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Journal of the British Interplanetary Society

VOLUME 72 NO.2 FEBRUARY 2019 General interstellar issue

DIRECT FUSION DRIVE for Interstellar Exploration S.A. Cohen et al. INTERMEDIATE BEAMERS FOR STARSHOT: Probes to the Sun’s Inner Gravity Focus James Benford & Gregory Matloff REALITY, THE BREAKTHROUGH INITIATIVES and Prospects for Colonization of Space Edd Wheeler A GRAVITATIONAL WAVE TRANSMITTER A.A. Jackson and Gregory Benford CORRESPONDENCE

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38 DIRECT FUSION DRIVE for interstellar exploration S.A. Cohen et al.

51 INTERMEDIATE BEAMERS FOR STARSHOT: Probes to the Sun’s Inner Gravity Focus James Benford & Gregory Matloff

56 REALITY, THE BREAKTHROUGH INITIATIVES and Prospects for Colonization of Space Edd Wheeler

62 A GRAVITATIONAL WAVE TRANSMITTER A.A. Jackson and Gregory Benford

70 CORRESPONDENCE

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JBIS Vol 72 No.2 February 2019 37 JBIS VOLUME 72 2019 PAGES 37–50

DIRECT FUSION DRIVE for interstellar exploration

S.A. COHEN1, C. SWANSON1, N. MCGREIVY1, A. RAJA3, E. EVANS1, P. JANDOVITZ1, M. KHODAK3, GARY PAJER2, T.D. ROGNLIEN4, STEPHANIE THOMAS2, MICHAEL PALUSZEK2 1Princeton Plasma Physics Laboratory, Princeton NJ, USA; 2Princeton Satellite Systems, Plainsboro, NJ, USA; 3Princeton University, Princeton, NJ, USA; 4Lawrence Livermore National Laboratory, Livermore, CA, USA.

Email [email protected] (corresponding author)

The Direct Fusion Drive rocket engine (DFD), based on the Princeton Plasma Physics Laboratory’s Princeton Field Reversed Configuration machine, has the potential to propel spacecraft to interstellar space and to nearby solar systems. This paper discusses a design for a starship that would be well suited to a variety of solar system and interstellar missions. DFD employs a unique plasma heating system to produce nuclear fusion engines in the range of 1 to 10 MW, ideal for human solar-system exploration, robotic solar-system missions, and interstellar missions. This paper gives an overview of the physics of the engine. Its innovative radiofrequency (RF) plasma heating system and the fuel choice are explained. The thrust augmentation method is described along with results of multi-fluid simulations that give an envelope of expected thrust and specific impulse. The power balance is described and the subsystems needed to support the fusion core are reviewed. The paper gives the latest results for the system design of the engine, including just-completed work done under a NASA NIAC study. A mass budget is presented for the subsystems. The paper then presents potential interstellar missions. The first are flyby missions. One is the proposed 550-AU mission that would use the Sun as a gravitational lens for exoplanet research. This mission can be done without a deceleration phase. Next, flyby missions – requiring major technological advances – to the nearest star are described. Finally we sketch a mission to orbit a planet in either the Alpha Centauri A or Alpha Centauri B systems. The mission analyses include a communications system link budget. DFD can operate in an electric-power-only mode, allowing a large fraction of the fusion power to be used for the payload and communications, enhancing the scientific return. All of the missions start in low Earth orbit.

Keywords: Interstellar Mission, Solar Sail Spacecraft, Post-Newtonian Gravitational Theory

1 INTRODUCTION NOMENCLATURE The idea to use fusion power for spacecraft propulsion has a B = magnetic field long history [1, 2], with its support arising from the high ener- β = ratio of plasma pressure to magnetic-field energy density gy density of the fuel and the high velocity of the fusion prod- c = speed of light ucts. Early proponents of fusion rockets that provided steady – rather than pulsed or explosive – propulsion based their de- cs = ion sound speed signs on the fusion devices that were then in vogue, tokam- E = ratio of plasma FRC plasma core length to diameter aks [3, 4], mirror machines [5] and levitated dipoles [6]. The γLH = Lower-hybrid drift instability growth rate experimental results of that period in fusion history indicated Ip = plasma current that the plasma’s anomalous transport, meaning poor plasma energy confinement, and instability would necessitate low β, Isp = specific impulse D-T burning, large and powerful machines, many meters in MT = metric tonne, 103 kg diameter, producing over a gigawatt in power and requiring a q = plasma safety factor meter or more of neutron shielding. Such large and massive rs = FRC core plasma radius devices could not be launched fully assembled; upwards of 100 launch vehicles would be needed. Such daunting and expensive s = 0.3 rs/ρi proposals never proceeded beyond the conceptual stage. S* = rs ωpi/c

Th = Thrust Recently, new designs of fusion devices, bolstered by exper- τA = Alfvén time ~ rsE/cs imental successes on prototypes, have raised optimism for the prospect of considerably smaller fusion-powered rockets that ωpi = ion plasma frequency are far lighter, less radioactive, and less costly. Commensurate

38 Vol 72 No.2 February 2019 JBIS DIRECT FUSION DRIVE for Interstellar Exploration with their reduced size, these rocket engines would produce only megawatts of power [7], nevertheless ample for a wide variety of missions in the solar system and beyond. The common feature of these rockets is the geometry of the magnetic field that confines the plasma. The “family name” for these fusion-reactor designs is field-reversed configuration (FRC), a label derived from the original plasma-formation method, not the shape of the field, as commonly thought.

Importantly, FRCs have more than 10x higher β than tokamak devices, the leading contender for terrestrial fusion power production. The high β, coupled with the FRC’s quasi-linear geometry, reduces the required peak magnetic field by about a factor of 3 compared to a tokamak’s. Lighter weight magnets are possible, important for spacecraft. The higher β also allows the use of so-called aneutronic fuels, e.g., D-3He, whose main reac- Fig.1 FRC sketch. tion produces far fewer neutrons than D-T fusion. Accordingly, less shielding (mass) is required. One member of the FRC family – the inductively driven, liner-compression Pulsed-High-Den- the scrape-off layer, SOL. sity (PHD) device – was designed to operate in a pulsed mode with D-T, producing an average power of 70 MW. Another FRC To form the closed magnetic-field lines, a strong plasma cur- family member is the Star Thrust Experiment (STX) [8], a 1-m rent is needed, perpendicular to the FRC’s magnetic field. On plasma radius design, formed and heated by an RF technique axis, the direction of the magnetic field created by the plasma called rotating magnetic fields [9, 10] (RMF). An STX rocket current, Ip, is opposite to that of the open field lines which are engine was predicted to be able to produce steady propulsion at created by external coils. If the axes of the two fields are not ex- a power level near ½ MW/m of length. actly parallel, MHD theory [13] predicts that the configuration will strongly tilt and destroy itself. In the following subsection In this paper we describe a third member of the FRC fami- we shall describe how RMFo generates the current and heats ly, the D-3He-fueled Direct Fusion Drive (DFD) rocket engine the plasma ions and electrons in such a way as to allow smaller [11]. Similar to STX in employing RMF, the DFD differs in ma- devices with excellent stability, not susceptible to the tilt mode. jor ways, ones that would result in a more practical rocket engine. Important differences are: 1) the DFD RMF method has 2.1. Macro-stability different symmetry [12] (odd parity versus even parity, RMFo vs RMFe), providing improved energy confinement hence al- MHD theory has shown itself to be accurate in predicting the lowing plasmas with 4-8 times smaller linear dimensions and stability of plasmas that are fluid-like [14]. Fluid-like plasmas 100-400x smaller volume and mass; 2) The smaller radius DFD are prone to several classes of instabilities. Criteria that deter- plasma is far more stable than the larger STX plasma; 3) the mine whether a magnetized plasma is fluid-like are collisional- smaller radius of the DFD plasma allows a method to improve ity and the ratio of particle gyro-radii to machine size. Highly the properties of the rocket exhaust, with specific impulse, collisional, that is, cold and dense, plasmas are fluid-like. Plas- 4 Isp, to 2 x 10 s (and beyond) and thrust, Th, to 10 N/MW; 4) mas where the ion gyro-radii, ρi, are small compared to the DFD operation reduces neutron wall fluxes more than a factor plasma radius, rs, are likely to be fluid-like. For an FRC, the of 1000 compared to D-T devices, thereby reducing neutron size criterion is defined by two nearly equivalent dimensionless * shielding thickness by a factor of 10 and increasing engine parameters: s 0.3 rs/ρi and S ≡ rs ωpi/c, where ωpi is the ion lifetime; and 5) increased attention to the engineering details plasma frequency and c the speed of light [15]. By choosing a of the complete rocket engine, such as improving energy-re- small, high-temperature FRC, neither fluid criterion is satisfied covery systems, raising specific power, and optimizing plasma and the plasma is said to be kinetic rather than fluid-like. Why a heating and fueling systems. kinetic plasma is stable against the tilt mode can be understood by considering the axis-encircling orbit of a single charged par- In section 2 of this paper we describe the physics of the ticle in a magnetic field, a stand-in for a hot plasma. An axial DFD’s fusion core, explaining how the novel RMFo method push to the particle, in an attempt to tilt its axis, causes the par- improves energy confinement, current drive, plasma heating, ticle to translate along B, not to tip over. No tilt occurs. More and plasma stability. Section 3 describes the choice of fuel, the complicated explanations can be extracted from Steinhauer’s neutron production rate, and the power balance. Section 4 de- review [16]. It should be noted that several FRCs [17, 18, 19, scribes how the energy in fusion products produced in the core 20] have achieved stable plasmas for durations 103 to 105 times is converted into directed momentum for propulsion. Section 5 longer than predicted by MHD theory, the Alfvén time, τA. describes two missions relevant to interstellar exploration. (Stability is predicted [15] for S*/E < 3.) The plasma durations were limited by power supply capabilities not instabilities. 2 THE DFD ROCKET ENGINE CORE We now address how RMFo heats particles and allows the The region where abundant fusion reactions take place is the size of the FRC to be relatively small. high temperature (ca. 100 keV), moderate density (ca. 5 x 1014 cm-3) plasma region named the core. For the FRC, this region is 2.2. Confinement inside a magnetic separatrix, an imaginary closed surface that demarcates open magnetic-field lines, those that leave the de- There are several reasons why energy confinement in FRCs can vice, from closed magnetic-field lines, ones that stay fully inside be good, that is, better than in tokamaks. We first discuss how the device, see Figure 1. The open field-line region is also called to keep the FRC confinement from becoming poor!

JBIS Vol 72 No.2 February 2019 39 S.A. COHEN ET AL

Fig.3 Magnetic field lines when a larger amplitude odd-parity magnetic field, tB = 0.04, is added to a Solov’ev FRC. (a) Closed field Fig.2 A very small-amplitude (Bt = 0.005), uniform, transverse lines in the y – z plane show expansion and contraction but remain magnetic field (even parity) is added to a Solov’ev FRC with B0 = 1. closed. (b) Projection of field lines originally in the x – z plane onto Two field lines are mapped. Though both field lines are long, they that plane show little change in shape. This FRC’s major axis is are clearly open. This FRC’s major axis is vertical [12]. horizontal [12].

The net magnetic field caused by the external coils and the Secondly, Rostoker [26] and others [27] noted that hot ions plasma current creates a nested set of closed field lines inside and runaway electrons in tokamaks had far better confinement a separatrix; each closed field line circles the plasma current than thermal electrons. The reasons proposed for the large once poloidally before closing on itself. Closed field lines are improvement was lower collisionality and less scattering by good for confinement since they encourage charged particles fluctuations because, like large ships in a choppy sea, the large to stay within the device. Open field lines allow particles and gyro-radii of these energetic particles made them less suscepti- their energy to flow out of the device,i.e. , confinement is poor- ble to small-scale fluctuations. er for open field lines. The addition of RMFe to an FRC causes the field lines to open, see Figure 2, while application of RMFo 2.3 Plasma current drive and plasma heating maintains the closure of field lines, Figure 3. One explanation is that the FRC, by itself, is of odd parity [21]. Mixing parities, RMFe was proposed to drive current in the plasma, not to heat such as by adding RMFe, causes all of an FRC’s field lines to it to fusion-relevant temperatures. The current-drive mecha- open, hence confinement to degrade. One experimental team nism was explained as being of second order, specifically, the [22] has compared even and odd-parity electron heating on the time-varying RMFe magnetic field (in the r and φ directions) same device and found a factor of 4 improvement in the energy confinement time. Another team [19] achieved 5 to 10-fold increases in electron temperature, Te, with RMFo compared to other machines, e.g., Reference [9], of the same size and heating power operating with RMFe.

Neoclassical theory [24] predicts that energy losses scale as (1 + q2). For tokamaks q ≥ 3 while for pure FRCs q = 0. Accordingly, FRCs should have about 10x better confinement. Sheffield prepared a survey of confinement quality in tokamaks which Kesner and Mauel updated; the results are shown in Fig- ure 4. The point denoted as C-2 represents data from a TriAl- pha FRC, clearly better than the tokamak results. Whether the same improvement occurs in FRCs at higher ion temperatures needs to be tested.

There are reasons to believe this improvement will occur. First, the main culprit expected [25] to cause anomalous energy transport in FRCs is called the lower-hybrid drift instability (LHDI), predicted to create mm- to cm-scale turbulence that increases transport. The LHDI growth rate, γLH, is the ratio of the electron drift speed to the ion thermal velocity. As ions get Fig.4 Confinement quality vs ion temperature, Ti. The TriAlpha hotter and the plasma denser, γLH gets smaller and the LHDI C-2 FRC device has shown better confinement quality, β/χ, than should become less important. tokamaks. (Adapted from Sheffield [23], by Mauel and Kesner.)

40 Vol 72 No.2 February 2019 JBIS DIRECT FUSION DRIVE for Interstellar Exploration created a z-directed electric field (alongB ), hence a current in that direction, Jz. From the JxB term in the fluid momentum equation, Jz interacted with Br, resulting in the desired azimuthal current Jφ.

In contrast, RMFo current drive is first order because of its Bz near the FRC’s midplane. The time variation of that field creates an azimuthal electric field, φE , near the O-line magnetic null, Figure 5, directly accelerating charged particles into betatron orbits near the null, Figure 6a). More precisely, the trajectories are punctuated betatron orbits, separated by periods in cyclotron motion. As the particles are accelerated along the null, they gain then lose energy, Figure 6b), because the (slowly Fig.5 Snapshot of the azimuthal electric field in the FRC’s midplane rotating) Eφ reverses direction halfway around. The more en- created by RMFo. This field rotates with the RMFo. ergetic the particles get, the further away from the null they can circulate. In the RMFo’s rotating frame, Figure 6c), these punctuated betatron-orbit electrons form a crescent, hence move, be far from the ion cyclotron frequency (at the FRC’s center) on the average, with an azimuthal velocity equal to that of the to allow quasi-resonances, particularly at higher harmonics, is RMFo [28]. seen in Figure 7b). Importantly, for both electron and ion heating, the non-uniformity of the FRC’s magnetic field, especially In an FRC reactor, these current-carrying electrons will the presence of nulls, causes orbits to lose track of the phase have very high peak energy, about 5 times greater than in D-T of the RMF, introducing stochasticity into the motion hence tokamak fusion reactors, consequently their collisionality will net energy gain [29]. Near Maxwellian distributions may de- be more than 10x less. This contributes strongly to the high velop, though usually the distributions are truncated at higher efficiency of RMFo for driving current. Away from the O-line energy. Note that the required RMFo strengths, to ~ 200 G, and null, the more massive ions will carry an appreciable part of the frequencies, 0.3-3 MHz, to achieve ion energies of 100 keV are current and diamagnetism will also provide a substantial part well within the capabilities of conventional RF equipment. Of of the required current. course, improvements in RF amplifier efficiency and reduction of amplifier mass would provide important benefits. Ion heating results from the same physical process, acceleration by Eφ, with an additional contribution from the RMFo-cre- Having a small FRC, allowed because of the better energy ated z and r electric fields. That the RMFo frequency should not confinement, makes RMFo operation better. It improves the

Fig.6a Punctuated betatron orbit near the Fig.6b As the electron moves against Eφ it Fig.6a In the frame rotating with the RMFo, FRC midplane. At the start and end of gains energy; as it moves with Eφ, it loses the punctuated betatron trajectory appears the betatron segments, the orbit becomes energy, resulting in the spikes in energy. as a crescent, with the betatron segments cyclotron. (Bo = 20 kG, r¬s = 10 cm, ωRMF/ “inside” the cyclotron segments. ωci = 0.5).

