Scientific Return of a Lunar Elevator T.M. Eubanksa, C.F. Radleya aAsteroid Initiatives LLC, P.O. Box 141, Clifton Virginia 20124 Abstract The concept of a space elevator dates back to Tsilokovsky, but they are not commonly considered in near-term plans for space exploration, perhaps because a terrestrial elevator would not be possible without considerable improvements in tether material. A Lunar Space Elevator (LSE), however, can be built with current technology using commercially available tether polymers. This paper considers missions leading to infrastructure capable of shortening the time, lowering the cost and enhancing the capabilities of robotic and human explorers. These missions use planetary scale tethers, strings many thousands of kilometers long stabilized either by rotation or by gravitational gradients. These systems promise major reduction in transport costs versus chemical rockets, in a rapid timeframe, for a modest investment. Science will thus benefit as well as commercial activities. Keywords: space elevator, lunar exploration, large space structures 1. Introduction tether deployed as a static or orbiting struc- ture stretching from a celestial body out The long term exploration and develop- into space [2]. In order for a space elevator ment of space would greatly benefit from to remain static (stationary with respect to the use of planetary-scale tethers, both the surface of the body it is attached to) its as dynamic tools and for space elevators center of mass must be in a stationary orbit, [1, 2, 3]. with the force of gravity on the tether be- Free-flying tethers must rotate to stay in ing balanced by either the centrifugal force tension. Those that rotate so as to cancel of rotation (for a terrestrial elevator) [6] or the relative motion between the tip and a tidal forces (for a lunar elevator) [7] on the planetary or satellite surface are called ro- mass of the tether plus any counterweight tovators [4]; such tethers may be used to above the center of mass. set up transportation systems moving ma- arXiv:1609.00709v1 [physics.pop-ph] 31 Aug 2016 The proposed Deep Space Tether terial to and from planetary surfaces at low Pathfinder (DSTP) mission is intended to relative velocities and without the expen- both test the technology of the prototype diture of fuel [1, 5]. A space elevator is a LSE and provide a substantial scientific return by doing touch-and-go sampling Email addresses: of a selected area on the lunar surface. [email protected] (T.M. Eubanks), The rotation of the DSTP would be used [email protected] (C.F. Radley) to match the relative velocity between Preprint submitted to and accepted by Space Policy; currently in press. September 5, 2016 its lower tip and the Moon 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- Shackleton 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 Earth 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 rocket 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 space elevator 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 most 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- Space Tether Pathfinder ple collection process will help to assess the nature and distribution of volatiles, even The DSTP would be spacecraft 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 solar 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.