Crawford, Smith: MoonLITE MoonLITE: A UK-led mission to the Moon Downloaded from https://academic.oup.com/astrogeo/article/49/3/3.11/218588 by guest on 24 September 2021

Ian 1: Farside view of the Moon Crawford as seen by the and Alan Clementine spacecraft. Smith Penetrators discuss the launched by the MoonLITE orbiter scientific would allow surface objectives of investigations in areas not visited by Luna, the proposed Surveyor or Apollo missions. MoonLITE mission. (NASA/JPL/USGS)

hile the surface missions to during the Apollo programme (see Wiec- the Moon of the 1960s and 1970s zorek et al. 2006, for a review). Moreover, the Wachieved a great deal, scientifically recent remote-sensing missions have themselves much was also left unresolved. The recent Abstract raised questions that will require new surface plethora of lunar missions (flown or proposed) measurements for their resolution, of which reflects a resurgence in interest in the Moon, not MoonLITE is a proposal for a UK-led one of the most important is the circumstantial only in its own right, but also as a recorder of mission to the Moon that will place four evidence for water ice, and by implication other the early history of the Earth–Moon system and penetrators in the lunar surface in order volatiles, within permanently shaded craters at of the interplanetary environment 1 AU from the to make geochemical and geophysical the lunar poles (Feldman et al. 1998). Sun (e.g. Spudis 1996, Crawford 2004, Jolliff measurements that are impossible from In order to make significant further progress et al. 2006, NRC 2007). Although the Clemen- orbit. It has the potential to make major in lunar science, and to make better use of the tine and Lunar Prospector missions have greatly contributions to lunar science, while at lunar geological record to understand solar sys- added to our knowledge of the geochemical and the same time providing knowledge that tem evolution more generally, it will be neces- mineralogical makeup of the lunar surface, and will be of central importance in planning sary to return to the surface. It will be especially these observations will soon be supplemented by future human missions to the Moon. Plus, important to make geophysical and geochemi- results from Kaguya, Chang’e-1, Chandrayaan-1 MoonLITE will demonstrate technologies cal measurements from areas not visited by and Lunar Reconnaissance Orbiter, our knowl- that will have wide applications for the previous missions, including the poles and the edge of the lunar interior is limited and relies exploration of other solar system bodies. farside. MoonLITE (Moon Lightweight Inte- largely on geophysical measurements made rior and Telecommunications Experiment) is a

A&G • June 2008 • Vol. 49 3.11 Crawford, Smith: MoonLITE

Throwing light on MoonLITE

(a) (b) Downloaded from https://academic.oup.com/astrogeo/article/49/3/3.11/218588 by guest on 24 September 2021 (c) (d)

