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Mercury in Modern Times

The invention of the telescope allowed astronomers to get a closer view of the planets, the Sun and the Moon. In 1609 the Italian scientist Galileo developed the first telescope suitable for astronomical observation, and turned this new instrument toward the Moon, Jupiter and Venus. A year later he made the first telescopic observation of Mercury, but found himself unable to get a clear view of the planet. In 1639 another Italian, Giovanni Zupus, studied Mercury’s orbit with the aid of a more powerful telescope. He discovered that, like the Moon and Venus, Mercury has phases. This provided additional evidence for Copernicus’ heliocentric view – that Mercury and the other planets orbit the Sun.

By the late 1700’s Johann Hieronymus Schroeter, working in Germany, had observed and sketched Mercury’s surface. Some 100 years later, Giovanni Schiaparelli observed Mercury over a seven year time frame and, based on the day-to-day positions of surface features, concluded that Mercury’s rotational period is the same as the period of its orbit around the Sun. Schiaparelli claimed that both the rotational period and the orbital period were 88 Earth days long, so that the same side of Mercury always faces the Sun (just as the same side of the Moon always faces the Earth). This “fact” remained unchallenged for almost 75 years.

The theory that Mercury is in synchronous rotation with its orbital motion was shattered in 1965. Astronomers Gordon Pettengill and Rolf Dyce used a radio telescope to make radar observations of Mercury’s spin rate. They found that Mercury’s rotation period is only 58.6 Earth days, not the 88 days accepted for nearly a century.

NASA made the first plans to send a spacecraft to the inner planets in 1971, when it was realized that three years later Venus and Mercury would be positioned so that a single probe could reach both with minimal cost and fuel. The resulting $98 million mission, named Mariner 10, made its first flyby of Mercury on March 29, 1974 at a distance of about 700 kilometers. This probe provided us with the first close-up photographs of Mercury’s surface. In three separate flybys, Mariner 10 was able to image slightly less than half the planet’s surface, revealing topography much like that of our Moon. But unlike the Moon, the photographs showed regions of huge scarps (cliffs) ranging as high as 3 kilometers. On-board instruments indicated that other features of Mercury are also not very Moonlike: It has a large iron core, a trace atmosphere, and a weak magnetic field. Mariner 10 also found that Mercury is more nearly spherical than is the Earth. Learn more about this mission by visiting NASA’s Mariner 10 website.