LLunar Opportunities for SETI
Eric J. Michaud * , , Andrew P. V. Siemion , , , Jamie Drew , S. Pete Worden University of California Berkeley, Berkeley, CA 94720 SETI Institute, Mountain View, CA 94043 University of Malta, Institute of Space Sciences and Astronomy The Breakthrough Initiatives, NASA Research Park, Bld. 18, Moffett Field, CA, 94035
A white paperNational Academy of SciencesPlanetary Science and Astrobiology Decadal Survey 2023-2032 * Corresponding author. Phone: +1 (925) 596-8844. Email: [email protected] a r X i v : . [ a s t r o - ph . I M ] S e p bstract A radio telescope placed in lunar orbit, or on the surface of the Moon’s farside, couldbe of great value to the Search for Extraterrestrial Intelligence (SETI). The advantageof such a telescope is that it would be shielded by the body of the Moon from terrestrialsources of radio frequency interference (RFI). While RFI can be identified and ignoredby other fields of radio astronomy, the possible spectral similarity between human andalien-generated radio emission makes the abundance of artificial radio emission on andaround the Earth a significant complicating factor for SETI. A Moon-based telescopewould avoid this challenge. In this paper, we review existing literature on Moon-basedradio astronomy, discuss the benefits of lunar SETI, contrast possible surface- and orbit-based telescope designs, and argue that such initiatives are scientifically feasible, bothtechnically and financially, within the next decade.
Since the 1960s, many have recognized theunique opportunities for radio astronomy pre-sented by the Moon [1, 2, 3, 4, 5]. In 1986,Jack Burns et al. declared that “the Moon isvery possibly the best location within the in-ner solar system from which to perform front-line astronomical research” [2]. The Moon’scharacteristics (its lack of an atmosphere, lowseismic activity, long nights, etc.) make it at-tractive for a variety of astronomical projects.Notably, it would be a revolutionary platformfor low frequency cosmology, which cannot beconducted from the surface of the Earth dueto the shielding effects of our ionosphere [4,6]. The search for extraterrestrial intelligencecould be similarly transformed by a lunar ra-dio telescope [7, 8]. The primary advantagefor SETI is that the body of the Moon pro-vides an excellent shield against terrestrial ra-dio frequency interference [9, 10, 11].Searches for signs of alien technology, mostnotably for narrowband radio signals, arecomplicated by the abundance of humantechnology around the Earth. When human-produced radio emission is detected in highvolume in SETI observations, it becomeschallenging to attribute any particular signalto extraterrestrial intelligence. While SETI
Figure 1: Orbits of over 2000 active satellitesaround the Earth (ignoring inclination), gener-ated from the UCS Satellite Database [12] .astronomers have developed observing strate-gies and specialized software for approachingthis challenge [13], it is worth exploring amore radical solution – conducting observa-tions from an area of space with minimal ex-posure to radio pollution.According to Zarka et al. “the farside of theMoon is during the Lunar night the mostradio-quiet place of our local Universe” [3].This assertion is supported both by computersimulations of radio-wave diffraction aroundthe Moon [9, 10], and by radio observations1 igure 2: Terrestrial radio interference as observed by the RAE-B satellite from lunar orbit. Notethe clear drop in the power of terrestrial radio emission as the satellite passes behind the Moon.This shielding from terrestrial RFI would be ideal for SETI observations. Taken from [11] conducted in lunar orbit [11].In 1968, NASA’s RAE-A (Radio AstronomyExplorer) observed that from its Medium-Earth orbit, “radio emissions from the Earth– both natural and man-made – were verycommon and often very intense” [11]. Thisfinding motivated the placement of the sub-sequent