The Hartebeesthoek Radio Astronomy Observatory (HartRAO) is in the process of constructing a Lunar laser rangefinder, to widen its area of scientific research. The project is being developed with the assistance of the Côte d’Azur Observatory, in France, and the US National Aeronautics and Space Administration (Nasa). “It’s very much a shoestring development,” says HartRAO acting MD Professor Ludwig Combrinck. “The French have donated the telescope and Nasa is helping develop the laser.”

The instrument will be dedicated to lunar rangefinding, but, as it will not be worthwhile to use it for this when the moon is low in the sky (the earth’s atmosphere will cause the laser beam to diverge too much to be useful), it will have a secondary task of satellite laser rangefinding. This latter task will employ a 0.5 milliJoule laser which will be switched on to the telescope. In contrast, the Lunar rangefinding laser will have a pulse length of 80 picoseconds and power of 100 milliJoules, running at a rate of 20 Hz.

Further, the Lunar range-finding laser will also be used to measure the highly elliptical orbit of the Russian-led Radioastron space radio telescope, which goes out beyond the moon’s orbit and which, as a result, cannot be measured by dedicated satellite laser rangefinders, which are only able to measure that part of the space telescope’s orbit that is closest to earth. HartRAO already has a dedi-cated satellite laser rangefinder and will get a second, to support Russian satellites, during the first semester of next year; this will be in cooperation with Russian space agency Roscosmos, with whom a contract has already been signed.

“The telescope is a 1 m [diameter] Cassegrain telescope. The laser will fire through the wide aperture of the telescope, to keep the beam collimated,” he explains. “Otherwise, the beam will diverge by the time it reaches the moon. A divergence of one arc/second gives a 2 km wide beam on the moon! “The laser beam will fall on suitcase-sized reflectors on the moon, placed there by the US Apollo manned and Russian Lunakhod unmanned missions; there are five in all.”

The fundamental mission of the Lunar laser rangefinder will be to continue to test Einstein’s Theory of General Relativity. (There are modern theories which suggest that General Relativity might break down at certain levels; ever more accurate measurements are needed to see if they may be correct.)

Lunar laser rangefinding allows the earth and moon to be used in a very big experiment. The two worlds can be taken as two masses falling around the sun. “The question is: do they fall at the same rate? With the orbit of the moon very accurately measured, we will be able to see if there is a very slight divergence from what is predicted,” elucidates Combrinck.

A niche has opened for South Africa is this area. The US no longer has an instrument dedi-cated to this research (although there is one which is used part-time). Germany and Italy are developing this capability, but it is not yet operational. Russia has not been releasing data. Only France currently has a facility dedicated to this research. Moreover, all these instruments are, or will be, in the Northern Hemisphere. The HartRAO instrument will be the only one in the Southern Hemisphere, and will thus complement, and not rival, the others. “It’s a very affordable way of doing advanced science,” he highlights. “It will also be very useful for training postgraduate students.”