destination moon - laboratory for atmospheric and space ......space launch system, the orion...

8 PHYSICS TODAY | JULY 2019

FROM THE EDITOR

Destination MoonCharles Day

on 26 March the National Space Council met at the US Spaceand Rocket Center in Huntsville, Alabama. There, the council’schair, Vice President Mike Pence, announced the administration’s

new goal of returning American astronauts to the Moon by 2024. Sevenweeks later the Trump administration added an extra $1.6 billionto its FY 2020 NASA budget request to fund the mission, which NASAhas named Artemis after the twin sister of the Greek Sun god, Apollo.Far more funding will be needed in the next four annual budgets.

Five years is unlikely to be enough time. On 30 May the USGovernment Accountability Office (GAO) issued its 11th annualassessment of NASA’s biggest projects. Three of them—theSpace Launch System, the Orion Multi-Purpose Crew Vehicle,and Exploration Ground Systems—are essential to landing as-tronauts on the Moon. The GAO auditors found that, together,the systems are $1.8 billion over budget and 38 months late.NASA’s average launch delay, at 13 months, was the longestthe office had found since 2009, when it first started reviewingthe space agency’s performance.

Whereas the GAO is skeptical of NASA’s ability to meet itsown deadlines, Pence repeatedly stressed the need to revisitthe Moon soon. “Urgency must be our watchword,” he told hisHuntsville audience. “Failure to achieve our goal to return anAmerican astronaut to the Moon in the next five years is not anoption.” NASA, he said, had to become leaner, more account-able, and more agile.

Given what it will take to return astronauts to the Moon by2024, it’s worth examining just how urgent the goal really is.The scientific case is perhaps the easiest one to assess. In 2011the National Research Council published its most recent decadalsurvey of planetary science. When the committee membersevaluated scientific opportunities, returning astronauts to theMoon was not White House policy. Without the prospect ofpiggybacking on a manned mission, the Moon was considereda potential destination for robotic mis-sions along with all the other bodies inour solar system.

The decadal survey made recommen-dations for two classes of missions, flag-ship and the smaller yet still ambitiousNew Frontiers. Retrieving samples fromthe surface of Mars was the highest pri-ority among flagship missions, followedby visits to Jupiter’s moon Europa andthe planet Uranus.

Lunar science was the goal of one offive recommended candidates for thenext New Frontiers mission: Specifically,retrieving samples from the ice-rich,

deeply impacted Aitken basin at the Moon’s south pole. Thescientific payoff would be great. Indeed, the south pole is theintended destination of the 2024 Moon shot. But the next NewFrontiers mission, to be announced later this month, will be either to Saturn’s moon Titan or to comet 67P/Churyumov–Gerasimenko.

What of other, nonscientific cases to return astronauts to theMoon by 2024? To his credit, Pence did not equivocate. The USmust remain first in space, he said, because the rules and valuesof space will be written by those who get there first and committo staying. He’s likely correct. In 1979 the United Nations Officefor Outer Space Affairs promulgated a treaty to establish regula-tions for the use of the Moon and other celestial bodies and togrant the UN jurisdiction over them. Eighteen countries haveacceded to or ratified the treaty; China, Russia, and the US arenot among them. Mining oxygen from lunar rocks and usingnuclear power to extract water from permanently shadowedcraters—two activities that Pence mentioned in his Huntsvilleaddress—contravene Article 11 of the Moon Treaty, which forbidsthe appropriation of lunar resources by states and companies.

Who might reach the Moon before the US? On 28 November2018, Dmitry Rogozin, head of Russia’s national space agency,announced Russia’s intention land a human on the Moon by2030. Two years earlier, Zhang Yulin, the deputy commanderof the China’s manned space program, announced the coun-

try’s intention to land a human on theMoon by 2036.

Does it matter if NASA goes all outto return to the Moon by 2024? Yes, Ithink it does. In its report, the GAO notedthat the combination of NASA’s existingoverruns and the addition of Artemiswill strain NASA’s budget: “NASA willhave to either increase its annual fund-ing request or make tradeoffs betweenprojects.” Those tradeoffs could includethe scientifically fruitful robotic missionsthat the decadal survey identified. Ifavor returning American astronauts tothe Moon, just not at any cost. PT

TURN TO PAGE 22FOR DAVID KRAMER’S

REPORT ON USING THE MOON

AS A WAY STATION TO MARS


P resident Trump, NASA’s leadership,Congress, and advocates for humanspace exploration agree that Mars

should be the ultimate destination forthe US spaceflight program. But will theadministration’s plan to send astronautsback to the Moon advance a Mars mis-sion, or could the lunar program drawresources away from Mars and thusdelay an excursion to the red planet?

In March of this year, Vice PresidentPence announced the administration’s de-cision to move up by four years, to 2024,its target date for sending astronauts, in-cluding the first woman, to the Moon. Butcongressional appropriators’ rejection of the administration’s request to add$1.6 billion to NASA’s fiscal year 2020budget to accelerate the Moon landingprogram casts doubt on the 2024 goal.

Trump’s December 2017 executiveorder, Space Policy Directive 1, acknowl-edged the goal of getting to Mars even asit ordered a return to the Moon. The 2017NASA authorization act—which doesnot provide funding—also confirmedMars as the ultimate destination forhuman exploration.

Regardless of exactly when it mayhappen, is putting humans back on thelunar surface truly a prerequisite forgoing to Mars? “I wish I could give youa really crisp, black and white answer,but it is a bit nuanced,” says Scott Hub-bard, who was director of NASA’s AmesResearch Center and NASA’s first Marsprogram manager.