Fig.7a Maximum ion energy versus RMFo frequency for different Fig.7b Early time evolution of ion energy for two values of the RMF RMFo strengths in a 10-cm radius, 20-kG FRC. strength, 2 and 20 G. The quasi-resonances at higher harmonics, 3-5, are evident, as is the stochastic nature of the heating.

JBIS Vol 72 No.2 February 2019 41 S.A. COHEN ET AL

penetration of the RMFo field to the FRC’s null line where current drive is more efficient; it requires higher RMFo frequency, which results in higher ion energies because Eφ ∂B⁄∂t. As we shall shortly see, other important benefits accrue, ones that result in far lower neutron wall load.

3 FUEL CHOICE, NEUTRON PRODUCTION, AND POWER BALANCE

The production of neutrons by fusion is particularly problem- atic for spacecraft propulsion. Neutrons cause damage and activation of nearby materials and structures, limiting their lifetime, necessitating maintenance, and increasing the mass needed for shielding. Neutrons are hard to “direct”, hence may contribute little to the thrust required of a rocket engine. Hav- ing all the fusion products be charged particles solves these problems at the added cost of requiring higher plasma temperatures because of the lower fusion cross sections of the “advanced” aneutronic fuels. Of the two aneutronic fuels most Fig.8a T+ trajectory projected on the midplane of an FRC. The +T commonly discussed, we choose D-3He instead of p-11B. The slowing down by electron drag in the SOL is accelerated to show the low energy release from p-11B fusion, plus the lower fuel densi- transition of the orbit from betatron to figure-8 to cyclotron, the ty possible at fixed magnetic field (because of the higher nucle- latter lying fully in the SOL, the region between the red and green ar charge) and the higher temperatures required, makes p-11B circles. a dubious choice. However a penalty must be paid for selecting D-3He. There are neutrons from one D-D fusion branch and possibly from the T fusion product of the other D-D fusion branch. Methods must be found to ameliorate these effects.

A small FRC allows solutions to these problems. The surface-to-volume ratio scales as 1/radius. For a 25-cm radius FRC, a 32-fold improvement is obtained compared to an 8-m tokamak. Additionally, fusion products born in a small FRC will have their orbits pass through the cool SOL of the FRC where the electron drag is strong. By an “airbrake”-like effect, see Figure 8a), fusion products which pass through the SOL even for a small fraction of their birth orbit, will rapidly cool, from 1 Mev to 100 keV, and their orbits will become cyclotron-like, lying fully in the SOL, Figure 8b). PIC studies [30] of the slowing down indicate that this process will occur in under 10 ms, far quicker than the estimated 20-s T burn-up time. Once in the SOL, the T+ will be exhausted out the nozzle with the cooler propellant, to be described in section 4. Only those neutrons produced by D-D fusion will remain a problem. Fig.8b Close-up view of the cyclotron segment of the T+ orbit, showing that the orbit eventually lies fully in the SOL [30]. The third step in reducing neutron production is to increase the ratio of 3He to D in the plasma [31]. This does reduce the 2 power density approximately linearly but the percentage of xs /2, where xs = rs/rc. Table 1 presents the results of our model 20 power in neutrons quadratically. with rs =30 cm, E = 6, Te = 30 keV, Ti = 100 keV, ne = 3x10 m-3, and a 2:1 3He to D ratio. Flat temperature and density pro- From a neutron-production perspective, the net effect of files are assumed. From these, it is relatively straightforward to these 3 measures should be in excess of a thousand-fold [32] calculate β, fusion power, magnetic field strength, and plasma reduction of neutron power flux to the first wall. The thickness current, Ip. The next step is to calculate volumetric losses from of the neutron shielding, 100% 10B, would be 10-30 cm, based radiation. Though Bremsstrahlung losses may also be calculat- on the duration of the mission and of the fusion-power pro- ed accurately, this is not the case for synchrotron losses because duction. of plasma absorption and wall reflections. Our model assumes full emission from a 3-cm thick shell just inside the separatrix 3.1 Power balance and rocket subsystems and no wall reflection. Further into the core the magnetic field is lower, hence the frequency lower; absorption of that emitted In this section we analyze a point design for a DFD rocket en- radiation occurs in the aforementioned shell. The Bremsstrahl- gine, focusing on power balance, to see if a consistent solution ung and synchrotron power will be absorbed in the neutron exists within the stability, energy confinement, and low radi- shielding. That energy is recovered with an efficiency of 60% oactivity constraints described above. We begin by specifying by a Brayton cycle cooling system. Power flow into the gas box the plasma density, ion and electron temperatures, plasma ra- ionizes the propellant there. The energy cost is typically 50-100 dius and elongation, and the external coils inner radius, rc. The eV/ion, with higher values required at lower densities. Of that latter, determined by the thickness of the SOL and of the shield- power, 80-90% is deposited on the gas box walls and recovered ing, sets the plasma β through the Barnes relation, <β> =1- by the Brayton cycle system. The power flows are depicted in

42 Vol 72 No.2 February 2019 JBIS DIRECT FUSION DRIVE for Interstellar Exploration

Figure 9, which, for this DFD, is providing primarily thrust. If more electrical power is required for station keeping or communications, the thrust power can be diverted to generating electrical power. The distribution of masses is shown in Figure 10. This assumes a conservative permitted neutron flux on the superconducting coils, below 1018 n/cm2 and below 10-4 DPA, resulting in a 10-cm-thick 10B shield, sufficient for 1 year at full power. Increasing the shielding thickness to 22 cm would increase the superconducting coil's lifetime to 13 years.

TABLE 1 Parameters for a 2-MW DFD rocket engine Parameter DFD

rs (m) 0.3 Elongation, E 6

Ba (T) 4.3

Ip (MA) 8.0 Ion species D-3He 3He/D 2

-3 20 ne (m ) 3 x10

Te (keV) 50

Ti (keV) 100 <β> 0.84 Fig.9 Power-flow diagram of a 2-MW DFD. PRMF (MW) 0.5 ωR (radians/s) 1.6x106

BR/Ba 0.003

Pf (MW) 2.13

Psynch (MW) 0.7

PBremss (MW) 0.32

PGB (MW) 0.1

classicalτEi /τE 2.7 s (T+) 2.3 s (4He++) 2.2 S*/E 2.8

γLH 0.02

ψRMF penetration 34 Isp (s) 2.3x104 Thrust (N) 12.5

Bnozzle (T) 20 % power in neutrons 1.1

2 -3 Wall load (MW/m ) 2x10 Fig.10 Mass budget of the DFD engine.

We now examine the consistency of this design point with specified as acceptable in D-T tokamaks, a sizeable improve- energy confinement, stability, and propulsion. The ratio of ment. The amount of thrust power lost in the neutron chan- the classical confinement time, classical τEi, to the required nel is small, 1%, though could be lowered by increasing the energy confinement time is 2.7, consistent with the improve- 3He/D ratio. ment in energy confinement seen by C-2 and PFRC-1. The two stability criteria are also satisfied: the LH micro-stability The spI predicted for the DFD depends on the propellant criterion, γLH, is < 1 and macro-stability criterion, S*/E, is less species and injection rate into the gas box. For Table 1 we than 3. One further criterion [10] worth mentioning, named have selected a low propellant injection rate, one that pro- 4 ψRMF penetration, is that for the RMF field to penetrate in the core duces an Isp above 2 x 10 s. For higher propellant flow, Isp of the FRC. This parameter was derived for RMFe not RMFo, so would drop and the power required in the gas box would its applicability is questionable. For RMFe ψRMF penetration must be increase along with the thrust, topics we describe in more greater than 1 for penetration. For the DFD, this parameter is detail in section 4. above 30, an encouraging margin in light of the possible lack of direct applicability. A pictorial representation of the subsystems is shown in Figure 11 and an artist’s rendition of a DFD module is in The neutron wall load for this plasma is 2,500x below that Figure 12.

JBIS Vol 72 No.2 February 2019 43 S.A. COHEN ET AL

Fig.11 Block diagram of DFD major subsystems.

Fig.12 Artist’s rendition of a 2-MW DFD module.

4 THE SCRAPE-OFF LAYER (SOL) AND ROCKET EXHAUST process and is predominantly transmitted to the electrons via fast-ion drag. The random thermal energy in the SOL electrons The SOL of the DFD is quite different than that of any oth- is transferred to the cool SOL ions through a double layer at the er fusion device. In tokamaks, for example, the SOL is heated nozzle and via expansion downstream, thus being converted and populated by diffusive transport across the separatrix of into directed flow. both energy and particles. The heat transport into it is local, that is, described by Fick’s law, by the local flux-surface-nor- Because of the relatively low temperature (< 100 eV) and mal gradient in pressure. Because this diffusive transport is high density (> 5 x 1019 m-3) of the SOL, resulting in a colli- slow compared to the flow along the magnetic field, the SOL sional mean-free-path of the thermal (majority) electrons less is onion-skin thin, δSOL, compared to the plasma’s radius. For than 50 cm, it is appropriate to use a fluid model for the SOL example, ITER’s SOL is predicted to be ½-cm thick while the between the gas box and the nozzle. Results from one UEDGE -4 plasma outer radius at its midplane is 9 m, δSOL⁄rs 6 x 10 . [33] fluid-code simulation are shown in Figure 13. In each, the In the DFD, the density profile of the SOL is determined by gas box is 1-m long, at the far left, the electron heating occurs the orifice to the gas box and the field expansion between the in the central 2 m, and the nozzle is located at z ~ 2 m. The gas box and the plasma midplane. For the DFD in Table 1, the inputs were Pi =1 MW of power and = 0.08 g/s of D2 gas into SOL would be about 7 cm thick, δSOL⁄rs >0.2 . Energy is depos- the gas box. (The gas input is equivalent to a current, eI = ited across the entire SOL cross section by the large gyro-radii NA e/amu ~ 3.85 kA, where NA is Avogadro’s number and e the fusion products. Thus the DFD, the SOL + FRC, is more like a charge on an electron.) From Emax = Pi/Ie, one can then readily navel orange, with a very thick rind. The energy is deposited estimate the upper limit of ion energy to be 260 eV. As Fig- in the SOL directly from the fusion products via a non-local ure 13c) shows, only half that value is reached. The culprits are

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Fig.13a Electron density, ne, contours. Fig.13b Electron temperature, Te, contours. Fig.13c Ion energy contours.

Fig.14a Thrust vs gas feed for powers of 0.25 to 7 MW. Fig.14b Exhaust velocity vs gas feed for powers of 0.25 to 7 MW. radiation and ionization losses and plasma energy brought to instrument is an infrared telescope capable of looking back the gas box walls by plasma transport. The results of an exten- toward the Sun to assess the solar system dust that causes IR sive number of simulations are presented in Figure 14, showing extinction as we look outward from Earth. It was too heavy thrust reaching 10 N/MW. for the Innovative Interstellar Explorer mission. The instruments are given in Table 2 over the page. The Exoplanet Im- 5 MISSIONS aging instrument would be a 1-m telescope with a large focal plane with a 0.4° field-of-view. The baseline communications We describe two missions, one to place a 1-m telescope at 550 system is a 40-GHz, Ka-band system with a 4-m-diameter AU where it can use the sun as a gravitational lens to image transmit dish and 500-kW power. The data rates as a function exoplanets and the second to deliver a 1 MT payload to Alpha of distance are shown in Figure 15, sufficient to return a 1080p Centauri. HDTV image every 6 seconds. (A 1-μ laser could increase the data rates 100-fold.) 5.1 550-AU mission The exoplanet telescope focus extends semi-infinitely. A The 550-AU mission will carry a camera and other instruments 1-meter telescope, with coronagraph components, could re- to a distance of 550 AU (and beyond) from the Sun. At that solve 3-km features on a planet 30 parsecs away. The light from distance, unique interstellar and solar system observations can the exoplanet appears as a ring around the sun, whose disc of be conducted. Using the Sun as a gravitational lens for imag- light is blocked. There are many complexities [38] to the data ing exoplanets is the one considered here. Conventional rocket analysis: pointing; focal properties of the sun are different in technology would result in a 30-year transit to 550 AU; data the radial and azimuthal directions; signal to noise; the ex- collection would start in 2060. Using the Direct Fusion Drive oplanet moves across the field of view; etc. The spacecraft is (DFD), the transit time to 550 AU would be 13 years. Even translatable perpendicular to the focal length vector to pro- accounting for development time, data collection will start 15 duce an image. years earlier than with conventional technology, in 2045 rather than 2060. In transit and on arrival, the DFD would provide High-spectral-resolution spectroscopic data is available for a megawatt of power for science, communication, and sta- every 3-km pixel. The unprecedented spectral resolution allows tion-keeping. Furthermore, DFD allows a much smaller launch LANDSAT-like characterization of the exoplanet surface. Ge- vehicle to be used, reducing mission costs substantially. ological and material features of the 3 km x 3 km areas can be determined. Weather patterns can be tracked in real time. If The mission objectives include the objectives of the Inno- the target exoplanet were Earth, the extent of industrial and vative Interstellar [34] and the 550 AU mission [35, 36]. One agricultural use would be available for each 3 km x 3 km area.

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TABLE 2 Instrument packages [37]* Acronym Instrument Mass Power Data rate (kg) (W) (bps) MAG Magnetometer 8.81 5.30 130.00 PWS Plasma wave sensor 10.00 1.60 65.00 PLS Plasma parameters 2.00 2.30 10.00 EPS Energetic particle 1.50 2.50 10.00 spectrometer CRS-ACR/GCR Cosmic-ray 3.50 2.50 5.00 spectrometer: anomalous and galactic cosmic rays CRS-LoZCR Cosmic-ray 2.30 2.00 3.00 spectrometer: electrons/positrons, Fig.15 Data rate for a Ka-band communication system. protons, helium CDS Cosmic dust sensor 1.75 5.00 0.05 NAI Neutral atom detector 2.50 4.00 1.00 ENA Energetic neutral 2.50 4.00 1.00 atom imager LAD Lyman-alpha detector 0.30 0.20 1.00 EXOI Exoplanet Imager 20 100 3 x 106 IRD Infrared camera for 10 100 3 x 106 solar system dust Total resources 35.16 229.40 6 x 106 *The power is that necessary to operate the instrument, not for communications. Fig.16 Example of what the telescope might be capable of resolving from 30 parsecs [39]. A list of select spacecraft specifications is shown in Table 3. The fuel mass does not include that for the outgoing spiral. The “efficiency” is the fraction of power that goes into thrust; the fuel “tank fraction” is the ratio of its mass to that of the fuel. Once the spacecraft departs from Earth, it takes 13 years to reach 550 AU. The same spacecraft could be put into solar orbit at 550 AU in 18 years. The additional time is due to deceleration. Orbiting at 550 AU could not be done with a solar sail or laser light sail.

