Celestial light switch reveals lunar ranging data

Sarina Tracy, sarina.tracy@photonics.com

When certain members of the universe play a linear game of hide-and-seek, humans pay attention: Many don special solar glasses; others stay up late to catch sight of a burnt-orange moon. Solar and lunar eclipses are sights to behold, appearing just a few times a year and visible only to certain geographies. The power of an eclipse, however, can be harnessed for uses other than wonder: On Dec. 21, 2010, a group of scientists exploited the unique qualities of a lunar eclipse for the sake of solving a mystery – and saving its research.

Dr. Tom Murphy, a physicist at the University of California, San Diego, and his team at Apache Point Observatory in Sunspot, N.M., have been aiming laser beams at suitcase-sized reflectors placed on the moon by Apollo astronauts and unmanned Soviet rovers. By timing the laser beam’s return to Earth, the scientists can measure the distance to the moon with millimeter precision. The process, called lunar ranging, has revealed the moon’s slow, spiraling path away from us. The group uses those measurements of the lunar orbit’s changing shape to test Einstein’s theory of general relativity.

The laser beam heads toward the eclipsed moon, reflecting off a thin layer of clouds about 7 km above the lunar reflector site. Photo courtesy of Jack Dembicky.

Under normal conditions, a returning beam signal is faint, with just one photon coming back from a pulse of more than 100 quadrillion photons. In addition, the Earth’s atmosphere nudges some photons off target, hitting the lunar soil, while the reflectors slightly diffract the returning beam. Most photons miss the telescope when they return. These losses are expected and come with the territory of beaming a laser more than 450,000 miles round-trip.

However, Murphy and his team discovered that, over time, those already-faint signals returned by the reflectors had faded even further. They recorded 10 times fewer photons than expected, suggesting something amiss. On full-moon nights, the results were even worse, dropping to just 1 percent of the predicted performance. The researchers dubbed this “the full-moon curse.”

“For a while, we thought we were just victims of bad luck,” Murphy said, “but the trend continued month after month.”

Accumulated moon dust on the reflector’s surface was blamed for the diminished returns, as light must pass through the surface of each prism twice – once on the way in and once on the way out. Although there is no wind on the moon, electrostatic forces and a constant bombardment by tiny meteorites kick up lunar dust, coating the clear glass prisms arrayed in each reflector. Murphy calculated that a dusting covering 50 percent of the glass would be enough to account for the dimming of the return signal they observed on most nights. What the dust didn’t explain, however, was the beam’s dramatic failure during a full moon.

The Apollo laser shines its beam from a telescope enclosure. Photo courtesy of Tom Murphy.

The explanation was surprisingly simple: heat. The prisms are sunk into cylinders so that the sun fully illuminates them only when it shines straight in. Because the arrays face Earth, that happens only on full-moon nights. When it does, the dark dust of the lunar regolith heats up, creating a thermal gradient between the surface and the depths of the prism. The performance is degraded by altering the refractive index, turning the prism into an unintentional lens and shifting the returning light so that even fewer photons return to the telescope.

To test this theory, the team had to turn off the sun. Metaphorically speaking, an eclipse does just that. If the poor performance on full-moon nights resulted from the prisms heating up, then turning off the light should boost the beam’s signal.

And so, for 5 1/2 hours on that night in December 2010, Murphy’s team ranged lasers from the observatory to four reflector arrays on the surface of the moon. When the Earth’s shadow passed across each prism, the researchers saw their predicted tenfold spike in performance, restoring the signal to normal levels.

“Getting rid of the sunlight in an ‘on, off, on’ test was an ideal way to test our primary suspicion,” Murphy said. “And we didn’t have to pay movers to put the moon in front of the sun – one of nature’s freebies.”