts ice atone side quicker and also creates a slightly inclined borehole. Aiming the water jet is easier, butmay not be enough.

Pathfinding and obstacle avoidanceThe fastest way to the ocean is, most likely, straight down trough the ice. The IPR computer will

therefor monitor its inclination and change it when its direction is not in the same direction as thelocal gravity vector. Moving straight down is only not possible when there are obstacles in the ice.It is important for the IPR to detect such obstacles and determine its size and distance from thecurrent position of the IPR. This can be done with reflecting acoustics (sonar) or electromagnetic(i.e. radar or laser) waves[23].

Both the ice penetrating radar and the sonar need transmitter and receiver antenna in the nose.

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Figure 13: CRUX GPR (ground penetrating radar) electronics board [23].

An ice penetrating radar based on the Miniature Ground Penetrating radar, developed byJPL for a Mars or Moon mission[23], seen in Figure 13, could be used. The electronics can allfit in the main electronics section (see Figure 4) while parts of the nose shell could function asreceiver/transmitter antenna.

Figure 14: The Tritech Micron Sonar[24], in the left placed in the Maya UAV [25].

A miniature profiling sonar may be fitted in the centre of the nose. The only sonar found thatis small enough, is the conceptual sonar of the MEMS [15]. The dimensions of this concept sonarare not defined, but they fit in the 7 cm diameter nose of the MEMS. A different profiling sonar isthe Tritech Micron DST sonar[24] made for the Maya Earth based autonomous mini-submarine.It can be seenin Figure 14. Rotating this 2D sonar around its axis would provide a 3D image ofthe ice below the IPR.

The central processing unit (CPU) of the IPR has to interpret the radar or sonar image anddecide when action is necessary. It should make its own altitude and position control decisions,control the active water jet direction and/or heat element thermal isolation to steer the IPR aroundthe obstacle.

When autonomous decision making is not possible, the IPR needs a braking system to stop itsmovement and wait for instructions from Earth (after sending the radar/sonar images to Earth).Since the heat elements of the IPR can’t be turned off, braking needs to be done with mechanicalhooks that pick in the ice and can support the whole IPR in a Europa gravity environment. Beingfixed in the ice may cause thermal problems because no cold water flows around the probe totransport the RTG heat. This is an extra design problem that should be avoided by creating afully autonomous system.

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5.4 Power

The transponder units and the main electronics in the IPR need independent electrical powersources. The transponders a miniaturised ∼100mW source and the main unit a ∼25W source.The four main power options are a battery, fuel cell, nuclear fission power generator and RTG(Polonium-238 alpha radioactive decay as heat source, see Figure 15).

Figure 15: Image of a glowing red hot pellet of plutonium-238 dioxide to be used in a radioisotopethermoelectric generator for either the Cassini mission to Saturn (planet) or the Galileo missionto Jupiter. [26]

A fuel cell or battery would be too large. A hydrogen fuel cell would need more than 80 Lhydrogen. That alone takes 4.5 m of a 15 cm diameter probe. A battery would be even longer.And nuclear fission power generator becomes only energy efficient in the 10 kW range.

The RTG is the most compact power source for long(years) continuous power supply. The lowconversion efficiency of 5-10% [27] is no waste in this case because the ”waste heat” can be usedto melt the ice and thermally control the system. Before it enters the ice, the RTGs have to becooled; they can not be switched off. Radiators around the IPR, during space travel, is the mostlikely cooling system. This is inconvenient for the lander-IPR configuration for it becomes larger.Once in the ice, there is no need for cooling systems and large radiators because the IPR shell andice/water around the IPR will function as a cooling system.

Figure 16: left: Thermoelectric Converter Module [28]. right: Multi-Watt RTG concept, 15 W[28].

IPR multiwatt-RTGPlutonium-238 (PuO2), that is already used in RTGs[27], produces a thermal power of 390

W/kg[26][29]. With a thermal-electric conversion efficiency of 8% and a density of 10.0· 103 kgm3 it

would require 100 cm3 of Plutonium-238 to power the IPR (∼25 W). When the cooling is doneoutside the probe, it could easily fit in 20 cm of a 12 cm diameter tube.

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Wireless transponder milliwatt-RTGThere are multiple feasible conceptual designs of milliwatt-RTGs [30][28]. One is shown in

Figure 17. 1 cm3 Plutonium-238 oxide as heat source and a small P-N type thermoelectric powerconvertor(see Figure 17) could fit in a 12 cm diameter, 1.5 cm thick disk. It would produce0.100 We and there would still be enough space for transmitter electronics and capacitors in thetransponder.

