June 29, 2016

Laser shootings to the Moon

Measurement of the Earth-Moon distance

By using laser shootings to the Moon, physicists accurately measure the distance which separates the Earth from our satellite. This provides valuable information to detect possible shifts.

Improvement of the planetary method initially suggested by Newton requires a good knowledge of the distance which separates the Earth from the Moon. In this context, during the first lunar landing in 1969, the astronauts of Apollo 11 deposited a reflector on the Moon. Altogether, five reflectors, placed on the Moon by robotized Russian missions and by American astronauts between 1969 and 1973, allow to precisely measure the Earth-Moon distance by laser telemetry.

The design of these reflectors, which physicists name "corner cubes", is the same as the one used on bicycle reflectors: they reflect light in the incidental direction. By concentrating powerful laser beams on these reflectors, physicists receive 2.6 seconds later photons which travelled to the moon and back, in spite of the huge attenuation of 1021 (1019 photons are sent to each shooting and only one photonis received every 100 shootings).

The timing of the travel is thus an accurate measurement of the distance between a point on the Moon (one of its reflectors) and a point on Earth (the telescope that emits and receives the laser beam) which can be evaluated with less than two centimetres. Not bad for a body which is at an average distance of 360,000 kilometres of Earth...

Reflector for the laser
Reflectors placed during the Apollo 15 mission in 1971.

Modification of the lunar orbit

By knowing the Earth-Moon distance by a few centimetres, physicists can perform a precise test of the equivalence principle.

The the Moon's motion is extremely complex. Disturbances of the lunar orbit compared to a circular orbit are separated in various contributions which generate small oscillations of the lunar orbit. One of these contributions, the "parallactic inequality", results from the effect of the solar tide on the Moon's motion compared to the Earth. The impact of this disturbance on the Moon is that the lunar orbit shifts slightly towards the Sun. The well-known amplitude of this shift is 110 kilometres.
As the Moon and Earth are of different composition, a violation of the equivalence principle should have an impacy on the way in which the Sun attracts these two bodies, hence on the lunar orbit around the Earth. In practice, such violation would create an additional shift of the parallactic inequality. By knowing the Earth-Moon distance with a two-centimetre accuracy, physicists compared the measured orbit and the theoretical orbitn which led to validate the equivalence principle with a relative accuracy better than 10-12.

After the first laser telemetry measurements made from the Pic du Midi at the end of the seventies, France developed and operated a laser station on the Calern Plateau dedicated to the measurement of the Earth-Moon distance. After 30 years of activities nearly exclusively dedicated to this measurement, the station was renovated in order to open it to new activities.

Now called MéO (optical metrology), the station activity is centred on optical links. The station is involved in several projects like the optical time transfer (T2L2), the detection of space debris, coherent links (collaboration with Paris Observatory), adaptive optics (collaboration with ONERA) and obviously laser telemetry and more particularly the measurement of the Earth-Moon distance, which remains the instrument's priority.

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Laser shooting from the Calern Plateau, France