Clockwise from right: McDonald Observatory’s 107-inch telescope fires a laser at the Moon; a laser lights up the Apache Point 3.5-meter telescope; part of the Apollo MOONSHOTS 15 retroreflector; Earth rises above the Moon as seen from . 40 years after the first astronauts walked on the Moon, Apollo’s last experiment is still probing the Moon, Earth, and much more

By Damond Benningfield

om Murphy scrambles up a narrow about the thickness of a paperclip. ladder into a concrete vestibule be- Murphy’s observations could add one more Tneath the 3.5-meter telescope at the “giant step” to Apollo’s accomplishments by Apache Point Observatory in southern New showing that ’s theory of grav- Mexico. A blue metallic cone punctures the ity is wrong. Such a result could explain dark center of the tiny room, part of the support energy, provide the first support for string structure for the 45-ton telescope. But Mur- theory, and unify two fundamental fields of phy is there to check on a boxy electronics physics — general relativity and quantum cabinet in the corner. A new addition to the mechanics. room, it measures the up-and-down flex of the “It sticks in the craw that the two pillars of 9,200-foot mountain peak below the telescope physics don’t get along,” Murphy says. “When in response to changes in air pressure, tides you try to merge them, it simply doesn’t work. in Earth’s crust, and even the crash of storm- You get pathologies in that marriage that make driven waves on shores thousands of miles scratch their heads.” away. “If the site moves up or down by half a Testing general relativity was one of the NASA (2) millimeter, we can sense that,” says Murphy, original goals of the laser experiment, which an associate physics professor at the Univer- has operated continuously since just days after Observatory; sity of California-San Diego. Apollo 11 touched down on the Moon on July

Point

che Such a tiny distance is critical because Mur- 20, 1969 — the only Apollo experiment that phy is making some of the most precise as- is still in operation. Scientists also hoped it

Murphy/Apa tronomical measurements ever attempted. He would help plot any wobbles in the rotation om T is using the telescope as a giant laser pointer, of the Moon or Earth, reveal details about the bouncing its light off of special reflectors left Moon’s interior, and determine whether the

Observatory; on the Moon by three Apollo missions and Moon is moving away from us. And over the

Donald

c M a robotic Soviet rover to measure the Earth- decades, the experiment has accomplished all Moon distance to within one millimeter — that and more.