“This debate has been going on fordecades,” says Hubbard. “You can makea solid case that you can send people toMars with only minimal testing at theMoon.” As far back as 1991, aerospaceengineer Robert Zubrin and colleaguesat Martin Marietta (now Lockheed Mar-tin) floated a Mars Direct plan, which es-

chewed a return to the Moon and the as-sociated components of NASA’s proposedlunar and Martian flight architecture.

Hubbard points to another proposalby three scientists at NASA’s Jet Propul-sion Laboratory (JPL) in 2015. It reliedheavily on a set of elements already builtor planned by NASA, such as the SpaceLaunch System (SLS) heavy-lift rocket,the four- person Orion capsule, a deep-space habitat, and a 100 kW solar- electric-propelled “tug” for transportingsupplies ahead of a human landing. Theplan entailed few if any operations on thelunar surface and avoided complicateddevelopment programs such as nuclear-thermal propulsion. The JPL proposalenvisioned an initial human missionlanding on Phobos, the larger of Mars’stwo moons, in 2033, with a Mars touch-down in 2039.

More recently, SpaceX has proposedflying humans directly to Mars aboardits planned “starship.” Paul Wooster,SpaceX’s principal Mars engineer, toldthe Humans to Mars Summit (H2M) inMay, “It’s not unreasonable” that the

company will put people on the planetby the mid 2020s.

Jonathan Lunine, a Cornell Universityastronomer who cochaired a NationalAcademy of Sciences (NAS) review ofNASA’s human spaceflight program in2014, says that “from a strictly engineer-ing point of view,” a direct-to-Mars ap-proach is feasible. “But you increase therisk tremendously, from two points ofview: One, you’re not going to be test-ing a lot of technologies until you actu-ally get to Mars; and two, politically, because you don’t have an intermediategoal in a program that is going tostretch significantly in time beyond whatApollo was.”

Returning to the Moon would buildmomentum in a human spaceflight pro-gram that hasn’t ventured beyond low-Earth orbit since the Apollo programended in 1972. “If we wait until Mars, thewhole government spaceflight programwill collapse of its own weight,” saysJohn Logsdon, emeritus professor ofspace policy at George Washington Uni-versity. “There’s a pretty convincing case

Quo vadis, NASA: The Moon, Mars, or both?Fifty years after Apollo 11,the US spaceflight programis juggling political andtechnological factors as itmoves toward the red planet,its ultimate destination.

ISSUES & EVENTS

NASA ADMINISTRATOR JIM BRIDENSTINEstands in front of an artist’s depiction of a lunarlander as he addresses an industry forum on theagency’s lunar exploration plans.

NASA

for making the Moon a first goal, but notthe last goal.”

Ken Bowersox, deputy associate ad-ministrator for NASA’s human explo-ration and operations mission directorate,told H2M attendees that “everythingwe do [on the Moon] is intended to inform our journey to Mars.” A timetablefor when humans could make such atrip could come as soon as 2025, he said.

An alternate route“Mars is the ultimate destination forhuman exploration of the inner solar sys-tem; but it is not the best first destina-tion,” concluded the 2009 report of anadvisory committee commissioned bythe Obama administration. The findingsof the panel, chaired by retired LockheedMartin CEO Norman Augustine, led tothe administration’s decision to excise theMoon as a destination for NASA’s explo-ration program (see PHYSICS TODAY, De-cember 2009, page 25). The committeeadvised that alternate destinations—alunar orbit, an asteroid, or a Lagrangepoint—were equally as useful as the sur-face of the Moon.

Obama chose an approach, outlinedin the report, of sending a crewed space-craft into a stable orbit near the Moon,from which a manned mission would

embark to a small asteroid. The rockwould be physically redirected into anorbit near the Moon. In addition to beingless expensive than landing on the Moon,a lunar orbiting spacecraft, the Augus-tine committee noted, could be a launch-ing point for a Mars mission that wouldavoid the energy and fuel required to es-cape the Moon’s gravity. But the asteroid-redirect plan garnered little supportfrom scientists.

Obama science adviser John Holdrensays the administration concluded that“there was little point in putting astro-nauts on the Moon again, more than 50 years after we did it the first time, un-less we were going to do significantlymore when we got there—meaning inour view setting up a crewed base.” Atthe time, NASA estimated the cost ofputting a crewed base on the Moon at$60 billion to $80 billion, he says. “We sawno prospect of such a sum materializingon any time scale of planning interest.”

Although the Augustine panel saidno viable human spaceflight programcould be carried out for less than a $3 bil-lion addition to NASA’s budget, Holdrensays Obama decided that the asteroid-redirect route could at least be started foran extra $1 billion per year, the amountof additional funding Obama was will-ing to request from Congress.

Holdren estimates NASA will have tofind an additional $5 billion each year tomeet its 2024 Moon-landing target.

A proving groundTo NASA administrator Jim Bridenstine,who assumed NASA’s helm in April2018, the Moon is “the proving ground”and “the path to get to Mars in the safest,fastest way possible. When we acceleratehumans to the Moon we are by definitionaccelerating humans to Mars,” he told theH2M conference. In following Trump’sdirective, NASA plans to establish a per-manently staffed outpost on the lunarsurface in 2028.

William Gerstenmaier, NASA associ-ate administrator for human explorationand operations, told the House Science,Space, and Technology Committee inMay that the Moon “provides an oppor-tunity to demonstrate new technologiesthat we will use on crewed Mars mis-sions: power and propulsion systems,human habitats, in-space manufacturing,life support systems, and in situ resourceutilization.”

Clive Neal, a University of Notre Dameengineering professor and lunar explo-ration advocate, says going directly toMars risks a repeat of the Apollo expe-rience. Despite its success, Apollo wascanceled due to its expense, and NASA

JULY 2019 | PHYSICS TODAY 23

THE LUNAR ORBITER proposed by the Trump administration would be a human habitat and a staging point for Moon landings.

