Jupiter
Because Jupiter has always been seen by the naked eye, no history of discovery exists for it. At certain periods, it is brighter than any other celestial object except the Sun and Venus. Certain facts concerning the planet, however, have been discovered by a host of astronomers.
The most well-known feature of Jupiter, its reddish colored cloud bands, were discovered as early as 1664. Credit for their detection has been given to several scientists, including Italians Niccolo Zucchi and Gian Dominico Cassini, Englishman Robert Hooke, and Belgian Gilles-François Gottigniez. Cassini determined the planet's rotation as 9 hours and 56 minutes, a surprisingly short period for such a huge planet. He also found the planet flattened by the ratio of 1/5, meaning the distance from pole to pole was significantly different from the equator's diameter.
After these initial calculations Jupiter was largely ignored, due in part to the spectacular discovery of Uranus by William Herschel in 1781. In 1842, however, German astronomer Friedrich Bessel, calculated the planet's mass. He determined that Jupiter had 388 times the mass of Earth, but had only 1.35 times the density of water, thus revealing that the huge planet was surprisingly light. Bessel further speculated what seemed to indicate a white hot planet that continuously heated its swirling clouds.
Another discovery added to this picture of Jupiter as a hot planet. In 1878 Wilhelm Tempel observed a huge red spot measuring 30,000 miles (48,270 km) long and over 7,000 miles (11,263 km) wide. Many felt this had to be a reflection of surface lava recently erupted from the planet. However, when historians looked carefully at past drawings of Jupiter, they discovered that the spot had been detected by astronomers as far back as the 1600s. The idea of a lava flow, therefore, seemed unlikely.
In the 1870s Hermann Vogel (1842-1907) used a device to measure the planet's spectrum. He found it was the same as the Sun's, which proved the light was reflected sunlight. But the measurement also contained a few bands which could not be identified. Someone started a rumor that one of these bands was typical for chlorophyll, so speculation started that there was rich floating aerial plankton in a hot, steam-laden atmosphere. Later these lines were identified with ammonia and methane, but they still did not indicate what composed the bulk of Jupiter's atmosphere. Most astronomers believed that hydrogen and helium were the major components, both of which can remain in a gaseous state at very low temperatures. This idea was proved correct in 1952 by W. A. Baum and A. C. Code when Jupiter moved in front of a star, the light of which shone through the planet's atmosphere, allowing it to be measured by a spectroscope.
The first person to detect moons of Jupiter and correctly identify them as satellites of the planet was Galileo who viewed them through his telescope in 1610. A contemporary of Galileo, Simon Marius (1580-1624), gave them the mythological names by which they have become known--Io, Europa, Ganymede, and Callisto--though Galileo preferred the much simpler J-I, J-II, J-III, and J-IV. Much debate ensued over the years as to shape and coloration on these worlds. However, they are so small and so far away that very little was determined. A fifth moon, J-V or Amalthea, was discovered some three centuries later in 1892 when American astronomer Edward Emerson Barnard (1857-1923), using an advanced telescope, blocked out the glare of the planet itself. Fellow American Seth Nicholson (1891-1963) is considered the primary discoverer of further moons. Using long-exposure photographs, he detected four additional satellites orbiting Jupiter, and by 1951 the total count of the planet's moons totaled twelve.
Four American spacecraft explored Jupiter's neighborhood and radioed back an astounding amount of information. Pioneer 10 in 1973 and Pioneer 11 in 1974 mapped out Jupiter's intense magnetic field and gave us detailed information about its gravity field. These measurements proved that Jupiter is a vast fluid planet; no solid surface appears to exist under the perpetual cloud cover.
Dramatic discoveries concerning Jupiter occurred in 1979 when the space probes Voyager 1 and 2, passed by the planet. Speeding by at 45,000 miles per hour (72,405 kph), they sent back spectacular photographs of the Great Red Spot as well as pictures which enabled scientists to reconstruct the planet's complex cloud motions. The probes also found a ring that encircles the planet and the first volcanic activity outside the Earth on the moon Io. They discovered new moons and lightning storms on Jupiter.
On October 18, 1989, the spacecraft Galileo was launched, headed for a 1996 rendezvous with Jupiter. Reaching Jupiter in 1995, Galileo released an atmospheric probe that descended into Jupiter's gaudy clouds, analyzing their structure and composition. The results were unexpected, revealing a chemical composition deficient in helium and water, and with a different vertical structure that had been hypothesized. Winds in the Jovian atmosphere were also much greater than expected, and persisted to greated depths than had been previously thought. The spacecraft itself settled into orbit in the Jovian system, swinging repeatedly by the planet and its numerous moons, returning pictures of unprecendented detail. Scientists controlling Galileo from the Jet Propulsion Laboratory in Pasadena, CA, carefully planned each orbit to maximize the scientific return. Often Galileo skimmed within 200 miles of the various moons. Important questions concerned the origin of the moons and their composition, as well as the nature of the frequent volcanic activity apparent on Io.
In July, 1994, Jupiter was in worldwide headlines as the various fragments of Comet Shoemaker-Levy 9 slammed into its southern hemisphere over a six-day period. The comet, which had been torn into pieces by Jupiter's strong gravity on a previous orbit, impacted with spectacular effect. The impact was observed by virtually every ground-based telescope that could see it, as well as Galileo and the Hubble Space Telescope. Studies of the huge blemishes in Jupiter's clouds were made to determine the chemical composition of the clouds and the impacting fragments. Thousands of amateur astronomers tuned in their telescopes as well, to see the spectacular show and to be eerily reminded that devastating impacts do happen from time to time in our solar system.
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