A Self-Reproducing Interstellar Probe

1. INTRODUCTION

In the classical "Bracewell probe" contact scenario [2, 7], the automated device enters our Solar System, detects radio emissions of an unnatural character emanating from Earth, and subsequently positions itself in some convenient parking orbit around our planet. Upon receiving some arbitrary human transmission, the intelligent probe beams an identical copy of the message back to the transmitter site in hopes of gaining our attention. Once accomplished, language lessons soon follow; hopefully, meaningful discourse and cultural exchange between humanity and the automated alien ambassador ultimately take place.

Project Daedalus, a preliminary design study of a flyby probe mission to Barnard's star recently completed by members of the British Interplanetary Society [8], has demonstrated the feasibility of this approach to interstellar exploration and communication using foreseeable human technology sometime in the next century. The automated probe strategy has received further support from Freitas [3], who shows that transmission of information across interstellar distances using energy-markers (photons) or matter-markers (probes) may be energetically and alternatives for highly developed technological civilizations Only such advanced societies realistically can afford either radio beacons or starprobes, and secondary distinguishability criteria suggest the possible superiority of intel ligent automata for contact and communication missions between extraterrestrial civilizations. The search for alien space artifacts in our own Solar System has already begun [9].

A major alternative to both the Daedalus flyby and "Bracewell probe" orbiter is the concept of the self -reproducing starprobe. Replicating spacefaring machines recently have received a cursory examination by Calder [4] and Boyce [5], I but the basic feasibility of this approach has never been seriously considered despite its tremendous potential. In theory, each self -reproducing device dispatched by the launching society would become an independent agent, slowly scouting the Galaxy for evidence of life, intelligence and civilization. While such machines might be costlier to design and construct, given sufficient time a relatively few replicating starprobes could search the entire Milky Way.

The present paper addresses the plausibility of self-reproducing starprobes and the basic parameters of feasibility. A subsequent paper [10] compares reproductive and nonreproductive probe search strategies for missions of interstellar and galactic exploration.

2. THEORY OF SELF-REPRODUCING AUTOMATA

There is little disagreement among cyberneticians that full machine self -reproduction is possible in principle. Quite a number of techniques have been explored by modern theoreticians; however, it was von Neumann [11]-[12] who first investigated several different models of automata replication.

The simplest of these schemes is usually called the "tessellation model." Space is divided into cubical cells, and each part of the machine and each piece of raw material from which it constructs itself occupies exactly one cell. All processes are quantized in time as well as space - activity occurs in step with regular clock cycles. In the tessellation model examined by von Neumann, space becomes two-dimensional for simplicity, with the machine occupying a connected set of squares. Each square is in one of a specified set of states. The machine is surrounded by inert squares which it must organize into an exact duplicate of itself.

Von Neumann's simplest example calls for a box consisting of 80x400 contiguous cells plus a long linear "tail" of coded instructions 150,000 squares in length. The box consists of a specific arrangement of three kinds of parts - neurons, transmission cells, and muscle cells - and performs two primary functions. First, it follows the instructions encoded in the tail, in sequence. Second, it copies the tail, which contains a coded description of a tailless box. Reproduction is complete when the box organizes the inert units around it into a copy of the tailless box, then copies its own tail and attaches this to the second box.

The second von Neumann scheme, commonly known as the "kinematic model," is somewhat more abstract and generalized. It involves the activity of a Universal Turing Machine, a device designed conceptually by Turing [13] which is able to process any specific algorithm. Since -its output is pure information, von Neumann conceived of a Universal Automaton capable of building material objects using any specific construction algorithm.

Imagine three Universal Automatons A, B, and C. If the description of A is fed into B, the output of B is the behaviour of A. If the behaviour of A is the construction of C then the output of B is the machine C. But if C is identical to B, then B is producing copies of itself - in essence, it is reproducing. Of course, for B to function properly it must have access to a stockroom well supplied with all component parts necessary to build each Universal Automaton.

It may be argued that the above is merely a design for a general purpose assembler robot, whose output happens to be copies of itself. This may be true, but still the process is properly termed "self -reproduction." Given access to the proper environment, B can indeed replicate itself. This is not cheating, for human beings also must have access to a very specialized environment in order to reproduce - a chemical "stockroom" full of air, water, and food containing assorted proteins, fats, carbohydrates, vitamins, minerals, and so forth. Lifeforms, like Universal 'Automatons, cannot produce order out of complete chaos. Rather, each can only transform more simply organized matter into more complexly organized matter.

