June 20, 2011

1970: first accurate measurement of the Earth-Moon distance


In order to precisely measure the Earth-Moon distance, Jean Rösch, head of the Pic-du-Midi Observatory came up with the idea for combining a telescope and lasers, the technology of which was being developed in the 1960's, to perform laser shots towards the moon.

In this purpose, he called for a laser specialist, Alain Orszag, from a laboratory of the Polytechnic School. He thus proposed him to use the Pic-du-Midi's 1 m telescope to make the measurements.

Tests had been carried out in the United-States by laser shooting the natural surface of the moon but the surface's relief did not allow accuracies of better than 150 m.

At the same time, especially in France, the space geodesy was developing thanks to Jean Kovalevsky of the Bureau of Longitudes, Jacques Blamont, Roger Bivas of the Aeronomy Service of the CNRS and many more.

In order to precisely pinpoint the satellites' orbits, laser retroreflector were mounted on their surfaces. So the first retroreflectors were mounted on the CNES' "Diademe" satellites D1C and D1D, launched in February 1967. The first metric distance measurements were made possible by this technology. These successes resulted in the creation of the CNES' Research Group on Space Geodesy in 1971.

CNES supplied the laser retroreflectors for the first lunar rovers

The Soviet Lunakhod's French retroreflector of the LUNA 17 and 21 missions featured fourteen 10.6 cm triangular prisms for a combined surface area of 680 cm² and a weight of 3.5 kg.

After the success of the Diademe missions, the Soviets asked CNES in 1968 to supply the laser retroreflectors for the Lunakhod 1 & 2 lunar rovers which were planned to land on the moon in the early 1970's.

Jean-Claude Husson, who was then the space geodesy program scientist at CNES put the Sud Aviation company of Cannes (now Thales Alenia Space) in charge of building the retroreflectors.

On November 17th 1970, the LUNA 17 mission was conveyed to the Mare Imbrium (Sea of Rains) region.

In the Moon race environment with the US, the Soviets did not mentioned that the mission would embark a movable vehicle and the transmission of the landing coordinates to CNES was made in the complete secrecy.

The first laser shots were carried out by night from the Pic-du-Midi Observatory on December 5th and 6th, 1970. A. Orszag used a powerful ruby laser, pushing the envelope of the knowledge of its time.

The laser shots were only possible a few days per month

The experimental conditions were tough:

  • since the link budget was very poor (even today, only 1 to 2 out of 1,020 photons are detected back), the measurements were only an option by night with the Lunakhod.
  • since the telescope could only be tracking tipped (on craters) in the illuminated part of the Moon (far from the rover), the 2 to 3 km diameter laser spot finally had to be blindly placed on the retroreflector by aiming for like a seamark more than 600 km away. The prior calculation of this deviation had thus to be very accurate.
  • since each measurement required from 70 to 200 laser shots over a period of 10 to 20 minutes, the moon displacement had to be taken into account by the pointing for the whole shooting session.

In 1965, the duration of each laser pulse was 50 ns, which led to a dozen meter uncertainty. This precision then kept improving. In practice, the laser shots were possible only a few days per month when a thin bright crescent of the moon was visible and only in good weather. Nevertheless, these first shots were successful and the first echoes from the moon were received.

But very quickly, the Lunakhod 1 stopped emitting echoes and various interpretations were given at the time:

  • wrong coordinates of the rover?
  • accidental cover closure?
  • retroreflector misalignment?
  • retroreflector deterioration resulting from the very demanding day-night thermal cycle for the optical components?

It was only in 2010, since the Lunar Reconnaissance Orbiter of NASA accurately localised the rover, that a laser shot from New Mexico could detect an echo. The problem was thus probably a wrong localisation of the rover.

Now, 5 lunar retroreflectors have been placed on the moon: two Franco-Soviet ones and these of the Apollo XI, XIV and XV missions. This ultra-large one was 10 times heavier than the French retroreflectors. However, in terms of efficiency, the rovers' retroreflectors proved to be of very good quality. Nowadays, the Apollo VX's retroreflector is being used more than the others and is the target of almost 80% of the laser shots towards the moon.

The moon moves away from Earth by 3.8 cm per year

The ruby laser have been progressively replaced by neodymium lasers (Nd-Yag) which were more powerful and able to emit shorter pulses (0.1 ns). The distance measurement accuracy kept improving from the metre in the 70's to 20 cm in the early 80's and a few millimetres now.

Besides, the results of the laser shots from the Pic-du-Midi Observatory showed that this site, if it was perfectly adapted (or even the only possible one) in terms of image quality to achieve a feasibility demonstration of laser-moon telemetry using the technology available at this time, was not the best position anymore to enhance and continue its first successes because of its geographic situation and unfavourable weather. That is what motivated the science community to develop a dedicated instrument which would be set up on the plateau of Calern, near Grasse (France) in the early 80's. In fact, even if the measurement accuracy is important, measurement regularity over several years or decades is also capital, in particular in order to observe the variations of some parameters over long periods. It has thus been determined, for example, that the moon is moving away from Earth by 3.8 cm per year.

The investigators' intuition of the 60's for accurately measuring the Earth-Moon distance worked very well. As proof, half a century later, laser shots towards the moon are still being performed from several observatories around the world and especially the Calern's one. These accurate measurements made the study of very complex motions of our natural satellite possible. They also gave access to some parameters of the moon and Earth's internal structure as well as fine motions of our planet (pole motions, continental drifts, etc.).

See also