Launch windows for gravity-assisted missions can be decades apart while a direct flight does not require any particular launch window since it does not employ any flybys of the planets. It can be launched as soon as it is ready. Figure 17 shows a transfer (flyby) trajectory. The Earth departure spiral requires 400 kg of fuel from an ISS orbit and is shown in Figure 18. The Fig.17 Flyby trajectory parameters [41]. spacecraft total mass of 5282 kg is low enough to be launched on any currently available launcher, as shown in Table 4. The of the two and is cryogenic. It has a cryo-cooler to recirculate spacecraft is in the inner radiation belt for 11.7 days. boil-off. Helium-3 is stored as a gas in the smaller tank. The antenna, (small blue vertical panels) and radiators (large black Achieving the spacecraft performance values listed in Table horizontal panels) are deployable. 3 will be challenging. The specific impulse corresponds to 2.6 keV deuterons. In the lab [40] magnetic nozzles have produced 5.2 An interstellar mission only ~100 eV ions, though at considerably lower power (density) than the DFD. Higher Isp studies would require kinetic codes Interstellar missions require much longer burn durations, and rather than fluid ones because of the reduced collisionality. higher Isp and specific power than the 550 AU mission. Fig- ure 20 shows the rendezvous distance as a function of specific 5.1.1 Spacecraft design power and thrust for a 325-year-duration mission. The power is fixed at 100 MW. The exhaust velocity is found by solving the The spacecraft design is shown in Figure 19. The 4-m-dia Ka- power equation: band high gain antenna dominates the spacecraft. A single (1) DFD engine is used. While a second engine would give the system some redundancy, it may be better just to fly two space- where η is the power to thrust efficiency, ~ 0.3. Figure 21 shows craft. For other missions, multiple engine modules offer strong the same but for a flyby. The maximum distance at each specif- benefits, noted later. The solar panels are for the spacecraft LEO ic power is achieved for different thrust levels. At low specific checkout phase. The deuterium (propellant) tank is the larger powers, higher thrusts send the spacecraft further. This is not

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TABLE 3 Spacecraft specifications for the 550 AU flyby Parameter Value Units Final position 555.60 AU Final velocity 479.10 km/s Final time 13.00 yr Fuel 3217.60 kg Mass Total 5282.00. kg Mass Engine 1700.00 kg Mass Payload 300.00 kg Exhaust Velocity 510. 00 km/s Power 1.70 MW Thrust 4.00 N Fig.18 Earth departure spiral [39]. Specific power 1.0 0 kW/kg Efficiency 0.30 Tank fraction 0.02

TABLE 4 Launch vehicles to put spacecraft into LEO Family Launch Vehicle LEO (kg) ISS (kg) Atlas 401 9,800 8,910 411 12,030 10,260 431 15,260 13,250 501 8,210 7,540 511 11,000 10,160 531 15,530 14,480 551 18,850 17,720 Delta IV Medium 9,190 8,510 Fig.19 Spacecraft design. The large tank is for liquid D, the smaller Medium+ (4.2) 12,900 12,000 tank is for gaseous helium-3. Medium+ (5.2) 11,060 10,220 true at high specific powers. At a specific power, the maximum Medium+ (5.4) 13,730 12,820 distance is achieved with 4 N thrust, not 8 N. There will be Heavy 28,370 25,980 an optimal thrust for every duration and specific power. The Falcon 9 Block 1 9,000 8,500 exhaust velocity assumed is a sizeable fraction of that of the full energy of the fusion products. The distance as a function Block 1.1 13,150 12,420 of time for intercept is given in Eq. (2) below, where ms is the mass at switch time, is the mass flow rate, is the propel- The switch time is found from the quadratic equation: lant exhaust velocity, , , and is (4) the velocity at switch time: where .

(3) The solution for ts that is less than tf is the correct solution.

Fig.20 Rendezvous distance in 325 years for different thrusts and Fig.21 Flyby distance after 325 years of constant thrust [39]. specific powers [39].

(2)

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Fig.22 Approach to Alpha-Centuri [39].

Entry into the star system is similar to entry into a plane- formance improves with the logarithm of the mass of the tary orbit within the solar system. The approach geometry is extra stages [42]. Employing 100 DFD units, a flyby of Al- shown in Figure 22. The final orbit adjust maneuver is shown pha-Centuri within 350 years is then achievable at a specific in Figure 23. By the time such a mission is launched, accurate power of 5 kW/kg, an improvement compared to requiring information about planetary orbits should be available so that 30 kW/kg for a single 100 MW DFD, as depicted in Figure the maneuvers can be planned in advance. Once in orbit the 21. spacecraft would have up to 100 MW of power to transmit data • Make superconductors that can last 300-500 years in the back to earth. The data rate from interstellar space using a 95 face of neutron bombardment. MW laser transmitter is shown in Figure 24. • Make superconductors that retain their superconducting properties at higher temperatures, to reduce the need for If the engine burns for 500 years it could go further, reach- cryo-coolers. ing Alpha Centauri, with specific power of 25 kW/kg, in 500 • Lower mass structures. years. This is shown in Figure 25 for a rendezvous. Currently • Increasing 3He supply, perhaps by T-suppressed D-D fusion our best estimates of attainable specific power are from 0.3 to reactors. The currently available 3He supply is (x1000) inad- 1.5 kW/kg, woefully inadequate for these missions. To achieve equate for a 100-MW-power, 300-year mission. the high numbers in these plots would require a number of rev- • Closed cycle method for recycling propellant/coolant dur- olutionary improvements, such as: ing electrical power generation mode of operation, to reduce the system mass. • Replace the Brayton cycle heat engine with a method of direct conversion from x-rays and waste heat to radio-fre- Figures 27 and 28 show example trajectories for the 325-year quency power. Direct conversion of heat to electricity is rendezvous and flyby missions. The parameters for these cases done now but is only about 5% efficient. Direct conversion are: a constant thrust of 4 N; a specific power of 100 kW/kg; of x-rays is done in x-ray machines but the efficiency is very engine power of 160 MW; and an exhaust velocity of 24,000 low. km/s. Note the switch time is beyond the halfway point as the • Use DFD staged modules – consume then jettison. This is spacecraft continues to become less massive. Using multiple similar to chemical rockets today, with the significant dif- engine modules, and jettisoning them and empty propellant ference that all remaining DFD modules provide thrust un- tanks along the way, could reduce the required specific power til they and their propellant tanks are jettisoned. The per- a factor of 10 while keeping the trip duration and payload the

Fig.23 Final orbit adjust [39]. Fig.24 Data rate from interstellar space for a 95-MW transmitter.

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Fig.25 Data rate from interstellar space for a 95-MW transmitter. Fig.26 Data rate from interstellar space for a 95-MW transmitter.

Fig.27 Sample flyby trajectory for T = 4 N [39]. Fig.28 Sample rendezvous trajectory for T = 4 N [39]. same. These jettisoned modules could act as relay stations for has the potential to reduce the cost and increase the scientific communications, increasing the data rates enormously. return for most solar system robotic missions and human missions to nearby planets. 6 CONCLUSIONS The paper illustrates, once again, the critical relationship The physics basis for low-radioactivity, FRC fusion reactors between specific power and mission performance. The current has steadily grown over the last two decades, with innovative estimated DFD specific powers are between 0.3 and 1.5 kW/kg. contributions from theory, modeling, and experiments. Im- Far higher specific powers are desired for missions to other star portantly, stability limits, once thought to be a major issue, systems, ones that will also require much better methods of re- have been exceeded by a factor of 105 and energy confinement cycling waste energy and components that are far less sensitive quality, seen in experiments and measured by the ratio of β to neutron irradiation. to plasma thermal conductivity χ, is a factor of 10 better than in the mainline fusion reactor designs. Scaling predictions Near-term work includes the completion of the PFRC-2 to hotter FRC plasmas is favorable. More recently, attention ion heating experiment, detailed mission analysis, and sub- has been given to technical and engineering aspects, such as system designs for the engine components. Design work on reducing the weight of subsystems, increasing electrical effi- higher efficiency RF heating systems and on superconducting ciency, and identifying components with high resistance to magnets is underway. Design of PFRC-3 will begin once the radiation damage. ion heating experiments are complete. This will be about 50% larger than PFRC-2 and aims at higher plasma temperatures From this foundation, we extrapolate to a Direct Fusion and pressures. The succeeding facility, the PFRC-4, is aimed Drive rocket engine that would permit high scientific-return at demonstrating fusion power generation with D-3He. Ad- interstellar research missions in the 2030 time frame, provid- ditional work will be done on the integration issues of multi- ed advances are made in achieving fusion and producing the ple engine modules, including the effect of one engine's field predicted thrusts and specific impulse levels. A DFD-powered on another. A critical point is that the engineering challenges spacecraft could be used for the 550-AU gravitational lensing in the DFD design, though large, are greatly reduced, com- mission. An Alpha Centauri flyby and orbital mission would pared to all previous fusion rocket engine concepts, because require a ten-fold increase in mission duration and place far of its small, clean, steady-state, high-β and modular nature. more difficult demands on the technical components. DFD The DFD design allows ambitious missions throughout and

JBIS Vol 72 No.2 February 2019 49 S.A. COHEN ET AL outside the solar system. Direct Fusion Drive has the poten- Acknowledgements tial to revolutionize space exploration. Near term research and This work was supported by the US Department of Energy development aim to move the technology to operational status Contract No. DE-AC02-76-CHO-3073 and NASA grant NNX- by 2030. 16AK28G.

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Received 9 April 2018 Approved 17 October 2018

50 Vol 72 No.2 February 2019 JBIS JBIS VOLUME 72 2019 PAGES 51–55

INTERMEDIATE BEAMERS FOR STARSHOT: Probes to the Sun’s Inner Gravity Focus JAMES BENFORD1 & GREGORY MATLOFF2 1Microwave Sciences, 1041 Los Arabis Lane, Lafayette, CA 94549 USA; 2Physics Dept., New York City College of Technology, CUNY, 300 Jay St., Brooklyn NY 11216, USA

Email [email protected] / [email protected]

The Starshot technology development program will build a modular Beamer system that will incrementally achieve steadily higher launch speeds. We examine what an intermediate-level Starshot Beamer system would be like and the relative merits of Beamer technologies in nearer-term missions. We quantify one such intermediate destination for robotic probes, the Sun’s Inner Gravity Focus. A constellation of such probes would each see a “pixel” of the image plane. We describe cost-minimized Beamers driving probes to ~100km/sec using laser, millimeter-wave and microwave sources and antennas/ optics. Such systems would cost of roughly $1 billion at present costs. Substantial progress on driving down laser and/ or millimeter-wave costs is essential to near-term system cost reductions. Power density in the sail varies substantially among the Beamers, with lasers giving the lowest sail temperatures.

Keywords: Beam-driven sails, Starshot, Microwave beam, Laser beam, Millimeter-wave beam, Solar gravitational focus, Interstellar precursor mission

1 INTRODUCTION TO PROJECT STARSHOT sideration of earlier missions and destinations nearer than the Centauri system is in order. Project Starshot is perhaps the most audacious proposed near- term space mission. Essentially, a huge terrestrial laser array As the Starshot technology develops, velocity regimes be- projects its beam upon a highly reflective photon sail attached yond anything available now will be attained. This will include to a pico-spacecraft. After the sail remains in the beam for a flyby probes of the outer solar system planets and moons, ex- few minutes, it exits the solar system at ~0.2c in the direction ploration of the Kuiper belt objects and interstellar precursors of Alpha/Proxima Centauri [1]. to investigate beyond the heliopause. All these missions have the advantage of not requiring any deceleration as the objective Starshot envisions a wafer-thin spacecraft with a payload is reached. mass of about 1 gram accelerated at >>10,000g over a distance of millions of kilometers by radiation pressure from a ~100 Here we examine what a specific intermediate-level Starshot Gigawatt laser. The Beamer would be situated in the Southern Beamer system would be like and the relative merits of technol- hemisphere as close as practically possible to a latitude of 60 ogies in these nearer-term missions. degrees because Alpha Centauri is visible from the southern hemisphere and at 60 south latitude Alpha Centauri will be 3 SUN’S INNER GRAVITY FOCUS MISSION high in the sky and atmospheric beam losses will be minimized. One such possible intermediate destination is the Sun's Inner Before the sail (which has dimensions of a few meters and a Gravitational Focus [2,3]. As a consequence of General Relativi- mass of a few grams) is inserted in the beam, it is ejected and ty, electromagnetic radiation emitted by celestial objects occult- deployed from a mothership near the apogee of a highly ellip- ed by the Sun is greatly amplified and concentrated into a focal tical Molyniya-type orbit. This orbit is geosynchronous with a cylinder extending from about 550 AU to infinity. Since the low perigee and a high apogee. At or near apogee, the velocity gravitational focus is a line focus, not a point focus, images can of a spacecraft in a Molyniya orbit is low relative to the ground. be retrieved over time as the spacecraft flies on into interstellar space. The diameter of this line focus is a cylinder only about All of these requirements push near-term technology far be- 1 kilometer. Initially, the amplification produced by the Sun's yond current limits. It is therefore a worthy objective to consid- Gravitational Focus was considered as a tool for SETI (Search er intermediate goals for the basic Starshot concept. for Extraterrestrial Intelligence) radio astronomers. Amplified radio leakage from ET's home system could be detected as well 2 INTERMEDIATE LEVEL MISSIONS as beamed signals in this manner.. This approach can also facil- itate communication with our more distant interstellar probes. There are many technological issues with the full Starshot mission parameters and some of these might preclude a launch in Of course, a current-technology solar-photon sail using a the 2030s. But the on-going Starshot technology development Sundiver maneuver could certainly perform a mission to the program will build a modular Beamer system which will in- inner Sun’s gravity focus within a human lifetime [4-5]. Appli- crementally achieve steadily higher launch speeds. Thus con- cation of desorption at perihelion could certainly reduce travel

JBIS Vol 72 No.2 February 2019 51 JIM BENFORD & GREGORY MATLOFF time [6-8]. With Sundiving, missions to the Sun's gravity focus the Beamer cost optimization method developed by Benford might be the limit of solar sail technology. [10], which minimizes the total system cost. We take sailcraft parameters from Parkin’s Starshot System Model, a thin-film Beamed photon sailing, however, does have the potential of circular photon sail with a mass of 4 grams, a payload of 1.5 reaching the stars. Applying it on a precursor mission might be grams, a diameter of 5 meters and a thickness of about 0.1 mi- a lot easier and a nice proof-of-concept for Starshot technology. cron (0.2 g/m2, in the range of graphene) [9]. Acceleration is unconstrained. As in the baseline Starshot mission, the wafer-thin spacecraft is released at or near the apogees of a Molyniya-type geo- This relation below minimizes cost by trading diameter stationary orbit and is accelerated to its cruise velocity. To re- against power while keeping transfer efficiency fixed at 97%. duce atmospheric absorption, the laser wavelength is assumed For a discussion of transfer efficiency in power beaming, see to be 1 micron. A 30-year flight time to 600 AU is assumed; reference [11]. The cost optimization method requires that we the required interstellar cruise velocity is about 100 km/s. Of specify the sail mass m, velocity V0 and diameter Ds, and the course additional velocity is required to leave the gravity well frequency f of the Beamer source: of the sun. From Earth that is ≥14 km/s.

4 COST-OPTIMIZED BEAMERS (1)

Here we consider a specific application of the basic Starshot concept, to fly a mission to the Sun’s Inner Gravity Focus. The gravitational focus begins at about beyond 550 AU and ob- The areal cost coefficient a($/m2) includes cost of the an- serving a good focus will require a longer distance in order to tenna elements (optic, in the case of lasers), its supports and reduce various background optical noise contributions. Reach- sub-systems for pointing and tracking and phase control. ing these distances in less than decades requires velocities that Radiated power cost coefficient p($/W) includes the source, make stopping there impractical. So it is a flyby mission, pro- power supply, cooling equipment and prime power cost. The ceeding outward along the focal line while continuing a cam- sail reflectivity is η. We also include in the following Tables paign of imaging. the cost of the electricity for the Beamer per launch, which is negligible compared to the capital cost of the Beamer and the The mission is to receive light from an exoplanet which is Starshot probes. distorted into an Einstein Ring, a narrow annulus of a few kilometers thickness. Thus imaging is done on pixel by pixel basis. Note that at the optimum (minimum) cost, the capital costs opt opt of radiator and aperture, CP and CA , are equal. For ~100 Geoff Landis has suggested launching many individual Star- km/s missions the cost (both capital cost and operating cost) shot sails is one way of providing a constellation, each one col- of electricity is negligible compared to the capital cost of the lecting the light at a single spot in the focal plane [3]: Beamer.

“The “Breakthrough Starshot” project, for example, envi- Minimum capital cost is achieved when the cost is equally sions high-velocity laser-pushed sails, each several meters in divided between antenna gain and radiated power. The opti- diameter, and extremely low in mass and cost. It would be mum power is: possible to send hundreds, possibly thousands, of such small probes out and use each one as a lightbucket to collect light (2) from a single spot on the focal plane. Each would require an equal number of occulting spacecraft, a coronagraph or an occulting disk (“starshade”). Each one was positioned extremely And the relation between radiator power and the radiating precisely to exactly block out sunlight. aperture is:

Landis also points out the difficulties of conducting this (4) mission [3]: " The difficulties include the required pointing, the size of the image on the focal plane, the speed of motion of the image across the focal plane, the requirement for an occulter Results from the optimization technique in equation 1 can to remove the brightness of the Sun itself from the image, the be compared to those computed using Parkin’s Starshot Sys- interference of the brightness of the primary star of the target tem Model, which performs nested optimizations to guaran- planet, the signal to noise ratio produced by the brightness of tee required final velocity for the spacecraft while minimizing the solar corona and the fact that the inherent aberration of the the overall cost [9]. Equation 1 is a simple analytic expression lens means that the focal blur of the image may be equal to half which produces results within a factor of 2 of the Starshot Sys- the diameter of the planet imaged. tem Model. Therefore Equation 1 can be considered a first ap- proximation, which can then be made more precise by use of The difficulties are not necessarily fatal flaws; clever ap- the full System Model computational software algorithm. proaches may make it possible to use this large telescope and avoid some or all of the problems. In particular, an approach is Relativistic effects are negligible at these velocities, as is the pointed out whereby examining slices of the Einstein Ring, the cost of energy and the cost of energy storage. (However, in surface of the planet might be computatively reconstructed." Parkin's Starshot System Model for 0.2 c velocities relativistic effects matter and energy storage is a large if not the largest 5 BEAMER COST component of the capital cost.) For this first-level optimization, we do not assume any upper limit on the power density In order not to choose the system parameters arbitrarily, we use on the sail.