An example of a modern approach is given by Laing [14] who has investigated two machine reproduction schemes involving complete self -inspection. In the first version the original machine is visualized as a one-dimensional string of components consisting of a constructor sequence (initially active) and a sequence containing the instructions to build an analyzer (initially passive). The constructor operates on the passive set and builds a working analyzer which, acting independently, produces a complete description of the original machine. Using this description, the original constructor builds an exact duplicate of the original machine, and reproduction is complete. In the second version the original machine consists of two parallel strings, each containing a working constructor and analyzer. The first string to be activated analyzes its passive partner and reconstructs a copy of it; the second string is next activated, resulting in a copy of the first. Thus the system has reproduced itself without recourse to an external self -description.

Much has appeared in the literature affirming the possibility of automata replication and development [15-27]. Computer programs and numerical patterns that reproduce themselves have been created [28-29] and several simple but ingenious physical machines capable of self-reproduction in specialized favourable environments have already been designed, constructed, and successfully operated by Penrose [30-32], Jacobson [33] and Morowitz [34]. Other machines have been built over the years which demonstrate the ability to feed, metabolize, learn, respond to stimuli, recognize the self, and move about in physical space with goal-oriented behaviour [35-38]. Surprisingly, many of these devices are quite modest in complexity, sometimes requiring as few as 30 bits [39] for complete physical description.

A sophisticated self -reproducing starprobe must be able to function in highly generalized environments. It will not be able to pick up its parts (or bits of structural information) "free" from the environment, hence it must carry with it much more descriptive data than any replicating machine built to date. But there is little doubt that such a machine can, in theory, be designed.

3. MISSION PROFILE

REPRO is designed from the payload up. To contain the necessary reproductive equipment the payload of the original Daedalus probe was approximately doubled, causing the vehicle mass to increase from 5.45x10⁷ kg to 1.07x10⁸ kg. While Daedalus is a flyby mission, REPRO must slow to a halt in the target solar system so that reproductive activities may commence. The double-Daedalus configuration must be used to decelerate the 10⁶ kg payload from interstellar cruise velocity down to parking orbit velocity. Hence, to accelerate double-Daedalus up to interstellar cruise velocity will require a "0th Stage" much larger than Daedalus itself. Using the same vehicle launch mass to payload ratio as the BIS starship, about 100:1 [40], the total fueled mass of REPRO must be 1.07x10¹⁰ kg.

Assuming humanity is the launching civilization, REPRO is constructed and fueled in Jovian orbit much like Daedalus. After inspection and certification for flight readiness the Oth Stage is ignited, propelling REPRO to about 12%c after a burn time of 4 years. Following a coast phase of 43 years to Barnard's star, the empty Oth Stage hulk is jettisoned and the remaining vehicle structure is rotated 180° so that it points backwards along the direction of flight. Stage I and Stage 2 are ignited in turn, dropping the payload down to interplanetary velocities (<10 km/sec) in about 4 years.

REPRO has a number of subprobes much like Daedalus but these are not constrained to perform simple flyby explorator~ missions since REPRO has fully decelerated. Orbiter, balloon/floater, rocket plane, and even surface lander missions on interesting planets or moons in the target solar system are possible. Detailed planetological data may be accumulated and processed, and a variety of xenobiological investigations under taken in the search for alien life and intelligence [41]. Sophisticated messenger probes of the kind envisioned by Bracewell could eventually be dispatched to parking orbits around selected planetary bodies in the target system.

Theory suggests that most single star systems should be accompanied by at least one jovian planet, possibly more [42]. In order to reproduce itself, REPRO needs 1.01x10¹⁰ kg of fusion fuel mined from a jovian atmosphere and 5.60x10⁸ kg of nonfuel mass. Upon arrival in the target system, the vehicle uses its remaining 2nd Stage fuel to guide itself into orbit around a small moon of a jovian gas giant. About half the payload, 4.43x10⁵ kg, is designated SEED. SEED is deorbited to the surface of the jovian moon where, over the next 500 years, it builds and launches a number of interplanetary probes. Its primary function, however, is to produce FACTORY, an automated manufacturing complex whose output (following rearrangement of its modular building-blocks [43]) is exactly one new REPRO every 500 years.

