The nucleus of Comet Hyakutake, observed by radar at a distance of 16 million kilometers (10 million miles) on March 24 and 25, is apparently 1-3 kilometers (less than 2 miles) across, a NASA radar scientist reported.
"This is the first and, so far as I know, only direct detection of the nucleus of comet Hyakutake," said Jet Propulsion Laboratory radar astronomer Steven J. Ostro. "We have touched the heart and soul of the comet," he added.
The observations were made with the 70-meter (230-ft) antenna at the NASA/JPL Goldstone Deep Space Communication Complex. The radar telescope also detected particles flying away from the nucleus at speeds of at least 10 meters per second (22 mph).
Ostro pointed out that five other comets have been detected in the NASA radar astronomy program, but this is the first comet radar detection since Halley was observed from the Arecibo radar telescope in 1985.
Several transmit-receive cycles were made on each of the two nights, he said. The echoes were received an average of 104 seconds after the 480-kilowatt radar signal was beamed at the comet. The power in the echo received from the comet was less than one billionth of a billionth of a milliwatt.
The radar echoes reveal that the comet's coma, the large visible cloud, must contain a great many particles not much smaller than a centimeter (about half an inch). Ostro noted that there seems to be about ten times as much radar echo power from these particles as from the nucleus itself.
The radar astronomy sessions were sandwiched between radio communication passes for the Galileo and Voyager missions, and were limited by system difficulties and the faintness of the radar echoes. The Goldstone antenna is part of the Deep Space Network, developed and operated for NASA by JPL.
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