52 Vol 72 No.2 February 2019 JBIS INTERMEDIATE BEAMERS FOR STARSHOT: Probes to the Sun’s Inner Gravity Focus

6 LASER BEAMER TABLE 1 Laser Starshot Intermediate Mission parameters* Lasers with Lasers with Lasers are the Beamer in the baseline Starshot concept. Using present-day costs optimistic costs the above optimization method (Eq. 1-3) to calculate system Power 21 MW 36 MW parameters for various Beamer technologies, we begin with the 2 2 most optimistic cost assumptions, those for the Starshot laser. Optic aperture 0.0020 km 730 m Parkin’s Starshot System Model assumes that cost of lasers can Optic aperture diameter 52 m 30 m be driven down to $.01/W and the optic aperture for lasers can Sail acceleration 47 m/s2 81 m/s2 2 be reduced to about 500 $/m , the current cost of computer Acceleration distance 1.1 10 8 km 6.3 107 km screens . These are far from contemporary costs, which Parkin estimates as $100/W and 1M $/m2. Acceleration time 2,100 sec 1,200 sec Laser power cost factor p 100 $/W 0.01 $/W In Table 1 we compare laser-based system parameters with Laser aperture cost factor a 1 M $/m2 500 $/m2 optimistic and present day laser cost assumptions. Capital Cost $4.25 B $0.73 M 7 MILLIMETRE-WAVE BEAMER * for 100 km/sec, 3 gram, 5-meter diameter sail, perfect reflectivity. Wavelength is 1-micron. Next we estimate costs for the cost of millimeter-wave driven Starshot intermediate missions and then extrapolate to use of economies of scale, which lowers cost for larger systems. Thus TABLE 2 Millimeter-wave Starshot Intermediate Mission far, millimeter-wave devices at high power ~ 1 MW are avail- parameters * 2 able at $6/W and 10,000$/m . No large market has developed Millimeter wave Millimeter wave for millimeter-wave devices, so economies of scale have not with present-day with economies been firmly established. We assume the learning curve of mil- costs of scale limeter-wave tubes will be approximately that of similar tube Power 3.4 GW 1.5 GW devices, such as klystron, for which the learning curve is well Optic aperture 2 km2 11 km2 established. Table 2 compares millimeter wave systems for mission parameters for ~ 100 km/sec missions with wavelength Optic aperture diameter 1.6 km 3.7 km 3mm (100 GHz). Sail acceleration 7,600 m/s2 3,300 m/s2 Acceleration distance 660 km 1500 km 8 MICROWAVE BEAMER Acceleration time 13 sec 30 sec Finally, we estimate cost for microwave-driven sails. Micro- Power cost factor p 6 $/W 0.70 $/W wave costs have reached true economies of scale and are now Aperture factor a 10,000 $/m2 100 $/m2 2 available in quantity at about 0.01 $/W and about a100 $/m . Capital Cost $41 B $2 B Consequently, there is no need to extrapolate future microwave * for 100 km/sec, 3 gram, 5-meter diameter sail, perfect reflectivity, 3 cost because present costs are low enough to work with. Thus mm wavelength. there is no ‘microwave with economies of scale’ column.

9 ECONOMIES OF SCALE TABLE 3 Microwave Starshot Intermediate Mission The components we’re modeling here, sources of microwave, parameters * mm-wave and laser beams, antennas and optics, may be pro- Microwave duced in large quantities for the large scales of directed ener- Power 29 GW gy-driven sails. High-volume automated manufacturing would drive costs down. Such economies of scale are accounted for by Optic aperture 2.9 km2 the learning curve, the decrease in unit cost of hardware with Optic aperture diameter 1.9 km increasing production [10]. Sail acceleration 6.4 104 m/s2 Acceleration distance 79 km Neither lasers nor millimeter waves have found markets that demand more than a few hundred sources. Parkin has extrap- Acceleration time 1.6 sec olated the cost of these devices from known learning curves Power cost factor p 0.01 $/W of similar technologies [12]. True commercial application of Aperture factor a 100 $/m2 lasers or millimeter waves would greatly reduce the price of Capital Cost, CC $0.58 B sources and objects, as occurred long ago for microwave technologies. At present the largest application for a megawatt-level *for 100 km/sec, 3 gram, 5-meter diameter sail, perfect reflectivity, 0.3m millimeter-wave sources is the ITER fusion project, which re- 10 Ghz wavelength. quires hundreds of devices.

An emerging near-term application for millimeter–wave er: from small mm-scale wafers at ~1 W power to larger ~500 W technologies is for 5G WiFi. Although the power levels will be lasers with long coherence length (a key constraint in operating low because of the short-range requirement, mass manufacture an array). Cost elements include emitters, optics and amplifiers. of millimeter-wave transmitters and apertures may enable substantial cost reductions to be realized in the next few decades. Lasers are being used for LIDAR in autonomous vehicles and at powers of 10-100 W, cost of order $10,000 dollars. So There are several options for the technology of the laser beam- the contemporary value for lasers at low power is $100 –$1000

JBIS Vol 72 No.2 February 2019 53 JIM BENFORD & GREGORY MATLOFF

$/W. Figure 1 is Parkin’s extrapolation to larger numbers of units. (Fiber laser cost projections are based on 1 kW unit sold for $150k in 2003.)

The near-term applications we quantify here would need only small numbers of the sources and antennas/optics. The lasers in Table 1 would require 36,000 to 320,000 units of 1 kW [12]. COURTESY KEVIN PARKIN fiber lasers. The millimeter-wave Beamer of Table 2 would require a few thousands of sources, which are currently available commercially at $6/W.

10 SAIL COST

The large number of sails needed to provide a useful image of an exoplanet means that we must take into consideration the cost of sails. Previous studies of beam-driven sails have neglect- ed this cost because each sail will cost far less than the Beamer. Here we will estimate the cost of such sails. Fig.1 Unit costs of laser and gyrotron (millimeter-wave) sources We can estimate the cost of large numbers of sail probes by drop when they are built in larger numbers. realizing that these will be very sophisticated materials, possibly silica nitride, which will be processed to produce structures that allow the remarkable characteristics such sails require. where τ is sail fractional transmissivity, reflectivity is η. For the They will be several meters in size, so we have to estimate what case of an opaque sail, τ = 0 and emissivity reduces to 1-η. (Note the cost of microfabrication will be. A comparable system may that these relations are true only for ε, τ and η measured at the be contemporary high-end peripheral processor units (PPU), same wavelength. In general, the infrared emissivity will peak which come in sizes of about a square centimeter. Costs of con- at a different wavelength than the Beamer wavelength.) Using temporary Intel microprocessors run from a few hundred to the Stefan-Boltzmann Law [15], the sail radiates power to at a thousand dollars, with areas of 20-60 square centimeters. A least equal the power density from the beam, at temperature: 2-meter sail could cost approximately 0.1-1 M$ each. There- fore 100 sails would be $10 M$ to 100 M$ and a thousand sails (6) would cost $100 M$ to 1 B$. And the requirement for an equal number of occulting spacecraft drives the cost higher, perhaps by a factor of 2. Clearly, the acceleration is temperature-limited, ~T4, even at speeds of ~100 km/sec. (This fact means that solar sail materi- Note that the cost of fabrication will depend upon the fea- als, which have low melt temperatures (Al, Be, Nb, etc.) cannot ture size required on the sail. The micron laser scale, the milli- be used for fast beam-driven missions. For example, aluminum meter-wave scale and the centimeter scale of microwaves differ has a limiting acceleration of 0.36 m/s2, which is <4% of grav- very much and therefore their costs will differ a lot and should ity.) The invention of strong and light carbon mesh materials be substantially lower. Therefore we cannot extrapolate laser has made laboratory sail flight possible because carbon has no scale costs to the millimeter and microwave. liquid phase, and sublimes instead of melting. Carbon can operate at very high temperature, up to ~3000 K, and graphene We conclude that cost of sails and the occulting spacecrafts, could well operate above 4000 K. in terms of production, launch and mission control will be non-negligible compared to the Beamer. Table 4 shows the temperatures of the various concepts, as- suming 1% absorption, α= 0.01, reflectivity η = 0.9, therefore 11 SAIL THERMAL ISSUES emissivity = 0.1. For lasers, temperatures are moderate, ~1,000 K. For millimeter waves, T ≈ 3,000 K, which may be above the The above examples assume perfect reflectivity, but in reality melting point of graphene. Therefore lower absorption, per- some reflection and absorption can occur. What is the tem- haps α= 0.001, is required. For microwaves, temperatures are perature the sail material will have to sustain if the reflectivity very high, so extremely low absorption is necessary. is not perfect? As noted in other work, for a sail that absorbs some fraction of the beam power, the incident power density of 12 COMPARING BEAMERS the beam on the sail, and hence the acceleration, will be limited by the maximum allowable temperature of the sail [13]. In Table 3 we summarize the capital cost, power densities on the sail and sail temperatures for the 5 Beamers we have quantified. Of the power incident on the sail, a fraction αP will be ab- The basic points are: sorbed on area A. In steady state, this must be radiated away • Launching to ~100 km/sec requires a Beamer system cost from both sides of the film, with an average temperature T, by on the order of $1 billion at present costs. the Stefan-Boltzmann law: • Costs of Beamer technologies, both radiation sources and (4) apertures, must be sharply reduced by new technologies and/or economies of scale. where σ is the Stefan-Boltzmann constant and e is emissivity, • Millimeter wave costs are comparable to lasers with pres- defined for the general case of a partially transmissive sail [14]: ent-day costs if economies of scale occur in the millimeter technology. (At present no widespread application is evident (5) for these sources.)

54 Vol 72 No.2 February 2019 JBIS INTERMEDIATE BEAMERS FOR STARSHOT: Probes to the Sun’s Inner Gravity Focus

TABLE 4 Comparison of Beamers for Starshot Intermediate Missions Beamer Capital Cost Power density on sail Sail Temperature, α = 0.01 Lasers with Present Day Costs $4.25 B 1.07 MW/m2 1,000 K Lasers with Optimistic Costs $0.73 M 1.83 MW/m2 1,100 K Millimeter wave with Present Day Costs $41 B 173 MW/m2 3,500 K Millimeter wave with economies of scale $2 B 77 MW/m2 2,900 K Microwave $0.58 B 1.47 GW/m2 6,000 K

• Reducing the cost of power will be more important than 1) The requirement for on-board propulsion to maneuver to reducing the cost of antennas: intercept the very narrow cone of greatly focused electromagnetic energy from a celestial object occulted by the Sun, at a (7) distance >550 AU from the Sun [14]. Could a suitable deep- space engine (such as the radioisotope-electric drive) be suf- • The microwave aperture is large, as is the power, and that ficiently scaled down to fit within the 1.5-gram payload mass? makes it less cost competitive than the millimeter-wave devices, if we assume economies of scale for the latter. 2) Perhaps the greatest disadvantage of using a Starshot Beam- • Power density on the sail varies substantially among the er to direct a probe to the Sun’s gravity focus to study Proxima Beamers, with the lasers giving the lowest sail temperatures, as- b Centauri and other hypothetical planets within the Centau- suming the same fraction of incident power is absorbed. How- ri system is logistics. Because the anti-Alpha-Centauri point ever, absorption will actually be very technology (and wave- of the celestial sphere is on the other side of the Earth from length) dependent. Most of the above temperatures might be the Centauri system, a second northern-hemisphere location realizable with development. would be required as the site for the required laser array. High • ~100 km/sec intermediate missions give such a small altitude, dry locations improve performance but are not essen- Doppler shift, it's likely that materials such as silicon nitride tial. Alaska provides several good locations, with short distanc- can be engineered to produce essentially perfect reflectivity es between low loss beam facility sites and the sea. (α~10-5) at a specific frequency. Therefore temperature may not become an issue for near-term sails. Acknowledgments We gratefully acknowledge funding of the Breakthrough Star- Even if one or more of these design possibilities are adopted, shot project by the Breakthrough Prize Foundation and tech- there are additional disadvantages to using Starshot to launch nical discussions with Kevin Parkin, Geoff Landis, Paul Gilster laser-boosted ‘wafersat’-type probes to the Sun’s gravity focus: and Gregory Benford.

REFERENCES

1. P. Lubin, “A Roadmap to Interstellar Flight”, JBIS, 69, 40 (2016). 8. E. Ancona and R. Ya. Kezerashvili, “Orbital Dynamics of a Solar Sail 2. “Direct Multipixel Imaging of an Exo-Earth with a Solar Gravitational Accelerated by Thermal Desorption of Coatings”, IAC-16-C1.6.7.32480. Lens Telescope”, Slava G. Turyshev et al., JBIS 71, pp. 361, 2018. Presented at 67th International Astronautical Congress, Guadalajara, Mexico, 26 September, 2016. 3. G. A. Landis, “A Telescope at the Solar Gravitational Lens: Problems and Solutions”, JBIS 71, pp. 369, 2018 . 9. K. Parkin, “The Breakthrough Starshot System Model", in press, Acta Astronautica 2018. 4. G. Vulpetti, “Sailcraft-based Mission to the Solar Gravitational Lens,” Space Technology and Applications International Forum (STAIF-2000), 10. J. Benford, “Starship Sails Propelled by Cost-Optimized Directed Albuquerque NM, AIP Conference Proceedings 504, 968 2001, doi: Energy”, JBIS 66, pg. 85 2013. http://dx.doi.org/10.1063/1.1290893 11. J. Benford, “Space Applications of High Power Microwaves”, IEEE Trans. 5. G. Benford and J. Benford, “Desorption Assisted SunDiver Missions”, on Plasma Sci. 36, 569, 2008. Proc. Space Technology and Applications International Forum 12. K. Parkin, private communication, 2011. (STAIF-2002), Space Exploration Technology Conf, AIP Conf. Proc. 13. G. A. Landis, “Microwave Pushed Interstellar Sail: Starwisp Revisited,” 608, ISBN 0-7354-0052-0, 462, 2002.. paper AIAA-2000-3337, 36th Joint Propulsion Conference, Huntsville 6. G. Benford and J. Benford, “Acceleration of Sails by Thermal Desorption AL, 2000. of Coatings”, Acta Astronautica, 56, 593 2005. 14. G. L. Matloff, Deep-Space Probes, 2nd ed., Springer-Praxis, Chichester, 7. R. Y. Kezerashvili, “Space exploration with a solar sail coated by UK 2005. materials that undergo thermal desorption”, Acta Astronautica 117, 231 15. G. L. Matloff, “Hesperides: Solar/Nuclear missions to the Sun’s gravity 2015. focus,” Acta Astronautica, 104, 477 2014.

Received 12 September 2018 Approved 23 February 2019

JBIS Vol 72 No.2 February 2019 55 JBIS VOLUME 72 2019 PAGES 56–61

REALITY, THE BREAKTHROUGH INITIATIVES and Prospects for Colonization of Space

EDD WHEELER 3598 Midvale Cove, Tucker, GA, 30084-3208, USA

Email [email protected]

Although Einstein considered truth as independent of human beings, reality for us is tied to human experience and the five senses. It is the context in which lives, aspirations, and plans develop or fail. Understanding reality is essential in laying markers for progress or lapse in momentous undertakings such as the Breakthrough Initiatives begun in 2015 and in man’s possible attempt to colonize space. This article examines the progress made in these two endeavors in context of reality and dangers. Breakthrough Initiatives to date has resulted at best in moderate success, but has yet to run half its at least decade-long course. Emphasis has switched from its unfocused Breakthrough Message program to the “Starshot” space probe, intended to launch by 2050 scores of tiny sail-craft, propelled by coordinated emissions from a field of lasers, to unprecedented velocity of .192 c. In a 22-year journey to nearest star system, Alpha Centauri, nanocraft will investigate if exoplanet Proxima Centauri b is habitable or has identifiable life. Among the challenges is whether craft can withstand the many thousands of g (gravitational) forces exerted during acceleration. Sustaining public interest for 26 years between launch and receipt of data may be difficult. A brief and compelling statement by the Initiatives to the public is needed on why space exploration is in their interest and how it could be decisive in the future of mankind.

