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Home , Dark Ages Radio Explorer, Lunar space elevator, Sinus Medii, Surveyor 6, Surveyor program

arXiv:1609.00709v1 [physics.pop-ph] 31 Aug 2016 rpitsbitdt n cetdb pc oiy current Policy; Space by accepted and to submitted Preprint [email protected] [email protected] o oetivsmn.Sinewl hsbnfi swl scommer as well as benefit thus rock will Keywords: chemical Science versus grad investment. costs gravitational modest transport a by str for in or capabilit tethers, reduction rotation the scale by major enhancing promise planetary either and stabilized use cost long missions kilometers the These inf lowering to explorers. leading time, human missions technolo the considers current ma shortening paper with tether This of built in be polymers. improvements can tether considerable th available however, but (LSE), without bec Tsilokovsky, Elevator possible perhaps Space to exploration, be back space not for dates would plans elevator near-term space in a considered of concept The Abstract iueo ul[,5.Asaeeeao sa is elevator space A 5]. [1, expen- fuel the of without diture low and at velocities surfaces planetary relative from to ma- and moving used to systems terial be transportation may up tethers set ro- such called a [4]; are and tovators surface tip satellite cancel the or to between planetary as motion so relative rotate the that Those tension. elevators space both for 3]. and 2, tethers, [1, tools planetary-scale dynamic from of as benefit greatly use would the space of ment Introduction 1. refligttesms oaet tyin stay to rotate must tethers Free-flying develop- and exploration term long The mi addresses: Email pc lvtr ua xlrto,lresaestructures space large exploration, lunar elevator, space a seodIiitvsLC ..Bx11 lfo igna20 Virginia Clifton 141, Box P.O. LLC, Initiatives Asteroid cetfi euno ua Elevator Lunar a of Return Scientific CF Radley) (C.F. Eubanks), (T.M. ..Eubanks T.M. a ..Radley C.F. , omthterltv eoiybetween velocity relative used surface. the be lunar would match the DSTP the on to of sampling area rotation selected The touch-and-go scientific a doing substantial of by a prototype provide return the and of technology LSE to the intended test is both mission (DSTP) Pathfinder mass. of any center plus the the tether above on the [7] elevator) of lunar mass or a [6] (for elevator) forces terrestrial tidal a force (for be- centrifugal rotation tether the of the either on by balanced ing of force orbit, stationary the a its in with to) be attached must is mass it of center body the to of elevator respect surface space with the a (stationary out static for remain order body to In celestial [2]. a space into from struc- stretching orbiting or ture static a as deployed tether h rpsdDe pc Tether Space Deep proposed The yi rs.Spebr5 2016 5, September press. in ly a t,i ai timeframe, rapid a in ets, ueatretilelevator terrestrial a ause nsmn huad of thousands many ings yuigcommercially using gy ilactivities. cial yaentcommonly not are ey et.Teesystems These ients. 124 atutr capable rastructure e frbtcand robotic of ies eil Lunar A terial. its lower tip and the during a molecular-weight polyethylene (UHMWPE) flyby, allowing for the collection of surface (brand name Dyneema R ) [8] and poly- samples from a suitable scientific target, phenylenebenzobisoxazole (PBO) (brand in the default mission from the floor of name Zylon R ) [9] are inexpensive and avail- Crater in the lunar South polar able in large quantities, ample for LSE teth- region. The collected material would then ers. Even a prototype LSE, deployed by a be returned to by the release of a single launch of an existing launch vehicle, return capsule roughly one half rotation would serve as the linchpin in a lunar deliv- period later, when elevator tip velocity is ery service, the LSEI, capable of transport- appropriate for a direct return trajectory. ing up to 5 tonnes of material to and from After sample release, the DSTP would the lunar surface per year and supporting a continue into deep space, allowing for long wide variety of scientific research, including term observations of the performance and on and near the lunar surface, at the L1 La- micrometeorite resistance of the tether in grange point, and deep into cislunar space the space environment and the first test at the counterweight. of kilometric radio interferometry in deep The ∆V required for a to ascend space. from lunar surface to EML-1 is 2.7 km The proposed LSE Infrastructure (LSEI), s−1. Goff [10] showed that the typical pay- the first true on any celestial load mass fraction for such a rocket is 34%, body, is planned as a follow-on to the DSTP. ∼1/3. A rocket which puts the 49 tonnes The LSEI would be a very long tether ex- LSE at EML-1 would otherwise be capable tending from the lunar Surface, through the of depositing 16 tonnes on to the lunar sur- Earth-Moon Lagrange L1 point (EML-1) face. So for LSE payload of 0.1 tonnes, this 56,000 km above the Moon, and on into cis- is equivalent to 16/0.1 = 160 payload land- lunar space. The LSEI prototype, scaled to ing cycles, which is the number of cycles be deployable