Solar System Exploration Augmented by In Situ Resource Utilization: System Analyses, Vehicles, and Moon Bases for Saturn Exploration

Human and robotic missions to Saturn are presented and analyzed with a range of propulsion options. Historical studies of space exploration, planetary spacecraft and astronomy, in situ resource utilization (ISRU), and industrialization all point to the vastness of natural resources in the solar system. Advanced propulsion is benefitted from these resources in many ways. While advanced propulsion systems were proposed in these historical studies, further investigation of nuclear options using high-power nuclear electric and nuclear pulse propulsion as well as advanced chemical propulsion can significantly enhance these scenarios. Updated analyses based on these historical visions are presented. At Saturn, nuclear pulse propulsion with alternate propellant feed systems and Saturn moon exploration with chemical propulsion and nuclear electric propulsion options are discussed. Issues with using in situ resource utilization on Saturn’s moons are discussed. At Saturn, the best locations for exploration and the use of the moons as central locations for Saturn moon exploration are assessed. Environmental issues on Titan’s surface may pres-ent extreme challenges for some ISRU processes. In-space bases for moon-orbiting propellant processing and ground-based processing will be assessed.


Introduction
Exploration and utilization of the outer solar system have always been a goal of the planetary science community and spacecraft engineers. Saturn with its fantastic ring system and many diverse moons has been the focus of exploration by the two Voyager flyby missions and the orbiting Cassini spacecraft. These space vehicles have identified areas of great scientific interest and also places on the moons where water, cryogenic ices, and other natural resources can be gathered. Using in situ resource utilization (ISRU) will allow more extensive exploration of the planet and its moons. This chapter presents analyses of the transportation options in the Saturn system, such as nuclear electric orbital transfer vehicles and chemical propulsion

Saturn and its moons
Saturn is the second largest planet in the solar system. Its orbit has an average distance from the Sun of 1.433 million km. The ring system surrounding Saturn is very extensive and spectacular and has a rich set of resonances and dynamics. The Cassini mission spacecraft instruments gathered a rich set of data throughout its lifetime. Saturn also has powerful radiation belts both near the planet and far beyond the ring system. Due to the radiation environment, the moons are a more important location for any spaceflight operations.
Based on the observations of the Voyager and Cassini spacecraft, the moons of Saturn contain a rich set of ices. Spectroscopic data show the nature of these ices to be water ice with other frozen gases: nitrogen, methane, etc. The moons' temperatures are in the range on 75-130 K. Table 1 provides the density of the major moons. The relatively low density also implies that the moons are primarily composed of frozen ices. Figure 1 shows the semimajor axes of the moon. Iapetus is the most distant at 3.56 million km from Saturn. The most proximate major moon is Mimas, which is only 185,000 km from Saturn [14][15][16][17][18][19]. Additional references on the moons of Saturn are provided at the end of this chapter [20][21][22][23][24][25][26][27][28][29][30].

Enceladus
Enceladus is a small icy moon near the outermost ring of Saturn; its radius is 248 km. Its semimajor axis is 238,020 km. Its gravity level is 2 × 10 −2 of Earth's gravity. This moon is particularly exciting as it is sewing water into space, and its water is feeding mass to Saturn's rings. The water plume emanates from the so-called tiger stripes in the southern hemisphere. The temperature of the tiger stripes is 10-100°K warmer than the surrounding icy surface. The Cassini spacecraft had made multiple flybys of Enceladus and detected organic molecules in the water plume. Thus, this moon may harbor the precursors of simple life forms.

Titan
Titan is the largest moon of Saturn with a radius of 2576 km. Its semimajor axis is 1.2218 million km. Titan has an appreciable atmosphere of 98.4% nitrogen, 1.4% methane, and other trace gases. Its gravity is 0.14 of Earth's gravity. Because of its dense clouds, radar must be used to gather data from space. The Cassini spacecraft observed lakes on Titan; these lakes are composed of liquid methane and ethane. These lakes are approximately that size of the Great Lakes of North America. The surface of Titan is approximately 94°K, and the surface is a complex crust of water ices and frozen hydrocarbons. Simulations and gravity data suggest an ocean of liquid water about 100 km below the frozen surface. Table 1. Saturn moon density, modified from Ref. [14].

Figure 1.
Semimajor axes of the seven major moons of Saturn.