4. ESTIMATION OF REPRO ELEMENT INVENTORY

In order to plan the replication of REPRO the material requirements must be determined with some specificity. This requires a detailed knowledge of the structure of the vessel to an accuracy of definition at least as good as the Daedalus vehicle. Unfortunately, no such design study exists for a star ship the size of REPRO, so its ultimate feasibility remains unknown. However, since a fleet of ~200 Daedalus vehicles could place 10⁶ kg of payload in orbit around Barnard's star or one of its planets, then assuming we accept as valid the techniques and conclusions of Project Daedalus the basic plausibility of REPRO cannot seriously be questioned. The technical details are not yet available, but apparently the mass/energy/time requirements are satisfied adequately by the gross physical parameters suggested above for the REPRO vehicle.

To determine an element inventory for REPRO the various Oth Stage component masses were calculated from the basic Daedalus starship model on the reasonable basis of comparative propellant mass ratio. This procedure was deemed acceptable in lieu of a sophisticated 10,000 man-hour design study since about 2/3 of the nonfuel Daedalus mass is directly related to the handling of fuel or the distribution of propulsive force. If M₀, M₁ and M₂ are the REPRO propellant masses for the 0th, 1st and 2nd Stages and C₀, C₁ and C₂ are specific component masses in the three Stages, respectively, then the scaling for C₀ given C₁ and C₂ is:

(1)

Taking M₀ = 10¹⁰ kg from the previous calculation involving mass ratios, and M₁ = 9.20x10⁷ kg and M₂ = 8.16x10⁶ kg as estimates from the figures given by White and Parfitt [44] the above expression reduces to:

The estimator given in Eqn. (2) was applied to every major component system in the Daedalus design. The results are in Table 1. No detailed analysis of the technical feasibility of these derived masses was attempted, although a quick check of two particular subsystems (engine reaction chamber, fuel tank mass and refrigeration hardware) indicated substantial agreement with the engineering constraints assumed for Daedalus.

The material requirements for a new REPRO are determined by breaking down the vehicle mass distribution into its elemental constituents. In many cases the precise alloys and materials to be used for a Daedalus component are specified in the summary report [44-48], and these values are adopted for REPRO. However, about 55% of nonfuel REPRO mass is unspecified in this fashion. Unspecified mass is treated in two categories: (1) Unidentified nonfuel "Vehicle Other" (labeled V.O. in Table 1), and (2) unidentified SEED or "FACTORY Other" (labeled F.O. in Table 1).

The elemental distribution of V.O. is determined in a similar manner. Each element was carefully considered on the basis of its most common use and classified either as aerospace/electronics/optical Or as nonaerospace/electronics/optical. Elements in the latter category were arbitrarily demoted to 0.1% of their annual US consumption value. Elements in the former category retain their original consumption values, as these substances are more likely to comprise a larger relative fraction of the starprobe vehicle mass. The revised consumption figures were again reduced to fractional abundances by weight for each element.

The final element inventory estimate for REPRO appears in the last columns of Table 2. These totals were obtained by summing the following three quantities: (1) Mass of specific elements specified in the Daedalus report, plus (2) Mass of unspecified "Vehicle Other" (3.08x10⁸kg) multiplied by each of the V.O. element abundances, plus (3) Mass of unspecified "FACTORY Other" (the 4.43x10⁵ kg SEED) multiplied by each of the F.O. element abundances.