2:Stages in the development of MoonLITE. (a) Cross-section of the type of penetrator to be used, with payload bays at the rear. (MSSL/ QinetiQ) (b) Full-scale penetrator outer body (with tip removed) for impact trials. (QinetiQ) (c) Impact trials showing penetrator-like object passing through 2 m of concrete. (QinetiQ) (d) An artist’s impression of the MoonLITE orbiter just after the release of one of its penetrators. (SSTL) proposed UK-led lunar science mission that To obtain “ground truth” geochemical data seismometer network, which would address the ● will contribute to these objectives by emplac- to complement orbital remote-sensing observa- following scientific questions. ing four scientific penetrators at widely spaced tions; and Size and physical state of the lunar core. Such ● localities on the lunar surface (Gao et al. 2008). To collect in situ surface data that will help in knowledge of the lunar core as we have has been ● In addition, a telecommunications experiment the planning of future lunar exploration. obtained from studies of the Moon’s moment of (the “TE” in “MoonLITE”) will be used to These top-level science objectives require that inertia and physical librations, and electromag- develop expertise in Moon–Earth communica- the penetrators emplace instruments capable of netic induction studies (Wieczorek et al. 2006). tions that will benefit UK involvement in future contributing to at least four different areas of These studies favour a small (R < 400 km), lunar missions. scientific investigation: seismology, heat-flow, partially liquid core, with suggested composi- In 2007 MoonLITE was considered by the geochemical analysis, and volatile detection/ tions ranging from iron–nickel, Fe–FeS alloy, BNSC–NASA Joint Working Group (JWG characterization. These are discussed in more to molten silicates. Confirmation of the size, 2008), which was established to explore avenues detail below. composition and physical state of a lunar core for UK–US collaboration in space exploration would have profound impacts on our under- following the signing of a statement of intent Seismology standing of the Moon’s origin, mantle evolu- in April. This report strongly endorsed the Seismology is the most powerful geophysical tion and magnetic history. For these reasons, MoonLITE concept, describing it as an “inspi- tool available for determining the interior struc- constraining the nature of the Moon’s core is a rational” project and “the primary mission for ture of a planetary body. However, the only top scientific priority of the penetrator-deployed potential [UK–US] cooperation”. Cooperation object other than the Earth where it has been seismic network. with other partners is also a possibility. In successfully applied is the Moon, where the Deep structure of the lunar mantle. One of ● the coming months, MoonLITE will undergo Apollo seismometers yielded important infor- the main contributions lunar studies can make an assessment of the science case by an inter­ mation on the Moon’s natural seismic activity, to planetary science is an enhanced understand- national peer review panel and a formal Phase-A and the structure of the lunar crust and upper ing of the internal differentiation processes that technical study. If approved for implementation mantle (Goins et al. 1981, Lognonné 2005). take place immediately after the accretion of a it will fill an important gap within the proposed However, the deep interior of the Moon was terrestrial planet. Magma oceans are likely to international lunar mission portfolio and help only very loosely constrained by Apollo seis- have been a common phase in the early evo- facilitate the future scientific and ultimately mology because the network was geographi- lution of rocky planets and, in contrast to the human exploration of the Moon. cally limited (essentially an equilateral triangle more evolved mantles of the larger terrestrial on the centre of the nearside between the Apollo planets, the structure of the lunar mantle may Scientific objectives 12/14, 15 and 16 sites; figure 3), so the informa- preserve a record of these early times. Seismol- The principal scientific objectives of the Moon- tion obtained on crustal thickness and mantle ogy may help elucidate these processes by con- LITE penetrator mission are: structure may not be globally representative. straining the initial depth of the magma ocean To further understanding of the origin, differ- There is now a pressing need for a more widely and its mineralogy (Lognonné 2003). Again, ● entiation, internal structure and early geological spaced network of lunar seismic stations, new, and more widely spaced, seismic data are evolution of the Moon; including stations at high latitudes and on the required if this record is to be deciphered. To obtain a better understanding of the origin farside. Penetrators delivered from orbit are Thickness of the farside lunar crust. Reinter- ● ● and flux of volatiles in the Earth–Moon system; ideally suited as a means of emplacing a global pretations of the Apollo seismic data have