NASA


ISSUES & EVENTS

lacked any follow-on program. “You’llwind up doing a one-and-done,” Nealsays. “There won’t be longevity or sus-tainability in a program.” Unlike distantMars, he adds, the Moon offers opportu-nities for commercial participation.

NASA in late May awarded 10-yearcontracts totaling $250 million to threecompanies to begin transporting nearlytwo dozen payloads of instruments andother equipment to the lunar surface inlate 2020. The agency’s FY 2020 budgetrequest included $1 billion for develop-ment of lunar landers by the private sec-tor. Billionaire Jeff Bezos recently un-veiled a mockup of a lunar lander beingdeveloped by his company, Blue Origin,although he provided no design details.

The poles of the Moon could hold, inpermanently shaded craters, millions oftons of water ice that could be used toproduce liquid oxygen and hydrogen tofuel a Mars-bound spacecraft, Neal andother experts say. Developing that re-source could obviate the need to trans-port fuel from Earth. Additionally, sur-rounding a spacecraft with a meter-thickcoating of water could protect astronautsfrom radiation on the way to a Mars orbit,says Neal.

NASA plans to use the Moon pro-gram, which it calls Artemis, to demon-strate several major components of theproposed Mars mission architecture.They include the lunar- orbiting com-mand and control platform, to be assem-bled in space, from which reusable land -ers would embark from and return to theMoon and where astronauts would bestationed for months at a time. The gate-way, as the platform is known, could alsobe useful for assessing the psychosocialand physical effects of long-durationspace travel beyond near-Earth orbit.NASA officials envision initial crew vis-its of up to 30 days to the gateway andlonger visits as additional modules aredelivered. NASA in May awarded a$375 million contract to Maxar Technolo-gies of Colorado to build the first sectionof the gateway, the power and propulsionelement. It’s due for launch in 2022. Atleast one other section will be needed toaccommodate the planned 2024 landing.

Last year, the Sixth CommunityWorkshop for Achievability and Sustain-ability of Human Exploration of Mars, agroup of 70 experts on lunar and Martianexploration and science operations, com-piled a list of technologies required for

Mars that would benefit from experiencegained from lunar operations. Among thetransportation and propulsion needs werecryogenic propellant management, land -ers, and vehicle servicing and refueling.Operations on the Martian surface thatcould be advanced with knowledge fromthe Moon included human health and bio-medicine, power systems, manned explo-ration rovers, and space suits. Others werein situ resource utilization—essentiallyliving off the land—communications, andhabitats and labs. The 2014 NAS reportlisted entry, descent, landing, advancedin-space propulsion and power, and radi-ation safety among key requirements fora Martian mission.

The proposed 2024 Moon landingwill use the SLS and the Orion crew ve-hicle. Both were designed with lunartravel in mind. The first crewed flight ofthe SLS–Orion system is planned to orbitthe Moon in 2022. The Government Ac-countability Office (GAO) reports that asof September 2018, the cost of the SLS,which NASA had scheduled for its initiallaunch last November, had grown by$1 billion, or 10% over its 2014 baselineestimate, and will not meet its re -scheduled June 2020 launch target. NASAofficials remain hopeful of an SLS launchlate next year. Orion, which was sup-

posed to fly uncrewed atop the SLS lastfall, was at least $379 million, or 6%, overbudget as of mid 2018, according to theGAO. Prime contractor Lockheed Martinexpects further cost growth.

Maintaining focusThe NAS report stressed that systemsdeveloped for the Moon or other inter-mediate destinations should keep theMars mission in mind. Lunine and oth-ers worry that relevance to Mars may be“traded away” in a sprint to get to theMoon by 2024. “The danger is that wewill end up repeating an Apollo stylelanding on the Moon as an accomplish-ment in itself, and once again that will bethe end,” Lunine says, mirroring Neal’sconcern. Once humans return, “peoplewill say that’s great, what’s next? And thewhat’s next is you would have to startfrom scratch, and there’s no impetus tostart from scratch.”

Casey Dreier, chief advocate and se -nior space policy adviser at the PlanetarySociety, agrees. “You have to have verydisciplined, focused, and deliberative de-cisions made on what to do if Mars is yourlong-term goal. If you say we have toland in 2024, do you really have the timeor ability to focus on how that will workin a Mars environment? Probably not.”

THIS SELF-PORTRAIT OF THE MARS CURIOSITY ROVER at a location known as MountSharp shows the dusty and rocky terrain that future astronauts may encounter. For scale, therover’s wheels are 50 centimeters in diameter and about 40 centimeters wide.

NASA


Going to the Moon “would still repre-sent a remarkable increase in capabilityfrom what we have right now for humanspaceflight,” Dreier says. “I’ll happily seehumans walking on the Moon if thatmeans getting out of low-Earth orbit.”

Another problem with NASA’s cur-rent course, says Hubbard, is the highcost of maintaining humans in space, asevidenced by the more than $3 billionNASA spends on the International SpaceStation (ISS) each year. The maintenanceburden on NASA’s budget will growmuch greater if a permanent habitationis set up on the Moon, and that will leavefar less money for a Mars developmentprogram, he notes.

Key differences between Moon andMars environments won’t allow for directtransfer of some elements, such as landersand manned rovers. Martian surfacegravity is 38% of Earth’s, compared withthe Moon’s 17% terrestrial fraction. Mars’satmosphere provides some protectionfrom radiation, whereas the Moon’s doesnot. Although dust is a hazard for hu-mans and equipment on both bodies, duststorms occur only on Mars.