5. MATERIALS ACQUISITION STRATEGIES

An interesting alternative to jovian propellant acquisitionis the cometary capture technique. In this scheme, REPRO must seek out and rendezvous with a large long-period (> 100 years) cometary body in a low-eccentricity solar orbit. While cometary nuclei are believed to average 1-10 km in diameter, a few exceptionally large objects (P/Schwassmann-Wachmann I, Haley's Comet) are known to have diameters from 50 - 100 km and masses from 10¹⁷-10¹⁸ kg. The exact chemical composition is unknown but is thought to represent original. condensate from the protostellar nebula at roughly "cosmic" abundances, perhaps with a slight depletion of volatiles. The main body of the nucleus may consist of perhaps 74% ices (water, ammonia, methane, and solid clathrate hydrates) and 26% stony-iron dust, for a net density of about 1100 kg/m³ [52]. The cosmic abundance by mass of He³ may be computed from Cameron [53] as 2.67x10^-5 and as 2.50x10^-5 for deuterium. Even assuming the helium isotope is depleted by as much as three orders of magnitude, a large comet might still contain 10¹⁰ kg of He³ and 10¹³ kg of D, enough to permit reproduction of at least one REPRO vehicle. Further comparison of nonfuel materials requirements with cometary "cosmic" abundances indicates that all needs may be satisfied by completely cannibalizing one large body of the kind discussed above.

It seems likely that there are two main cometary belts, an Inner Cloud within 40 AU and the Oort Cloud out to about 100,000 AU. Recent estimates place the total population Of the Inner Cloud at about 3x10⁶ bodies [54]. These remain in relatively circular orbits until deflected into the inner Solar System by planetary gravitational perturbations. Orbital inclinations of Inner Cloud members are random [55], so starprobes arriving along arbitrary trajectories should be able to find suitable candidates reasonably close to the plane of entry. Assuming one comet of every thousand is sufficiently large for utilization by a reproductive probe, then about 3000 suitable objects exist within a heliocentric search volume of radius 40 AU. If REPRO must locate at least one large comet within 5 years of target system entry, starprobe sensors must be capable of scanning 10^-6 AU³ sec^-1, necessitating a field diameter of at least 1 arcsec scanning to a depth of 40 AU with an integration time of 100 seconds. Within this range a 100-km-wide comet with albedo 0.1 has an apparent magnitude of +23 or brighter, observable using a 2-metre search telescope somewhat smaller than NASÁs 2.4-metre Space Telescope [56] or Daedalus's 5-metre optical telescopes [41].

Once it has been decided that the bulk of the mass of REPRO must be drawn from a jovian planet in the target star system, efficiency dictates that the base site be located somewhere in jovian orbit. Heavy elements comprising the nonfuel mass can most easily be drawn from one of four sources - atmosphere, jovian moon, jovian trojan asteroids, or the asteroid belt [52, 57-58].

Element abundances for "average asteroidal" material were assumed to include 74.7% silicate, 19.6% nickel-iron, and 5.7% troilite after Mason [59], and appear in Table 3. Data for the rare gases AT, He, Kr, Ne, and Xe are estimated from Heymann [60], and the value for H is averaged from data in Heide [61] and Brown [62]. A recent study of four large Trojan asteroids of Jupiter and two of the outer Jovian moons Himalia (J6) and Elara (J7) indicates an abnormally low surface albedo a result which calls into question the validity of the current "ice model" for these bodies [63]. Although the blackening may be only a surface effect, the highly irregular shape of Hektor, the largest Trojan body, implies that it cannot be composed of solid ice because ice has insufficient structural strength to support the irregular shape against collapse. Hartmann [64] calculates that if Hektor is composed of chondritic material it should be stable against internal crushing. If this conclusion is applicable to Himalia and Elara, smaller bodies but similarly low in albedo, then asteroidal capture becomes a more likely explanation for the origin of the outer Jovian moons.

Jovian atmospheric heavy element abundances generally fall about two or three orders below "average asteroidal, " assuming "cosmic" abundances [53] for the gas giant. (See Table 3). There are only eight elements for which jovian abundance may be higher than the asteroidal; Ar, C, H, He, Kr, N, Ne Xe. Extraction of these elements thus should be delegated to the aerostat balloon factory system.

Transportation out to the asteroidal belt or to the trojan clusters from the target system jovian is far more expensive if time and fuel than transportation of an equal mass to an outer jovian moon. In our Solar System, there are four known such moons orbiting at a distance of about 1.1x10⁷ km from Jupiter with diameters 170 km (J6), 80 km (J7), 40 km (J10), and 20 km (J13); another five moons orbit about 2.2x10⁷ km from Jupiter, with diameters 50 km (J8, J11), 40 km (J9), 30 km (J12), and 20 km (J14).

In this paper it is assumed that a jovian moon about 100 km in diameter having "average asteroidal" composition can be located and colonized by REPRO.