with one launch of a heavy lift to recoup the LSE launch cost. For sample vehicle, would be able to lift roughly 5 tons return, another factor of three applies, so of lunar samples per year, and deploy a sim- ∼53 sample return cycles would recoup the ilar quantity of equipment onto the lunar launch cost. surface. The LSEI would enhance a crewed While the initial LSEI would not be able Deep Space Habitat (DSH) at EML-1, for to deliver human passengers to and from the a small fraction of the total DSH cost by, lunar surface, a functioning LSEI prototype for example, supporting tele-robotic explo- would enhance the capabilities of humans ration on the surface. Similar scientific work in a Deep Space Habitat (DSH) in a Lis- could be accomplished by a farside LSE, sajous orbit around EML-1, as envisioned which could also provide real time commu- in the 2011 Global Exploration Roadmap nications to the farside, opening an entire [11, 12]. The LSEI would: enable astro- lunar hemisphere to exploration. nauts to deliver rovers and instruments to Of the possible near-term space ele- the lunar surface, teleoperate that equip- vator deployments (Earth, Moon, Mars), ment from only 56,000 km altitude, lift se- a lunar nearside elevator is undoubtedly lected surface samples to EML-1, evaluate the technically feasible. Modern those samples, and use that evaluation to high strength polymers such as Ultra-high- direct the acquisition of further samples. 2 2. The Scientific Goals of the Deep surface imagery returned during the sam- Pathfinder ple collection process will help to assess the nature and distribution of volatiles, even The DSTP would be with a if sample return is not successful. (Por- 5000 kilometer long tether, with a tether tions of the Shackleton Crater rim are in mass of 2228 kg and a total system mass sunlight at any time of month [16], provid- of 3043 kg, rotating every 2.44 hours with ing illumination of the crater bottom that a sampling probe on the far tip [13]. The is typically several times full-Moon illumi- DSTP would flyby the Moon as a rotova- nation on Earth.) The search for lunar tor [4] to collect lunar samples in a touch- volatiles ranks high in the decadal surveys of and-go manner, followed by a cruise in deep planetary science [14], and the Permanently space as an engineering test of the tether Shadowed Regions (PSRs) on the Moon are technology needed for the first LSE [13]. arguably the easiest such locations to ac- The DSTP would be the first tether actually cess in the system. The PSRs con- deployed as a rotovator, rotating to match tain an important scientific record of the the velocity of its sampling tip with the history of volatiles in the inner Solar Sys- lunar surface, which would enable sample tem, and a potential resource for future eco- acquisition from a scientifically interesting nomic development [17, 18]. These regions region, such as the permanently shadowed have been the target of intense scientific in- regions at the lunar poles. Approximately terest in the last decade, and were the target 2 hours after sample collection the DSTP of the LCROSS impactor [19], but surface would use its rotational velocity to sling- sampling by landers or rovers is complicated shot the sample back to Earth for a ballis- by the lack of solar power and direct com- tic reentry with a minimal expenditure of munications with Earth in a PSR. fuel. The DSTP would then continue on Figure 1 shows the general DSTP tra- into deep space for a long-duration expo- jectory near the Moon in a 2-body gravi- sure test of the radiation and micromete- tational simulation, while Figures 2 and 3 orite resistance of the tether’s design, and show the DSTP tether positions one hour also a test of kilometric radio interferome- before and just after the time of sampling, try in deep space [13]. respectively. Figure 4, an enlargement of The primary scientific justification of the Figure 3 (inverted so that the crater floor DSTP mission would be lunar sample re- is at the bottom), shows that the tether de- turn; its lunar science objectives address scends almost vertically at the lunar sur- every one of goals in the “Lunar Polar face; to a surface observer the motion of the Volatiles and Associated Processes” white probe up and down inside the crater would paper submitted to the 2011 Decadal Sur- appear to be almost completely vertical, en- vey [14]. Current DSTP mission planning abling sampling from topographically rough has focused on sampling volatiles on the regions. In addition, there is a clear line- shadowed floor of Shackleton Crater at the of-sight back to the main spacecraft at the lunar South Pole, which is a cold-trap and other end of the tether, allowing for direct should collect substantial amounts of sur- relay communication with Earth at the time face volatiles from collisions and out-gassing of sampling. on other areas of the Moon [15]. Near- Shackleton Crater sits on the boundary 3 Tether at 0.003 hours Counterweight 15000 15000 Counterweight Probe Probe 10000 10000 5000 5000 Moon Moon 0 0 -5000 -5000 -10000 -10000 -15000 -15000 Perpindicular to Lunar orbital plane (km)

