Moon's mystery to be solved with lasers and mirrors

An American astronomer hopes to answer to within a millimetre a question first posed by the jazz trumpeter Dizzy Gillespie: how high the moon?

Tom Murphy, of the University of Washington, will use a billion watt laser "bullet", a telescope at Apache Point, New Mexico, to measure the distance, along with mirrors that were left on the moon 30 years ago by Apollo astronauts and Russian landing craft.

He will fire a pulse of laser light at a reflector surface on the moon, wait for it to bounce back, and then use the known speed of light in a vacuum to calculate precisely how far the moon is from the earth.

Long before the Apollo programme, astronomers had worked out that the centre of the moon was about 384,150 kilometres (238,700 miles) from the centre of the earth. The astronaut Edwin "Buzz" Aldrin left the first laser reflector on the moon in 1969.

Within a month scientists knew the distance to within 4.5m (15ft). Within a few years, researchers had narrowed the margin of error to within 25cm (10in). By the mid 1980s, the accuracy had closed to within an inch.

In 30 years of laser measurements, scientists have confirmed that day length alters by about a thousandth of a second in a year, because of the drag of tides and the atmosphere. And they have measured the movement of the continents - at the speed at which fingernails grow. They have also confirmed that the moon is receding from the earth at about 4cm a year.

But Dr Murphy plans to spend five years narrowing the uncertainty still more. He will aim a series of inch long, one-gigawatt pulses of laser light through his telescope to the five mirrors now on the moon.

The earth's atmosphere will be the first problem. It will distort the tightly focused beam so that by the time it gets into space it will be more than a mile wide; so most of the light will miss the mirror.

"It's very tall odds," saidDr Murphy. "And then for a photon to make it back to the telescope, the odds again are about one in 30m."