Keywords: Objective Reality, Breakthrough Initiatives, Exoplanet, Space Colonization, SETI

1 INTRODUCTION tificial intelligence, which, however cunning or immeasurably distant from the universe, would be subject to cosmic events like Paul Davies’s view, accepted by many scientists, is that “science supernovae, large asteroids, and other primal forces of nature. is empirical, and…must be grounded, somehow, in reality [1],” although notably Stephen Hawking disagreed in asserting: The above being stated, it should be acknowledged that Teg- “I don’t know what it [reality] is. Reality is not a quality you mark’s suggestion of uncertainty that the laws of science, as we can test with litmus paper. All I’m concerned with is that the know them, may not be universal occurred also to the British theory should predict the results of measurement” [2]. Thus, astronomer and cosmologist Fred Hoyle, who once entered a the borders of this paper - physical reality, consciousness, a specific editorial footnote that, while Einstein’s theory of rel- worldwide scientific monitoring for potential signals from in- ativity states “no material body in our particular locality can terstellar space, and plans for possible colonization of planets move faster than light; a distant body is not so limited” [5]. and investigated star systems - are set as lacking unanimity, and countered by plausible arguments of theorists and multiverse As the following will show, physical reality is the context of advocates that not everything in the universe is measurable or, existence, despite occasional ambiguities of its sources or di- even if quantifiable, may not with certainty be universal. mensions and the “somewhat chaotic and often mutually in- consistent pictures” confronting us [6]. But in the objective Max Tegmark proposes four types of multiverse scenarios world and in human pursuits, including the Breakthrough “somewhere within an infinite stack of parallel worlds”: “‘Lev- Initiatives and future efforts to colonize space, physical reality el IV’ multiverse contains completely disconnected universes, will be knowable through consciousness as formed by human governed by different laws or mathematical structures…[with] experience and senses. Roger Penrose importantly observed assumption…that any mathematically possible universe must there seems “something non-algorithmic about our conscious exist somewhere” [3]. However, Tegmark’s theory is not widely thinking” and that ability to “intuit…truth from falsity…in ap- accepted by scientists, and generates little interest either within propriate circumstance…is the hallmark of consciousness” [7]. Breakthrough Initiatives or feasibility studies on man’s efforts toward colonizing space. The theory lacks support due to what 2 REALITY Thomas Huxley called “The great tragedy of Science – the slay- ing of a beautiful hypothesis by an ugly fact” [4], that fact being Richard Dawkins believes that we “know what is real in one of the implausibility that laws of physics and other sciences do not three ways…directly, using our five senses, or indirectly, using apply elsewhere in either the universe or hypothetical “multi- our senses aided by…instruments such as telescopes…[and verses,” whether domain of mortal creatures or conceivably ar- that]…other things like hatred or love are real enough but de-

56 Vol 72 No.2 February 2019 JBIS REALITY, THE BREAKTHROUGH INITIATIVES, and Prospects for Colonization of Space pend for their existence on the brain.” “Ultimately,” he writes, space journalists, one of whom reports the number of estimated “it always comes back to the senses” [8]. Dawkins and co-au- planets in the Milky Way “could outnumber its stars, and [the thor McKean are hardly alone in attributing great importance galaxy is] home to billions of potentially habitable worlds” [17]. to the senses in formation of perceptions leading to our consciousness of physical reality; however, they fail to acknowl- However, despite Hawking’s similar and more informed edge the essentiality also of human experience in discerning view, broad optimism seems not yet supported by scientific evi- what is real. dence. Findings opposing optimism were published recently by a team of three scientists who pointed to “the apparently lifeless Other scientists and thinkers specifically include experience universe we in fact observe,” and that “incorporating models of as bedrock in forming the awareness of reality. Alfred White- chemical and genetic transitions on the paths to the origin of head posited that “actualities of the Universe are processes of life…[shows] extant scientific knowledge [which] corresponds experience, each process an individual fact” [9]. Pragmatist to uncertainties that span multiple orders of magnitude” [18]. Francis Bradley wrote that “seeking for reality, we go to experi- Their conclusion: “there should be little surprise when we fail ence…What we discover…is a whole in which distinctions can to detect any signs” of ETI. be made, but in which divisions do not exist” [10]. This paper will not attempt to weigh truth in Bradley’s assessment, but em- Although the Initiatives have a database of more than 3800 phasizes that, in human thought, summations are not always known exoplanets, with near- term plans by NASA and the EU conclusive or permanent. As the great pragmatist, in psycholo- to catalogue thousands more, Breakthrough Initiatives’ route gy and philosophy of science, William James, acutely inquired, toward success confronts five hard realities. “Is the Kosmos an expression of intelligence rational in its inward nature, or a brute external fact pure and simple?” [11]. i. The uncomfortable fact is that, despite tens of thousands of star systems monitored since 2015 by the 100-meter Nation- A cursory aside on reality might illumine. Bradley seems al Radio Astronomy Observatory in West Virginia, Australia’s correct in assigning human experience, as filtered by intelli- 64-meter Parkes radio telescope in New South Wales, and the gence, to be essential in understanding reality, more vital even University of California Lick Observatory’s three-meter reflec- than sensory perceptions. Second, though pervasive through- tor telescope and 2.4-meter Automated Planet Finder, after out both mind and matter, reality is in flux and ultimately tran- over three years of wide effort, not a single credible signal from sitory. The same is true for its place in the mental process. James space has been recorded. The silence experienced in search for reminds that “Thought is in constant change” [11]. Hence, both ETI (SETI), begun in 1966 with Carl Sagan’s book Intelligent human thought and its products, such as plan or design, are Life in the Universe, enters its sixth decade. What has been de- perishable. Regarding the material world of molecules, J. Rob- risively, if unfairly, called the Great Martian Chase continues. ert Oppenheimer warned that “Matter, as we know it, consists of impermanent objects.” [12]. ii. There is also the bothersome fact that, while NASA presently has confirmed more than 3800 exoplanets [19] and 3 THE BREAKTHROUGH INITIATIVES similar success (over 3500 exoplanets) is registered by EU catalogue [20], it remains true that exoplanet Kepler-452b, at An exoplanet is an Earth-like planet in a distant star system’s distance of 429.45 parsecs and discovered in 2015 using four “habitable zone,” and has rocky interior and surface conditions years of NASA’s Kepler space telescope data, is the only exo- favorable for liquid water. At beginning of 2015, after moni- planet to date identified as the “closest twin” of Earth outside toring over 150,000 stars and assorting more than 4000 exo- the solar system [21]. planet candidates for further study, NASA’s Kepler Space Tele- scope verified its 1000th planet beyond the solar system [13]. iii. Although the Initiatives’ “Starshot” space probe, planned By year’s end, European Exoplanet had catalogued 1919 plan- to launch by 2050, will be to planet Proxima Centauri b, clas- ets, but only one as possibly Earth-like, Kepler-186f in 2014. sified in the habitable zone of nearest star Proxima Centauri The NASA Exoplanet Science Institute had listed 1832 planets, (distance of 1.34 parsecs), and Proxima b is expressly referred with just 30 identified as “potentially habitable” [14]. Within to as “a viable candidate habitable planet,” the exoplanet none- this background of growing numbers of known planets, the theless receives “30 times more EUV radiation than Earth and Breakthrough Initiatives were announced at London’s Royal 250 times more X-rays,” as well as more than 2000 times more Academy in July 2015. A letter by many of the world’s leading “stellar wind pressure” from solar winds [22], suggesting that scientists was presented, strongly endorsing need for the Initia- Proxima b seems a marginal location to develop or sustain an- tives. Among its co-signers was Stephen Hawking, who set the ything resembling intelligent life as we know it. tone for the historic quest: “In an infinite Universe, there must be other life. There is no bigger question. It is time to commit iv. Discovery in early 2017 of the Trappist-1 star system’s sev- to finding the answer” [15]. en exoplanets, at 12.6 parsecs, stirred interest and reveals the system may have conditions favorable for liquid water and like- Funded for at least a decade by Yuri Milner, Russian billion- ly has planets with “thick atmospheres, oceans, or ice crusts” aire and physicist, Breakthrough Initiatives uses three of Earth’s [23]. A study finds that the atmosphere of Trappist planet b ex- most powerful telescopes to monitor for possible Extra-Terres- ceeds “the runaway greenhouse limit,” five planets “are unlikely trial-Intelligence (ETI) signals from the million nearest stars to have an enriched atmosphere (e.g. CO2) above a bare core,” and closest 100 galaxies. The Initiatives monitor three major and two planets probably “have rocky interiors” [24]. However, programs: Breakthrough Listen, Breakthrough Message, and nothing indicates that Trappist-1 has planets resembling even a “Starshot” space probe, the first two structured to investigate distant cousin to Earth, much less a potential “twin.” closely whether ETI exists. Max Tegmark estimated “probably at least 1020 habitable planets in our Universe alone” [16]. v. It may concern the Initiatives’ leaders and advocates to His optimism regarding this vast number perhaps transferred have received wide publicity on their efforts for more than to high expectation for exoplanets. The optimism is shared by three years, but not yet achieving the informed debate, inter-

JBIS Vol 72 No.2 February 2019 57 EDD WHEELER national or otherwise, which by announcement the program sought for its Breakthrough Message sector. Whether a signal from Earth is to be structured and possibly transmitted has not been discussed by the Association for the Advancement of Sci- ence, the National Academy of Sciences, the Royal Academy of London, the Aspen Institute’s International Partners program, or by other recognizably influential groups. Nor has there been discernible effort to consult with India and China, which col- lectively represent more than one-third of Earth’s population, in the Breakthrough Initiatives’ charting of a program that manifestly could affect humankind. Fig.1 Breakthrough Initiatives’ “Starshot” Nanocraft (illustration by Dennis McCabe). It is not to scale: the sail (4 x 4 m) is over 31,700 Starshot’s robotic spacecraft will be exceedingly small, and times larger in area than the postage stamp-sized microchip (21 x was designed to weigh one gram [25], but since changed to per- 24 mm) at the center of the craft. haps four grams [26]. At center will be a sail-mounted microchip, postage stamp in size but containing an atomic battery, sub-gram photon thrusters, sensors, computer and micropro- pact events such as large asteroids, and expressed concern also cessors, miniature megapixel cameras, and transmission radi- about climate change, GM (genetically modified) viruses, and os [25]. The microchip (504 square mm) is surrounded by sail nuclear war. In 2016 he gloomily warned on BBC that plane- of about 16-square meters, highly reflective (.99995), made of tary disaster “becomes a near certainty in the next thousand or graphene or graphene-like material 50 nm thick and designed 10,000 years,” giving time for humans to “spread out into the to weigh a half gram. A 35 GW laser beam on sail will be the cosmos,” and noted that “We will not establish self-sustaining photon “wind,” accelerating the craft for 247 seconds to 57,500 colonies in space for at least the next hundred years, so we have km/second. During acceleration the photon wind will be pro- to be very careful in this period” [31]. vided by a large laser array on Earth that is 4 x 4 km in area [25], thereby subjecting a 4-gram craft to gravitational force “of Hawking’s alarm and pessimism are not shared by scien- the order of 60,000 g” [27], at which time the spacecraft would tists like Edward Wilson, Freeman Dyson, and Leeds Uni- have an equivalent mass of 240 kg (four time the resultant mass versity geologists who conducted a 2018 measurement of ice of an accelerating one-gram craft). melt in Antarctica. Wilson, preeminent biologist and perhaps the world’s foremost naturalist, states optimistically: “It is an The engineering challenges are enormous to produce a na- especially dangerous delusion if we see emigration into space nocraft, along with durable sail designed only atoms thick. as a solution to be taken when we have used up this planet… Graphene is not reflective and reflective materials such as met- Earth, by the twenty-second century, can be turned…into a als currently are ruled out as too heavy [28]. However, since permanent paradise for man” [32]. Dyson, who has written sail reflectivity will key in achieving .2 c velocity (215.8 million for more than four decades on potential colonization of space, km/h), it may prove essential to construct the sail of a metal foil emphasized early that multi-billion-dollar space ventures like- such as titanium or, if feasible, to apply a reflective micro-coat- ly are not the best approach. He has long advocated that “it’s ing of titanium on the sail, even at disadvantage of increasing its possible to go into space on a much smaller scale. A cost on the mass. While a reflective metal would increase weight, it should order of $40,000 per person would be the target to shoot for… add to needed strength for the craft to withstand hyper-G forc- that would make it comparable to the colonization of America” es during acceleration. For example, a .02 mm thick titanium [33]. He added perceptively: “…if you give people the choice of foil as sail or a titanium micro-coating of same thickness to being brothers or going into space, that could provide the im- the sail would increase spacecraft weight about tenfold (45.4 petus for colonization…often in the past a journey that looked grams) [29]. However, it should significantly increase probabil- like exile from one point of view has turned out to be an oppor- ity that most of the perhaps scores of nanocraft to be launched tunity from another.” Few probably share Dyson’s preference (one from the southern hemisphere every 24-48 hours [25], for asteroids as among the best sites for space colonies, but his presumably for months) will eventually reach exoplanet Prox- incisive knowledge of mathematics and astrophysics, and intu- ima b. itiveness about human motivation, were remarkable 40 years ago and remain so. The Starshot space mission seems boldest of Initiatives’ endeavors. The potential bidders and contractors were provided Before his death in 2018, Stephen Hawking repeatedly en- comprehensive instructions on what is expected, none being dorsed commercial space travel, advising others to “look to the stated more clearly than the imperatives to “Launch within 30 stars, not down at our feet” [34]. He cited global warming as years at an affordable cost” and “GO FAST!” [25]. a major and imminent threat to humans: “We are close to the tipping point when global warming becomes irreversible… 4 PROSPECTS FOR COLONIZATION OF SPACE [pushing] Earth over the brink, to become like Venus, with a temperature of 250 degrees, and raining sulfuric acid” [35]. The most influential proponent for space colonization was Ste- However, Hawking’s alarm about a near-future, irreversible phen Hawking, who for more than two decades prior to his “tipping point” in Earth’s climate is not supported by recent re- death, repeatedly urged that mankind faces grave threats, in- search and measurements of Antarctic ice melt [36]. Combin- cluding those from development of artificial intelligence: “The ing satellite observations of the Antarctic Ice Sheet, 1992-2017, genie is out of the bottle…I fear AI may replace humans al- scientists documented that it “lost 2.72 +/- 1.39 billion tonnes together” [30]. Hawking also warned of mass extinction “per- of ice…which corresponds to an increase in mean sea level of haps sooner rather than later,” and urged that we begin serious 7.6 +/- 3.9 millimeters (errors are one standard deviation)… thought about relocating permanently to other planets. He be- ocean-driven melting…for West Antarctic increased from 53 lieved the chief threats are from low-probability but high im- +/- 29 billion to 159 +/- 26 billion tonnes per year…and for

