Sir

Your News Feature 'Heating up the heavens' (Nature 452, 930–932; 2008) discusses experiments using the High Frequency Active Auroral Research Program (HAARP) facility. I would like to clarify the goal of the lunar-echo experiments.

The high power and low-frequency range of HAARP provide radio waves that can penetrate the lunar surface because of the low electrical conductivity of the lunar regolith, allowing investigation of the subsurface. Parallel research efforts currently use radar sounders on satellites orbiting the Moon. In its radar mode, HAARP is the most powerful of the few Earth-based facilities that can participate in such investigations.

Our lunar-echo experiments began in 2001, when HAARP was not yet at full power capability, but in the latest experiment we succeeded in receiving a lunar echo at the lowest frequency obtained so far by an Earth-based radar (4.8 megahertz). Our initial results will be presented at the General Assembly of the International Union of Radio Science in August 2008.

The fact that radio amateurs can record the HAARP transmissions and lunar echoes is a result of free-space propagation of the radio waves. In the lunar-echo experiment of January 2008 that you mention, the schedule of the HAARP transmissions was provided so that they could listen in during the research activities. That opportunity was enthusiastically and successfully taken up by radio amateurs in many countries and has produced interesting data in its own right, related to world-wide propagation effects on the two-way (Earth–Moon–Earth) radio signals — for example, on ionospheric refraction, scattering and scintillation.