Spacecraft Exploration

Voyager 2 is the only spacecraft to have encountered the Neptunian system. This spacecraft and its twin, Voyager 1—both launched in 1977—originally were slated to visit only Jupiter and Saturn, but the timing of Voyager 2's launch gave its trajectory the leeway needed for the spacecraft to be redirected, with a gravity assist from Saturn, on extended missions to Uranus and then to Neptune.

Voyager 2 flew past Neptune and its moons on Aug. 24-25, 1989, observing the system almost continuously between June and October of that year. It measured the planet's radius and interior rotation rate and detected its magnetic field, determining that the latter is both highly inclined and offset from the planet's rotation axis. It confirmed that

Neptune has rings and discovered six new moons. Neptune previously had been thought too cold to support active weather systems, but Voyager's images of the planet revealed the highest atmospheric winds seen in the solar system and several large-scale storms, one the size of Earth.

Because Neptune was Voyager 2's last planetary destination, mission scientists risked sending the spacecraft closer to it than to any other planet during the mission. Voyager passed about 5,000 km (3,100 miles) above Neptune's north pole. A few hours later it passed within 40,000 km (24,800 miles) of Triton, which allowed it to gather high-resolution images of the moon's highly varied surface as well as precise measurements of its radius and surface temperature.

As of 2009, no future missions to Neptune are planned.

Pluto, the Küiper belt, and beyond

Neptune is the last planet in the solar system. However, beyond Neptune is not empty space but a plethora of unusual icebound worlds, the Kuiper Belt and its most notable member, the dwarf planet Pluto.

Pluto was formerly regarded as the outermost and smallest planet. It also was considered the most recently discovered planet, having been found in 1930. In August 2006, however, the International Astronomical Union, the organization charged by the scientific community with classifying astronomical objects, voted to remove Pluto from the list of planets and give it the new classification of dwarf planet. The change reflects astronomers' realization that Pluto is a large member of the Kuiper Belt, a collection of debris of ice and rock left over from the formation of the solar system and now revolving around the Sun beyond Neptune's orbit.

Pluto is not visible in the night sky to the unaided eye. Its largest moon, Charon, is close enough in size to the dwarf planet that it has become common to refer to the two bodies as a double system. Pluto is designated by the symbol B.

Named for the god of the underworld in Roman mythology (the Greek equivalent is Hades), Pluto is so distant that the Sun's light, which travels about 300,000 km (186,000 miles) per second, takes more than five hours to reach it. An observer standing on Pluto's surface would see the Sun as an extremely bright star in the dark sky, providing Pluto on average 1/1,600 of the amount of sunlight that reaches Earth. Pluto's surface temperature therefore is so cold that common gases such as nitrogen and carbon monoxide exist there as ices.

Because of Pluto's remoteness and small size, the best telescopes on Earth and in Earth's orbit have been able to resolve little detail on its surface. Indeed, such basic information as its radius and mass have been difficult to determine; most of what is known about Pluto has been learned since the late 1970s as an outcome of the discovery of Charon. Pluto has yet to be visited by spacecraft, though the U.S. spacecraft New Horizons departed Earth for the Pluto-Charon system in 2006 and will arrive there in July 2015; many key questions about it and its environs can be answered only by close-up robotic observations.

0 0

Post a comment