Perpindicular to Lunar orbital plane (km) -30000 -20000 -10000 0 10000 20000 30000 -30000 -20000 -10000 0 10000 20000 30000 Earth-Moon Radial Direction (km) Earth-Moon Radial Direction (km)

Figure 1: Trajectories of the two tips of the DSTP Figure 3: The DSTP 10 seconds after the time during the entire lunar sample return period, as of the touch-and-go sampling, from the simulation seen from a selenocentric reference frame [13]. The shown in Figure 1. main spacecraft, assumed to include the upper stage as a counterweight, is considerably more mas- sive than the probe and is thus closer to the tether center-of-mass, which executes a smooth ballistic motion. This Figure represents 6 hours of total mo- tion.

3

Sunlight 2

1

0

-1 Height km

-2

-3 Tether at -1.000 hours Crater Profile 15000 Counterweight Tether Probe Probe 10.0 seconds after contact 10000 -4 -15 -10 -5 0 5 5000 Shackleton Crater X Axis (km) Moon 0 -5000 Figure 4: A cross sectional view of Shackleton -10000 Crater from Lidar data [20], with the DSTP tether -15000 and probe 10 seconds after closest approach, when Perpindicular to Lunar orbital plane (km) -30000 -20000 -10000 0 10000 20000 30000 it is ∼100 m above the crater floor [13]. This image Earth-Moon Radial Direction (km) is from the same simulation sampled at the same time as for Figure 3. The green horizontal arrows indicate the maximum illumination of the crater Figure 2: The DSTP one hour before the touch- by the Sun; the crater interior below these lines is and-go sampling, from the simulation shown in Fig- permanently shadowed (the sampling probe is in ure 1. shadow for ∼2 minutes).

