Lunar Eclipse Makes Laser Ranging More Accurate
February 12, 2014

Measuring The Distance Between The Moon And Earth Easier During Lunar Eclipses

April Flowers for - Your Universe Online

The full moon has long been linked to strange events and even stranger behavior, even though careful scrutiny has dispelled any association, according to Susan Brown of UC San Diego. So when scientists bounced signals off the lunar surface on full moon nights and only received faint echoes in return, they went looking for an explanation based in reason, not superstition. The most compelling evidence, though, arrived during another event known to evoke irrational fears — a night when Earth's shadow eclipsed the full moon.

Scientists like Tom Murphy, a UC San Diego physicist, aim laser beams at suitcase-sized reflectors that were placed on the moon by Apollo astronauts and unmanned Soviet rovers. Murphy is able to measure the distance from here to the moon with millimeter precision by timing the light's return to Earth.

These measurements are called lunar ranging, which has revealed that the moon is slowly spiraling away from the Earth. Lunar ranging also suggests that the moon has a molten core. Using precise measurements of the changing shape of the lunar orbit, Murphy and his team are subjecting Einstein's theory of general relativity to the most stringent test yet.

The signals, faint to begin with, have faded over time. Murphy leads a project at Apache Point Observatory that sends laser pulses of 100 quadrillion photons. On average, only one lonely photon returns from each pulse, if any return at all. Some of the photons are nudged off target by Earth's atmosphere so that they hit the lunar soil, and the beam is slightly diffracted by the reflectors so that most miss the telescope when they return.

Murphy's team records ten times fewer photons than expected, even accounting for the losses caused by atmosphere and diffraction. The return rate on full moon nights decreases even more, dropping to just one percent of the predicted performance. On these same nights, other observatories were unable to detect any returned signal.

As a joke, the team started calling this "the full-moon curse," Murphy told Brown. "For a while we thought we were just victims of bad luck, but the trend continued, month after month."

Accumulated dust could be the culprit behind the diminished returns, according to Murphy. If true, this could spell bad news for future plans to place telescopes on the surface. There is no wind on the moon, but electrostatic forces and a constant bombardment by tiny meteorites could have kicked up some of the lunar dust, making it cover the surface of the clear glass prisms arrayed in each reflector.

Light passes through each prism twice — once on the way in and once on the way out. A 50 percent coating of dust would be enough to account for the loss of signal on most nights, but not on full moon nights. Murphy thinks the cause of the full moon loss is heat.

Because the prisms are sunk into cylinders, the sun only fully illuminates them when it shines directly into them. The arrays face Earth, so the full illumination only happens on full moon nights. A thermal gradient between the surface and the depths of the prisms is created as the dark dust of the lunar regolith heats up. This would change the refractive index, degrading their performance further by turning the prism into an unintentional lens and diverging the returning light. Even fewer photons would return to the telescope.

This idea is met with some enthusiasm because it creates a thing that scientists love: a testable prediction. If, as Murphy thinks, the diminished returns on full moon nights are caused by heating of the surface of the reflector cubes, then turning off the light should boost the signal as soon as the surface cools and temperatures throughout the cube are uniform.

Simple, right? You just need to turn off the Sun. Or, wait for a lunar eclipse when the Earth passes between the Sun and the moon. Murphy and his team were able to have decent observing conditions during a lunar eclipse on the night of December 21, 2010. They ranged lasers from the three Apollo reflector arrays and an array mounted on a Soviet rover for five and a half hours as the edge of the Earth's shadow passed by each of them in turn and as they re-emerged into the sunlight.

The team observed a tenfold spike in performance as the Sun's light was blocked, just as Murphy predicted. The signal return was restored to the levels seen on other nights. The results of their experiment were described in the journal Icarus.

Some skeptics ask, if moon dust is moving about and obscuring the arrays, why can the boot prints left by astronauts decades ago still be seen? According to Murphy's calculations, at the rate of deposition that occurred to obscure the reflectors, covering the boot prints would take tens of thousands of years.