Laser ranging studies gravity change

17 Jun 2002

US researchers are to make the most accurate measurement of the distance between the moon and Earth.

Moon and back

By using laser pulses sent through a 3.5 m telescope, a team of US-based researchers plans to measure the distance to the moon with millimeter accuracy. The results should produce fundamental physics data of unprecedented precision.

The APOLLO (Apache Point Lunar Laser-ranging Operation) project will use the large-aperture telescope housed within the Apache Point Observatory, US, to make their measurements.

An Nd:YAG laser operating at 532 nm will send out 100 ps pulses at 115 mJ per pulse at a rate of 20 Hz. This beam will then be expanded to fill the telescope and aimed at fused-silica retro-reflectors placed on the moon by the Apollo 11, 14 and 15 lunar missions.

Tom Murphy, a member of the APOLLO team based at the University of Washington explained: "The atmosphere distorts the collimated beam and at the lunar surface, the beam diameter is typically 2 km. Back on Earth, out of the 3 x 10¹⁷ photons sent, we'll detect on average 5 to 10 photons per pulse."

An integrated array of avalanche photodiodes (APDs) will detect the weak return signal, which is gathered within the same telescope that transmitted the pulses. The detectors will measure the round-trip travel time of the laser pulses to a precision of a few picoseconds, corresponding to millimeter precision in the distance to the moon.

"Each detected photon will pin down the lunar distance to maybe 10 cm. By accumulating many, we will reduce this down to millimeter precision," said Murphy.

The two primary goals of the project are to test Einstein's equivalence principle, which in turn will scrutinize general relativity, and to look for variations in the strength of gravity over time. Murphy told Optics.org that "lunar laser ranging is the only technique that can be used to study these parameters and our chances of turning up something interesting are not altogether negligible."