3.12 A&G • June 2008 • Vol. 49 Crawford, Smith: MoonLITE now constrained the thickness of the nearside least of the abundances of the major rock-form- both of which will be ejected before impact. anorthositic crust to between about 30 and ing elements (e.g. Mg, Al, Si, Ca, Fe and Ti). Each penetrator will impact the lunar regolith 40 km (e.g. Wieczorek et al. 2006, Lognonné This could be achieved by penetrator-deployed at a speed of ~300 m s–1 (equivalent to a free 2003). However, the thickness of the farside X-ray fluorescence spectrometers. As well as fall from 30 km onto the lunar surface). It is crust has not been seismically constrained. teaching us much about the geology of the sites entirely feasible for an instrumented package Estimates based on gravity data are typically in that have yet to be sampled, such measurements to survive an impact at such speeds and a vast the range 70–90 km (Wieczorek et al. 2006) but would provide additional “ground truth” for amount of resource has been devoted to such these are non-unique, and farside seismic meas- the calibration of remote-sensing instruments conditions within a defence context. “Penetra- urements are required to determine the average on forthcoming lunar orbital missions. tors” are common-place within that sector lunar crustal thickness, which has significant and a (limited) range of components are avail- implications for understanding the bulk com- Polar volatiles able off-the-shelf that will survive impacts of position, and thus origin, of the Moon. As is well known, the Lunar Prospector neu- >50 000g (MoonLITE expects up to 10 000g). Studies of natural moonquakes. Understand- tron spectrometer found evidence for enhanced This expertise is by no means purely empirical ● ing natural lunar seismicity, and especially the concentrations of hydrogen at the lunar poles, in nature; a very sophisticated predictive mod- relatively strong (up to magnitude 5) shallow which has been widely interpreted as indicating elling capability also exists. The MoonLITE Downloaded from https://academic.oup.com/astrogeo/article/49/3/3.11/218588 by guest on 24 September 2021 moonquakes, is important both for our knowl- the presence of water ice in the floors of perma- project will tap this capability for a scientific edge of lunar geophysics and the planning of nently shadowed craters (Feldman et al. 1998). end. Moreover, Mars 96 (Surkov and Kremnev future exploration activities (Neal 2006). If water ice is present, it is most likely to have 1998), Deep Space-2 (Smrekar et al. 1999, been derived from comets hitting the lunar sur- 2001), and Lunar‑A (Mizutani et al. 2001) Heat-flow face. The confirmation of water ice (and other penetrator development programmes have over- Measurements of surface heat-flow provide valu- volatiles) at the poles would be important for come many key problems and demonstrated able constraints on the composition and thermal what it will reveal about the flux and composi- survivability in ground tests. evolution of planetary interiors. The lunar heat- tion of cometary volatiles into the inner solar flow was measured at the Apollo 15 and 17 sites system (which is of significant astrobiological Lifetime (Langseth 1976). However, these measurements interest), and also because such volatiles could Each penetrator will be designed to operate for have been subject to numerous reinterpretations be a very valuable resource in the context of one year below the lunar surface. This has sig- (Wieczorek et al. 2006), and in any case may future human exploration of the Moon. We nificant consequences for total energy require- not be representative of lunar heat-flow as a consider that volatile detectors, deployed on ment. It is not proposed to have a detached whole. An important measurement would be to penetrators and landed within permanently body surface element (unlike DS-2), therefore determine the heat-flow as a function of distance shadowed craters, would be a powerful and all power must be generated internally. Moreo- from the Procellarum KREEP