Iapetus
Iapetus is one of the most distant large moons from Saturn, and its radius is 712 km. Its gravity level is 2.4 × 10 −2 of Earth's gravity. This moon has a ridge that girdles a significant fraction of the equator. One hemisphere has very dark material, with an albedo is 2-6%, while the other hemisphere in comparison is very bright. The composition of the dark material is perhaps organic material and carbon. Simulations have suggested that the dark materials may have been deposited by numerous particle collisions. An alternative theory is that magma from the interior has risen to the surface and that magma may be visible in photos as materials that have filled in numerous craters. Table 2 provides the gravity levels of the major moons. While Titan has a surface gravity level of 14% of Earth, the remaining moons have gravity levels of 1 × 10 −2 Earth gravity or less. Such low gravity levels may make transportation, operations, and industrial processing on the moon's surfaces very challenging. On the other hand, the low gravity is helpful is reducing the delta-V needed to transport large masses to and from the moon's surface. Table 3 provides the delta-V to reach low orbit about the moon and the moon's escape velocities. As Titan is the largest moon, the delta-V for escape is the largest: 3.17 km per second. These delta-V values will be important in selecting the most attractive moons for ISRU processing. The planetary gravitational constants and radii were found in Ref. [19].

Exploration vehicles
As a prelude to human missions, a detailed survey of the major Saturnian moons is planned. The survey is driven by ISRU requirements: identification, prospecting, resource capturing, and utilization. The final steps will be power generation and manufacturing on the surface of the moons.
A first survey of the major moons will include orbiters and small landers. A suggested set of spacecraft would be nuclear electric propulsion transfer vehicle and a small chemical propulsion lander. The NEP transfer vehicle would use a complement of science instruments to assess the ices and regolith on the moons' surface. A megawatt class radar system aboard the orbiter would provide data on ice thicknesses and regolith composition. Once an attractive site is identified, the chemical propulsion lander can descend to the surface and perform a series of in situ assessments. Chemical laboratories delivered to the surface can sample the ices and regolith. Samples may even be returned to orbit for caching and planned return to Earth.
The NEP vehicle has a specific impulse of 5000 seconds with a propulsion system efficiency of 50%. The power level of the reactor was 10 MWe, and the reactor specific mass was 10 kg/kWe. The propellant tankage dry mass fraction was 5%, and the fixed mass was 20 MT [31]. Additional mission assumptions are discussed in the next sections.
The chemically propelled oxygen/hydrogen propulsion landers were described with a mass scaling equation. In sizing the chemical propulsion landers, a vehicle mass scaling equation was used [31][32][33][34][35]: where M dry,stage is the stage dry mass, including residual propellant (kg); M dry,coefficient is the B mass coefficient (kg of tank mass/kg of usable propellant mass); M p is the usable propellant mass (kg).
In almost every case, the chemical propulsion landers had a B coefficient of 0.4. With the very large delta-V missions at Titan, the B coefficient was 0.2. The lander specific impulse was 480 seconds [31,[36][37][38]. Table 3. Landing, launch, and escape of delta-V for the seven major moons of Saturn.

Mission delta-V results
Examples of the chemical propulsion lander masses are shown in Figures 2 and 3: Titan and Enceladus, respectively. At Titan and Enceladus, the smallest 1 MT payload landers would be for ISRU prospecting and exploration. The 50 MT payload landers would be for industrial scale ISRU propellant production plant delivery.
The NEP vehicle delta-V values are shown in Tables 4 and 5. The delta-V was computed with a low-thrust trajectory estimation algorithm. Table 4 shows the round-trip delta-V values for trips between the seven major moons; the table shows that using Dione as a central exploration moon, the total delta-V for a fleet of OTVs is minimized. An option with only six moons was also investigated, noted in Table 5. As Iapetus is the most distant moon in the system of moons, a separate analysis was conducted excluding Iapetus. For both the seven moon and the six moon options of Tables 4 and 5, the moon Dione shows the minimal fleet delta-V. Though this is the moon showing the minimal delta-V for the entire fleet, the influence of the OTV and lander mass may change the optimal (or minimal mass) solution.