4 MARCH/APRILJULY/AUGUST 20092008 Clockwise from left: S t a r D a t e 5 “All the goals we identified have been from Earth, though, giving NASA enough con- any longer, fraction of the laser light actually struck How Far? realized,” says Carroll Alley, lead scientist fidence in the experiment to schedule a reflec- then you went the LR3. The process was repeated as this First McDonald for the Apollo 11 laser experiment and a re- tor for the first manned landing, in 1969. to bed, and trickle of light reflected off the instru- Laser Detection search professor in physics at the University Alley’s group designed an array of 100 ret- you got up and ment and returned to Earth. of Maryland-College Park. “The fact that it’s roreflectors, each 1.5 inches wide, housed in a went back to So while each laser pulse consisted August 19, 1969 lasted for 40 years has greatly increased the metal frame. The tray was as big as an attache work again. But of trillions of particles of light, known precision of the measurements. The longer case and its 10-by-10 array of reflectors, which we made it. We as photons, it took several shots to Roundtrip Laser the experiment lasts, the more we can say were recessed to protect them from direct had it ready.” get a single photon to return to Travel Time about our original questions.” sunlight, looked like an egg carton. The ex- During their the telescope. Instead, the telescope 2.49596311 seconds Many of those questions were born in the periment required no power, communication, two-and-a-half- was filled with background light days after the Soviet Union launched the first or moving parts, making it easy to set up and hour moonwalk, from the Moon and other sources. Calculated Distance artificial satellite, Sputnik 1, in late 1957, operate. Apollo 11 astronauts So scientists needed to know just to the Moon when Alley was a member of a research group Neil Armstrong and when to look so they could filter 374,135,457.91211219 at Princeton. Group leader Robert Dicke sug- hile the scientists prepared the Edwin Aldrin gath- out the stray light and identify meters gested that shining giant searchlights on satel- lunar end of the experiment, Alley ered about 50 pounds the laser photons. “If you didn’t lites studded with special reflectors could yield also looked for the terrestrial end: of rocks and soil, set know the range, the background 232,476 miles, 5,246 feet, W new insights into the physics of . 5.485 inches an astronomical observatory. Several rebuffed up a sheet of metal light was horrific,” says Silver- The idea gained momentum with two de- his overtures because their telescopes were foil to gather particles berg. velopments in the early 1960s: the invention too busy or because they feared that the of the solar wind, and Lick Observatory recorded Estimated Error of the laser and plans to send both machines high-power laser could damage a telescope’s deployed a seismometer the first successful return, on ±.000000003 seconds and people to the Moon. The Moon would reflective coating. The University of Michi- to listen for moonquakes. August 1. McDonald didn’t 4.5 meters provide a bigger and more stable platform for gan agreed to host the experiment on a new Armstrong also set up the see its reflection until Au- 14 feet, 9 inches studying tiny effects of gravity, while lasers telescope under construction in Hawaii, while Lunar Ranging Retro Re- gust 19. would provide a stronger signal than conven- Lick Observatory signed up for a few weeks flector (LR3), NASA-speak Lick dropped out a Total McDonald tional light sources with a smaller investment of work on its 120-inch (3-meter) telescope, for the laser experiment, few weeks later, leaving Lunar Ranges of energy. then the world’s second largest. about 70 feet away from the the field to McDonald. Apollo 15 lunokhod 2 (April 2009) Alley and his colleagues developed the con- In early 1969, though, Michigan backed out lunar lander, Eagle. The scientific team re- Approximately 6,560 cept of lunar laser ranging, in which a space- of the deal, leaving Alley with no long-term McDonald and Lick both fined its techniques and (each ‘range’ incorporates the craft lands an array of “retroreflectors” on the home for the lunar laser ranging experiment. took aim at Tranquility Base began regular ranging results from many individual laser Moon and an astronomical telescope fires a “A colleague here at Maryland told me that within minutes, but without Apollo 14 Apollo 11 experiments in early shots) laser beam at it and measures the reflection. there was a new telescope coming on line at success. Scientists weren’t 1970. “All of a sudden, NASA (5) Precise timing of the beam’s round trip the McDonald Observatory,” Alley says. “The sure just where Eagle had land- we knew how to do it,” reveals the distance between Earth and 107-inch was then the third-largest telescope ed, they had little experience says Silverberg, who From top: Apollo 11 laser the Moon far more accurately than any in the world. I got in touch with the director, at aiming a telescope at such a became project man- reflector, Lunokhod before other technique. Harlan Smith, and his response was very posi- rapidly moving target, and the ager when McDon- launch, the four working A retroreflector looks like the inside of tive. He even provided a plane to meet us in distance to the Moon was known ald took over the retroreflectors pinpointed on an Apollo image of the a cube that’s been cut in half from corner El Paso and fly us to McDonald.” NASA had only within a half-mile or so. NASA contract Moon, Apollo 14 reflector, to corner. “You take three mirrors and funded the telescope, which was dedicated in Scientists needed precise three- from the Univer- and Apollo 15 reflector. arrange them so they’re at right angles late 1968, to support its ambitious program of dimensional coordinates because of sity of Maryland. to each other, and that forms a corner,” solar system exploration. the way the laser worked. Apollo 14 left explains Jerry Wiant, assistant manager By the time Maryland and Texas worked out At McDonald, the Korad laser a second reflec- for laser ranging at McDonald Observa- the details, though, time was running short. It shined its red beam on the telescope’s tor on the Moon tory, which has been firing lasers at the was spring, and Apollo 11 was scheduled for primary mirror, which reflected the in early 1971, Moon since 1969. “Any light that comes launch in mid-July. Alley dispatched a team light into space. Each shot consisted of and Apollo 15 in bounces off those surfaces and comes of scientists, engineers, and technicians to a single pulse just three billionths of a added a third back out toward the source. It’s a beauti- install and test the laser and its instrumenta- second long. That created a “pancake” of that was three ful concept.” Retroreflectors are used on tion on the telescope, which itself was still in laser light that was 107 inches wide and a times bigger cars, bicycles, and highways to reflect a shake-down mode. few feet thick as it left the telescope. As it than the oth- car’s headlights, allowing drivers to see “There was no alternative — you would traveled through the atmosphere, though, ers. The So- Schematic shows the obstacles more easily. be ready when they landed,” recalls Eric the pancake spread out. By the time it viet Union McDonald 107-inch NASA turned down a proposal to attach a re- Silverberg, a member of the Maryland team reached the Moon, the beam was a mile or a t t a c h e d telescope and the flector to the last of a series of unmanned mis- who later oversaw the McDonald laser effort. two across, but any targeting error meant it reflectors arrangement of the laser 3 to the Lu- components. sions to explore the lunar surface. The craft’s “About all I remember of that first few weeks is could miss the LR . And even if the beam television camera did detect laser beams fired that you worked until you couldn’t stay awake did hit its target, only an infinitesimally small nokhod 1