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East Antarctic, with average rate of…5 +/- 46 billion tonnes expressed by Bertrand Russell, who warned that man “is a per year being the least certain.” This data from a 2018 article ferocious animal,” and “we had better leave the moon in peace. in Nature obviously is imprecise in part, but appears to raise As yet, our follies have been only terrestrial; it would seem a no emergency warning that Earth will soon wither to multi- doubtful victory to make them cosmic” [41]. Whether perspec- ple Sahara Deserts or devolve to a shrouded planet like Venus. tives on space such as those of Eiseley and Russell in the future Space colonization pioneer Gerald O’Neill at Princeton Uni- prove wise or troublesome for colonizing is not indetermina- versity was among the first to write serious proposals for space ble, merely as yet unknown. colonies. By 1974 he attracted national attention, and afterward recommended ways to construct large “almost Earth-like col- In attempting to look to the future of Breakthrough Initia- onies” from the materials and energy found in space. O’Neill tives, SETI, and space colonization, a glance backward is ap- envisaged that “By 2050, some two hundred million people propriate. Although not designed to further SETI, the Initia- may be making annual trips out into space and back again… tives’ Breakthrough Listen program is of great interest to SETI, [some] who will maintain two homes, one in space and one on which has failed to date for reasons set forth by several critics. earth”[37]. Although his prediction seems now in the distant An American astronomer pointed out over 16 years ago that future, it was ingenious and may yet be realizable: self-suffi- SETI labored under the probable misconception that Earth was cient colonies, in size ranging from about 1000 to maybe 50,000 still unknown by ETI, when in reality our planet was almost people, living as families in aluminum spheres about a mile certainly identified as a “living world” long before evolution in diameter, with two immense windows to reflect abundant of mankind [42]. The fact that no contact has been made may sunlight for solar-power energy to industry and agriculture, in be similar to reasons humans do not attempt to communicate order to enable a self-sustaining space colony. The windows, with wildlife on the Serengeti. along with addition of what O’Neill simply called “five to six feet of plain old dirt,” would prevent entry of lethal cosmic rays. Two other notable critics observed that SETI “is ill-founded and…will yield only negative results” because of flawed reli- There is a quality at once commonsensical, pioneering, en- ance on “beamed broadcasts” and listening stations devised by trepreneurial, and familial in O’Neill’s vision of man’s future in human intelligence [43]. They seemed doubtful that man pos- space, which is distinct from somewhat grandiose plans, or im- sesses the required intelligence to conceive effective signaling aginings, for staged periods by decade and involving perhaps or to carry it through. In brief, they essentially proposed that thousands of passengers in behemoth “worldships” and mining humankind exhibits little value for the “Intelligence Principle” craft, capable of .025c (7500 km/s) cruising speed, each “with articulated by Steven Dick, astronomer and NASA chief histo- dimensions on the order of a kilometer in length and…[assem- rian for seven years until 2009, whom they quote: “the main- bled by fleets as] multi-million-tonne vehicles of the cruiser tenance, improvement, and perpetuation of knowledge and worldship size, and…multi-billion-tonne naturalistic world- intelligence is the central driving force of cultural evolution, ships,” all organized for “formation flying…[or] to physically and…to the extent intelligence can be improved, it will be im- dock worldships side by side…[through] delicate manoeuvre” proved” [44]. Other similar reservations have been expressed [38]. about SETI and its founding concept, which was evaluated as “a failure of imagination” in making “the scope of their project… The purpose of these titan-like, futuristic craft presumably too narrow” [45]. Its initial and fundamental mistake was will be to enable billionaire investors like Elon Musk and Jeff judged to be failure “to reach out to the scientific community” Bezos to mine other planets and eventually to establish col- for in-depth discussion of the project onies on them. Musk, “a ravenous reader of science fiction,” intends such colonies as necessary for humans to become “a 5 CONCLUSION multiplanet species, and create a backup hard drive for the human race” [39]. He believes the “best bet” is colonization of SETI doubtless has been too criticized and wrongly lampooned. Mars. Individuals such as Musk and Bezos are called plutocrats Its wide ambitions, though, are hardly more aspiring than by some, but considered as well deserving of esteem by most those of Breakthrough Initiatives, which enjoys much greater of society. scientific and financial support than did SETI. Whether public enthusiasm and backing for the Initiatives will be sustained The great evolutionist Loren Eiseley, however, would hard- throughout the coming half century, awaiting the anticipated ly have been among admirers. He pointedly wrote that it was results of Starshot’s mission, is not at all certain. The reality is “lengthened family relationship between adults” that gave rise that neither is man’s future, in space or generally, beyond ques- to man and “enables us to call him human” [40]. Thus, he finds tion. Bertrand Russell perhaps was right in part by assessing it both ironic and dangerous that this same “creature professes that mankind thus far has survived often through “ignorance to pierce the sham of life and to live by tough-mindedness… and inefficiency…[with] aims no more lofty than those -pur [including man’s] desire to fly away to Mars, still warring, still sued by tyrants in the past.” He predicted man will become ex- haunted by his own black shadow…the adolescent escape tinct if “we develop cleverness without wisdom” [41]. mechanism of a creature who would prefer to infect the outer planets with his problems than to master them at home” [40]. Amid uncertainties, this article offers no roadmap for as- Though his view is probably shared by few scientists, we could sured scientific success. Instead, it closes with recommendation acknowledge that many scholars, serious thinkers, and other to revisit relevant studies by Charles Darwin and a little-known reflective persons may agree to some extent with Eiseley, pos- modern scholar, Walton Hamilton. Darwin first proposed sibly even more so than with wealthy and prophetic advocates “natural selection” as the main determinative in survival of of “hard drive” colonization of space in order to preserve, by animal and plant life. However, he later embraced “the adap- their vision and hand, mankind as an identifiable multi-planet tationist explanation of evolutionary change,” which holds that or cosmic species. animals and plants that adapt best to natural surroundings are most likely to survive [46]. Hamilton showed that Darwinism Doubts similar to Eiseley’s about human intelligence were applies also as “rule for survival for the person” and for institu-

JBIS Vol 72 No.2 February 2019 59 EDD WHEELER tions and “that one must adapt or perish.” Indeed, he observes, for setbacks or failures. With expanded knowledge of poten- the “national economy itself is the creation of an endless stream tial “new worlds,” Breakthrough Initiatives could even lead the of adaptations” [47]. way in researching and addressing two unanswered and sel- dom-asked questions. First, to what extent, if any, will explora- The lesson, if any, in the foregoing is that neither the Initi- tion of interstellar space reveal reason and the laws of science atives nor space colonization is likely to follow the milestones as we know or construe them? Second, will deep space con- and progress currently planned. They inevitably will be altered sistently be shown as cold and empty regions of endless en- by realities encountered, whether anticipated or unexpected. ergy, and for man brutish matter seemingly unrelated to our For example, should “Starshot,” with planned launches at least concept of intelligence but dominant over it? Answers to these 30 years in the future, urge its scientists and contractors to “GO questions could transform our view of reality and perhaps also FAST”? Can tiny spacecraft, with transparently thin “sails” influence future intentions to colonize worlds which may prove merely atoms thick, withstand the tens of thousands of g forces to be totally alien and where man, lacking immunity, becomes to be exerted on them at launch without structural damage or hard pressed for success or even survival. disintegration? What will be the effect of -57 Celsius temperature on graphene? Will hundreds of lasers, in array across a Whether public support for space programs will continue is 16- square km area, effectively coordinate their firings to accel- uncertain. Establishing memorable and favored causes in the erate smallest of spacecraft, each propelled to distance beyond public memory can be difficult. The publisher of theInterna - the moon, at speed never previously envisaged by human engi- tional Herald Tribune once advised that the “need to condense” neering (215 million km/hour), without inflicting structural or an intended message in “very few words” is essential [50], and computer ware damages that compromise the mission? Could best achieved by reducing “them down into about 5%” of to- there be, in Elon Musk’s wry phrase, instances of “rapid un- tal wordage. Science is generally unconcerned with creating scheduled disassembly”? public support or devising clever phrases. However, if public support for the Initiatives is to be won, and especially retained In addition, who can begin to predict what NASA and EU possibly for decades, significant efforts are needed to form -co scientists may discover? NASA’s Transiting Exoplanet Survey gent thought in expression of a phrase that clearly establishes Satellite (TESS) telescope replaced the Kepler Space telescope in public consciousness why space exploration is compellingly in April 2018. Since 2009, Kepler found more than 2300 ex- important both to the national public and humankind. An im- oplanets, enabling astronomers to estimate the galaxy has “at proved phrasing of something perhaps in vein of the follow- least two billion potentially habitable planets.” NASA scientists ing could resonate with the public and might be considered by “hope to spot 500 Earth-size planets and perhaps 20,000 new Breakthrough Initiatives: worlds in total,” but their “real hope is that we start finding things that we didn’t expect” [48]. With four wide-field 16.8 DEEP SPACE: MANKIND’S GREAT STRIDE FORWARD megapixel optical cameras, TESS will “survey 85% of the sky – an area 400 times larger” than did Kepler [49]. Knowledge is power. It can also be exalting as well as hum- bling. If the Initiatives gather unexpectedly important informa- Through such improvements, in coming decades and pri- tion or insights, especially the type of knowledge in foregoing or to receiving the first substantive “Starshot” data, at least 50 paragraphs, they will have registered one of the most important years from now, it seems prudent to develop contingencies of possible signals to Earth.

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Received 20 November 2018 Approved 26 February 2019

JBIS Vol 72 No.2 February 2019 61 JBIS VOLUME 72 2019 PAGES 62–69

A GRAVITATIONAL WAVE TRANSMITTER

A. A. JACKSON & GREGORY BENFORD Triton Systems Houston Texas; Department of Physics & Astronomy, UC Irvine

Email [email protected] / [email protected]

We consider how an advanced civilization might build a radiator to send gravitational waves signals by using small black holes. Micro black holes on the scale of centimeters but with masses of asteroids to planets are manipulated with a super advanced instrumentality, possibly with very large electromagnetic fields. The machine envisioned emits gravitational waves in the GHz frequency range. If the source to receiver distance is a characteristic length in the galaxy, up to 10000 light years, the masses involved are at least planetary in magnitude. To provide the energy for this system we posit a very advanced civilization that has a Kerr black hole at its disposal and can extract energy by way of super-radiance. Background gravitational radiation sets a limit on the dimensionless amplitude that can be measured at interstellar distance using a LIGO-like detector.

Keywords: CETI, Gravitational Wave Transmitter

“My rule is there is nothing so big nor so crazy that one out of made it so, along with others. That resolve by Marconi provoked a million technological societies may not feel itself driven to do, a world we now enjoy. We now listen for signals from other provided it is physically possible.” minds across vast distances, using technologies similar to ours.

Freeman Dyson The Search for Extraterrestrial Technology 1965 Space-time is stiff, incredibly so. Producing the slightest tremors in it demands enormous amounts of mass-energy. 1 INTRODUCTION Even with LIGO’s state-of-the-art equipment, two ordinary stars orbiting each other don’t emit gravitational waves at a This paper attempts a preliminary estimate of radiated gravita- measurable level. To see gravitational waves demands close, tional wave signals. While this demands large sub-stellar mass- grazing interactions of neutron stars or black holes. es moving in close orbits or high speeds, it seems possible that this emission mechanism might be used by life forms whose Perhaps intelligences elsewhere now command a complex vast resources we cannot now envision beyond estimates, but energetic instrumentality and can send possible messages to whose signals we may register with an evolved detection tech- civilizations such as ours. Their motives we cannot know. Per- nology. haps they have reason to prefer to speak to those who have mastered the far more difficult task of sensing gravitational waves, The LIGO and the VIRGO detectors see black holes and compared to the vastly simpler detection of electromagnetic neutron stars merging, using software templates derived from signals in a myriad of possible wavelengths. This we now have. detailed, strong relativistic calculations. Pulling a good signal out of the vast sea of noise demands filtering from the many This study stands in the tradition of Dysonian ideas. More sources of noise. In future, gravitational wave astronomy will than a half-century ago, Freeman Dyson proposed that SETI combat such noise problems with ever-more detailed methods agendas should look at technologies for harnessing an entire to tease out even fainter signals. Perhaps, for motives we can- star’s energy, building on ideas of the legendary writer Olaf not well imagine, other smart beings will even encode signals Stapledon. Dyson suggested that we should look not only for in the gravitational waves that might wash through our space- signals, but for side effects like infrared emission, on scales that time every moment. do not contradict physical law, but are beyond conceivable human engineering. We do similarly here for signals in gravita- A similar possibility emerged in the 19th century, after Max- tional waves emitted for a purpose, following on the SETI ideas well predicted electromagnetic waves moving at the speed of evolved since the 1950s. light and Hertz, in a simple experiment using electrical circuits, detected them in radio wavelengths. Hertz thought sending sig- 2 GRAVITATIONAL WAVES AND KARDASHEV nals would never happen; his waves were too weak and diffuse CIVILIZATIONS in spectrum, he thought. An Italian teenager heard of Hertz’s remarks and thought of sending messages with the waves and If one supposes that a civilization sends signals using gravita-

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TABLE 1: Advanced civilization transmitter located at 10,000 light years Dimensionless Mass converted to Kardashev Scale Gravitational Wave Amplitude h Energy (ergs) Civilization K [2] Receiver 10-22 ~0.1 Earth Mass 3.6 LIGO at 100 Hz 1027 grams 10-25 ~ mass of Ganymede 3.0 Advanced Gravitational Wave ~1026 grams Detector ~1GHz 10-33 ~ The mass of asteroid Ida ~ 2.4 ‘Planck’ Length Detector 1017 grams tional waves, there are two problems to be solved: The trans- acting masses. An example could be a small mass, m, falling mitter, and the receiver. The LIGO receivers have seen grav- or orbiting the big mass M. The factor f is a function of mo- itational radiation from natural objects. As a gravitational tion in the system and the and angular momentum, L, of a M. wave passes through matter it can change its geometry, namely The factor f ranges between .01 for falling in, to .5 for orbiting a characteristic length. If one measures a length L and it re- a Schwarzschild black hole to 2.0 for orbiting a rotating black sponds to a gravitational wave by ΔL, the ‘strain’ is measured hole. by h= ΔL/L. This dimensionless amplitude is very small indeed, due to the weakness of gravitational waves. LIGO can measure If one arrives at a circular orbit with excess kinetic energy h to the value of 10-22, or in approximate physical terms 1/1000 one inserts a Lorentz factor the diameter of a proton. This would be gravitational synchrotron radiation [3], with Physically, h is related to the transmitter by h~ΔE/r where the small mass injected into an orbit about the big mass. Taking ΔE is a burst of gravitational radiation energy and r is the dis- f = 1, [3, 4, 5]: tance from the transmitter. Take ΔE as the amount of energy produced in annihilation of a mass m, namely mc2, and take the (3) distance of the transmitter to be 10,000 light years. The amount of energy produced can be related to the characterization pa- To generate the radiated energy m needs to be ‘deep’ into rameter specified by the Russian scientist Nikolai Kardashev the region near M’s Schwarzschild radius, rs. Suppose this [1] (the amount of energy available to a civilization). Energy advanced civilization has found two primordial black holes, types are characterized by scales such as Type 1 ‘planetary’, PBH, of one Earth Mass and one tenth Earth mass. Say the Type 2 ‘stellar’ and Type 3 ‘galactic’. Let K 1, 2 and 3 denote Earth mass PBH is in orbit about the civilization’s home star these civilizations. Table 1 shows a calculated correspondence and they can maneuver the .1 earth mass black hole. (Remem- between dimensionless amplitude and amount of energy pro- ber we are talking advanced civilization here, maybe Kardashev duction for civilization located at 10,000 light years. 3.) One model of a transmitter could be a small mass m injected into orbit about a large mass M. The geometric size of the The annihilated mass is given in grams and representative small black holes can be computed from their Schwarzschild objects. At present LIGO could possibly detect a Type 3 plus radius, [3, 4, 5]: civilization at a distance 100 light years, but presently only in the frequency range of ~100 Hz. A more plausible signal may (4) lie in the GHz range. (Note: The Kardashev Scale ‘number’ is calculated from Sagan’s expression K= (log10P-6)/10, [2], where Let M be a small black hole of one earth mass which has P is power in watts, taken here to be generated in one second.) characteristic size of 1 centimeter, let the small mass m be of size .1 centimeters (that ~1027 and 1026 grams). To get the 2 GRAVITATIONAL WAVE PRODUCTION most energy out, or put another way the best amplitude at a receiver a long distance away, the small mass needs to orbit 2.1 Gravitational radiation process 1 as close to M as possible. The closest distance would be at the photon sphere, rp = 3rs. This is the radius, for a Schwarzschild Suppose an advanced civilization makes a device to transmit black hole of a circular photon orbit, which cannot be done information via gravitational waves. Communication means by a massive particle. Now use an aspect of motion in general reception as well as transmission. The LIGO experiment uses relativity, in which an unbound particle injected at a critical an observable parameter called dimensionless amplitude, h, impact parameter in a region less than 10 rs will experience given by [3, 4, 5]: non-Newtonian motion. A mass on an energetic unbound trajectory injected into a close orbit about a black hole can do (1) many orbits and then return to infinity. Aim the particle (of velocity v) at the last critical orbit, with β=v/c, the critical ra- where ∆E is the radiated gravitational energy and r is distance dius is given by, [6, 7]: between observer and radiator. (G is the gravitational constant and c the speed of light.) (5)

