aa r X i v : . [ phy s i c s . pop - ph ] F e b ‘Oumuamua is not Artificial J. I. KatzMarch 4, 2021
Abstract
I summarize evidence against the hypothesis that ‘Oumuamua isthe artificial creation of an advanced civilization. An appendix dis-cusses the flaws and inconsistencies of the “Breakthrough” proposalfor laser acceleration of spacecraft to semi-relativistic speeds. Realityis much more challenging, and interesting.
Recently, the popular press [1, 2] has discussed the hypothesis, promoted in apopular book [3], that ‘Oumuamua, the interstellar object that transited theSolar System in 2017 [4], is the product of an “alien” civilization, presumablyreconnoitering the Solar System, rather than a natural fragment (a “Jurad”,an asteroid or comet nucleus) that escaped from an extra-Solar planetarysystem [5]. There are several reasons why the alien civilization hypothesis isnot credible:
The Panstarrs system detected one such object in its few years of operation.A second interstellar object, clearly cometary, has also been detected. Thereis some controversy over whether ‘Oumuamua is asteroidal or cometary:imaging imposed very strict upper limits on its rate of outgassing, but non-gravitational acceleration has also been claimed [4] (disputed by [6]). Loeb[3] attributes this to the effect of Solar radiation or the Solar wind on anartificial structure with a low ballistic coefficient. However, Bialy and Loeb17] find that the reported acceleration would imply a thickness and ballisticcoefficient orders of magnitude less than those considered for Solar sails orfor laser-accelerated spacecraft [8].‘Oumuamua had a velocity, far from the Solar System but with respect toit, of about 26 km/s. The smallest credible distance of a sending civilizationis about 10 light years (this volume contains 10–20 stars, enough that if allthe possible optimistic assumptions are made it might contain an advancedcivilization; the closest extra-Solar star is about 4 light years away). Thetransit time from that distance would be about 10 years .A decision to launch toward our Solar System must have been made ∼ years ago. Even if one imagines the launching civilization capable of arbitrar-ily good observations, and able to infer from the state of the Earth then thata technological civilization worth monitoring, alerting or contacting wouldevolve in ∼ years, it could not predict the chronology of Panstarrs or asimilar system accurately. Hence, unless it (or we) have been unusually lucky,they must have launched ≫ such probes. This would be extraordinarilyinefficient. A flyby is an inefficient way to collect data and an unreliable way to attractnotice. As our space programs know, if you want to collect information abouta body, send an orbiter or a lander. There is no compelling argument againstthe presence of artificial orbiters in the Solar System, or against landers onany body other than the Earth. ‘Oumuamua was neither. ‘Oumuamua tumbled: its light curve was not periodic. This is unusual, butnot unprecedented, among asteroids. Internal friction damps tumbling, butslowly in rocky material at low asteroidal temperatures and even more slowlyat the interstellar temperatures to which ‘Oumuamua was long exposed. A The orbit of ‘Oumuamua has been extrapolated backward and does not approachany star and possible planetary system for much longer [9], but we must allow for thepossibility of a non-ballistic trajectory powered by an on-board ion engine and nuclearreactor.
Loeb suggests a network of space telescopes to image any future interstellartransiters (and more ordinary Solar System objects!). A diffraction-limited10 m aperture has a resolution of about 50 nrad in blue light. At a distanceof 1 AU that corresponds to a resolution of about 8 km, far too poor toresolve an object hundreds of meters across like ‘Oumuamua. Interferometrycan do better, of course, but we cannot be confident of an unambiguous dif-ference between the visibility functions of natural and hypothetical artificialinterstellar objects.It is technically feasible to send probes to fly by such intruders. Panstarrsdetects roughly one per year (and a future system likely many more). Velocityincrements of 10 km/s, achievable in minutes with chemical propellants (slowacceleration by ion engines would not be sufficient) would enable interceptorsstored in Solar orbits to fly by a significant fraction of interstellar intruders.Close-up imaging would be possible, or even collision whose debris could beanalyzed spectroscopically from Earth.
