As with the inner planets, most of what scientists first learned about the outer planets came from observation using Earth-based telescopes. The data collected from these observations was of limited value because the outer planets are so much farther away than are the inner planets. For example, Jupiter is about 430 million miles (720 million km) from Earth. By comparison, the minimum distance from Earth to Mars is only about 33 million miles (56 million km). The availability of spacecraft beginning in the 1960s provided a critical new tool for the exploration of the outer planets. Today scientists have an extensive collection of data about the chemical and physical properties of the outer planets as a result of a number of space missions to one or more of the planets.
The first such mission was Pioneer 10, launched on March 2, 1972. Pioneer 10's mission was to fly through the asteroid belt and around Jupiter, collecting data on the planet's magnetic field, radiation belts, atmosphere, and interior. After completing this mission on March 31, 1997, the spacecraft continued in its path toward the outer limits of the solar system. It continued to send back data on the edges of the solar system and interstellar space until April 27, 2002. At that point, its power source died out and the probe was unable to send further transmissions to Earth stations.
Pioneer 10 was followed by its partner, Pioneer 11, launched on April 5, 1973. Like Pioneer 10, its primary objective was a study of the planet Jupiter, although it added other items to its list of accomplishments. After reaching the giant planet on December 2, 1974, it took pictures of the planet's Great Red Spot and collected data that allowed scientists to calculate the mass of one of the planet's moons, Callisto. The space probe also studied hydrogen and dust particles in Jupiter's atmosphere and determined other properties of the atmosphere and surface. After completing its Jupiter mission, Pioneer 11 swung around the planet and used the gravitational boost it thereby gained to travel on to Saturn, where it became the first spacecraft to collect data from that planet. After passing beyond Saturn on September 1, 1979, it headed for the fringes of the solar system. Contact was finally lost with the spacecraft in November 1995.
Both Pioneer 10 and Pioneer 11 carried plaques showing what life on Earth is like, in the hope and expectation that any other life-form in the universe with which it might come into contact would know where the probe had come from and what its inventors were like.
The next mission to the outer planets consisted of two spacecraft, Voyager 1 and Voyager 2, launched on September 5 and August 20, 1977, respectively. The primary goal of the Voyager mission was to fly past Jupiter and Saturn, the two largest planets in the solar system, and to collect and transmit data on their atmospheres, interiors, and satellites. Voyager 2 was intended originally as a backup for Voyager 1, in case the earlier spacecraft malfunctioned or made discoveries of special interest that could be followed up with a second probe.
Voyager 2 was launched at an especially propitious moment in the history of the solar system: At the time four of the outer planetsJupiter, Saturn, Uranus, and Neptune—were aligned in such a way as to allow the spacecraft to fly past them all, providing scientists with their first close look at the next-to-outermost planets, Uranus and Neptune, in addition to the planned targets of Jupiter and Saturn. The alignment that permitted this special tour of observation occurs only once every 175 years, so the data provided by Voyager 2 about Uranus and Neptune has been of very special value to researchers.
The two Voyager spacecraft completed their planetary missions in late 1989, 12 years after they left Earth's surface. Even though their primary missions had ended, they did not lose their value to astronomers. They continued in flight beyond the orbit of Neptune, into the outermost reaches of the solar system. Scientists expect them to continue their flights, returning valuable data, until 2020 or later. Now called the Voyager Interstellar Mission, the two spacecraft will be collecting data on the most distant reaches of the Sun's influence. Scientists hope to learn more about the heliopause, the outermost region at which solar effects can be observed, about the solar wind and the Sun's magnetic field, and about the nature of interstellar space. As of the end of 2006, Voyager 1 was about 9.3 billion miles (15 billion km) from the Sun, and Voyager 2 was about 7.3 billion miles (12 billion km) from the Sun.
The fifth spacecraft to travel past Jupiter was Ulysses, launched on October 6, 1990. Ulysses's primary goal was a study of the Sun, but in order to obtain the momentum it needed to attain solar orbit, it was launched toward Jupiter first. As the probe orbited the planet, it picked up additional energy in a slingshot-like effect that had been used with other spacecraft, sending the probe back toward the Sun, where it attained orbit in June 1994. Ulysses passed around Jupiter on February 8, 1992, when it collected additional data on the planet's magnetic field and the dust particles in its atmosphere that had been detected by earlier spacecraft.
The spacecraft Galileo was launched from the Space Shuttle Atlantis on October 18, 1989, with the goal of achieving orbit around the planet Jupiter. It was the first space probe to enter and remain in orbit around one of the outer planets. On its way to Jupiter, Galileo passed through an asteroid field and took extraordinary photographs of the asteroids Gaspra (October 29, 1991), Ida, and Dactyl (both on August 28, 1993).