The NAS cautioned against wastingNASA resources and time on “dead-end”development programs that won’t be of use on Mars. Notably, the academylisted the single-use descent stage of thelander design for the 2024 lunar surfacemission.

Propulsion systems are likely to dif-fer from one destination to the other.Whereas the SLS–Orion system is con-ventionally fueled, NASA is eyeing both solar- electric and nuclear propulsion forMars travel. The NAS study recom-mended nuclear propulsion for Marstravel, saying the power levels of the best solar- electric systems are far too low touse in human transit. Specifically, itcalled for developing both nuclear- thermal, in which a fluid such as liquidhydrogen is heated to high temperatureto create thrust, and nuclear- electric, inwhich electricity generated by a nuclearreactor is used to drive a propellant athigh speed. Neither has been deployedin space.

The two technologies are separatefrom radioisotope thermal generators, anuclear technology that has poweredmore than two dozen spacecraft sincethe 1960s. Those devices generate ther-mal energy from the radioactive decay of plutonium-238, but aren’t powerful

enough for propulsion. (See PHYSICSTODAY, December 2017, page 26.)

Time-frame estimates for a crewedMars landing range from 2033 to the2040s and beyond. The launch windowto the quickest path to Mars opens onlyevery other year. The Science and Tech-nology Policy Institute (STPI), which sup-ports the White House Office of Scienceand Technology Policy, concluded that2037 would be the earliest feasible dateand 2039 the more likely date for a

launch to the red planet. It said that 2033,the date proposed in the 2017 NASA au-thorization act, “is infeasible under anybudget scenario and technology devel-opment and testing schedules.”

The NAS report committee estimatedthat the earliest crewed surface missionto Mars will occur between 2040 and2050, assuming that the ISS is extendedto 2028 and that the human spaceflightbudget is increased at twice the rate ofinflation.

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S ince its founding in 2011, the Insti-tute for Basic Science (IBS) in SouthKorea has largely lived up to its am-

bitious goals. It has attracted top scien-tists, produced world-class science, andmade inroads in internationalizing thecountry’s research community. For con-tinued success, however, the IBS mustwin over both the country’s other scien-tists and its current politicians and con-

vince them that the big federal invest-ment in a relatively small number of in-vestigators is worthwhile.

When the IBS was created, South Koreahad an impressive track record in appliedscience and manufacturing; the auto andelectronics industries are examples. Inlaunching the new initiative, the coun-try’s then president Myung-bak Leenoted that countries at the forefront of

MICROPARTICLES SUSPENDED IN A ROTATING DENSE FLUID self- organize into dynamic patterns. Researchers at the Institute for Basic Science Center for Soft and Living Matter inSouth Korea study these nonequilibrium systems to gain insight into symmetry breakingand pattern formation in rotational frames of reference. The four images are snapshots withdifferent rotational histories; they show the same mixture of three kinds of polyethylenemicroparticles that differ in density, size, and color.

The country’s network of curiosity- driven research centersis a scientific and cultural experiment.

Domestic quarrels cloud future of SouthKorea’s Institute for Basic Science

ISSUES & EVENTS

OLGIERD CYBULSKI AND BARTOSZ A. GRZYBOWSKI

The STPI put the total cost of a NASAspaceflight program leading to a Marslanding in 2037 at $217 billion, includ-ing $121 billion devoted to Mars- relatedhardware development. Of the total,$34 billion has been spent to date for theSLS and Orion programs. Lunine was lessdefinitive when he told a House hearingin May that it would require hundreds ofbillions of dollars.

Although Bridenstine and other offi-cials have repeatedly insisted that the costwill be shared with international part-ners, there have been few if any specifics.If the US wants to reduce the cost, saysLunine, “it will need the kind of interna-tional contributions that we have neverseen before in human- piloted programs.”For example, the US has borne 85% of thecost of the ISS and even pays for seats on Soyuz flights to the station. Moreover,he and others note, relations with Chinahave deteriorated to the point that coop-eration may not be possible. The otherbig challenge, Lunine adds, is how to co-operate with other nations without giv-ing away US technologies.

David Kramer

Bradley L. Jolliff and

Mark S. Robinson

This month marks the

50th anniversary of the Apollo 11

Moon landing. Together, the six

Apollo landings laid the foundation

for modern planetary science.


APOLLO

THE SCIENTIFIC LEGACY OF THE

PROGRAM

O


Brad Jolliff is the Scott Rudolph Professor of Earth and Planetary Sciences at Washington University inSt Louis, in Missouri. Mark Robinsonis a professor in the School of Earthand Space Exploration at ArizonaState University in Tempe and theprincipal investigator of the NASALunar Reconnaissance Orbiter Camera.

Each of the Apollo missions explored carefully selected land-ing sites and conducted a variety of experiments to probe thelunar interior and measure the solar wind. Well-trained astro-nauts made geologic observations and collected samples ofrock and regolith, the impact-generated layer of debris thatcomposes the lunar surface. Over a half century of study, thesamples have revealed abundant information not only aboutthe Moon’s origin and history but also about the workings ofour solar system.

Apollo 11Results from the Apollo 11 mission established key paradigmsof lunar and planetary science. After a harrowing descent tothe surface, Armstrong set the Eagle down on the crateredbasaltic plains of Mare Tranquillitatis. Extravehicular activitywas brief—just two and a half hours during that first mission—and included setting up surface experiments and exploring asmall cluster of craters near the lunar module and Little WestCrater some 60 meters away, as shown in figure 1. Aldrin’siconic Apollo 11 bootprint photo revealed much about the lunarsoil, including its fine-grained nature, its cohesiveness, and itsability to pack tightly together.