A fast way to estimate the radiated gravitational energy is given The aim point is determined by a critical impact parameter by [3,4,5,6]: given by [8]: (2) (6) where is m is a small mass << M, and m and M are the inter-

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These expressions are in units of the Schwarzschild radius. If TABLE 2: Parameters for GW Machine 1 the inbound orbit just shaves the critical orbit rc then it will Distance light years 1000. make turns about the central mass M. Mass 1 M (in Earth mass) 1 For a given injection energy the number of turns at the crit- Mass 2 m (in Earth mass) 0.1000 ical radius can be calculated. The critical impact parameter is Gamma γ (Lorentz Factor of orbit) 10.00 the aim point at which the small mass goes into ‘winding’ orbit Beta β (v/c) 0.9950 radiating gravitational waves. The orbits at the critical radius Schwartz radius of M (cm ) 0.8860 are unstable; after a number of turns the small mass returns to infinity. The amount of radiation can be approximated as grav- Schwartz radius m (cm ) .08860 itational synchrotron. If the impact parameter is b = bc (1+ δ), Impact Parameter (in rs ) 5.187 where δ is a very small displacement. The number of orbits can Critical radius (in rs ) 3.002 be approximated by [9]: Impact parameter delta E-07 (7) Winding Number estimate 10.52 Kinetic Energy Incoming (ergs) .5377E+49 At rc the orbital frequency is: Orbital Energy – Circular orbit (ergs) .4781E+48 GW frequency GHz 3.329 (8) Circular orbital period (seconds) .1039E-08 and the frequency of the gravitational radiation is then: Estimated GR Energy radiated at critical .2523E+47 orbit (ergs) (9) Dimensional Amplitude h, at 1000 Ly .8789E-23

rs = Schwarzschild Radius and the orbital period: (10)

Take the energy radiated to be enough to excite the LIGO 2.2 Gravitational radiation process 2 non-dimensional amplitude at 1000 light years. However the gigahertz frequency is higher than any current gravitational Orbital manipulation of earth mass mini black holes may be wave observatory. possible for a K3 civilization, but is extremely costly in energy. A second ‘machine’ suggests itself – an accelerated mass. Mag- The process would begin by injecting (using Kardashev 3 giore [8] calculated gravitational radiation of a mass under ar- technology) a small mass m into an unbound orbit about M, bitrary acceleration, the energy ΔE radiated in a short segment deep in the non-Newtonian region of the black hole, execut- of time Δt is approximately [10], ing several orbits—say, make it orbit ~10 times and return to a great distance. To keep the mechanism going, the small mass (11) m has to be artificially returned to orbit about the big mass. The energetics are enormous. A unique feature of this system where m is the accelerated mass and γ is the Lorentz factor for is how tiny it is! Black hole M is almost 1 centimeter and black the motion. The non-dimensional amplitude h is then obtained hole m is a millimeter. The whole orbital configuration is about from (1) if the distance is specified. one meter. The civilization must array an instrumentality that can precisely aim and hit the right impact parameter, monitor The adjustable parameters are the mass m, Lorentz factor and ‘trim’ the stability of the ‘operational’ orbit. The energy ra- γ and Δt. The source distance then places an upper limit on diated will cause decay of the circulating orbit and the small h. If one fixes h at an Advanced LIGO value of 10-28 [11], and mass m cannot be allowed to fall into the big mass black hole. the distance of the transmitter from the receiver is 10000 light The small mass must be cycled back to the big mass. The phys- years. This implies a ‘burst’ mass of 10,000 metric tons ‘boost- ics is allowed but the engineering physics is on the level of a K3 ed’ to a Lorentz factor γ of 1000 in an interval Δt of 1 picosec- civilization. ond (10-12 seconds). This transmitter mass would have to be a mini black hole with a radius of ~10-17 cm and a Hawking Radi- Table 2 is an approximate model of an artificial gravitational ation lifetime of ~3 years. Energy of ~10 solar luminosities are wave machine. With a given Lorentz factor γ, of 10, that is a β = implied! The process would have to be repeated and modulated v/c of .995, a small mass injected with the critical impact param- (in some way) to make a signal. We are back in the territory of eter will make ~ 10 revolutions and radiates a pulse of ~1047 ergs very advanced technology, a K3 civilization. If, in the future, each orbit every 10-8 seconds (10 nanoseconds). The wavelength Earth technology were able to measure the dimensionless pa- of the radiation is about 9 cm, while the exciting particle orbits rameter h at the Planck Length [12] one could see K3 transmit- at about 3 centimeters from the central black hole. This model ters with more modest energetic parameters than given above. has the ‘machine’ at 1000 light years. At 1000 light years range a detector would have to be an order of magnitude more sensitive 3 A CANDIDATE GRAVITATIONAL WAVE MACHINE than LIGO and in a much higher frequency range. 3.1 Zoom-Whirl As this model stands the orbital energy is about 1014 the solar output per second – about 1000 times the luminosity of We now examine the problem, as outlined in section 2.1, of the Galaxy! Note: because these are small black holes (possibly what instrumentality might an advanced civilization deploy to harvested primordial black holes), the scale of the ‘machine’ is realize a gravitational radiation machine. One guiding rule is small – possibly no larger than a meter! to follow Freeman Dyson, if the physics allows, it is possibly

64 Vol 72 No.2 February 2019 JBIS A GRAVITATIONAL WAVE TRANSMITTER technologically feasible. That does not mean a civilization will TABLE 3 The Orbital Gravitational Machine marshal the economic and sociological forces to realize such Parameters Value Units an extreme artifact. Mastering the energy requirement for this Distance 10000 Light Years kind of machine is extreme. But physics, as we now fathom it, defines the difficulties. Central Mass 1 Earth Mass Exciter Mass .1 Earth Mass 33 A civilization orbiting a solar type star at best has ~10 ergs/ Lorentz factor γ 2 sec on tap. (Other power sources will probably be less.) Set Percent the speed of light .866 C some parameters and see what kind of transmitter might be β built. First set the sensitivity of a ‘LIGO’ like receiver. Physical Schwarzschild radius central ~1 Cm considerations seem to imply one could measure the dimen- mass M sionless ‘strain’, h, 10-29 (private communication, Matzner[11]) Schwarzschild radius exciter ~.1 Cm in the not too distant future (h is approximately 10-22 at the mass m present). Take as a characteristic distance to an advanced civi- Critical Impact Parameter bc 5.7 Schwarzschild radii of M lization in our galaxy as 10,000 light years. Critical Impact radius 5.0 Schwarzschild radii M Aim point δ (for [bc (1+δ)]) 10-6 Suppose the civilization uses the mechanism in section 2.1 – a small mass flies by a larger mass at a very close distance. Number of turns at the ~9.35 critical radius From equation (3) the energy in a pulse is a function of the mass ratio and the mass of the flyby ‘exciter’. To transmit over Orbital period at the critical 10-7 Seconds interstellar distances the exciter should be of substantial mass, radius(~bc) some fraction of an earth mass, which must be maneuvered in Incoming Energy 1049 Ergs orbit. The exciter mass is injected into a close-encounter orbit Approximate orbital energy 1048 Ergs where it ‘winds’ for a finite number of revolutions (whirls) and at circular radius then returns to a large distance [13]. Changing the orbit of the Gravitational energy radiated 1047 Ergs exciter will require substantial energy. A trial and error calcu- per winding orbit lation using the modeling in section 2 shows that if the primary Frequency of the radiation 3.3 GHz -4 mass is one Earth mass then a 10 Earth mass exciter (about Note: The sun radiates 1033 ergs/sec the mass of Ceres) injected with a Lorentz factor of 2 can excite a detector (h~10-29) at 10,000 light years. Take the central mass as a mini-black-hole of radius ~1cm while the exciter has a ra- Fig.1 was produced using the equations of the website Rela- dius of ~1 micron. tivity 4 Engineers, plotted with a new color for every full orbit. (http://www.einsteins-theory-of-relativity-4engineers.com/rel- A scenario could be as follows. A Kardashev 2 civilization ativistic-orbits.html) The orbit ‘whirls’ around the black hole a has harvested primordial sub-stellar mass black holes. An earth few times and then 'zooms' out to the apoapsis and back again mass black hole is placed in orbit some distance away from the – a so-called whirl-zoom orbit. This happens when the periapsis home planet; call this the ‘primary’ of the gravitational ma- is very close to the black hole, between two to three times the chine. Then a smaller mass, the ‘exciter’, mass 10-4 of the prima- event horizon radius. If the orbiting particle comes much closer, ry, is injected to with excess energy (that is, an unbound orbit). it will either fall into the black hole, or it will escape completely, It shaves the critical radius at ~5 Schwarzschild radii (rs). At depending on the total orbital energy of the particle [13]. this radius a mass can wind for a finite number of orbits; below this radius it plunges into the primary. While riding this knife Making this happen demands delicate control of the orbit edge orbit, the exciter radiates energy, approximately 1041 ergs and energy, using methods we do not now know. The central per revolution as a pulse. While this orbit shaves the critical hole’s spin adds to the incoming hole’s angular momentum, radius by approximately 10-6 of its radius, the total energy ra- boosting the orbit back out. Zoom-whirl behavior is character- diated is 10-4 the orbital energy. After ~10 orbits it returns to istic of strong relativity and radiates harmonics in the gravita- ‘infinity’—back to the cycling processor. To repeat the process, tional waves—the key to imposing high-bit-rate signals on the the civilization must turn the exciter mass such that it has the outgoing gravwave train. same impact parameter as before, and so repeats the process.

It may be possible to configure the operational orbit such that it is periodic. With the periapsis and energy arranged, just so, one can obtain repeating zoom-whirl orbits (see below). The motion inside of 10 Schwarzschild radii is non Newtonian. The configuration is described in Table 3.

Generally, injecting the small mass into an eccentric orbit around a black hole means those orbits decay. However, a mass injected with kinetic energy can skim the knife edge at the photon sphere, ~3 Schwartzschild radii. There exists the possibility that a small black hole can be manipulated electromagnetically, through some kind of advanced technology. A small mass m with the right impact parameter can come in from infinity and can go into winding orbits that circle at 3m about 10 times and then escape. Relativistic orbits very close to black holes precess around the black hole and form nested sets. Fig.1 Nonlinear zoom-whirl orbits, color coded for each pass.

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As an example, consider a 0.1 Earth mass m forced into an orbit around a 1.0 Earth mass central body, M, both black holes of size less than a centimeter. The smaller mass m is injected into a nearly radial trajectory toward the central mass M. With a suitable angular momentum embedded into the smaller m, the mass twists into a class of zoom-whirl orbits, approaching no closer than 5 centimeters, i.e., well beyond the critical radius from which it cannot be recovered, ~ 3 cm. The orbital kinetic energy of this 5.4 x 10^48 erg/sec and potential energy of 4.8 x 10^48 erg/sec.

The zoom-whirl orbits emit gravitational radiation at frequency ~1 GHz while executing orbits in ~ 10-7 sec.

The zoom-whirl orbits last through an estimated number of loops made before escape of about 13.5 – full orbits before escape, the winding number. The total emission interval is a mi- crosecond. This means at least a thousand bits of information can be emitted before the smaller mass returns to the energy Fig.2 Characteristic shape of the waveform in a zoom-whirl orbit. source to be resupplied. The gravitational synchrotron loss is just less than kinetic energy. The civilization must make sure that m does not crash into M, so m flying out would have to 3.2 The Kerr Bomb Power Plant be deflected by the expenditure of energy, re-accelerated and sent back in again. This accelerator can be large, but the emit- To make such energies, now suppose the advanced civilization ting staccato ‘ticker’ is quite small, only centimeters in size. has 3 small black holes in its inventory. Two are the ‘orbital One might envision a number of small masses as the ‘exciters’ machine’, the larger central mass plus the exciter mass. The orbiting in a complicated convoy to make a more intense, sig- machine central and exciter black holes form a binary system nal-rich ensemble. These can then emit in concert, insuring a orbiting the home star. longer message. The third black hole is a rotating (Kerr) black hole – the Waveforms are modulated by the harmonics of zoom- ‘machine’ power plant. It exploits super-radiance [14, 15], to whirls, showing quiet phases during zooms and louder glitch- extract energy from the rotating black hole by scattering elec- es during whirls. Zoom-whirl behavior in spinning pairs is a tromagnetic radiation from it. Radiant Energy from the home common feature of eccentric orbits, despite the drain of gravi- star is diverted to the black hole bomb, where it gets ampli- tational radiation. Fine tuning of initial conditions at the outer fied by superradiance. Surround the power house black hole limit of the orbits, apastron, codes the emitted message. (Figure with a spherical mirror about 300 meters in diameter [11]. 2 shows an example obtained from a detailed numerical simu- Figure 3 shows a cutaway of the ‘bomb transmitter-mirror’ lation using full general relativity.) system. This scattered radiation will pick up a small amount of energy from the rotating event horizon. If the scattered ra- The orbit emits a periodic succession of high-amplitude/ diation is then confined, for merely a short length of time, high-frequency parts (coming from the whirling motion of the by a spherical ‘mirror,’ then an enormous amount of energy particle near the periastron) and intervening low-amplitude/ may be made available. Suppose the ‘power-house’ black hole low frequency parts (from the zooming in and out motion, apas- has a mass of Jupiter and the Kerr black hole is rotating at its tron). Quadrupole (ℓ = 2) emission dominates gravwaves [5]. maximum. This means the horizon would be spinning at ~1 GHz, as long as the impinging radiation has a frequency of Of course, managing the black hole and gravwave stresses less than or of the order of this it will be amplified. One notes in the central region of the emitter is crucial. The emitter sits an immediate problem with the mirror, if the radiation is to in the core of a relatively low-mass surrounding structure that be contained to a level 1016 times bigger than the sun’s output must flex and endure severe stresses. The wave stretches one then the mirror would have to have a tensile strength greater transverse direction while the other compresses. Because of than neutron star material! Figure 4 is the composite gravita- potential accident, the entire assembly should orbit well be- tional wave machine. yond any inhabited zones. If a Kardashev civilization of order 2 has mastered the ener- The total gravwave energy emitted in this example is about a gy output of its local star, that makes some of this energy avail- hundredth of an Earth mass. How can this be replenished? Ob- able (about .001 of its local star’s output) to transmitters located viously, a culture able to handle such masses and thus energies nearby the local Kerr black hole mirror. Then in ~1 second (the must have sources we do not know. The precursor to any such e-folding time) there is an amplification of 1017, producing the project must be using black holes to extract energy. Such ener- energy needed to manipulate the orbit of the exciter mass. The gy sources might be essential in mining operations throughout extra energy extracted from rotational energy of the Kerr black an outer solar system, where large, sporadic energies are useful hole can be used to maintain the orbital machine’s configura- in mass processing for industrial use. tion. Arrange the ‘bomb’ such that it pulses – that is, before the sphere can blow up the energy is transferred to the gravitation- One notes that besides giving the exciter mass a kinetic en- al machine. ergy of 1049 ergs, an orbital energy of 1048 must also be attained and modified. These energies are 1610 greater than a local solar For a sphere only hundreds of meters to a kilometer in di- type star output. Where would this energy come from? ameter one seemingly could have an almost solid sphere whose

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Fig.3 The Kerr power plant, a pulsating black hole bomb.

Fig.4 A schematic representation of total ‘gravitational wave machine’ system. Note the ‘Superradiance machine’ is inward of the gravitational wave machines; the Home Star is located somewhere in this system.

JBIS Vol 72 No.2 February 2019 67 A. A. JACKSON & GREGORY BENFORD stability would have to be controlled. This Kerr power plant The urge to propagate culture quite probably will be a univer- could supply the energy needed for both GW machines men- sal aspect of intelligent, technological, mortal species (Minsky, tioned above. There is a problem in that making the mechanics 1985). of the engine work since the civilization is generating almost a • The Funeral Pyre: A civilization near the end of its life an- supernova power output of electromagnetic energy. Contain- nounces its existence. ment of this energy could mean in the end product a ‘waste • Ozymandias: Here the motivation is sheer pride; the heat’ object of extraordinary brightness! Beacon announces the existence of a high civilization, even though it may be extinct, and the Beacon tended by robots. How such an instrumentality of this order could be realized • Help! Quite possibly societies that plan over time scales is mind boggling. A naïve estimate shows that the mirror has ~1000 years will foresee physical problems and wish to discov- to be made of something strong, about 15 orders of magnitude er if others have surmounted them. An example is a civilization greater than the tensile strength of Graphene—actually, more whose star is warming (as ours is), which may wish to move than the tensile strength of neutron star matter. So these de- their planet outward with gravitational tugs. Many others are mands stretch the credibility of any Dysonian engineer. But possible. other tricks to offset such pressure may arise from superior en- • Join Us: Religion may be a galactic commonplace; after all, gineering methods. Recall that buildings readily constructed it is here. Seeking converts is common, too, and electromagnet- now were thought impossible only a century ago. ic preaching fits a frequent meme. • Ineffables: Aliens quite likely have desires and philos- 4 ELECTROMAGNETIC LEAKAGE ophies inscrutable to us. These could be powerful and en- igmatic and have consequences we cannot anticipate. Possi- We must mention that for a gravwave-emitting society, not let- bly the motives listed above are common, but not the major ting the electromagnetic energy radiate to make its own beacon impulses of advanced, strange minds. We should expect the may be essential. unexpected.

