Mapping the Moon

According to

Vision for Space Exploration, astronauts will return to the Moon as early as 2015. On the Moon, astronauts will learn how to live on an alien world before attempting longer voyages to other planets such as Mars.

However, current maps of the Moon are not very precise. In some areas, near Apollo landing sites, the locations of craters and ridges are well known as they were extensively photographed by lunar orbiters and Apollo astronauts. But much of the lunar surface is known only approximately.

"If you ask 'where is a crater on the far side of the Moon?' chances are there's probably many kilometres of uncertainty in its true positioning," says David Smith, a scientist at NASA's Goddard Space Flight Center. Even on the near side of the Moon, Smith adds, errors in the true global position of features may be as large as a kilometre.

To improve this situation, NASA plans to send a high-precision laser altimeter to orbit the Moon and create a 3D map of its surface.

The laser, named "LOLA," (Lunar Orbiter Laser Altimeter) is scheduled to launch in 2008 onboard the Lunar Reconnaissance Orbiter spacecraft. LOLA works by bouncing pulses of laser light off the lunar surface as it orbits the Moon. By measuring the time it takes for light to travel to the surface and back, LOLA can calculate the roundtrip distance. LOLA is capable of timing pulses with a precision of 0.6 nanoseconds, corresponding to a distance error of no more than 10cm.

"In a sense, the Moon is an ideal object for making these kinds of observations because it has no atmosphere to interfere with the propagation of the laser pulses," says Smith, who is the Principal Investigator for LOLA.

LOLA will map the Moon for at least a year, orbiting from the Moon's north pole to the south pole and back every 113 minutes. As it orbits, LOLA will send out laser pulses 28 times per second. Each pulse consists of five laser spots in a cross-like pattern spanning about 50 metres of lunar surface. Altogether, LOLA will gather more than four billion measurements of the Moon's surface altitude.

After taking into account uncertainties in LOLA's orbit, the overall error in the true elevation of lunar features should be no more than a metre, while the true horizontal locations of those features should be known to within 50m or less. The next-best laser ranging map, made by the Clementine mission in 1994, had an error of 100m vertically and a horizontal resolution as coarse as 30km.

This new map, combined with high-resolution images of the lunar surface taken by a camera onboard the spacecraft, will, according to NASA, offer by far the best 3D model of the Moon ever created.