4 of the older and much larger South Pole- Aitken Basin, an ∼2500-km diameter im- pact basin which brought up material from deep inside the lunar interior. Given the surface albedo results from Selene [16], it is highly likely that a sample collection from the floor of Shackleton Crater would include rock or samples from the South Pole-Aitken Basin [21]. A proposed South PoleAitken Basin Sample-Return (SPA-SR) mission was highly ranked by the National Research Council Planetary Science Decadal Survey Figure 5: The components of the LSEI LSE, to [14] and was suggested for a New Frontiers scale, superimposed on a image of the Earth-Moon class mission, with a cost cap of $1.0 billion. system from the spacecraft. The DSTP would provide a first look at both South Pole-Aitken Basin material, dio Astronomy Explorer B [23] indicate that and at the volatiles in a PSR, within a at the lunar distance the Earth interference NASA Discovery mission cost cap. is typically about 2 orders of magnitude Another scientific goal of the DSTP is above the celestial background. The ter- to deploy the first radio interferometer for restrial interference should thus decline to a the kilometric spectral region between 10 manageable level when the DSTP is &3 mil- kHz and 1 MHz, which is largely unex- lion km from Earth, or ∼2 weeks into the plored for radio astronomy as these wave- extended deep space mission. lengths do not penetrate the iono- sphere. The proposed radio interferometer, 3. The Prototype Lunar Space Eleva- the Dark Ages Pathfinder (DAP), would tor Infrastructure consist of two 10-km dipoles attached to ei- ther tip of the DSTP, providing an interfer- The LSEI currently is planned to be ex- ometric baseline of ∼5000 km and allowing ecuted in a single Discovery class mission, for rotational synthesis as the tether rotates. starting with the delivery of 58,500 kg of At 1 MHz this baseline would allow for an Zylon HM fiber plus associated equipment angular resolution of approximately 1◦, al- to the EML-1 Lagrange site. Figure 5 shows lowing detection of candidate point sources to scale the major components of LSEI, and limited mapping of extended sources. the string, the Landing Platform (LP), the The DAP would complement the proposed supply depot at EML-1, and the Counter- (DARE) [22], Weight (CW), while Table 1 provides basic which is to operate over a higher frequency information about the default LSEI for both radio bandpass of 40-120 MHz. The DAPs lunar hemispheres. ability to distinguish solar system, galactic The LP attached to the tether descends and cosmological sources from terrestrial in- to the lunar surface in the initial prototype terference will improve as the distance from deployment. After landing, we refer to it the Earth increases. Observations with Ra- as the Landing Station (LS); the planned 5 LS location is , near 0◦ Latitude and Longitude on the lunar nearside. Fig- ure 6 shows the topography of Sinus Medii from the [24]. There are 3 natural locations for long-term scientific observations from the LSEI, the LS on the surface, the deployment platform at EML-1, and the Counterweight (CW) at the far end Figure 6: Sinus Medii from Surveyor 6, taken about of the elevator. All 3 locations should be 44 km from the proposed landing site (Figure 7-41 instrumented, both for the scientific return from [24]). and to monitor the elevator’s performance. The primary initial science goal of the tains concentrations of lunar volatiles which LSEI prototype mission is the return of the can be used for propulsion and life sup- lunar samples to Earth. LSEI will take a port. The LS will also become an important core sample upon landing and will deliver transit point for long distance lunar rovers one or more microrovers to the lunar sur- which will recover samples over large dis- face to assist in collecting surface samples. tances much more cheaply than using rocket LSEI will return up to 100 kg of samples landers. in the first Lift from the lunar surface, us- ing a reusable solar-powered lifter. Sample LSEI plans to use Single Cube Retrore- returns can be done without fuel using a flectors (SCR) as Laser ranging targets [29] nearside LSE, as material (in a suitable re- for navigation during deployment of the turn capsule) could be simply released at LSE and the Landing Platform. The SCR the right moment for a direct reentry tra- would become a permanent addition to the jectory to a desired landing location; any- Lunar Laser Ranging (LLR) retroreflector thing separated from the LSE an altitude network. The LSEI would thus augment & 220,670 km above lunar surface will re- LLR studies of basic physics, lunar dy- enter the Earth’s atmosphere in ∼1.4 days namics, and Earth-Moon celestial mechan- at a velocity of ∼10.9 km s−1 without any ics [30]. expenditure of fuel. This same technique Poorly understood electrostatic levitation can be used to return high value ore sam- and transport of dust happens in regions ples or mining products from a lunar mining near the lunar terminator [31], and may be enterprise. important in the covering of PSR volatiles There are some important sites with ma- over geologic time. If electrostatically- terials of economic interest near to the levitated dust is present LSEI will sample EML-1 LS. The nearby crater Lalande is it in situ with passive Aerogel collectors or known to have some of the highest con- electrets (permanently charged materials), centrations of surface KREEP deposits on deployed at altitude during periods with a the Moon [26], as well as 19 impact melt elevator lift is not scheduled. pits, possible sites for volatiles [27]. Also, The CW can observe the Earth from over nearby mare areas appear to have elevated 100,000 km away, outside of the existing concentrations of Helium-3 [28]. Regard- satellite constellations. It will be able to less of landing site, lowland regolith con- observe the magnetic and charged 6 Parameter Elevator Location Nearside Farside Tether Material Zylon PBO Zylon PBO Length 278544km 297308km Mass 48700kg 48700kg Surface Lift Capacity 128 kg 110 kg TotalTaper(max/minarea) 2.49 2.49 Maximum Force 517 N 446 N Landing Site 0◦ E 0◦ N 180◦ E 0◦ N

