The Most Accurate Measurement of Mercury

For eleven minutes the great 85-ft.

radio telescope at Goldstone in the Mojave Desert beamed a burst of powerful microwaves at the sky. Then scientists from the Jet Propulsion Laboratory turned off their transmitter and switched on a sensitive receiver. Almost at once a glowing line on an oscilloscope screen broke into dancing ripples. The waves of the transmitter had traveled 60.5 million miles into space and bounced back from the tiny planet Mercury.

This was man's first radar contact with the distant planet. It is a tough target to hit, for it is only 3,010 miles in diameter, not very much bigger than the moon, and its orbit keeps it close to the troublesome sun. When Goldstone's radar waves set out for Mercury, they had an effective strength of 25 billion watts. By the time they straggled back, they mustered only five ten-thousandths of a billionth of a billionth of a watt. They had lost the even regularity of oscillation with which they had started, and now they were wiggling wildly because of the beatings they had taken on the surface of Mercury.

But the new irregularities contained new knowledge about the target planet. JPL's radar contact measured the distance of Mercury with an error less than 100 miles--an accuracy that is not possible in optical astronomy. It also timed Mercury's slow rotation, which has the same speed as its 88-day orbit around the sun. Most of the results agreed with predictions. But there was one surprising variation: the surface of Mercury proved to be unexpectedly rough. "We're not talking about vast mountains and valleys," says JPL Radio Astronomer Richard Goldstein. "We're talking about something the size of a pile of rocks or ocean waves."

There could surely be no water waves, since the temperature of Mercury on its permanently sunlit side is 1,800DEG F. The roughness of Mercury could be due to any irregularities more than a foot or so in width--large enough to scatter the 5-in. waves of the Goldstone transmitter. If Mercury's surface were smoother than that, the radar waves would be reflected from a small highlight in the center of the disk. Instead, the planet is radar bright all over, which means that its whole surface must have irregularities that bounce radar waves back toward the earth.

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