Factory analysis
The ISRU factory will allow the OTV to be refueled from water ices on the moons. Initially, the OTVs, their science landers, and factory lander are delivered to the centric moon. The OTVs are delivered with no NEP propellants and only   oxygen and hydrogen propellants for the science landers (with a 1 MT payloads) and 50 MT payload factory lander. Once in orbit about the centric moon, the factory lander with the 50 MT ISRU factory lands. The mining and conversion of water ice to oxygen and hydrogen for the factory lander's ascent to orbit are conducted. Additional hydrogen is created on the centric moon and that hydrogen is delivered to the orbiting OTVs. The OTVs can then be dispatched on their moon exploration flights to the other remaining moons. The factory lander will have to perform several flights to deliver the full propellant loads for the orbiting OTVs. Figure 4 presents the estimated masses of a series of propellant factories. The factory masses are based on the lander Isp and the factory design, which is a function of the level of integration with the lander.
For example, a light propellant factory has separate tanks for lander and factory. A heavy propellant factory has separate tanks for lander and factory but has higher masses for enclosures that protect against the elements, winds, micrometeoroids, etc. Also, higher masses are included for foundations for cryogenic surfaces (creating a stable structure for the base). For a super lightweight propellant factory, propellants and all fluids are fed to and stored in lander tanks. Appendix A delineates the masses in the heavy configuration.
In Figure 5, the masses of the ISRU factory and the OTV propellant masses needed for the Saturn moon survey are compared. With the cases without Iapetus, six moons are surveyed. The mass of the ISRU factory is 50 MT. In all cases, the total OTV propellant load is significantly higher that the factory mass. Thus, the use of the factory can enable not only the first survey of the moons but many more. Typically, nuclear reactors have been designed for a 7-year life at full power and a 10-year overall life (operating for the last 3 years at a reduced power level). Given the typical 7-year lifetime of a space nuclear reactor [13], the number of OTV flights can be 6-7.

Mission scenario results and interpretations
To fully explore the moons, a fleet of OTVs and landers were conceived. One OTV and one lander would visit each moon. At the central or centric moon, a 50 MT ISRU factory is landed. The 50 MT payload includes an ISRU factory and a set of empty propellant tanks. The factory creates the propellant for all of the NEP OTVs and landers. The propellant tanks will be filled with the moon-derived ISRU propellants.
The lander was designed to bring that propellant into orbit to refuel the OTVs. The lander propellant tanks are fueled with oxygen and hydrogen. The lander then ascends to the escape conditions of the moon's orbit. Then the lander performs a rendezvous with the OTV(s) and refuels OTV(s); it may also refuel the smaller exploration landers aboard the OTV(s). The newly fueled OTV with its exploration lander completes the orbit transfer to another moon. After completing its exploration, the OTV returns to the moon with ISRU propellant factory.
An additional case for Titan with a 100 MT lander payload was included. This case was added as the lander delta-V for Titan is the highest of all of the moons; therefore, a larger propellant mass is needed for each flight. A larger lander payload case might be attractive in reducing the number of flights needed for refueling the OTVs.
An overall fleet mass of the OTVs and the landers was then estimated. The fleet consisted of one NEP OTV and lander designed to visit a specific moon. Each OTV carried a lander with a 1 MT payload. For the seven moon options, seven OTVs and seven exploration landers were sized. Each OTV was sized to operate from a central moon. As noted above, an additional lander operating from the centric moon was added to provide the ISRU refueling capability for the OTV fleet.
Using the mission delta-V values and vehicle sizing data, the following optima for moon exploration were found. In Table 6, the total mass of the OTV  and lander fleets using a central moon are shown; the minimal mass is 958 MT for the moon Dione. Figure 6 also presents the fleet mass data. While Dione represents the minimal fleet mass, the optimum is fairly broad over a set of four moons: Enceladus, Tethys, Dione, and Rhea. After further exploration and consideration of currently imprecisely known factors (ice composition, regolith strength, ease of access to any subsurface moon oceans, etc.), an optimal moon can be selected. Table 7 presents the same analyses, excluding Iapetus. In Table 6, the minimal fleet mass is found for the moon Dione. In Table 7, the minimal fleet mass is for the moon Tethys: the fleet mass is 785 MT. While Tethys represents the moon with the minimal fleet mass, the optimum is fairly broad over a set of four moons: Enceladus, Tethys, Dione, and Rhea.
By excluding Iapetus, the total fleet mass is significantly reduced; for Dione, the fleet mass is reduced from 958 to 800 MT. Further restrictions of the moon exploration fleet (e.g., from six to four moons) may be needed to fit within the payload capacity of future interplanetary transfer vehicle (ITVs). As Iapetus is more remote than the other moons, it is possible that a separate ISRU space base may be more attractive than a centralized ISRU space base. Due to the low gravity level of the moons, a moon-orbiting space base may be attractive. Processing control of ices in a low-gravity environment may be very difficult. It is suggested that an artificial gravity space base in orbit about the moon may be the best location of resource processing. After a series of small missions have been conducted, a large ISRU space base may be established. The base might be a refueling point for extensive exploration. The water ices from the moons would be brought to the space base for processing, to allow the refueling of the landers and the NEP OTVs.