The Early Apollo Scientific Experiment Package contained,among other instruments, a passive seismometer and a laser-ranging retroreflector. Although designed to work for onlythree weeks, the seismometer provided a first key look at lunarseismic data. The seismometers brought to the Moon during theApollo 12, 14, 15, and 16 missions were used as a larger network

O n 20 July 1969, Apollo 11astronauts Neil Armstrongand Edwin “Buzz” Aldrinlanded on the Moon whileMichael Collins orbited in

the command module Columbia. “Tranquility Basehere. The Eagle has landed” became one of themost iconic statements of the Apollo experienceand set the stage for five additional Apollo landings.

NASA/GSFC/ASU


APOLLO PROGRAM

to probe the interior structure and measure thousands of moon-quakes that would eventually be detected.

The retroreflector on Apollo 11 was the first of five eventu-ally delivered to the Moon. Active laser ranging still preciselymeasures the Moon’s distance as it slowly recedes from Earth.Some 22 kg of samples were collected during that mission.(Collectively, the missions returned a total of 382 kg of material,and the last, Apollo 17, carried 111 kg.) The regolith, used asfiller in the rock box, was separated from the rocks back onEarth and analyzed for its contents. The fine-grained particles,labeled 10084 and known as “Armstrong’s packing soil,” maybe the most studied geologic sample in history.

The rocks turned out to be largely basalt—volcanic rockformed by partial melting in a planet’s (or moon’s) interior.They contained higher concentrations of titanium than anybasalts on Earth but were otherwise made of familiar minerals,primarily the Mg-Fe-Ca silicate mineral pyroxene, the Ca-Alsilicate mineral plagioclase, and theFe-Ti oxide ilmenite.

Radiometric dating found the basaltsto be more than 3.5 billion years old,and isotopic relationships betweenrock and regolith materials suggestedthat the Moon itself is ancient, havingformed earlier than 4.4 billion yearsago.1 Although the volcanic rocks con-tain vesicles, indicative of gas releaseupon eruption, they lack evidence ofany other alteration and are nearly de-void of water, carbon dioxide, andother volatiles. (See the Quick Study byLindy Elkins-Tanton, PHYSICS TODAY,March 2011, page 74.) Lunar rocks arealso completely barren of any signs of life.

The regolith samples proved in-valuable in the rich variety of materi-als contained within them (see, for example, figure 2). Meteorand asteroid impacts, pervasive in lunar history, ejected bits ofrock tens to hundreds of kilometers in all directions. Volcanicglasses, impact glasses, and breccias—rock fragments that be-came mixed during those impacts—were all part of the regolith.So were agglutinates, a new type of welded soil particle pro-duced by micrometeorite impacts in the regolith. Mixed in withthat local material were small fragments of plagioclase-richrock (anorthosite) from the distant highlands.

In 1970 geologist John Wood and others inferred thatanorthosite crystals floated toward the surface of a magma ocean,where they accumulated to form a plagioclase-rich crust.2 Denserminerals such as pyroxene and olivine, by contrast, sank toform the lunar mantle. The Moon thus formed hot and under-went differentiation early in its history. (See PHYSICS TODAY,February 2008, page 16, and the article by Dave Stevenson, No-vember 2014, page 32.) That early history was unraveled fromonly a handful of small rock fragments found in the regolith.

Building on successApollo 12 followed quickly in November 1969. The lunar mod-ule Intrepid executed a pinpoint landing within walking dis-tance of the pre-Apollo Surveyor 3 spacecraft. The landing site

afforded the possibility of sampling not only local rocks andregolith but also materials ejected from Copernicus Crater, 350km away. Part of Apollo 12’s payload included a seismometer,magnetometer, solar-wind spectrometer, and ion and dust de-tectors—all powered by a radioisotope thermoelectric genera-tor. In addition to taking hardware from Surveyor 3 for the tripback to Earth, the astronauts explored several craters and col-lected material excavated from different depths to establish astratigraphy of the subsurface.

Besides several types of basalts, the astronauts sampledrocks that were likely part of a spoke-like ray of materialejected from Copernicus Crater. Among the materials wereropy glasses and nonbasaltic rocks, which offered evidence thatthe crater had formed 800 million years ago.3 The inferred ageof Copernicus Crater and subsequent dating of other impactcraters and volcanic surfaces became the foundation for lunarchronology. The ground-truth data allow us to relate the size

and frequency of impact craters per unit area to the age of thesurface under study (see figure 3). And that relationship formsthe basis for the relative chronologies of impact and volcanicevents on the solar system’s other rocky planets—Mercury,Venus, and Mars.4

The Apollo 12 samples proved remarkably diverse. The ma-terial known as KREEP—rich in potassium, rare-earth ele-ments, and phosphorus—was found in impact-melt rocks andrare granites. Several types of basalt, distinct from those foundat the Apollo 11 landing site, came from the underlying se-quence of lava flows.

The Apollo 14 lunar module Antares was the first to land onterrain that differed from flat volcanic plains. Analysis of orbitalphotos of the location, known as the Fra Mauro formation, in-dicates that the rocks there came from the enormous Imbriumbasin-forming impact event, which occurred more than 600 kmto the north. Fra Mauro breccias were determined to have

Solar windcollector

Retro-reflector

Close-upcamera

Little WestCrater

Seismometer

Lunarmodule

US flag

TVcamera

10 m

FIGURE 1. THE APOLLO 11 LANDING SITE shows locations wherea US flag, television camera, and surface experiments were placedby astronauts Neil Armstrong and Edwin “Buzz” Aldrin. As they placedinstruments and walked around the landing site, the disturbed soilleft a visible path. (Image courtesy of NASA/GSFC/ASU.)