Table 1: Prototype Lunar Elevators [25]. environment in the Earths magnetopause as to deploy a multiline fail-safe system such the CW goes in and out of the Earths mag- as the “Hoytether” [33] to achieve a design netosphere twice per lunar month. life-time of 5 years; testing of the chosen In addition, EML-1 is a logical location micrometeorite protection system in deep for the observation of the nearside of the space would be one of the primary engineer- Moon. One fairly small optical telescope ing goals of the DSTP mission. (20 cm) could continuously observe the en- tire nearside, searching for meteorite im- 4. A Farside Lunar Space Elevator pacts and transient lunar phenomena, and An elevator on the lunar farside (with a also be able to detect and characterize the ◦ landing point at or near longitude 180 , lat- orbits of close lunar orbiters. ◦ itude 0 ) could fulfill many of the scientific Various factors could limit the useful life and logistical goals of a nearside LSEI, but of an LSE, with micrometeoroid impacts be- would also provide unique advantages of its ing an especially serious threat to tether own [25, 34]. longevity (in cislunar space there is no sig- nificant flux of man-made orbital debris). 4.1. Sample Return from the Lunar Farside The LSEI would be a very thin tether, with To date, all lunar sample returns have a radius of ∼0.2 mm if it were just a sin- been from from 9 sites on the lunar near- gle strand. Such a strand would be broken side [35]. The LSE in Table 1 assume nat- if impacted by a meteorite with a mass as ural elevator landing sites (i.e., directly be- small as 10−5 gm. A variety of methods neath the ), as these seem have been used to determine the microm- most appropriate for a initial elevator de- eteorite flux in near-Earth and near-lunar ployment. An EML-2 LSE would thus pro- space [31, 32]; the cumulative flux of me- vide an immediate sample return from a teorites of this mass and larger is ∼10−8 previously unsampled region (and, indeed, m−2 s−1. The LSEI is sufficiently long that from a previously unsampled hemisphere). its surface area would be ∼105 m2; if the The EML-2 landing site (Figure 7) is near LSEI were made from a single strand it Lipskiy Crater and just North of Daedalus would have a micrometeorite impact life- Crater in very rugged and heavily cratered time measured in hours. The LSEI will have terrain in the lunar highlands. 7 elevator as a antenna tower [34]. Decamet- ric and kilometric radio astronomy could be conducted during the lunar night, when ra- dio interference from the Sun is also blocked and when solar powered climbers would not be using the near surface part of the LSE. The farside is recommended as a radio quiet zone by the International Telecom- munications Union under ITU-R RA.479. Maccone [37] has proposed a more exten- sive Protected Antipode Circle [PAC], a more extensive protection zone than that proposed by the ITU. The PAC centered around the antipode on the farside spanning an angle of 30 in longitude and latitude in Figure 7: Image AS11-44-6607, taken by as- all radial directions from the antipode. The tronaut Michael Collins during the mis- sion, July, 1969, from an altitude of ∼110 km. This PAC is the most shielded area of the farside, image shows the very rough highland terrain at the with an expected attenuation of man-made farside elevator LS (the default LS is towards the RFI of 100 dB or higher. Neither the PAC upper left of this image). This part of the lunar nor ITU farside rules have been adopted by terrain has never been explored or sampled by any any law making body; in any case a farside surface mission. LSE would have to avoid interference with farside radio observatories, whether at the 4.2. Farside Radio Astronomy LS or installed elsewhere by other missions. The farside of the Moon is totally shielded from terrestrial radio transmissions, and is 4.3. Other Farside Science arguably the best place in the solar sys- An EML-2 LSE would enable a variety of tem for a radio astronomy base. The Earth other farside science, including the monitor- is a major source of radio noise and in- ing of and fields in near interplan- terference, both natural and artificial, and etary space at EML-2 and at the far end its ionosphere blocks ground based observa- of the elevator, which would enable deep tions at frequencies . 10 MHz. The farside monitoring along the Earth magnetotail at LSE LS at 180◦ E longitude point would Full Moon. From a EML-2 station almost be an ideal location for such a farside radio the entire farside could be monitored for astronomy base, far from terrestrial inter- meteor impacts, complementing the terres- ference and with a view of the entire sky. trial monitoring of the nearside for impacts Many have proposed radiotelescopes there, [38]. The monitoring of the time and lo- e.g. Jester and Falcke [36]; an EML-2 LSEI cation of farside impacts will be especially would considerably reduce the cost of build- important if a nearside lunar seismological ing and supplying a lunar farside radio tele- network is re-established, as impacts on the scope system, enabling both the installation farside will provide seismic waves traversing of antennas on the surface at the LS, as well the lunar core to nearside seismometers. A as vertically using the lower portion of the EML-2 LSE would extend the lunar seis- 8 mological network to the farside itself, pro- In addition, the velocity of the counter- viding a truly global lunar monitoring net- weight of a farside LSE would provide sig- work. Even one seismometer at the farside nificant ∆V for injection into a trans-Mars LS would enable the seismic study of the en- orbit. Material lifted past EML-2 could be tire lunar interior [39] in combination with sent to Mars with a minimal investment in the locations and times of nearside impacts fuel, helping to support long-term colonies provided the ongoing terrestrial monitoring on that planet. of meteorite impacts visible lunar surface [38]. 6. Conclusions

