Jupiter (planet)

C

The Outer Satellites

Prior to 1999, two additional families of small satellites, located in inclined elliptical orbits at large distances from Jupiter, were known. The first family, Leda, Himalia, Lysithea, and Elara, orbit at average distances of about 11 million km (about 6.6 million mi). These satellites, along with the inner and Galilean satellites and Jupiter’s rings, revolve about Jupiter in the same direction that the planet rotates on its axis. The second family, Ananke, Carme, Pasiphae, and Sinope, orbit at average distances of about 21 to 23 million km (about 13 to 14 million mi) and revolve in the opposite direction. Since 1999, 47 more distant small moons have been found, bringing the total number of known satellites to 63. Most of these new members are also in elongated, tilted orbits and are less than 10 kilometers (6 miles) in diameter. The nature of the orbits of the outer moons suggests that they are trapped asteroids or fragments of larger bodies that were broken up by collisions with asteroids or comets.

VII

Spacecraft Missions

An era of detailed observations of Jupiter began with NASA’s Pioneer 10 spacecraft, launched in March 1972. Pioneer 10 was followed in April 1973 by Pioneer 11. These simple spinning spacecraft carried instruments that provided excellent information on Jupiter’s gravitational field, magnetosphere, and upper stratosphere. The next NASA spacecraft explorations of Jupiter were the Voyager 1 and Voyager 2 missions of 1979. The Voyager craft were designed to maintain a stable orientation in space, so that onboard cameras and other imaging instruments could be used to map Jupiter in ultraviolet (UV), visible, and infrared (IR) light. The visual images provided detailed maps of Jupiter’s cloud deck, the IR data produced information about how heat escaped and the relative abundance of materials in Jupiter’s upper atmosphere, and the UV data provided information on the interaction of Jupiter’s magnetic field with the solar wind and the upper atmosphere. In 1990 NASA launched the spacecraft Ulysses from an orbiting space shuttle to study the Sun from an orbit passing over its poles. To get Ulysses into that unusual orbit, astronomers aimed the spacecraft to swing twice around Jupiter, using the planet as a gravitational slingshot. While flying by Jupiter in 1992 and 2004 Ulysses took measurements of Jupiter’s magnetosphere and gravitational field.

In 1989, prior to the launch of Ulysses, NASA launched the Galileo spacecraft on a mission to Jupiter. The Galileo spacecraft took a slower route to Jupiter, reaching the planet in 1995. Unlike previous spacecraft that merely passed by Jupiter, Galileo entered orbit around the planet in order to engage in longer-term study. The spacecraft also launched a remote probe into the planet. The probe plunged through Jupiter’s opaque cloud deck, and the orbiting Galileo spacecraft relayed information the probe gathered to Earth. The probe transmitted its readings until it reached a depth in Jupiter’s atmosphere where the pressure was 20 Earth atmospheres, at which point high temperatures caused its transmitter to fail. Galileo’s remote probe provided direct measurement of the relative abundance of the elements in Jupiter’s outer atmosphere and the strength of its winds, revealing an unexpected low level of water in the clouds and high wind speeds. The Galileo spacecraft continued to gather and transmit information about Jupiter’s magnetic field, atmosphere, and moons until 2003. NASA dove the spacecraft into Jupiter’s atmosphere when Galileo’s fuel dwindled in September 2003. Galileo was traveling so fast that friction with the atmosphere burned up the spacecraft.

More data on Jupiter was collected by the Cassini/Huygens spacecraft, which flew by Jupiter in December 2000 on its way to a rendezvous with Saturn in 2004. Cassini’s mission to Saturn was similar to Galileo’s Jupiter mission: to orbit Saturn and drop the Huygens probe, built by the European Space Agency, onto Saturn’s moon Titan.

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The New Horizons spacecraft made a flyby of Jupiter in February 2007 to gain a gravitational boost from the giant planet, accelerating the probe on its path to fly by Pluto in 2015. New Horizons took images and collected data about Jupiter, its magnetosphere, its rings, and its moons at the same time that the Hubble Space Telescope and the Chandra X-ray Observatory viewed the planet. The combined information gave scientists a more complete picture of Jupiter during the encounter. New Horizons was able to take detailed motion pictures of Jupiter’s atmosphere as the planet rotated and was also able to study Jupiter’s magnetic tail for a much greater distance beyond the planet than any previous probe.

VIII

Directions for Future Studies of Jupiter

Planetary scientists are interested in studying Jupiter further to learn about its interior structure, chemical composition, atmospheric circulation, heat loss, and aging processes. The next major mission planned for Jupiter is the Jupiter Polar Orbiter or Juno, as part of NASA’s New Frontiers Program of lower cost missions that began with the New Horizons flyby of Pluto. The Juno spacecraft will go into a polar orbit around Jupiter to study the composition of the planet and of its atmosphere, as well as the structure of its magnetic field. Special areas of research include whether Jupiter has an ice-rock core and how the Jovian magnetic field is generated. A major goal of the mission is to better understand how Jupiter and the other giant planets formed in the early solar system. The Juno mission is scheduled for launch in 2011 to reach Jupiter in 2016.

Astronomers have detected more than 200 planets orbiting other stars in widely varying orbits. Jupiter can serve as an accessible laboratory as scientists try to understand the limited data that can be obtained from these distant worlds. Exploration beyond the Juno mission calls for a careful study of the most important factors that can be measured, the technology required to do the job, and realistic budgetary projections. New ways to explore more efficiently are needed, including improved spacecraft power systems, miniaturization of instruments, and upgrades to Earth-based radio receiving equipment. Smaller craft could be used to make detailed maps of Jupiter’s gravitational and magnetic fields, sample its atmosphere, explore the Galilean moons, or perform other tasks.

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