The varying EM fields will be plausibly of lower frequency. Even today, with easy electromagnetic communications, They are not changing the zoon-whirl orbits, they're power- some prefer hard copy messages, with a secure, often human, ing the slower accelerations, so may be an order or magnitude carrier—especially in financial matters. Blockchain methods lower in frequency, to 100 MHz or so, but powerful. So then try to decrease interception risk, but many prefer hard copy. the emitters could make such leakage uninteresting, by other Somewhat similar motives may well persist into cultures vastly methods. more powerful than ours, who prefer gravwave signals to the easier electromagnetic ones. They may choose gravwaves be- One idea is to capture the electromagnetic energy further cause they do not wish to be known to mere electromagnetic away, with a reflecting metallic sphere. Can this be the same civilizations. Such gravwave societies would probably be vast- sphere in the Kerr bomb? Best to make it a perfect conduc- ly old, compared with ours. They may have gone through eras tor, feeding energy back in, or absorbed/routed and saved for of electromagnetic communication with other societies and accelerating the next black hole to high energy. Capturing re- learned that early, easy technology correlates with aggressive flectors which store the electromagnetic energy until needed, cultures. These could send devastating relativistic missiles by routing it through waveguides and resonant storage cavities across interstellar space, of great threat. Then going to a harder can shape the next, needed accelerating electromagnetic ener- gravwave technology would have some appeal, along with utter gy wave that drives the black holes up to the needed energy. electromagnetic silence. Recycling such vast energies would be essential. Many other tricks to reuse photons may arise from superior engineering LIGO has opened a window that perhaps few societies in our methods. galaxy could manage, or wish to. It may show us more than astronomers expect. A further way to avoid seeming like an electromagnetic signal would be to deliberately blur the leaked emission. It is also possible that we have overestimated the technical Phase-delaying, merging and time-staging leakage strips it of difficulties. Received power can be enhanced in transmissions signal. This makes even a bright emission seem like an astro- if sources are made coherent. In gravitational radiation, this nomical oddity, rich in energy but not in meaning. would mean resonant paralleling of trajectories in the smaller masses, as they orbit the central mass. This might be pos- 5 CONCLUSIONS: sible, but will greatly complicate matters, as the zoom-whirl orbits are already highly nonlinear; adding a coherence con- The great equalizer in all communication across vast ranges is straint makes their management more difficult. One can also the speed of light and gravwaves alike. This leads to motives, imagine an array of more than one m/M system. This would once societies achieve technologies. The grand goals of alien mean spacing the large mass elements in order for their emis- minds can be imagined [16]. Briefly, they can be sions to align. Such an array then can direct emissions in a • Kilroy Was Here—memorials to dying societies. narrower spatial and perhaps frequency band, just as in elec- • High Church—records of a culture’s highest achieve- tromagnetic systems. Some efficiency improvements seem ments. The essential message is this was the best we did; re- possible this way. . If so, the threshold of gravwave emitters member it. A society that is stable over thousands of years may may be low enough to make it a commonplace of truly long- invest resources in either of these paths. The human prospect lived societies. has advanced enormously in only a few centuries; the lifespan in the advanced societies has risen by 50% in each of the last Acknowledgements two centuries. Living longer, we contemplate longer legacies. We are grateful for discussions with Martin Rees, James Ben- Time capsules and ever-proliferating monuments testify to ford, Freeman Dyson, Stephen Baxter and Richard Matzner. our urge to leave behind tributes or works in concrete ways. Thanks to Douglas Potter for technical illustrations.

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REFERENCES

1. N. Kardashev, “Transmission of Information by Extraterrestrial 9. Ya. B. Zeldovich and I.D. Novikov, Relativistic Astrophysics, University of Civilizations”, Soviet Astronomy 8: 217. (1964) Chicago Press, 1971. 2. C. Sagan, A. Druyan, F. Dyson, D. Morrison, Carl Sagan's Cosmic 10. M. Maggiore, Gravitational Waves: Volume 1: Theory and Experiments, Connection, Cambridge University Press Oxford University Press, 2007. 3. S. Detwiller, “Black holes and gravitational waves. I – Circular orbits 11. R. Matzner, Private Communication. about a rotating hole”, Astrophysical Journal, Part 1, vol. 225, Nov. 1, 12. C. Hogan, “Now Broadcasting in Planck Definition” - arXiv:1307.2283 1978, p. 687-693 [quant-ph] FERMILAB-PUB-13-685-A 4. S. Shapiro and S. Teukolsky, Black Holes, White Dwarfs, and Neutron 13. J. Levin and G. Perez-Giz, “A periodic table for black hole orbits”, Phys. Stars Rev. D 77, May 2008 5. C. W. Misner, K. S. Thorne, J. A. Wheeler, Gravitation, W. H. Freeman. 14. W. H. Press, S. A. Teukolsky, “Floating Orbits, Superradiant Scattering 1973 and the Black-hole Bomb”, Nature 238 , 211–212, 1972. 6. K.S. Thorne, Particle and Nuclear Astrophysics and Cosmology in the 15. V. Cardoso, Ó. J. C. Dias, J. P. S. Lemos, and S. Yoshida, “Black-hole Next Millenium, 160 (1995) bomb and superradiant instabilities”, Phys. Rev. D 70 26 August 2004 7. C. W. Misner, R. A. Breuer, D. R. Brill, P. L. Chrzanowski, H. G. Hughes, 16. Adapted from J.Benford, G. Benford, and D. Benford, “Searching for III, and C. M. Pereira, “Gravitational Synchrotron Radiation in the Cost Optimized Interstellar Beacons,” Astrobiology, 10:5 (July, 2010), Schwarzschild Geometry”, Phys. Rev. Lett. 28, 998, 1972 491-498; and J. Benford, G. Benford, and D. Benford, “Messaging with 8. S. Dolan, C. Doran, and A. Lasenby, “Fermion scattering by a Cost Optimized Interstellar Beacons,” Astrobiology, 10:5 (July, 2010), Schwarzschild black hole”, Phys. Rev. D 74, September 2006. 475-490.

Received 13 December 2018 Approved 26 February 2019

JBIS Vol 72 No.2 February 2019 69 CORRESPONDENCE

Correspondence

DR DAVID G. STEPHENSON P.O. Box 281, 312 Church Street, Merrickville, Ontario, Canada K0G 1N0

Email [email protected] in the Quarterly Journal of the Royal Astronomical Society [3], and which later attracted the attention of Grey [4] who was seeking the origin of the term ‘Fermi’s Paradox.’ After To: the Editor of JBIS funding for ionospheric research in Canada was reduced 3 Janaury 2018 in 1979 I returned briefly to the U.K., before joining a team developing instrumentation for the Out of the Ecliptic deep Dear Sir, space probe, later called the Ulysses probe, in Germany. There was little interest in SETI in Canada, and as far as I I read A.R. Martin’s paper published in the June 2018 am concerned the phrase ‘Fermi’s Paradox’ emerged entirely edition of the J.BIS with great interest. It brought back many from the discussions in the U.K. and centred on the BIS memories of the summer of 1976. I would like to clarify my during the summer of 1976. modest contribution to the emergence of the phrase ‘Fermi’s Paradox’ into general use. Why is the night sky dark? During the nineteen century this question became known as ‘Olber’s Paradox’ and was I was brought up in Yorkshire and south London and widely discussed among astronomers. It is a question that received my physics doctorate in Meteor Radar Astronomy is both simplistic and profound. Since the phrase Fermi’s at the University of Sheffield in 1970. By then I had accepted paradox emerged, ‘Where is Everybody?’ has been equally an offer of a research fellowship as part of a team studying simplistic and profound. It addresses the possible abundance the dynamics of the ionosphere at the Institute of Space and of technically capable, intelligent life in the universe. Atmospheric Studies at the University of Saskatchewan in Clearly our current concepts are inadequate. Scientific Saskatoon, Canada. Like a good ex-pat I returned to the developments over the past 40 years have only sharpened the U.K. during the summer of 1976 for an extended family paradox. Terrestrial evolution has produced an intelligent holiday and to attend meetings and workshops of the Royal species capable of asking Fermi’s question. The chemical Astronomical Society and the British Interplanetary Society. building blocks of life are found in interstellar space. It In his paper Martin describes the intellectual tides churning seems improbable that, given the wide variety and numbers the BIS that summer, and I was readily drawn into them. The of planets being detected across interstellar space, that Search for Extraterrestrial Intelligences (SETI) was widely technically capable species would not have evolved elsewhere discussed. Carl Sagan’s story of Fermi’s famous question was before the emergence of homo sapiens. Why are their works frequently mentioned, and the word paradox was common not apparent across interstellar space? currency. I met David Viewing, discussed SETI with him and visited his group’s installations at Peterborough. Wherever life emerges it will be moulded by evolution to take ever more complex forms. Comparative reproductive Canadian government policy at that time was that advantage drives the course of evolution. An individual or scientific research should be quickly applied and have an species that has a trait that allows it to exploit an available obvious economic impact, so esoteric speculation was not ecosystem niche more effectively than its competitors will encouraged. Nonetheless, after I returned to Saskatoon reproduce more successfully and prolifically than those I submitted my thoughts about SETI to JBIS. These were competitors. Offspring that carry that trait will, in turn, published in March 1977 [1] and contained the phrase reproduce successfully. Over the course of generations the “Fermi’s Paradox” for the first time in an archival publication. trait will spread and be enhanced. A species that enters a new During my visit of the summer of 1976. Fermi’s name and the ecosystem without competitors, or evolves to overwhelm word ‘paradox’ were so frequently used that I suppose it was local competitors is freed to express its comparative inevitable that they should come to be associated. reproductive advantage without restraint. That species becomes invasive and rapidly occupies the ecosystem at the Fermi’s Paradox was the subject of a discussion session at expense of any present occupants. After a limited period, a BIS conference in April 1977 and later published in JBIS however, physical limits will constrain further expansion, and 1979 [2]. That year I used the phrase in a paper published as the productivity of the reduced and overtaxed ecosystem

70 Vol 72 No.2 February 2019 JBIS CORRESPONDENCE

falls, the reach of the invasive species rapidly declines to a ‘Business As Usual’, and projected that during the year 2020 low level that can be supported by the now impoverished human civilization would burn fossil fuels sufficient to dump ecosystem. 10 billion tonnes of carbon, as 36.6 billion tonnes of carbon dioxide, into the atmosphere. Subsequent I.P.C.C. reports An opportunistic, physically adaptable species would have warned that these large emissions of greenhouse gas evolve intelligence in response to rapidly changing would inevitably lead to irreversible, severe climate changes conditions. Such a species could adapt its means of life in by the middle of the twenty first century. The effects on the a much shorter time than physical evolution would allow, productivity of the world’s ecosystems would be catastrophic. and therefore it would successfully establish itself in many In addition, as the proportion of carbon dioxide in the different ecosystems simultaneously. Sophisticated tool- atmosphere rose, soils and the oceans would become acidic making would accelerate this adaption. Tool-making human and inhospitable to many important species. beings successfully colonized and dominated the Americas from the Arctic to the Tropics and further to sub-Antarctica Demonstrably homo sapiens is hopelessly enthralled by in only a few thousand years. A species that could pose evolution’s primitive lusts: to greed and to breed. During Fermi’s question must have evolved not only the intelligence the latest United Nations’ Climate Summit at Katowice, to state that question, but also have developed the technology the Global Carbon Project research group published that utterly to dominate its natal planet. Free to express evolution’s during 2018 the world’s economies would burn fossil fuels prime directives that species would inevitably invade sufficient to dump over 37 billion tonnes of carbon dioxide and fully exploit every available resource and ecosystem into the atmosphere. During the almost seventy years since niche of that planet. The barriers to further expansion into Fermi asked his famous question, the population of homo space would be formidable. Establishing independent, self sapiens has tripled, and continues to increase. Humanity sustaining ecosystems on the harsh, alien surfaces of nearby has swarmed over the planet to scour its ancient resources celestial bodies would be extravagant and time consuming. as its ecosystems fever and choke on our wastes. I am An intelligent species could only escape the fate awaiting an truly surprised that more than 40 years after I first wrote invasive species if its advance were wilfully contained. Such the phrase “Fermi’s Paradox”, it continues to be the focus an intelligence would, however, be perverse, having denied for profitless speculation about the evolution, spread, and the primal drives for exploitation and reproduction that manifest destiny of intelligence both here and elsewhere in caused it to evolve. the galaxy.

The fossil fuel-powered Industrial Revolution freed Yours sincerely, humanity from the curbs of famine and pestilence placed on its expansion and exploitation of this planet. Farms became David Stephenson more productive, and large quantities of food could be moved quickly. Sanitation and medical science triumphed over infectious diseases. Eventually the physical sciences, engineering and electronics opened a wide window on the REFERENCES universe and made space flight feasible. During the early 1. Stephenson D.G. (1977) “Factors Limiting the Interaction between nineteen eighties I was challenged during a workshop at Twentieth Century Man and Interstellar Cultures”. JBIS 30: 105-108 the BIS headquarters: “Scientists deserve to be out of work. 2. (1979) “The Fermi Paradox: A Forum for Discussion”. JBIS 32: 424-434 They don’t think commercially!” In October 1994, chapter 3. Stephenson D.G. (1979) “Extraterrestrial Cultures within the Solar 11 of a report to the Canadian House of Commons by the System?” Quarterly Journal of the Royal Astronomical Society, 20: 422. Auditor General stated that current policy was to ‘make the 4. Grey R.H. (2015) “The Fermi Paradox is neither Fermi’s nor a Paradox”. prevailing culture of science... more management-orientated Astrobiology Vol. 15 No.3 and businesslike’. Four years earlier the first report of the United Nations Intergovernmental Panel on Climate Change (I.P.C.C.) had published a series of projections estimating how much carbon human civilization would emit into the atmosphere. The highest emissions estimate was labelled: Received 4 January 2019 Approved 11 March 2019

JBIS Vol 72 No.2 February 2019 71 ANNOUNCEMENT

Putting Astronauts in Impossible Locations

A one day technical symposium 9:00 a.m – Wednesday 27th November2019 BIS HQ, 27/29 South Lambeth Road. London

CALL FOR PAPERS

While the human exploration of the Moon and Mars has been extensively examined, serious technical consideration of the rest of the solar system has been largely ignored. This symposium is designed to explore the limits of where human exploration can go in the solar system and how to overcome the challenges involved. The symposium is open to papers on the transportation requirements, the practicalities of habitation in extreme environments and any other aspects of a solar system-wide civilisation. Submissions should be on the basis that there will be a completed paper delivered before the symposium, as well as giving a presentation on the day. All papers will be considered for publication in the Journal of the British Interplanetary Society.

Proposed papers should be described in an abstract of no more than 400 words, and submitted to the Society via [email protected].

Submission deadline 31st July 2019

72 Vol 72 No.2 February 2019 JBIS Have you got what it takes?

After two years spent successfully steeringJBIS towards its new look, Editor Roger Longstaff is moving on to fresh challenges. The Society is now looking for someone to replace him. This is a part-time position, typically taking two days a week, that would suit someone who is either in part- time employment, self-employed or retired but still takes a keen interest in the field of astronautics, and who has a background in related academia, astronautics or the space industry itself. Administrative help will be provided and the position attracts remuneration for each issue published. If you think you might fit the bill, please contact Executive Secretary Gill Norman at [email protected] for more details. Journal of the British Interplanetary Society

VOLUME 72 NO.2 FEBRUARY 2019

www.bis-space.com

ISSN 0007-084X PUBLICATION DATE: 29 APRIL 2019