6 JULY/AUGUST 2009 S t a r D a t e 7 and 2 rovers, although the Earth-bound driv- the two. As gathers 100 to 1,000 times more photons than the Sun” by solar gravity, Murphy says. If gen- Debunking ers parked Lunokhod 1 in the wrong direc- Earth and the the McDonald station. Combined with more eral relativity is correct, then the Sun will pull the Debunkers tion, so only Lunokhod 2 has provided any Moon circle precise measurements of the laser’s position on equally on both of them. If other theories of We b The lunar laser ranging data. each other, Earth’s surface, that is allowing Murphy and gravity are correct, then the Sun will pull on Teams in Hawaii, Arizona, Australia, and their gravity other project members to plot the Moon’s orbit them differently, so the Moon’s path will be McDonald Observatory ranging experiment mcdonaldobservatory.org not only tells us Germany also tried laser ranging, but with acts as a brake, about 10 times more precisely, he says. shifted toward the Sun by a tiny amount — no limited success. France got into the game, too, slowing the Such precision will allow them to better ad- more than 13 meters (40 feet). McDonald Laser Ranging Station about the Moon’s www.csr.utexas.edu/mlrs orbit, rotation, and although its laser system has been out of other’s rotation. dress the laser ranging experiment’s original Earlier laser ranging studies, which plot- and interior, it operation for several years, it has amassed This has locked goal: testing Albert Einstein’s theory of gravity, ted the Moon’s position with an accuracy of Apache Point Laser Experiment also proves that more moonshots than any group other than the Moon so that known as general relativity. about one centimeter (0.4 inch), show that www.physics.ucsd.edu/~tmurphy/ astronauts really did McDonald. it rotates at the So far, Einstein’s version of relatively has the Moon is just where general relativity says apollo/apollo.html walk on the Moon, Texas astronomers “ranged” three times a same rate at which withstood every test. Yet general relativity it should be. But by improving the precision Apollo Project Sites says Tom Murphy. day: when the Moon was halfway across the it spins on its axis, cannot accurately describe the behavior of of the measurements to one millimeter (0.04 www.apolloarchive.com “If the lunar landing sky and three hours before and after. “It was so the Moon always the universe at the smallest scales — the inch), APOLLO will test general relativity 10 www.nasa.gov/mission_pages/apollo/ business was a a gruesome schedule,” says Silverberg. But presents the same realm of — so scien- times more precisely. “Is that enough? Nobody index.html hoax, I’d know they “somehow, it just worked,” adds Peter Shelus, face to Earth. tists have a gut feeling that if they keep knows,” Murphy says. “There’s a scientific para- Apollo 11 Experiments were lying, because who has directed the McDonald laser project The Moon is trying testing, eventually it will fail. digm that expects a violation at any time now. history.nasa.gov/alsj/a11/ we wouldn’t get since the late 1970s. to do the same thing to “It doesn’t make sense that we’d get But there’s no guarantee that another order of a11EASEP.pdf any reflection,” he By 1985, though, NASA could no longer af- Earth through the tides. gravity right on just the second try,” says magnitude will put the theorists out of work.” Ra d i o says. So the 10 ford to fund the experiment on the 107-inch As Earth slows down, Murphy. “Newton tried it in the 1600s, If the equivalence principle fails, then so or so photons that telescope, and other astronomers were eager some of the energy of its but it wasn’t quite right. It doesn’t does general relativity. That might explain StarDate Series on Apollo 11 Mission Murphy gets from to get rid of the interruptions to their own rotation is transferred to make sense that Einstein would come dark energy — a discovery that the universe each “moonshot” July 16: Apollo 11 projects. So McDonald donated the original the Moon, which moves along and get it right the second time is expanding at a faster rate as it ages. It could July 17: Shooting the Moon confirms that the laser to the Smithsonian and built a perma- farther away from Earth. around. Nature is more complex than be a mysterious form of energy that is pushing reflectors are sitting July 18: Lunar Dust nent laser-ranging station with a 30-inch Laser ranging also has plot- that.” outward on space itself, or it could simply be a July 19: Lunar Artifacts on the lunar surface telescope