formed about 3.9 billion years ago. Because Imbrium is knownfrom relative stratigraphy to be one of the youngest of the impact basins, almost all the other basins must have formedbefore that time. Excavated from deep in the lunar crust, theImbrium rocks are rich in KREEP.5 Exposure ages of samplesejected from the nearby Cone Crater revealed the crater to be50 million years old, providing another key datum in lunarchronology.6

Apollo 15 and 16Launched in the summer of 1971, Apollo 15 was the first of theso-called J missions, which included the first lunar rover andlonger extravehicular activity—nearly 19 hours on the lunarsurface—during which astronauts collected some 77 kg of sam-ples and explored more complex geology. The lunar moduleFalcon landed on another flat mare deposit close to the spectac-ular Apennine mountains. Some peaks rise up to 4000 m abovethe landing site and are part of the rim of the Imbrium basin.A key mission goal was for the astronauts to traverse the baseof Mons Hadley Delta, one of the Apennine peaks, to searchfor ancient crustal material brought up from the depths whenthe basin was formed.

One of the most remarkable finds was a clod of green pyro-clastic glass beads, which represented material from deep inthe mantle brought up rapidly, without crystallizing, to the sur-face during the eruption of a massive fire fountain. Perhaps themost famous of the samples whose collection was enabled bythe rover was “Seatbelt Rock,” a highly vesicular basalt shownin figure 4 and discovered by mission commander David Scott.Knowing that the astronauts were short on time and that mis-sion control would not approve a stop to collect the rock, Scottused the excuse of stopping to fasten his seatbelt—hence thename—during which he quickly picked it up.7

Trained to look for coarsely crystalline rocks that might rep-resent deep crustal material, Scott and others recognized theimportance of yet another sample, “Genesis Rock,” by its lightcolor and coarse, reflective crystal facets. The rock proved tobe anorthosite, considered a plagioclase flotation cumulate ofthe magma ocean and thus a pristine sample of lunar crust. Iso-

topic analyses confirmed that the rock is indeed ancient—morethan 4 billion years old. But analyses also revealed a complexthermal and shock history that obscures when it actually formed.Collection and documentation of the rocks in their geologiccontexts, along with precise locations and descriptions by theastronauts, enabled the construction of exquisitely detailedmaps and cross sections of the landing sites.8

Another advance with the J missions was addition of theScientific Instrument Module (SIM) on the Endeavor. It en-abled systematic orbital remote sensing using panoramic andmapping cameras; x-ray and gamma-ray spectrometers, whichdetermined elemental compositions; and a laser altimeter fortopography. SIM bay observations by the Apollo 15, 16, and 17missions provided an approximately equatorial swath of datafor the lunar surface that researchers used to extrapolate from“Apollo-zone” areas to the entire Moon. The J-mission orbitalobservations had to last the scientific community until the 1990s,when the Clementine and Lunar Prospector spacecraft acquiredglobal remote sensing.

Apollo 16 was the only mission that explored lunar high-lands far from the maria. The lunar module Orion gently landednear mountainous terrain known as the Descartes highlands.The main scientific goal was to investigate the origin of theCayley plains, a region adjacent to those highlands and thought,prior to the mission, to have formed from silica-rich rocks andash deposits. The Cayley plains actually overlap the mountain-ous Descartes formation and are thus younger. From orbitalphotography, geologists interpreted the Descartes formation asejecta from the ancient Nectaris basin, whose rim is less than300 km away.

Apollo 16 astronauts took advantage of the sampling oppor-tunities afforded by two impact craters, North Ray and SouthRay, by landing between them. Using the rover, they sampledejecta from both to determine their ages—yet more data pointsfor the lunar chronology. The relatively smooth Cayley plainswere shown to have formed as an impact-related deposit, mostlikely by material ejected from Imbrium. Among the rocksejected from North Ray and South Ray Craters were impact-melt, fragmental, and regolith breccias. The latter, composed

Agglutinate

Agglutinate

Impact glass

Breccias

Plagioclase rich1 mm1 mm

Volcanicglass

Basalt

a b

FIGURE 2. SOIL PARTICLES FOUND IN THE MOON’S SURFACE DEBRIS, or regolith, during the Apollo 11 mission. (a) Shown here are rockfragments (impact breccias); volcanic and impact glasses; fused particles (agglutinates); a light-colored, plagioclase-rich fragment; and piecesof volcanic basalt. (b) The same rock particles are sliced optically thin for study by transmitted-light microscopy. (Images are from John Wood,Smithsonian Astrophysical Observatory.)


APOLLO PROGRAM

of lithified regolith, were significant becausethey provide a time-stamped snapshot of theoutput of the Sun via trapped solar-wind gasesat the time the regolith breccias formed.

The largest Apollo sample ever returnedwas a 12 kg breccia nicknamed “Big Muley,”after Bill Muehlberger, who led the Apollo 16and 17 field geology teams. The side of the rockthat faced up on the lunar surface is dottedwith an abundance of pits from its exposure tomicrometeorites. An important legacy of theApollo missions is the superb training that wasincorporated into the program. That trainingallowed the astronauts to work directly withscientists at mission control to optimize thefieldwork. The approach culminated with theinclusion on Apollo 17 of a geologist astronaut,Harrison Schmitt.

Peaks and valleysApollo 17 landed in the beautiful Taurus– Littrow Valley, completing the Apollo programin December 1972. The lunar module Challengerplaced the astronauts in a geologically com-plex area on the edge of the Serenitatis basin.The valley itself is defined by peaks, shown infigure 5, that tower 2500 meters above the basalt-flooded floor. Mission objectives included as-certaining the age of the basin, determining theage and composition of the basalts, and collect-ing pieces of ancient crust excavated duringthe basin’s formation.

