Laser Retroreflector Array

NASA has delivered a retroreflector array to ESA that will allow the Lunar Pathfinder misssion to be pinpointed by laser ranging stations back on Earth as it orbits the Moon. Such centimetre level laser measurements will serve as an independent check on the spacecraft as it fixes its position using satellite navigation from an unprecedented 400,000 km away from Earth – proving the concept of lunar satnav as well as telecommunications ahead of ESA’s dedicated Moonlight initiative. 

Laser retroreflectors are well-established space technology, normally used to precisely determine the orbit of satellites around the Earth. By measuring the time of flight for the laser pulses to travel from Earth to the satellite and back, its precise distance can be calculated – along the same lines as radio-based ranging, but achieving much higher accuracy because of the short wavelength of light.

 In approach they resemble the mirrored ‘cat’s eyes’ embedded in motorways to reflect light precisely back to its source, thanks to an intricate internal reflection setup – a total of 48 ‘corner cubes’ in the case of the LRA, which were individually and rigorously inspected and measured in the laboratory. The optical performance of the array was accurately measured at NASA's Goddard Space Flight Center.