6. FACTORY AND SEED

Upon arrival in jovian lunar orbit, the first step in the reproductive process is to deorbit SEED, all remaining functioning wardens, most of the 412 MW Stage 2 power supply, and other miscellaneous equipment totalling ~5x10⁵ kg to the surface of the jovian moon. In the weak 7 milligee gravity field this manoeuvre should require 290 MJ from an orbit 100 km above the surface. an eight hour trip using a 10 kW propulsion system. This is a deceleration of 4 milligees, well below the 13-113 milligee design limit [44] for all powered flight phases of the Daedalus mission.

Following moonfall, SEED and its wardens unpack and proceed slowly to erect FACTORY during the next 500 years. SEED operates under the direction of its own autonomous computer system which is activated upon landing, in coordination with the flight computer which remains in orbit aboard the 2nd Stage hulk. The 4.43x10⁵ kg SEED grows into a 3.46x10⁸ kg FACTORY, which is reorganized, then produces a 1.07x10¹⁰ kg REPRO starprobe during the second 500-year period. The mass growth rates are 1.3%, and 0.690%, respectively, fairly modest for modern automated industrial systems and an order lower than typical rates for biological organisms.

The actual reproductive apparatus consists of 13 distinct robot species, including Chemists, Aerostats, Miners, Metallurgists, Computers, Fabricators, Assemblers, Warehousers, Crawlers, Tankers, Wardens, Verifiers and Power Plants. These collectively perform all of the functions of a living system [65]. SEED carries representatives of only the first nine species mentioned. Several aging wardens are provided "free" from the derelict Stage 2, together with its 412 MW power supply. All Tankers and Verifiers must be constructed in situ from scratch.

6.1 Chemists

A subsidiary function of Chemists is to combine various elements and simple molecules into perhaps 100 basic reagents needed for SEED and FACTORY extraction, manufacturing and testing processes. Johnson and Holbrow [69] describe an aluminum processing plant for which 13% of the total refinery mass consists of five nonelemental process chemicals (H₂O, H₂SO₄, Na₂SO₄, NaCl, and LiCl. Preparation of industrial process chemicals generally is quite easy in comparison with element extractions from ores. Most of these substances, both inorganic and organic, are one or two step preparations, so the additional mass of SEED and FACTORY Chemists to perform this function should again represent only a few per cent of the total Chemist mass.

FACTORY must produce essentially all nonfuel REPRO mass in 500 years. The mass flow requirements for each element, based on the REPRO inventory in Table 2, are divided by the jovian moon elemental abundances in Table 3 and the total production time of 500 years to obtain the lunar ore throughput rates tabulated in Table 3. The total mass flow rate is 3860 kg/sec, so in 500 years FACTORY Chemists must process 6.06x10¹³ kg of jovian moon ore.

Fusion breeding [70-71] of the most troublesome elements from more abundant precursors was considered in an attempt to reduce Chemist mass. flow rates. Plausible breeding pathways were found for the production of Mo from Zr (4.24x10^-2 mole/sec), Rh from Ru (3.68x10^-4 mole/sec), W from Hf (8.14x10^-5 mole/sec), Au from Pt (3.88x10^-5 mole/sec), Ta from Hf (9.26x10^-6 mole/sec), In from Cd (1.88x10^-7 mole/sec), Cu from Fe (5.88x10^-3 mole/sec), Ti from Ca (5.56x10^-2 mole/sec), and Al from Mg (8.57x10^-1 mole/sec), for a total of 9.61x10^-1 mole/sec breeder throughput. Utilization of all nine processes reduces the mass flow rate from 3860 kg/sec to 1110 kg/sec of jovian moon ore.

Setting aside technological difficulty, the main problem appears to be energy. Each of the proposed breeding pathways requires each precursor atom to absorb 2-7 neutrons to achieve majority conversion, coupled with continuous extraction of desired isotopic species to prevent degradation. If on average 100 2.45-MeV neutrons must be liberated by a DD fusion reactor to transmute one precursor atom into one rare atom (2-7% transmutation efficiency), then to breed all nine elements requires a power supply of 2.27x10¹³ watts. Even assuming 10,000 watts/kg for the higher temperature DD reactor the mass of this supply is 2.27x10⁹ kg, nearly seven times the mass of FACTORY. It is concluded that fusion breeding is not feasible within REPRO mission constraints.

