Early Planetary Observations at Arecibo by Gordon H. Pettengill

With its 1000-ft, zenith-oriented reflector, the newly dedicated in late 1963 promised over 100 times the radar sensitivity of other research radar systems of the time. While radar observations of had been reported in the several years preceding, they were limited to simple detections of echoes at closest approach only; Arecibo was capable of seeing not only Venus over almost its entire orbit, but and as well (see Fig. 1).

Fig. 1. Path Loss and Delay of Planetary Radar Echoes But, of course, the limited sky coverage afforded by Arecibo’s zenith orientation meant that we had to wait till mid-February 1964 to see the inner planet Venus, and even later to detect echoes from Mercury. But when they came in, they were strong! It took many hours of accumulating weak echo data before MIT’s Millstone could claim a Venus detection, whereas we could see strong echoes from individual radar pulses at Arecibo on our first try on Feb 10, 1964. Venus observations were used for improving its ephemeris, as well as for verifying and improving earlier determinations of the astronomical unit of distance within the solar system (see Fig.2).

Fig. 2. Venus Rotation Period at Inferior Conjunction, 1964

1 Figure 3 plots the improvement in our knowledge of this important parameter (poorly determined from optical observations) that radar enabled in the early 1960’s.

Fig. 3. AU Determinations 1961-1966 Perhaps the most important of these early radar observations at Arecibo involved Mercury, which by April, 1965, had moved high enough in the sky at inferior conjunction to allow observation of the delay-Doppler distribution in its echoes – sufficient to infer its apparent rotation (see Fig.4).

Fig. 4. Delay-Doppler Bandwidth of Echoes from Mercury The observed rotational inference could be compared against the values predicted from assumptions of its intrinsic sidereal rate as shown in Figure 5.

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Fig. 5. Apparent Rotation Rate of Mercury for Various Sidereal Rates From the first few observations in early April we knew the classical optical determinations of Mercury’s sidereal rate as in one-to-one synchronism with its solar orbit were wrong, and a few weeks later in late April, we knew by how much. It turned out that there was indeed orbital synchronism, but at a 3/2 rate. In fact, Mercury presents opposite sides to the sun on alternate perihelia as shown in Figure 6.

Fig. 6. Orientation of Mercury as it Moves around the Sun in its Orbit

3 Who were some of these early radar astronomers at the site? I (see Fig. 7) was the first, having moved to in July 1963 as the finishing touches to the Observatory were set in place. Rolf Dyce (Fig. 8) joined us in January 1964 from the Stanford Research Institute, and Don Campbell (Fig. 9) came over from Sydney University in Australia in early 1965, as part of a reciprocal arrangement with Cornell for exchanging visiting scholars between those two universities. Ray Jurgens (Fig. 9) also joined our team in 1965.

Fig. 7. Gordon Pettengill in the Control Room, 1963

Figure 8. Rolf Dyce at the Primitive PDP 160 Computer in 1965

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Fig. 9. Ray Jurgens (left) and Don Campbell at Arecibo in 1965 I returned to MIT at the end of 1965, but Rolf, Ray and Don stayed on to observe planets using the ever improving radar capability – moving to a one megawatt transmitter in the early 1970’s at a shorter 12.6 cm wavelength from the original 150 kilowatt system at 70 cm. The combination of shorter wavelength and increased average power raised the radar’s sensitivity for planetary work by over a hundred times (20 dB), and enabled observations of Saturn’s rings and its satellite Titan, as well as of the Galilean satellites of Jupiter and many asteroids and comets!

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