one mountaintop away, and it’s still ted wobbles in the Moon’s Lunar laser ranging tests one of flaw in our understanding of gravity, which is July 20: Lunar Landing — placed there by the basic tenets of general relativity, July 21: A Giant Leap moonwalkers. operating today. rotation that are caused by based on general relativity. The new McDonald Laser Ranging Station, the gravitational tug of Earth, known as the equivalence prin- A flaw in general relativity would also pro- July 22: Moonbugs ciple. In Einstein’s theory, every July 23: Testing Moonrocks which fires shorter but more frequent pulses the Sun, and other astronomi- vide the first experimental support for string July 24: Return to the Moon of a blue-green laser, not only keeps tabs on cal bodies on the Moon’s lumpy object is gravitationally the same theory — a view of the universe in which all stardate.org/radio the Moon, it also shoots artificial satellites in surface. The wobbles have regardless of its composition. In forms of matter and energy consist of tiny, vi- low orbit, just as Robert Dicke had envisioned helped scientists plot the Moon’s other words, if you drop a ham- brating strings. And it would open the way to after Sputnik. Among other applications, shape, and have even revealed mer and a feather on the airless a new theory of gravity that would play nicely the satellite ranging provides information on secrets about its core. Moon, as astronaut David Scott with quantum mechanics. Earth’s rotation and the motions of its tec- “One of the fun surprises is that did in 1971, then the Moon’s “You can never anticipate what new, funda- tonic plates. we saw something funny in the gravity should pull them down mental insights into the world will provide,” Moon’s rotation — a rather strange to the surface at the same Murphy adds. “General relativity is a part of rtificial satellites don’t answer ques- dissipation in its energy,” Williams rate. (It did.) everybody’s lives. It’s used in the GPS system, tions about the Moon, though, or says. “It took 20 years to lock it “If that’s the case, then for example. If we didn’t understand general Aabout gravity. That still requires ob- down, but it’s telling us that the you can formulate gravity relativity, the entire system would fall apart in servations of Earth’s only natural satellite. Moon has a fluid core about 350 to as a property of space-time an hour. Those observations have helped planetary 390 kilometers in diameter.” There is itself — gravity is a geo- “General relativity departs from Newton’s scientists piece together many details about evidence that the liquid core surrounds metric concept,” Murphy theory of gravity by about one part in 100 mil- the Moon and its interactions with Earth. a solid inner core, although the observa- says. “It’s at the very heart lion. That’s irrelevant for most applications — “At the start, we were just learning about tions are inconclusive. of general relativity.” too many other things get in the way. But what the orbit of the Moon,” says James Williams, a The Moon’s interior could become Lunar laser ranging if the next breakthrough is at one part in 100 Left: The McDonald Top: A corner senior research scientist at NASA’s Jet Propul- clearer, though, with new data from Tom allows physicists to test million of general relativity? We don’t know laser station ‘shoots’ retroreflector like those sion Laboratory who has studied laser-ranging Murphy’s project in New Mexico: the the concept on a large what might come from that.” the Moon, which is results since 1971. “That’s not very exciting, Apache Point Observatory Lunar Laser- scale. But it would be quite a legacy for the last overexposed in this in the laser arrays on the M

c Moon. Above: Diagram but you need it to do other science.” ranging Operation (APOLLO). DONALD “Because the Earth working experiment of our first trips beyond image. shows how the corner One of the first things scientists learned is Thanks to its bigger telescope, more pow- and Moon are each Earth. reflectors always reflect a that the Moon is moving away from Earth at a erful laser, and more sensitive detectors, OBSERVATORY in orbit around the light beam in the direction rate of about 1.5 inches (3.8 cm) per year as a APOLLO is providing far more laser-ranging Sun, each is being Damond Benningfield is executive editor of from which it came. result of the gravitational interaction between data than any previous effort. The system accelerated toward StarDate.

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