Mission planners had identified a large, re-gional pyroclastic ash deposit in orbital images and wanted tofind and sample some of that material. A cluster of secondaryimpact craters, aligned along a ray from the 2400-km-distantTycho Crater, was seen in the valley along with a light mantledeposit, formed by avalanche, at the base of South Massif. Sci-entists hypothesized that the craters and the mantle depositformed as ejecta from Tycho Crater struck the area. Astronautssampled the light mantle deposit for researchers to determineTycho’s age, as had been done for Copernicus Crater duringthe Apollo 12 mission.

Additionally, Schmitt discovered a deposit of orange glassbeads as an exposed layer in the rim of Shorty Crater. That ma-terial was pyroclastic as well. The color was related to theirhigh Ti content, quite different from the very low Ti of theApollo 15 green glasses. Like the green glass, however, the or-ange-glass soil became one of the most important of the Apollosamples, oft sought for study because it represents one of themost pristine samples of the lunar interior, unmodified by crys-tallization processes.

Basalts of the Taurus–Littrow Valley formed 3.7–3.8 billionyears ago. Impact-melt breccias were sampled from bouldersat the base of North and South Massifs, their ages just a few tensof millions of years older than the breccias from Imbrium. Because of the considerably more advanced degradation ofSerenitatis basin, it was apparent that many impact basins hadformed in that time interval, amounting to a cataclysmic bom-bardment as also suggested by lead-isotopic analyses.9 Sam-

ples collected on the light mantle deposit and elsewhere in thevalley had exposure ages of around 110 million years, and thatage was assigned to the Tycho impact event.10 Ancient crustalrocks greater than 4.0 billion years old were also found amongthe Apollo 17 samples. They continue to provide the grist fortests of hypotheses about the origin of the Moon’s ancient crust.

Unlike its predecessors, Apollo 17 carried an active seismicexperiment designed to determine the subsurface structure bypicking up signals generated by explosive charges. Other ex-periments probed surface electrical properties, determined theeffects of exposure of biological materials to cosmic rays, andused a traverse gravimeter to help map out subsurface struc-ture. The orbiting command service module America carried amicrowave sounder, an IR radiometer, a far-UV spectrometer,mapping and panoramic cameras, and a laser altimeter. The or-bital data sets provided by those instruments would be the last

1

10−1

10−2

10−3

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AGE (10 years)9

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)k

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OrientaleA17

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A11 (“old”) A11 (“young”)

A16

Copernicus

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Cone

Serenitatis

FIGURE 3. LUNAR CHRONOLOGY is based on the ages of lunarsamples that represent surfaces on which impact crater size– frequency distributions have been determined. N(1) refers to thenumber of craters that are 1 km in diameter or larger, and the plotrelates that number to the crater accumulation time. Numbered labels “A” and “L” refer to Apollo and Luna missions, respectively.Measurements of crater size and frequency distributions come fromorbital photographs. (The method and plot are adapted from ref. 4;the age data are taken from ref. 5.)


direct measurements scientists would have from lunar orbit formore than two decades.

Surface geophysicsThe seismic array deployed by Apollo 12, 14, 15, and 16 contin-ued to transmit data to Earth until September 1977, when thearray and other instruments were turned off. More than 12 000seismic events were detected altogether. Some 7000 of themcame from deep moonquakes, which were correlated with tidalforces exerted by Earth’s gravity. Others were attributable tometeoroid impacts, the deliberate crashes of booster rockets,and shallow thermal moonquakes caused by the heating andexpansion of the crust.

Seismic data provided information about the thickness ofthe lunar crust, changes in the seismic velocity as waves crossedthe crust–mantle boundary, deeper seismic discontinuities inthe mantle, and a deep zone of seismic attenuation. Early workestimated the average crustal thickness at 60 km, but modernanalyses place it between 30 and 40 km.11,12

Ranging to the lunar retroreflectors from Earth continuestoday. The Moon’s irregular rotational motions indicate a par-tially fluid core. The 2011 Gravity Recovery and Interior Lab-oratory (GRAIL) mission confirmed a partially molten deep-mantle zone and constrained the size of the fluid outer and

solid inner core.13 (See PHYSICS TODAY, January 2014, page 14.)Coupled with the available Apollo seismic data, the new grav-ity measurements significantly improve our understanding ofthe Moon’s internal structure.

Samples and curationThe Apollo samples are broadly similar to Earth materials inmineralogy and chemical composition. But their chemistry isdistinctly lunar. Moon rocks formed under extremely low oxy-gen fugacity such that most of the iron they contain is divalent(Fe+2) and most samples contain at least a small amount of ironmetal (Fe0). The Fe-Ti oxides are mostly ilmenite (FeTiO3), butalso contain ulvöspinel (Fe2TiO4), armalcolite ((Fe,Mg)Ti2O5)(first found in lunar rocks and named after Armstrong, Aldrin,and Collins), and tranquillityite (Fe8(Zr,Y)2Ti3Si3O24), a newmineral named for the Sea of Tranquillity, where it was found.

The basalts provide insights into the lunar mantle and earlydifferentiation processes. Variations in basalt types reflect aheterogeneous mantle, which lacks a homogenizing processsuch as Earthlike convection. Owing to ground-truth samplesfrom Apollo, we can infer basalt types from other areas usingremote sensing. Volcanic glasses occur in regolith samples fromall Apollo sites, with a wide variety of compositions, spanningTiO2 concentrations from less than 1 weight percent to more

a

c

b

d

1 cm

1 inch

1 cm

1 inch

FIGURE 4. ROCKS COLLECTED during Apollo 15 and Apollo 16. (a) “Seatbelt Rock” 15016 is a vesicular (porous) basalt. (Adapted from NASAphoto S71-46632.) (b) “Genesis Rock” 15415 is made of ferroan anorthosite, a major rock type of the lunar crust. (Adapted from NASA photoS71-44990.) (c) A 1.8 kg sample of anorthosite, 60025. (Adapted from NASA photo S72-42586.) (d) This top surface of an 11.7 kg breccia,61016, known as “Big Muley,” contains numerous tiny impact craters, or zap pits. (Adapted from NASA photo S98-01215.)


than 16 weight percent. Those compositions reflect the hetero-geneity of the mantle and late-stage magma-ocean processes thatled to areas within the mantle of widely different Ti contents.

