2. The new, close-up view from space
Flybys, orbiters, landers and probes
We live at an incredible time, when all of the major planets, and most of their satellites, have been viewed close up with the inquisitive eyes of robotic spacecraft. They have perceived awesome, unanticipated features that are far beyond the range of human vision with even the best telescopes on the ground. No two of these fascinating new worlds are exactly alike. Most of them have been investigated many times, with increasingly sophisticated instruments.
This captivating voyage of discovery began close to home, in the late 1950s, when the first artificial satellites were lofted into orbit around the Earth and the Russian Luna 3 spacecraft swung once around the far side of the Moon, which had never been seen before. It is strongly deficient in the large, dark maria that characterize much of the near side facing Earth.
In all, there have been six manned landings on the Moon, beginning with Apollo 11 in July 1969 and ending with Apollo 17 in December 1972. The actual landings were performed by the bug-like Lunar Module that separated from the main spacecraft, while in orbit around the Moon, and returned to it. At first the astronauts traveled on foot, staying near to the Lunar Module, but they subsequently moved to more remote locations in roving vehicles. Altogether, 382 kilograms of rocks were brought back from the Moon for analysis in the terrestrial laboratory, determining the Moon's age, chemical composition, history and probable origin.
The first spacecraft to be launched on lengthy journeys beyond the Moon were flyby missions, the Mariners, Pioneers and Voyagers, that passed near the planets and their satellites to give us new vistas, unavailable from the ground, and making important discoveries in the process. In 1962, instruments aboard the Mariner 2 flight to Venus detected a perpetual flow of charged particles in interplanetary space, emanating from the Sun. Mariner 10, launched in 1973, made the first spacecraft photographs of Venus, and traveled on to reveal the heavily-cratered surface of Mercury.
In 1972-74, the Pioneer 10 and 11 missions to Jupiter showed that spacecraft could pass safely through the asteroid belt, blazing a trail for the extraordinarily successful Voyager 1 and 2 flyby missions whose itinerary included Jupiter (1979), Saturn (1980, 81), Uranus (1986) and Neptune (1989). Voyager 1 and 2 vastly improved our understanding of the atmospheres of the giant planets, and discovered unexpected rings, moons and magnetic fields. They also transformed the satellites of the giant planets into unique and distinctive places with diverse surfaces and in some cases atmospheres or magnetic fields.
The initial explorations using flybys were followed by orbiters that greatly increased the time available for detailed study, often for years at a time. They revealed many features that previous flyby missions had missed, and forever changed our view of the planets and their satellites.
The Viking 1 and 2 orbiters amplified and enhanced our new perspective of Mars in the late 1970s. Each Viking also had a nuclear-powered, 1-ton (1,000 kilogram) lander that was sent safely to the planet's surface, obtaining beautiful panoramas of the Martian surface and measuring the properties of the thin, freezing atmosphere. The Viking landers were also sent to search for both extant and extinct life on Mars, but the results were inconclusive.
More recently, the exploration of Mars has begun to shift from global to exceptionally close-up, high-resolution views taken with the Mars Global Surveyor. At the end of the 20th century and the beginning of the 21st century, it obtained images that show much finer detail than those obtained with the Viking orbiters, including layered deposits suggesting ancient lakes or shallow seas, and dramatic evidence for recent flows.
In the 1990s, the Magellan orbiter used radar to penetrate the thick, cloudy atmosphere of Venus, mapping the entire planet with a clarity and resolution not available for much of Earth. Since Venus is perpetually shrouded in clouds, this was the only way to detect its surface. Magellan's radar images have revealed an unearthly world that was resurfaced long ago by rivers of outpouring lava, and disclosed numerous volcanoes that now pepper its surface.
The Galileo orbiter-probe spacecraft, launched in October 1989, was so massive that no existing rocket had the power to launch it directly to Jupiter, its primary target. Instead, the spacecraft was placed on a looping trajectory that took it past Venus once and Earth twice. The gravity of these planets was used to accelerate and propel the spacecraft in slingshot fashion toward its eventual rendezvous with the giant planet. While the roundabout route took six years, in comparison to the direct, 21-month flights of Pioneer 10 and 11, it also took Galileo on close encounters with two asteroids along the way.
Galileo carried an entry probe that penetrated Jupiter's kaleidoscopic clouds, obtaining the first direct, or in situ, sampling of a giant planet's atmosphere. The main orbiting spacecraft looped around Jupiter for more than five years, until 2001, obtaining high-resolution images and analysis of the planet, its ring, and the four large moons.
The Cassini spacecraft was launched on its seven-year journey to Saturn on 15 October 1997, with arrival expected in June 2004. It includes an orbiter, whose instruments will study the planet's atmosphere, rings, satellites, and magnetic environment. The spacecraft also carries the Huygens Probe that will be parachuted into the hazy, dense atmosphere of Saturn's intriguing moon Titan, determining the properties of its Earth-like atmosphere and its mysterious surface below.
Comets are so tiny and so far away that you cannot detect them until they come near the Sun, and their center is then buried within the brilliant glare of fluorescing gases and reflected sunlight. As a result, no one had ever seen the bare surface of a comet's nucleus until 1986, when the Giotto spacecraft peered into the core of Halley's Comet. It found the nucleus to be a black, oblong chunk of ice and dust, roughly the size of Paris or Manhattan. At the moment of encounter, the comet was spewing out about 25 tons (25,000 kilograms) of water every second, propelled into sunward jets by the vaporizing ice.
There is a growing awareness of the similarities of the major planets and some moons, despite the differences that make each of them unique. They all exhibit common properties and similar processes, such as impact craters, volcanoes, water and atmospheres, reminding us of the basic elements in ancient Greek philosophy - Earth, fire, water and air.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University