The far future: human Saturn missions with nuclear pulse propulsion (NPP)
Historical analyses of human missions to the outer planets have included many nuclear propulsion conceptual designs. Nuclear pulse propulsion was investigated and was considered a practical alternative to any chemical propulsion options. The round-trip impulsive delta-V for such missions was approximately 60 km/s [34,[38][39][40][41].
Human missions to Jupiter and Saturn were suggested in the 1960s. Large-scale exploration missions with many astronauts were planned. The primary propulsion system considered was nuclear pulse propulsion. Many small nuclear packages were exploded behind the vehicle, propelling it onto a high-thrust trajectory. For a human Jupiter or Saturn mission, the delta-V was approximately 60 km/s. While the round-trip Jupiter missions were designed to orbit Callisto, at Saturn, Titan was selected. Titan is the largest moon of Saturn, the delta-V to land there is high, 2.2 km/s, and the escape velocity is 3.17 km/s. These values represent an all propulsive landing on Titan and include a 20% delta-V penalty for gravity losses [23]. Such a propulsive delta-V was selected to make the comparisons with the other Table 7. Total masses of the lander and OTV fleets, for moon-centric options, including six moons (without Iapetus). moon landers as consistent as possible. If a lander were to be used for many flights, repacking parachutes or reoutfitting a robotic lander with additional parachutes on orbit may be cumbersome. Figure 7 shows the mass of the Saturn missions for a range of delta-V of 60-120 km/s. While the 60 km/s missions represent a fast mission to Saturn (approximately 250-500 days), a fast mission in both directions may require 120 km/s. The vehicle mass for the 120 km/s missions is over 47,000 MT, while the 60 km/s mission requires less than 6000 MT.
In Ref. [23], using ISRU for refueling a fast Saturn mission was analyzed. Saturn's atmosphere was considered a likely source of the nuclear fuels: helium 3 and deuterium. After analyses of the ISRU transportation systems, it was found that Uranus was a more likely atmospheric fuel source. The complexity of moving OTVs from low Saturn orbit to an assembly point (such a Titan) and the trip time for the low-thrust transfers (for a round-trip delta-V of over 94 km/s) were very prohibitive. With the Uranus option, the nuclear fuels were mining and sent to Titan. The round-trip OTV delta-V for lifting the mined fuels to a moon of Uranus was approximately 32 km/s. While the delta-V for mining transportation is lower, the need for an interplanetary transfer vehicle (ITV) is apparent. A relatively low-energy Uranus to Saturn transfer is possible with an NEP ITV. While the ISRU option for the fast Saturn mission is attractive, the time for gathering the nuclear fuels may require 10 years of mining operations.

Concluding remarks
Saturn and its moon have always been fascinating. Ever since Galileo Galilei noted in his early telescopic observations that "Saturn has ears," the excitement  for Saturn exploration has been strong. Both Saturn and its moons are rich with resources: hydrogen and helium in the planet's atmosphere and ices on the moons. Using the resources of the outer planet moons, new options for exploration are possible. Multiple moons can be visited and explored. High-power OTV with nuclear power can reveal the nature of the ices and regolith of the moons.
Detailed exploration of the major moons can be completed with a set of small chemical propulsion moon landers and nuclear electric orbital transfer vehicles. A series of such OTV and landers showed several optimal locations for conducting a moon survey. Dione was an optimal location for the minimization of the OTV and lander fleet mass. A central moon location allowed a large ISRU factory to fuel many OTV and exploration lander flights.
Far future human exploration of the Saturn system may employ very highenergy nuclear propulsion systems. Nuclear pulse propulsion vehicles may allow fast transfers to Saturn and the delivery of robotic exploration vehicles and human explorers. Using ISRU for refueling a fast Saturn mission was analyzed. Saturn's atmosphere was considered a likely source of nuclear fuels: helium 3 and deuterium. After analyses of the ISRU transportation systems, it was found that Uranus was a more likely atmospheric fuel source. There are numerous benefits of ISRU in Saturn and Uranus systems. With ISRU, our exploration options are nearly endless, and the abilities to uncover the secrets of Saturn moons are awaiting our scientific investments.