After 40 years' reflection, laser moon mirror project axed

An experiment, begun when Apollo 11 astronauts Neil Armstrong and Buzz Aldrin left a mirror on the lunar surface 40 years ago to allow Earth-based astronomers to fire lasers at it, has been ended by American science chiefs.

The National Science Foundation (NSF) last week wrote to scientists working at the McDonald Laser ranging station at Fort Davis in Texas to tell them the annual $125 000 funding for their research project was going be terminated following a review of its scientific merits.

The decision means that four decades of continuous lunar laser research at the McDonald Observatory, run by the University of Texas at Austin, will be halted by the end of this year. Among the project’s unlikely achievements has been the discovery that the moon is moving away from Earth at a rate of 6,4cm a year.

The mirror’s existence, and the fact that astronomers can bounce lasers off it and detect the returning beam, has also provided Nasa and other scientists with compelling evidence to refute the claims of moon-landing deniers who claim the Apollo lunar mission were hoaxes filmed in an Earth-based studio.

“It is a bitter-sweet feeling to know this is going to come to end at McDonald,” said Peter Shelus, head of the laser ranging project. “We have done a great deal of important work using the moon mirrors but it is clearly time for it to end. However, we are hopeful that this work will be continued at other astronomy centres.”

The mirror left by Aldrin and Armstrong after they landed on the Sea of Tranquillity on 21 July 1969, was one of five known as “corner mirrors” or “retro-reflector arrays” that were taken to the moon in the later Sixties and early Seventies. Two other corner mirrors were brought to the moon by astronauts on later manned lunar flights, on the Apollo 14 and the Apollo 15 missions. In addition, a second pair were built by French scientists and flown to the moon by the Soviet Union on their robot Luna probes.

Corner mirrors are important scientific instruments because, when struck precisely by a laser beam, they reflect that beam in a parallel path straight back to the source of the laser.

“Essentially, we measure when that beam goes out and when it comes back,” said Shelus. “We know the speed of light, of course, so that timing allows us to calculate the moon’s distance with incredible precision.”

After these laser measurements were amassed for years, calculations by astronomers at the McDonald Observatory showed that as the moon orbits Earth, it creates a bulge of water that travels round the planet behind it. This bulge—which we experience as tides—exerts a gravitational pull on the moon, slowing it down as it circles Earth at a distance of 384 000km.

As a consequence of being held back by this pull, the orbit of the moon becomes altered and it moves slowly away from Earth—at a rate of 6,4cm a year. These measurements have, in turn, allowed scientists to carry out valuable tests of theories about relativity and gravity, added Shelus.

A spokesperson from the NSF said last week that, after carrying out two reviews, it had decided there was no longer “a strong science case” for continuing its 40-year support for the lunar laser ranging project. The spokesperson added that two other astronomy centres—at Apache Point in Texas and Observatoire de la Côte d’Azure in France—- were expected to carry out lunar-ranging experiments in future.

“These are very good centres,” said Shelus. “However, it does mean that the continuity of our measurements, which we have established since the Apollo missions, will now have to stop. It is, rather sadly, the end of an era.” -

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