The hypothesis that ‘Oumuamua is the product of an advanced civilizationdoes not resolve any previously inexplicable conundrum, the necessary justi-fication for a speculative hypothesis. ‘Oumuamua is entirely explicable as afragment expelled from its parent planetary system by gravitational interac-tion [5], at any time in the history of the Galaxy.The “Breakthrough” project has argued that it is feasible to accelerate,using lasers, spacecraft of low ballistic coefficient (sails) to semi-relativisticspeed, orders of magnitude greater than the observed speed of ‘Oumuamua;this is discussed in the Appendix. 3
Laser-Accelerated Spacecraft
The Breakthough project has suggested [8] that an advanced civilizationmay be capable, using a laser, of accelerating a spacecraft of low mass andvery low ballistic coefficient, to semi-relativistic speeds. Their analysis isneither quantitative nor detailed. Here I take their parameters of a 1 gspacecraft with A = 10 m sails composed of 300 ˚A thick Al foil, acceleratedto 0 . c in 10 cm by radiation pressure. The sails have an areal mass loading(ballistic coefficient) of 10 − g/cm and a mass of m = 1 g; larger ratios ofsail to payload mass produce only marginal increases in performance. Thenumber of spacecraft is not defined beyond “thousands”, nor is it statedwhether they are all to be accelerated simultaneously or one at a time; thelatter reduces the peak power requirement but, unless the initial injection(by conventional means) is into geostationary orbit, requires complex timingand orbital manipulation. I will assume the latter. A.1 Apertures
For 1 µ laser light this corresponds to diffraction limited apertures of about300 m; the requirement is to focus light at 10 cm onto a 3 m sail. A.2 Focusing
A 1 g spacecraft traveling at 0 . c has a kinetic energy of 1 . × ergs.A 100 GW laser operating for 120 s radiates 1 . × ergs. The requiredacceleration time is mcv/ P , where P is the laser power, or 90 s. The distancetraveled is ( mc/ P ) v = 2 . × cm, a few times greater than the nominal10 cm specified by Breakthrough. This demands more performance fromthe focusing system. A.3 Laser power
Power engineers (quite apart from laser designers) will find the requirement ofdelivering 100 /ǫ GW, where ǫ is the laser efficiency, formidable. Fortunately,the lasers can be distributed over much of a hemisphere of the Earth if thelaser acceleration begins from geostationary orbit. Still, it would require useof a large portion of the US electrical generating capacity for an hour toaccelerate even one 1 g spacecraft. 4 .4 Vaporization The sail will vaporize.
A.4.1 Thermal loads
The energy expended to accelerate a 1 g spacecraft is
P t or 9 × ergs.This should be compared to the latent heat of melting of 1 g of aluminumof ∼ ergs. If even 10 − of the incident laser energy is absorbed, thesail melts. This estimate is pessimistic because it ignores radiative coolingof the sail, though metals are poor radiators for precisely the reason theyare good reflectors, but it indicates a formidable problem. No metal has areflectivity higher than 0.9999. In other words, 100 GW over 10 m is 1MW/cm , about 10 times the intensity of sunlight, and several orders ofmagnitude times intensities known to vaporize metallic surfaces. A.4.2 Radiative cooling
Including radiative cooling in steady state (rapidly achieved for a very thinsail) the sail’s temperature is T = T bb ( ǫ laser /ǫ th ) / (1)where T bb is the gray-body temperature [ P/ ( Aσ SB )] / = 20000 K, A is thesail area (10 m ), σ SB is the Stefan-Boltzmann constant, ǫ laser is the sail’sabsorptivity (equal to its emissivity) at the laser wavelength (1 µ ) and ǫ th isthe sail’s emissivity at MWIR and LWIR wavelengths corresponding to itsequilibrium temperature. The ratio of emissivities is greater than unity be-cause metals are better reflectors (and worse emitters) at longer wavelengthsand because very thin metal layers have smaller emissivities than thick lay-ers. Hence thermal emission cannot save the sail from thermal destruction ifits absorptivity exceeds the unrealistic value of 10 − . A.4.3 Better radiator?