During the Apollo program, scientists had the foresight torecognize the value of the samples and established the Cura-torial Facility at Johnson Space Center. They set protocols forcuration, handling, and allocation in a way that would preserveportions of all samples for future generations, with special caregiven to the rarest and most important of them.

The protocols ensured that nearly 40 years after they were col-lected, samples would be available for analysis of indigenous OHand H2O in volcanic glasses, phosphate minerals, and melt in-clusions using new and highly sensitive analytical methods.Those studies revealed that the Moon did not form as depleted ofvolatiles as was once thought.14–16 (See also PHYSICS TODAY, Janu-

ary 2016, page 17.) Rather, the Moon heavily degassed the volatilesduring the magma-ocean and later volcanic stages. The precisemea surement of remanent magnetism in lunar samples revealedthat the Moon had an early core dynamo until sometime be-tween 3 billion and 4 billion years ago (reference 17; see alsothe article by David Dunlop, PHYSICS TODAY, June 2012, page 31).

Gateway to the solar systemThe Apollo era exploration and decades of study of lunar sam-ples laid a foundation of knowledge about Earth’s nearest neigh-bor and provided a cornerstone for planetary science. Apolloshowed the Moon to be ancient, some 4.5 billion years old andmade of materials similar to those on Earth, but consistent withthe Moon’s smaller size, lower pressure, lack of atmosphere,and lack of any obvious aqueous alteration. Its minerals androcks bore evidence of an early magma ocean and differentia-tion into a mantle and crust. Heating and remelting of the in-terior produced voluminous basaltic volcanism 3-4 billion yearsago. From study of Apollo samples and data came the conceptof the Moon’s formation via a giant impact on early Earth,which still stands as the leading hypothesis for the origin of theMoon. Apollo surface samples gave us our first look at alter-ation by exposure to galactic cosmic rays, energetic solar par-ticles, and meteorites, ranging from microscopic to asteroidal.

Perhaps the most far-reaching scientific legacy of Apollo isthe ongoing exploration of our solar system. The Apollo samplesprovided the first evidence of the so-called late, heavy bombard-ment of asteroids, thought to have spiked around 3.9-4.0 billionyears ago. Models of the early solar system’s orbital dynamicssuggest that shifts in the orbits of Jupiter and Saturn may havedestabilized early asteroid and cometary belts and led to thatcataclysm some 500 million years after the solar system formed.18

The Apollo samples and explorations showed that the keyto testing those dynamical models is on the Moon, awaiting thenext round of surface exploration and sample collection.

REFERENCES1. D. A. Papanastassiou, G. J. Wasserburg, D. S. Burnett, Earth Planet.

Sci. Lett. 8, 1 (1970).2. J. A. Wood et al., in Proceedings of the Apollo 11 Lunar Science

Conference, A. A. Levinson, ed., Pergamon Press (1970), p. 965.3. P. Eberhardt et al., Moon 8 104 (1973).4. G. Neukum, B. A. Ivanov, W. K. Hartmann, Space Sci. Rev. 96, 55

(2001).5. P. H. Warren, J. T. Wasson, Rev. Geophys. 17, 73 (1979).6. D. Stöffler, G. Ryder, Space Sci. Rev. 96, 9 (2001).7. D. E. Wilhelms, To a Rocky Moon: A Geologist’s History of Lunar

Exploration, U. Arizona Press (1993).8. P. Spudis, C. Pieters, in Lunar Sourcebook: A User’s Guide to the

Moon, G. H. Heiken, D. T. Vaniman, B. M. French, eds., CambridgeU. Press (1991), chap. 10.

9. F. Tera, D. A. Papanastassiou, G. J. Wasserburg, in Fifth Lunar Sci-ence Conference, Lunar Science Institute (1974), p. 792.

10. R. J. Drozd et al., in Eighth Lunar Science Conference, Lunar ScienceInstitute (1977), p. 254.

11. A. Khan, K. Mosegaard, J. Geophys. Res. Planets (2002), doi:10.1029/2001JE001658.

12. P. Lognonné et al., Earth Planet. Sci. Lett. 211, 27 (2003).13. J. G. Williams et al., J. Geophys. Res. Planets 106, 27 933 (2001).14. A. E. Saal et al., Nature 454, 192 (2008).15. F. M. McCubbin et al., Am. Mineral. 95, 1141 (2010).16. E. Hauri et al., Science 333, 213 (2011).17. B. P. Weiss, S. M. Tikoo, Science 346, 1198 (2014).18. R. Gomes et al., Nature 435, 466 (2005). PT

APOLLO PROGRAM

SouthMassif

NorthMassif

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SculpturedHills

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L5L6

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L3 L2 L1

L7 L8

S5S1

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Lunarmodule

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FIGURE 5. TAURUS–LITTROW VALLEY, the landing site for Apollo 17.(a) In this oblique, 18-km-wide scene looking generally to the west,Mare Serenitatis is at the upper right. North is to the right. (Imagecourtesy of NASA/GSFC/ASU.) (b) This view of the valley (the lowerleft part of panel a) shows the astronaut traverses. Numbered labels“S” and “L” refer to sampling stations and lunar-roving-vehicle stops,respectively. (Image courtesy of NASA/GSFC/ASU.)

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