4.4. Farside Communications Relay The DSTP and the LSEI are crucial first Communications has always been a severe steps in the development of space elevators, complication for the engineering of missions and in future tether missions, but can and to the lunar farside, as there is no direct must be justified on the basis of returned line-of-site between the Earth and any lo- science, in addition to their engineering re- cation deep in the farside ( bring turn. Building the DSTP is feasible, has an occasional line-of-sight to locations at the exciting scientific return and would be natu- farside-nearside boundary). A EML-2 LSE ral first step in developing an elevator-based would provide a communications mast vis- lunar infrastructure program, leading to the ible from almost any location on the lunar construction of the LSEI and in due course farside, and could thus serve as a relay for the development of a true transportation in- communications with the Earth. A farside frastructure for the inner solar system. LSE communications relay would literally open an entire lunar hemisphere for further References exploration. [1] H. Moravec, A non-synchronous orbital sky- hook, Journal of the Astronautical Sciences 25 5. Facilitation of Mars Exploration (1977) 307–322. [2] P. A. Swan, D. I. Raitt, C. W. Swan, R. E. Ishimatsu et al. [40] have shown that the Penny, J. M. Knapman, Space Elevators: An launch weight of missions to Mars can be re- Assessment of the Technological Feasibility duced by 68% by using oxygen derived from and the Way Forward, International Academy of Astronautics, 2013. the Moon as propellant. An LSE would al- [3] P. A. Swan, Opening up Earth- low relatively inexpensive transport of lunar Moon Enterprise with a Space Ele- regolith to a cislunar oxygen production fa- vator, New Space 3 (2015) 213–217. cility, further reducing transportation costs. doi:10.1089/space.2015.0025. [4] R. L. 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