Terrain (PKT) on economical means of determining whether or ver, the temperature 3 m below the lunar surface the northwestern part of the lunar nearside (Jol- not scientifically and operationally valuable is estimated to be between 250 K and <100 K liff et al. 2000). Remote sensing measurements deposits of volatiles exist at the lunar poles. depending upon location – the latter figure have determined that the heat-generating ele- referring to permanently shaded polar craters. ments (U, Th, K) are concentrated at the surface Development methodology Lithium-based batteries (providing 500 Watt- in this region, but the question remains whether MoonLITE is envisaged as both a lunar science/ hours) together with radioactive heating units this is a surficial effect (owing to excavation of exploration mission and as a “penetrator dem- (RHU) are proposed. Very-low-power electron- a global underlying KREEP-rich layer by the onstration mission” and the development meth- ics and power-saving operation strategies will Imbrium impact), or whether these elements odology reflects both of these aspects. While it also be employed. are indeed concentrated in the mantle below the is essential that the mission achieves its scientific PKT (Wieczorek et al. 2006, Hagermann and objectives, it is also anticipated that the tech- Communications Tanaka 2006). The latter scenario would predict nological developments therein will have direct A polar orbiting satellite will be used for two- a much higher heatflow in the PKT than else- application to other solar system bodies. The way communications between ground control where, and would have major implications for adopted development methodology is character- and each penetrator. For penetrators located our understanding of mantle evolution (Wiec- ized by the following: away from the lunar poles, communication zorek and Phillips 2000). There is thus a need A scalable, modular design around a core data passes will occur every 15 days with ~90 s of ● to extend these measurements to new localities and power distribution network; contact at each. For polar penetrators the fre- far from the Apollo landing sites (e.g. the polar Model-based impact stress prediction, vali- quency of contact will be much higher, but in ● regions and the farside highlands) and penetra- dated through impact trials, leading to a well- this case the amount of information is still lim- tor deployment of a global heat-flow network defined payload element environment; ited by the available transmitter power. Each would be an attractive means of achieving this. Inclusion of well-proven technologies brought penetrator will be able to transmit 10 Mbits of ● in from outside of the space domain; data during its one-year lifetime. A Lunar-A Geochemistry “Pick-and-mix” payload selection to match study (Mizuno et al. 2000) has analysed the ● The only places on the Moon from which sam- specific mission opportunities. likely communication effects of the overlaying ples have been collected in situ are the six Apollo lunar regolith and associated impact crater. landing sites and the three Soviet Luna sample- Impact return missions. No samples have been returned The penetrator delivery to the lunar surface will Payload from the polar regions or the farside, greatly take place in two stages: the penetrators will be In accordance with the scientific objectives laid limiting our knowledge of lunar geological transferred to lunar orbit as the payload of a out above, the baseline MoonLITE scientific processes. Although additional sample-return polar orbiting communications relay satellite, payload comprises: missions are desirable, this may not be practi- followed by release, de-orbit and descent (Gao et Accelerometers and tilt-meter. Three-axis ● cal in the short term. An alternative would be al. 2008). Each penetrator will have an attached accelerometers will be mounted at the head to make in situ geochemical measurements, at de-orbit motor and attitude control systems, and tail of the penetrator to provide a complete