Current ore processing technology for aluminum, REPRO's most-needed element, permits a 4750-ton refinery to produce 1.74 kg/sec Al metal using soil from Earth's Moon [69]. This material is only 5.6%, A] by weight so the factory must process 31 kg/sec soil to achieve the stated output. This is a net processing rate of 6.53x10^-6 (kg ore/sec)/kg refinery. Assuming one order improvement from technological advances, then if the values given for the aluminum plant are typical for element extractions FACTORY may be able to process jovian moon ore at the rate of 6.53x10^-5 (kg/sec)/kg. This gives a total FACTORY Chemist mass of (3860)/(6.53x10^-5) = 5.91x10⁷ kg, equivalent to about 124 individual 475-ton refineries. The power requirement for the original aluminum plant is 40 watts/kg; adopting this value, the total for FACTORY Chemists is 2360 MW.

SEED Chemists must process enough jovian moon ore to produce the entire FACTORY mass of 3.46x10⁸ kg. However, replacement mass must be added to take account of break downs and repairs. Daedalus carries 20 tons of repair shops and spare parts to service a 450-ton payload for 50 years, or 8.9x10^-4 (kg spares/kg serviced)/year. The REPRO-building phase lasts 500 years, so the extra mass of shops and spares needed to maintain FACTORY is estimated as (8.9x10^-4)(500)(3.46x10⁸)=1.54x10⁸ kg. The FACTORY-building phase also lasts 500 years; the logarithmic average mass of the growing SEED during this time is 1.24x10⁷ kg, so the extra mass of shops and spares needed to maintain SEED is estimated as 5.52x10⁶ kg by the same method as above. Including main tenance support, SEED Chemists must produce a total FACTORY mass M_F = 5.06x108.

It will be noted that the function of FACTORY is to produce REPRO, hence production of REPRO must be linear; on the other hand, since the function of SEED is to produce FACTORY, and since FACTORY consists of "universal" modular components of the same types used in SEED, then in effect SEED is building more SEED, hence production of FACTORY must be exponential. If M(t) is the mass of the growing SEED at time t, then the rate of growth dM(t)/dt = XM(t), where X is the mass of new SEED generated per unit time interval, per unit mass of existing SEED at time t. The equation of growth may be written as:

where M_s is the mass of the Original SEED at t = 0.

The value of X and the FACTORY mass M_f are determined as follows. From Eqn. (3) we have M(t) =M_s · exp(Xt) = M_t, where t is the 500-year production time of FACTORY. Also, X may be fixed by assuming that the SEED mass at t = t, it is not reorganized into FACTORY hardware capable of pro ducing REPRO, but rather retains its structure as fully mature SEED. This non-reconfigured SEED can produce more SEED components instead of new REPRO mass at a rate of M_t/t kg/sec, where M_t is nonfuel REPRO mass. However, this fraction must be corrected for the difference in element abun dances between the output of FACTORY (new REPRO, with REPRO abundances) and the output of the mature SEED (more SEED, with F.O. abundances). To obtain a relation for X, the above production rate must be multiplied by the ratio of the appropriate output abundances relative to pvian moon soil, and divided by the mass of existing SEED at t = t. Hence, X = (M_t/t)(R_t/R_f)/M_f, where R_t = S{(REPRO abundances)/(jovian moon abundances)} and R_f = S {((SEED or F.O. abundances)/(jovian moon abundances)}. Since M_t (5.60x10⁸ kg), M_s (4.43x10⁵ kg), and t (1.57x10¹⁰ sec) are flyby mission parameters, and Rg (Rg(1.10x10⁵) an R_f(2.68x10⁴)2.68x10⁴) are readily calculated from the data in Tables 1, 2, and 3, this leaves two equations and two unknowns.  Solving simultaneously, M_f = 3.46x10⁸ kg and X = 6.66/t.

The same value of the rate constant X may be used to calculate the individual SEED mass of each exponentially growing component robot species. The most general definitionof the component rate constant X. is mass flow output 01 component per unit mass of existing, component, so X_c = (M_tt) (R_t/R_f)(M_fc/M_f)/M_fc where M_fc is the mass of the particular component in FACTORY. The dependence on M_fc drops out so X_c = X  for all components. Since SEED components must produce M_Frather than Mf, SEED component mass Ms is given by:

Plugging in M_fc = 5.91x10⁷ kg, from Eqn. 4, M_sc =1.11x10⁵ kg for SEED Chemists. The initial power requirement is 4.4 MW.

