Frontiers of Science:

Jupiter’s Big Red Eye

By Dr. Sherwood B. Idso

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    Imagine looking through a powerful telescope and seeing a big red eye looking back at you. That is exactly what happens when viewing the planet Jupiter. What’s more, if you keep looking at it for several years, it may even wink at you! No, this isn’t science fiction—it’s science fact. And a fascinating fact at that.

    Besides being the largest planet in the solar system and having the greatest number of moons, Jupiter is perhaps best known for its unusual Great Red Spot. First seen by Robert Hooke in 1664, it was actually called the “Eye of Jupiter” by the astronomer Cassini the following year. As for its “winking,” it completely vanished and reappeared eight different times between 1665 and 1708. It was very bright in 1878 and almost vanished again in 1883. It then brightened, only to fade once more at the start of the 1900s. Today, however, it glares brightly back at those who view it with powerful telescopes.

    As its name implies, the Great Red Spot appears as a tremendous reddish orange oval located in Jupiter’s southern hemisphere. It is now about 20,000 miles long—big enough to enclose several Earths—and has at times been even twice that long. But what, exactly, is it?

    Scientists have puzzled over the nature of the Great Red Spot for centuries. Their current theory is that it is a gigantic hurricane-like storm. Indeed, they have determined that the spot rotates counterclockwise when viewed from the top, and that the swirling currents of air within it are generally rising. As a result, cloud tops within the Great Red Spot extend several miles above those of the surrounding atmosphere.

    Since wind speeds on Jupiter reach hundreds of miles per hour, the rising currents of air within the Great Red Spot are thought to create very turbulent conditions, more violent by far than those of any thunderstorm or tornado on Earth. It is also believed that great electrical discharges take place within the “hurricane,” making Earth’s lightning flashes mere sparks by comparison.

    In addition to its “Big Red Eye,” Jupiter has a number of smaller “freckles” or white spots, which also seem to be atmospheric storms. None of them are as stable as the Great Red Spot, however, but they are all longlived compared to any storms experienced on Earth.

    Most of what we know about Jupiter’s Great Red Spot has been obtained from Earth-based telescopes and from the spacecrafts Pioneer X and XI. Launched in March of 1972, Pioneer X came within 81,000 miles of Jupiter’s banded cloud tops in December of 1973. It was followed by Pioneer XI that was launched in April of 1973 and came within 26,000 miles of the cloud tops in December of 1974. Both missions were very successful and returned many good pictures of the Great Red Spot were obtained.

    We are soon to learn even more about this fascinating feature of our solar system’s major planet, however, for in August and September of 1977 the United States launched yet another pair of spacecraft on their way to Jupiter and beyond. Christened Voyager I and II, this latest spacecraft duo is scheduled to encounter the planet on March 5 and July 9, 1979. Both carry special scientific equipment that will allow scientists to get a good look deep into the mysterious eye of the greatest planetary storm we have ever known. It is sure to be a “weather report” that will stagger the imagination and one that none of us will want to miss.

    The planet Jupiter and its Great Red Spot as seen by Pioneer XI on December 2, 1974, from a distance of 715,000 miles. (NASA photo.)

    The closest picture of Jupiter’s Great Red Spot taken by Pioneer XI as it flew past the giant planet on December 2, 1974. Taken from a distance of 340,000 miles, it reveals streams of white clouds flowing both north and south of that central feature. Below the right end of the Great Red Spot is one of three white ovals that are usually about equally spaced around the planet. These “freckles” have been known to exist for only the last 30 years. (NASA photo.)

    A diagramatic timetable for the flights of Voyager I and II to Jupiter.

    An artist’s conception of one of the Voyager spacecraft aiming a group of scientific instruments at Jupiter as it passes by the giant planet and completes one important part of its mission. (NASA artwork.)

    A view of Jupiter’s north polar region, an enlargement of a portion of it. An array of comparatively smaller hurricane-like storms is visible. (NASA photo.)

    Photo of the north polar region, but from a greater distance. Many Earth-like thunderstorm cloud patterns are apparent. (NASA photo.)