The space probe neared its target in July 1995 and released a probe carrying instruments capable of measuring the temperature, pressure, and chemical composition of the atmosphere, as well as cloud characteristics, sunlight, and internal planetary energy. The probe survived for 59 minutes, during which it penetrated about 125 miles (200 km) into the Jovian atmosphere. At that point surrounding pressures caused the probe to melt and/or vaporize, and it lost contact with Earth stations.
Meanwhile, Galileo continued falling toward Jupiter until it was captured by the planet's gravitational field and attained orbit on
December 7, 1995. The probe remained in orbit for about two years, and during this time it discovered an intense radiation belt above the top of the planet's clouds and determined with some precision the composition of the planet's atmosphere. After completing its mission, the spacecraft disengaged itself from orbit and began falling toward the planet's surface. As it spiraled in toward the planet, Galileo flew past a number of Jupiter's satellites and photographed the moons Europa (December 1999) and Amalthea (November 2002). On September 22, 2003, it plunged into the planet's inner atmosphere and was destroyed. Among its many accomplishments, Galileo found 21 new Jovian satellites, bringing to 61 the total that orbit the huge planet. (That number has since increased to 63, as of early 2007.)
The most recent space probe sent to the outer planets is Cassini-Huygens, a joint project of the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). The spacecraft was launched from Kennedy Space Center on October 15, 1997. Saturn was its primary target, but in order to gain the energy needed to make this long trip the probe was first directed toward Venus. It swung around that planet on June 24, 1999, and then around Earth on August 18, 1999. From there it headed out toward Saturn, traveling with the double slingshot-effect energy obtained from the trips around the two inner planets.
The Cassini-Huygens spacecraft consists of two parts. Cassini is the orbiter, designed to attain orbit around Saturn, while Huygens is a space probe, designed to be released into the atmosphere of the planet's moon, Titan. NASA was responsible for the design and construction of the Cassini orbiter, while ESA was responsible for the Huygens probe.
On its way to Saturn, Cassini-Huygens passed by the asteroid Masursky (on January 23, 2000) and Jupiter (December 30, 2000). In early June it passed near one of Saturn's outermost moons, Phoebe, and there took photographs and collected data that promise to reveal important new details about that moon's origin. The spacecraft arrived in the Saturnian atmosphere exactly on schedule, on June 30, and sent back some of the most astonishing astronomical photographs of any solar system object ever collected. The Huygens probe detached from Cassini on December 25, 2004, and went into an orbit around Titan for 21 days before finally crashing through the moon's atmosphere and into its surface. Meanwhile, the Cassini orbiter was expected to remain in orbit until June 30, 2008.
The last planet-like object to be explored is Pluto. Since its discovery in 1930 by American astronomer Clyde Tombaugh (1906-97), Pluto has been considered one of the nine planets in the solar system. In August 2006, however, the International Astronomical Union established a new set of criteria for planets that Pluto did not meet. It is now considered a dwarf planet. In spite of its new designation, astronomers remain very interested in the chemical composition and physical characteristics of Pluto. Astronomers do have a fair amount of information about Pluto from ground-based observatories, the Hubble Space Telescope (HST), and the Infrared Astronomical Satellite. NASA's New Horizons space mission is expected to provide a great deal more detail about the strange and mysterious astronomical body. New Horizons was launched on January 19, 2006. It is expected to fly by Pluto and its satellite, Charon, in July 2015.
After completing its observations of the planet and its moon, New Horizons will continue outward into the Kuiper Belt, a region of space located beyond Neptune that includes Pluto itself. The Kuiper Belt consists of more than 70,000 bodies with diameters of 60 miles (100 km) or more. Its existence was discovered in the early 1990s, and scientists still know very little about its constitution, the physical and chemical properties of its bodies, or their orbital properties. Interest in the Kuiper Belt intensified in 2002 with the discovery of the largest nonplanet object in the outer solar system, a body that has been given the name Quaoar (pronounced "kwa-whar") in honor of a Native American creation god. Quaoar has a diameter of about 800 miles (1,250 km), making it about half the size of Pluto. It travels in a nearly perfectly circular orbit around the Sun at a distance of about 42 AU (astronomical units). By comparison, Neptune's orbit is located at a distance of 30.22 AU from the Sun, and Pluto's orbit is at a distance of 39.83 AU from the Sun. The diagram on page 132 shows the relative orbits of the outer planets and Quaoar.
© Infobase Publishing Orbit of Quaoar compared to orbits of outer planets
© Infobase Publishing Orbit of Quaoar compared to orbits of outer planets
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