RAE-B spacecraft into lunar orbit(at an altitude of 1100 km), where terrestrialnoise would be blocked for some fraction of itsorbit by the body of the Moon [5]. RAE-Bdid indeed observe “impressive occultations”[11] of such noise, as illustrated in Figure 2.Terrestrial radio noise was attenuated by 1-3orders of magnitude as the spacecraft passedbehind the Moon.Computer simulations tell a similar story, andindicate that a radio telescope positioned onthe surface of the lunar farside would be even more strongly protected from terrestrial RFIthan a telescope in lunar orbit. In one suchsimulation, Yuki Takahashi [9] found that ra-dio waves at frequencies as low as 50 kHzwould likely be attenuated by at least 10 or-ders of magnitude on the opposite side. Inanother study, Pluchino et al. [10] estimatedthat near the crater Daedalus (almost exactlyopposite the Earth), the power of geostation-ary satellite interference at 100 MHz and 100GHz would be attenuated by 7 and 10 ordersof magnitude respectively.Together, these studies indicate that the ra-dio environment around the Moon’s farside isradically different, and favorable in compari-son with, the Earth’s surface or orbit for thepurposes of SETI. Shielded from terrestrialinterference, a Moon-based telescope wouldreceive RFI only from satellites and rovers lo-cated on the Moon or beyond it. The number2 igure 3: Simulated diffraction of a 60 kHzwave, incident from the left, around the body ofthe Moon. Over the farside, we see a predicted10 order of magnitude attenuation of the signalpower. Taken from [9] of such radio-producing devices is far lowerthan around the Earth, and even if human ac-tivity around the Moon were to increase sub-stantially within the next decade, we shouldstill expect that the number of such devicesto be dramatically lower than the number ofsatellites in Earth orbit. (see Figure 1). Nev-ertheless, it would be wise to protect the lu-nar farside as a radio quiet zone to the extentpossible. Proposals like Claudio Maccone’s
Protected Antipode Circle should be seriouslyconsidered by the international community[10, 14].
Placing a radio telescope on the surface of thelunar farside would minimize its exposure toterrestrial RFI. Radio noise could be espe-cially mitigated if a crater were selected asa landing site. The craters Saha [15, 16],Tsiolkovsky [9, 17], Malapert [9, 18], andDaedalus [10, 14] have been chosen in past lu- nar radio-astronomy proposals. Crater wallscould block out interference originating fromthe Earth-Moon L4 and L5 points, as well asfrom lunar orbiters. The surface could alsoprovide a stable platform for the construc-tion of a larger dish, perhaps exploiting thecurved geometry of a crater itself [19]. A setof smaller devices could also be positionedacross the surface, forming an interferometersimilar to LOFAR. The surface would also al-low for longer continuous observations thanan orbiter. To minimize the radio noise fromthe Sun, observations should be conductedduring the ∼
14 day lunar night [3, 4].One possible constraint on a surface-basedmission is that a large battery pack wouldlikely be needed to power observations dur-ing the lunar night. Current solar-poweredinstruments on the farside, such as China’sChang’e 4 lander and Yutu 2 rover, shut downduring the night to conserve power. How-ever, a radio telescope would be operationalprimarily during the night, and hence con-straints on lander mass, and therefore on bat-tery capacity, may limit the active observa-tion time of the telescope to a fraction of thenight.Communication would also be a challenge fora surface-based radio telescope. The obvi-ous RFI benefit of never being in line-of-sightwith Earth also makes it impossible to com-municate without using a relay satellite. Cur-rently, the only such satellite is China’s Que-qiao, which orbits the Earth-Moon L2 point.In the next decade other options may becomeavailable for relaying communications.