Figure 17: left: NASA-ARC mini-RTG, 40mWe [28]. right: BiTe Thermoelectric Module15W[30].

5.5 Thermal control

Thermal control will be very difficult in one part of the mission life. The IPR has to travel throughempty space for years before it reaches the -170 C ice on Europa. The IPR contains a constant 1000W thermal power source. This heat has to be transported away from the IPR for it not to becometoo hot. The meltwater from the ice will take care of heat transport on Europa, but radiators willhave to radiate the heat into empty space while travelling to Europa. Adding radiators, mainly onthe nose, during flight to Europa has some power and size impacts on the whole mission.

The ice temperature at the surface is -170 C and decreases to -0 C at the liquid-ice boundary.The IPR is, however, always surrounded by a few mm of liquid water ( 0-20 C) of which thetemperature doesn’t change much with depth. The IPR will melt faster through the ice at lowerdepths and create the same water layer around it. Bringing the IPR to a complete hold mayoverheat it because the surrounding water becomes static and may function as isolation. Thermalmodels and experiments need to find how the IPR behaves thermally.

Additional heat units based on Plutonium-238 decay (see Figure 15 ) could be placed at placedthat become too cold. Electric heating strips may be placed in the shell of to prevent the waterfrom refreezing around the back of the IPR, during the coldest first part of the ice. The samemay be achieved with thermal conductive strips that guide heat from the IPR to specific placesalong the IPR. This has the advantage that it bypasses the low efficiency thermal to electric energyconversion.

5.6 Computer

The computer is the brain of the IPR. It is the central command post of the IPR and consists ofprocessors, memory, input/ouput channels, analog/digital convertors and other logic circuit boardsand electronics.

The computer receives input from the sensors, science payload and from Earth via its antenna.The computers processes this information and creates control commands for the actuators andprocessed housekeeping and science data for the antenna to be send back to Earth.

An overview of the different inputs and outputs generated by and for different subsystems isshown in Figure 18.

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Figure 18: State measurement and CPU control flow diagram of the main IPR segments.

Besides creating control commands and housekeeping data, the computer has to process andcompress science data (e.g. images) to decrease the bandwidth required to send the data back toEarth.

The computer is a critical part of the IPR and should be very reliable and highly redundant. It isespecially vulnerable for the high radiation environment around Jupiter. This requires appropriateshielding. Once in the ice, the ice will provide enough protection against radiation and shieldingis not needed anymore. This means that the shielding does not have to be part of the IPR thatmoves through the ice.

6 Conclusion and recommendations

This section concludes this essay with a short conclusion and recommendations.

6.1 Conclusion

A miniature, 12 cm diameter, 2 m long Ice Penetrating Robot (IPR) that can penetrate a 10 kmthick water ice layer on Europa, deliver a science payload and communicate with Earth is feasiblewith today’s technology. Such an IPR could bring a small submarine to a possible liquid oceanon Europa and relays its findings back to Earth. Melting through the ice would take 1-4 yearsand would require 1 kW thermal power in the form of a Radioisotope Thermoelectric Generator(RTG).

The major technological difficulties in the design of a IPR are the thermal control duringspaceflight, the communication through the ice and the long mission lifetime.

Thermal control during the spaceflight to Europa, which takes several years, is difficult becausethe only feasible power source is radioactive isotope that can not be switched off and alwaysproduces around 1 kW of thermal power. This has to be radiated into space when there is no iceto cool the system. This probably requires large radiators that can only be removed just beforeentering the thick ice layer.

Communication through the ice is difficult because a tether is no option due to movement in theice that could break it. Wireless communication is only possible if multiple wireless transpondersand disposed along the IPR’s way through the ice. These transponders need to carry a small RTGto provide electric and thermal power.

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The long and varied lifetime of the entire mission makes it difficult to design a reliable butsmall IPR. Testing is also more difficult for longer lifetimes and this makes the mission fairly proneto failure. It is, however, not impossible and the design could be used for other ice missions onMars, Enceladus and Earth.

6.2 Recommendations

Further research into the thermal behaviour of the IPR, small wireless communication transpondersand a thermal control/cooling system for the RTGs during spaceflight is needed.

Besides the experiments and research that can be done on Earth, an explorational mission hasto be send to Europa to characterise its ice layer. Most important are ice thickness, ice saltiness,ice dynamics and possible obstacles. Searching for a suitable landing zone for the IPR should alsobe a primary goal for an explorational mission to Europa.

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