One of the space program's longest-running experiments -- and one with a NIST connection -- celebrates its 25th anniversary this month by continuing to return data. During their pioneering moon landing on July 20, 1969, the Apollo 11 astronauts set up a laser reflector to make precise measurements of the distance between the Earth and moon. The still-operational experimental station reflects a powerful laser pulse aimed at it from telescopes on Earth. By measuring how long the pulse takes to return to Earth (the round trip takes about 2.5 seconds), scientists have defined the Earth-moon distance to within 2.5 centimeters (1 inch). The reflector was designed primarily by James Faller of the Joint Institute for Laboratory Astrophysics, operated cooperatively by NIST and the University of Colorado. It consists of a briefcase-sized aluminum panel studded with 100 corner reflectors (the corners of precision-ground glass cubes that have been cut off at 45 degree angles), each about 3.8 centimeters (1.5 inches) across. When a ray of light enters the cut-off surface, it is internally reflected from the three sides of the corner, exits the cut-off surface parallel to its entry path and then returns to its source. The same principle is used in bicycle reflectors. The Apollo 14 and 15 missions delivered two other Faller-designed reflectors, including one with 300 cube corners. All three reflectors are targeted almost nightly by scientists at observatories in Texas and France.