A telescope in lunar orbit would not facethe communications and power challenges of3 a) RFI from the UHF Satcom (b) RFI from Iridium Satellites
Figure 4: RFI observed from the MeerKAT telescope site in South Africa. The MeerKAT instru-ment will perform an unprecedented SETI survey of one million stars as part of the BreakthroughListen Initiative. The presence of RFI at the telescope will increase the complexity of the dataanalysis associated with the campaign. A lunar observatory would not be exposed to this interfer-ence. (a)(b)
Figure 5: Jupiter being an emitter of low-frequency radio waves, some observations may be bestconducted when both the Sun and Jupiter are obscured [3]. (a) displays altitude over time ofthe Sun and Jupiter for a telescope located at the lunar farside (0 o N, 180 o W). Green denotesperiods where both the Sun and Jupiter are below the horizon. The length of this window variesthroughout the year, as the relative positions of Jupiter and the Sun change from the perspective ofthe Earth-system. (b) depicts the fraction of time during which both bodies are obscured, averagedover 60-day windows, over the next ten years. Some periods of the year will be more favorable forobserving than others. v tocomplete, but also increases mission complex-ity and risk, as was recently shown in 2019 bythe Israeli ’Beresheet’ and Indian ’Vikram’lunar landing failures. An orbiter may alsobe able to support a larger antenna than alander of equivalent mass, since the weight-lessness of orbit obviates the need for certainstructural elements that would be necessaryat the surface under lunar gravity. There isalready precedent for large radio telescopesin space. The RAE-2 orbiter had an impres-sive antenna length of 229 meters [11]. Doc-uments leaked by Edward Snowden revealedthe details of high-altitude SIGINT satelliteslaunched by U.S. intelligence services, a fewof which feature a 20-30 meter unfurlable re-flector dish [20].There are some disadvantages to an orbiter-based SETI mission. RFI may might not beattenuated as much as on the farside surface.Also, due to the Moon’s gravitational lumpi-ness, most lunar orbits are inherently unsta-ble. For instance, the PFS-2 subsatellite (de-ployed during Apollo 16), lasted only 35 daysbefore crashing into the surface [21]. For-tunately, there exist several “frozen orbits”which could enable several years of observa-tions from lunar orbit [22, 23]. Further workshould be done on determining the best or-bital parameters of such a mission. When selecting between mission concepts,several additional questions should be con-sidered. A major priority should be obtain- ing a more precise characterization of the lu-nar RFI environment. Particularly, how hasChina’s Chang’e 4 mission affected the RFIenvironment of the farside? How could futurelunar missions, like NASA’s proposed Gate-way space station, interfere with SETI obser-vations? Also, what interference can we ex-pect from artificial satellites and robotic lan-ders elsewhere in the solar system? Further-more, what might the hardware look like ona lunar radio telescope?
Whether performed from the lunar surface orfrom orbit, Moon-based radio astronomy of-fers unique advantages for SETI. Critically,recent trends conspire to make such a missionnot only increasingly feasible, but also in-creasingly necessary. The reduction in satel-lite launch costs [24] and the popularizationof smaller satellite buses is leading to anever greater number of satellites being putin Earth orbit. SpaceX’s StarLink constella-tion alone may contribute tens of thousandsof new satellites to the already RFI-denseswarm around the Earth. This will furthercomplicate Earth-surface-based SETI obser-vation campaigns. However, the same eco-nomic and technological forces which are en-abling this ramping up of satellite launchesalso make a lunar SETI mission more feasible.Small organizations now routinely place rela-tively inexpensive satellites into orbit. Hawk-Eye 360, a small company based out of Vir-ginia, has managed to design, build, andlaunch three satellites for the purpose of de-tecting and precisely locating radio sourceson the surface of the Earth. These missionsand others form a rough blueprint for, andsignal the increasing feasibility of, sending asmall instrument dedicated to SETI to the5oon. Such a mission would enable a de-tailed survey of the lunar RFI environment,and act as a proof of concept for more sophis-ticated missions in the future. A lunar SETImission would mark the beginning of a newera in the history of SETI, where an increas-ing human presence in space is accompaniedby an expanding ability to discover extrater-restrial life other than our own.
We thank Claire Webb, David MacMahon,Steve Croft, Howard Isaacson, Julia De-Marines and especially Daniel Czech for theirsuggestions and feedback. Thanks to BraamOtto for contributing the waterfall plots forFigure 4. This work was supported as partof the Breakthrough Listen Initiative, spon-sored by the Breakthrough Prize Foundation.
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