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MOONSHOTS 15 Retroreflector; Earth Rises Above the Moon As Seen from Apollo 11

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MOONSHOTS 15 Retroreflector; Earth Rises Above the Moon As Seen from Apollo 11 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 Apollo 11. 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 Albert Einstein’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 physicists 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 TAR D ATE 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 gravity. 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.
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