6.2 Aerostats

Aerostats designed for operation on Jupiter for Project Daedalus have an empty mass of 180.2 tons, including ascent ferry mass, and consume 10 MW of power, all of it generated by the onboard reactor. Each factory can process 680 kg/sec of gas giant atmosphere, and is designed for fully automatic operation in the high-radiation, turbulent jovian environment.

From the jovian throughput rates in Table 3 it is clear that the mass flow for He³ is controlling. A total Aerostat fleet of (14,500)/(680) = 21.3 units must on average operate continuously for 500 years to provide all the He³ and D propellants for a single REPRO starprobe. Although Parkinson suggests a lifespan minimum of 8 years, industrial factories generally are assigned a maximum working life of 30 years. If the average of these is taken as typical for Aerostats, 19 years, then in 500 years of FACTORY operation the entire Aerostat fleet must be replaced 26.3 times. A total mission fleet of 560 units must therefore be constructed in order to maintain an average working fleet of 21.3 units in continuous operational status. The total mass of FACTORY Aerostats is 1.01x10⁸ kg.

Parkinson has suggested that workable devices might be designed on a somewhat smaller scale. If SEED carries two small Aerostats (each one 15% normal mass) to gather -fuel and other isotopes during the FACTORY -building phase, then the total mass of SEED Aerostats is 54.1 tons.

One minor difference between Daedalus and REPRO Aerostats is the incorporation in the latter of additional facilities for the extraction of the elements Ar, C, Kr, NN, Ne, and Xe because of their expected greater abundance in the jovian atmosphere than on the jovian moon. The extra production requires the continuous service of only about 10^-4 Aerostat, a negligible additional production load for the FACTORY fleet. All six elements are readily obtained using simple cold traps, since the extraction processes for recovering them need an efficiency of only ~10^-6 compared to He³ and D processors. Each full load of fusion fuel, 16.8 tons, thus is accompanied by ~260 gm of the six specified elements.

6.3 Miners

The Miners are semi-intelligent general purpose excavation robots, charged with the responsibility of locating and digging up jovian moon material for processing by Chemists. Physical functions include drilling, blasting, dozing, shoveling, stripping, and lifting large quantities of ore. Search functions are highly specialized, confined to locating surface ore veins and local pockets of mineralogical enrichment (survey satellites do most of the finding - see Section 6.5).

FACTORY Chemists must process 3860 kg/sec of jovian moon ore, so the Miners must dig it up. Typical performance figures for power shovels, bulldozers and shovel dozers are (3x10^-3 (kg excavated/sec)/kg machine and 4 watts/kg machine during continuous operation [72]. These values, derived from experience working in the 1 -gee terrestrial environment. may be somewhat pessimistic when directly applied to the 7 milligee jovian moon environment, but hopefully not unduly so. If they are taken as appropriate, with a duty cycle of 50% to account for delays due to the finding function, then the total mass of FACTORY Miners is 2.57x10⁶ kg with a power consumption of 10 MW.

SEED Miners must locate and excavate enough jovian lunar ore to provide the entire mass of FACTORY plus shops and spares. Using Eqn. (4), for M_fc = 2.57x10⁶ kg M_sc = 4820 kg with an initial power requirement of 19.3 kW.

6.4 Metallurgists

FACTORY Metallurgists must produce M_t/t = 3.57x10^--2 kg/sec in order to complete REPRO on schedule. Taking the glass refinery figures as typical, the total mass of FACTORY Metallurgists would be 29.7 tons. However, a subspecies of Metallurgist robots handles electronic-gade materials of the highest purity, performing specialized functions such as ultrapure crystal growth, bulk wafer fabrication, and selected doping and drifting prelithographic operations. Although the mass flow is low, machine complexity is high because of the extreme standards of purity. If electronics Metallurgists also have a mass of 29.7 tons, then the total mass of FACTORY Metallurgists is 59.4 tons with a power consumption ~130 kW.

SEED Metallurgists must process the entire mass of FACTORY plus shops and spares. From Eqn. (4), M.= I I I kg; this is pro