Wednesday, June 9, 1999 Published at 13:12 GMT 14:12 UK
Moon's tail spotted
The Earth passes through the sodium trail once a month
By BBC News Online Science Editor Dr David Whitehouse
The tail of sodium gas that streams out for great distances behind the Moon has been observed better than ever before.
The new observations were made on the nights following the Leonid meteor shower of November 1998.
The sodium atoms were blasted into space as tiny meteorites struck the lunar soil, it is believed.
The tail of sodium gas was seen to stretch for distances of at least 800,000 kilometres (500,000 miles) behind the Moon. Its appearance changed over three consecutive nights.
Since the Apollo Moon program, scientists have known that the Moon has an atmosphere, but it is extremely thin.
"It is one continuously being produced by evaporation of surface materials, and then continuously being lost by escape or impact back onto the surface," said Michael Mendillo, professor of astronomy at Boston University, where the new research was conducted.
Ten years ago, ground-based telescopes revealed that sodium gas formed part of the lunar atmosphere.
"There are less than 50 atoms of sodium per cubic centimetre in the atmosphere just above the surface of the Moon," says Jeffrey Baumgardner, of Boston University's Centre for Space Physics.
In contrast there are ten thousand million billion molecules per cubic centimetres in Earth's atmosphere at the surface.
The researchers found that when the Moon is new, it takes two days or so for sodium atoms leaving the surface to reach the vicinity of the Earth.
They are pushed away from the Moon by the pressure of sunlight and, as they sweep past us, the Earth's gravity pulls on them, focusing them into a long narrow tail.
If the Moon's sodium tail were perhaps a thousand times brighter it would be bright enough for the human eye to see. It would appear as a glowing orange cloud dominating the night-time sky.