A Radar Determination of the Rotation of the Planet Mercury

DURING the recent inferior conjunction of the planet Mercury in April, 1965, radar observations were obtained by the Arecibo Ionospheric Observatory in Puerto Rico (operated by Cornell University with the support of the Advanced Research Projects Agency under a Research Contract with the Air Force Office of Scientific Research). The system operated at a frequency of 430 Mc/s, with an antenna gain of 56 dB and a transmitted power of 2 MW. The resulting sensitivity was sufficient to obtain significant echoes not only from the nearest part of the planetary disk but also from more distant regions, removed by up to 0.06 of the planet's radius. By using short transmitted pulses of 500 µsec duration, it was possible to isolate the echo power from these more distant regions, and to carry out a Fourier analysis of their spectral composition.