This problem might be mitigated if the metal were backed with a thin layerof an effective (necessarily insulating) radiator. Supposing a black radiatorand an overoptimistic ǫ laser = 10 − yields T = 2050 K, still exceeding themelting point of Al of 933 K. Each of these assumptions is unrealistically5ptimistic (the actual emissivity at 1 µ of Au is 0.03 and that of Al 0.07), andignores the difficulty of fabrication. A.5 Mass of the sail
The force on the spacecraft F = 2 P/c = 7 X dynes. This must be trans-mitted from the sail to its compact 1 g payload. The payload dimensions arenot specified, but one thinks of an integrated circuit with area 10 cm . Thestress in the sail where it is attached to the payload is F/Ch , where C is thepayload circumference and h the sail’s thickness. For C = 20 cm and h = 300˚A the stress is > >
300 times the strength of aluminum. Makingthe sail thicker makes it more massive; the minimum mass is obtained if thethickness varies inversely as the distance from the payload. A simple calcula-tion shows that the sail mass is (2
P/c )( ρ/σ ) R , where ρ and σ are the densityand strength of the sail, ρ/σ ≈ − s /cm ) and R is the sail’s radius. Theresult is that the sail’s mass M ≈ F Rρ/σ . The hypothetical 10 m ( R = 1 . a wouldrequire R ≤ σ/ ( aρ ), which for the assumed a = 7 × cm/s and theparameters of Al would require R ≤
14 cm. The corresponding area wouldbe ≤ .
06 m , much less than the assumed 10 m . The laser intensity wouldthen be at least 170 times greater than that previously assumed, that wouldalready exceed the tolerable heat load on the sail. The proposed design isnot self-consistent. A.6 Utility?
How does a 1 g spacecraft carry any useful sensors? How can it determinewhat is around α Centauri (presumably the interest is in habitable planets)?
A.7 Data return?
How can a 1 g spacecraft transmit any detectable signal back to the Earthfrom a distance of 4 X cm? 6 .7.1 Signal strength A half-wave dipole antenna, using a hypothetical 100 W of power obtainedfrom photocells illuminated by the parent star of the planet studied, wouldproduce 10 − W/cm at the Earth at 1 pc distance, or 10 − W in a 1km array. For a bandwidth B the corresponding temperature would be ∼ /B ) mK, that may be compared to present-state-of-the-art antennatemperatures ≈
20 K. Extensive coherent integration would be required todiscriminate the signal from the incoherent thermal cosmic 3 K backgroundand the thermal emission of the parent star. A high gain transmitting an-tenna would improve these values, but would be difficult to include within a1 g mass budget.
A.7.2 Visible light?
An optical system could collimate the beam, but at the price of quantumnoise more than 10 times greater than at 100 MHz, and folding such opticsinto a 1 g spacecraft would be a formidable problem. A diffraction- limited1 m aperture would increase the received intensity by 10 compared to adipole, or 10 − W in a 10 m telescope, or 3 photons/s. It is difficult enoughto detect planets, reflecting 10 W of visible light (giving the same brightnessas the hypothetical diffraction-limited aperture transmitting 10 W) nearstars.
A.8 Relays
The total power required to transmit a signal may be reduced by a factorof about N if a relay chain of N equally spaced transceivers is establishedalong the transmission path; the power at each transceiver is reduced by afactor of about N . These transceivers would be in deep space where the onlyplausible power source would be reactors, and the requirement of achievingcriticality implies minimum masses of tens of kg each. Decay of isotopes like Sr and
Cs, with half-lives of decades, produces ∼ ∼
30 km/s would require tens of thousands of years toreach the nearest stars at distances of ∼ eferences [1] accessed February 12, 2021[2] accessed February 12, 2021.[3] Extraterrestrial: The First Sign of Intelligent Life Beyond Earth , Loeb,A. (Houghton Mifflin Harcourt, 2021).[4] https://en.wikipedia.org/wiki/’Oumuamua accessed February 14,2021.[5] Hansen, B. and Zuckerman, B., Res. Notes Am. Astr. Soc. 1, 55 (2017)arXiv:1712.07247.[6] Katz, J. I. Ap. Sp. Sci. 364, 51 (2019) arXiv:1904.02218.[7] Bialy, S. and Loeb, A. Ap. J. 868, L1 (2018) arXiv:1810.11490.[8] https://breakthroughinitiatives.org/initiative/3https://breakthroughinitiatives.org/initiative/3