A&G • June 2008 • Vol. 49 3.13 Crawford, Smith: MoonLITE motion history (position and orientation) dur- undisturbed under the lunar surface for a vast investigator for the MoonLITE Phase-A study. ing impact. A tilt-meter will be essential to period of time (probably hundreds of millions Acknowledgments. We thank our colleagues in the provide for the interpretation of heat flow and of years) and so represent the ultimate time UK Penetrator Consortium (http://www.mssl.ucl. seismic data. capsules. One possibility would be to engage ac.uk/planetary/missions/UK_Lunar_Penetrator_ Seismometer. A three-axis MEMS-based the public (both within the UK and abroad) in Consortium.doc) for their many contributions in ● microseismometer is proposed, based on novel deciding what legacy we might wish to leave on developing the penetrator concept. MoonLITE micromachined technologies being developed at the Moon in the form of information encoded as a mission concept took initial form in a study Imperial College (e.g. Pike and Standley 2005). on microchips carried in the penetrators. undertaken by Surrey Satellite Technology Ltd These will have a sensitivity and bandwidth (SSTL) on behalf of STFC and we wish to thank comparable to that provided by the Apollo mis- Conclusions Sir Martin Sweeting, Andy Phipps, Yang Gao and sions (see fig. 5 of Gaoet al. 2008). By deploying a range of instruments (including their colleagues at SSTL for their important and Geochemistry package. A miniaturized X‑ray seismometers, heat-flow probes, X-ray spec- ongoing contributions. We further thank Dave ● fluorescence spectrometer is proposed that will trometers and volatile detectors) to diverse loca- Parker (BNSC) for his pivotal role in initiating detect and quantify the major the MoonLITE study, and Chris rock-forming elements in the local Castelli (STFC) and other members Downloaded from https://academic.oup.com/astrogeo/article/49/3/3.11/218588 by guest on 24 September 2021 regolith (e.g. Na, Mg, Al, Si, K, of the UK–NASA Joint Working Ca, Ti and Fe) together with diag- Group for their help and support in nostic minor and trace elements. developing the concept within both A drill is proposed to bring sam- the UK and internationally. Part of ples of the local lunar regolith the work presented here has been 21 into a common analysis chamber 2 funded by the UK STFC. 17 17 24 for both the geochemistry and 15 References volatile detection instruments. 20 Water/volatile experiment. Crawford I A 2004 Space Policy 20 ● 5 16 91–97. Several techniques are proposed 13 11 Feldman W C et al. 1998 Science 281 as options for this important 6 1496–1500. 9 16 measurement, including: neutron 3 Gao Y et al. 2008 Planet. Space Sci. 56 12 14 368–377. spectroscopy, mutual impedance 1 Goins N R et al. 1981 J. Geophys. Res. probe; calorimetric analyser; pres- 86 5061–5074. sure sensor; optical spectrometer; Hagermann A and Tanaka S 2006 and miniature ion-trap mass spec- Geophys. Res. Lett. 33 L19203. trometer. Jolliff B L et al. 2000 J. Geophys. Res. 7 Heat flow experiment. To 105 4197–4216. ● Jolliff B L et al. (eds) 2006 New Views measure the heat flow in the of the Moon Rev. Min. Geochem. 60 lunar regolith, both thermal pp721. gradient and thermal conductiv- JWG 2008 Joint Working Group ity measurements are required. Report on Lunar Exploration, http://www.bnsc.gov.uk/content. The current baseline choice for aspx?nid=7208. penetrator structural material is 3: The approximate landing sites of Apollo (green), Luna (red) and Surveyor (blue) Langseth M G et al. 1976 Lunar Planet. aluminium, which represents a on the nearside of the Moon. The Apollo seismic network (white) was deployed by Sci. Conf. 7 3143–3171. major challenge to thermal gra- Apollos 12, 14, 15 and 16; heat-flow measurements were made by 15 and 17. Note Lognonné P et al. 2003 Earth. Planet. the geographically restricted nature of these measurements; MoonLITE would dient measurements since the Sci. Lett. 211 27–44. extend coverage to the poles and the farside. (Moon photograph Ken Florey) Lognonné P 2005 Ann. Rev. Earth. penetrator itself is manifest as a Planet. Sci. 33 571–604. thermal “short”. Alternative approaches are tions on the Moon from which geochemical and Mizuno T et al. 2000 IEEE Trans. Aero. Elec. Sys. 36 being studied to overcome this problem, includ- geophysical measurements have not yet been 151–161. ing a trailing thermal probe, external thermal obtained (including the poles and the farside), Mizutani H et al. 2001 Penetrometry in the Solar insulation and deployed needle probes. the MoonLITE penetrators have the potential System (Austrian Academy of Sciences Press, Vienna) 125–136. Descent camera. Part of the descent module to to make major contributions to lunar science. ● Neal C R 2006 Geotimes 51 (July) 30–35. provide context images prior to impact. At the same time, they will provide knowledge NRC 2007 Report on the Scientific Context for the Additional instruments (e.g. to measure sur- (e.g. of lunar seismicity and polar volatile con- Exploration of the Moon. http://www.nap.edu/catalog. face magnetic and electrical properties) may centrations) that will be of central importance php?record_id=11954. Pike W T and Standley I M 2005 J. Micromech. also benefit from penetrator-deployed instru- in the planning of future human missions to the Microeng. 15 S82–S88. ments, and will be considered during the Moon, and will also demonstrate a technology Smrekar S et al. 1999 J. Geophys. Res. 104(E11) Phase‑A study. that will have wide applications for the explora- 27013–27030. tion of other airless bodies throughout the solar Smrekar S et al. 2001 Penetrometry in the Solar Public engagement system. Last, but not least, MoonLITE offers System (Austrian Academy of Sciences Press, Vienna) 109–123. MoonLITE has already received significant the potential for enhancing public interest in Spudis P 1996 The Once and Future Moon (Smithsonian public and media interest. Lunar exploration science and technology. Institution Press). ● by its nature is accessible to all and inspirational Surkov Y and Kremnev R S 1998 Planet. Space Sci. to many. MoonLITE offers many opportunities Ian Crawford ([email protected]), Birkbeck 47(8/9) 1051–1060. Wieczorek M A and Phillips R J 2000 J. Geophys. Res. for public engagement and these will be pursued College London, and Alan Smith ([email protected]. 105 20417–20430. throughout the programme. For instance, the ac.uk), UCL Mullard Space Science Laboratory, Wieczorek M A et al. 2006 Rev. Min. Geochem. 60 MoonLITE penetrators will remain relatively are respectively project scientist and principal 221–364.

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