Solar systems

Until recently, the solar system was the only known example of a planetary system, although it was widely believed that other comparable systems did exist. A number of such systems have now been detected, although the information available about them is very limited. See extrasolar planet for more information.

[edit]
Discovery of the solar system
The nature and structure of the solar system were long misperceived, for at least two reasons:

The Earth was considered stationary, and the motion of objects in the sky was therefore taken at face value: the sun was thought to orbit the Earth, for example. (This conception of the universe, in which the Earth is at the center, is called the Geocentric model; geos means "Earth" in Greek.)
Many solar system objects and phenomena cannot be perceived at all without technical aid.
Over the last several hundred years, conceptual and technological advances have helped us understand the solar system much better.

The first and most fundamental of these advances was the Copernican Revolution, which proposed that the planets orbit the sun—models of the solar system with the sun in the center are called heliocentric. (Helios means "Sun" in Greek. And solar means "having to do with the sun" in English; hence the term solar system.)

But despite the name, the most striking Copernican realization was not that the sun was central but that the Earth was peripheral, orbital: planets had been considered merely points in the sky, but if the Earth itself was a planet, perhaps the other planets were, like Earth, huge solid spheres. (Moreover, the lack of perceptible stellar parallax, despite the Earth's orbital motion, indicated the extreme remoteness of the fixed stars, which prompted similar speculation that they could be objects like the Sun, perhaps with planets of their own.)

[edit]
Exploration of the solar system
Since the start of the space age, a great deal of exploration has been performed by unmanned space missions that have been organized and executed by various space agencies. The first probe to land on another solar system body was the Soviet Union's Luna 2 probe, which impacted on the Moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on Venus in 1965, Mars in 1976, the asteroid 433 Eros in 2001, and Saturn's moon Titan in 2005. Spacecraft have also made close approaches to other planets: Mariner 10 passed Mercury in 1973.

The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in 1973. Pioneer 11 was the first to visit Saturn, in 1979. The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980–1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989. The Voyager probes are now far beyond Pluto's orbit, and astronomers anticipate that they will encounter the heliopause which defines the outer edge of the solar system in the next few years.

Pluto remains the only planet not having been visited by a man-made spacecraft, though that will change with the launching of New Horizons by NASA in January 2006. It is scheduled to fly by Pluto in July 2015 and then make an extensive study of as many Kuiper Belt objects as it can.

Through these unmanned missions, we have been able to get close-up photographs of most of the planets and, in the case of landers, perform tests of their soils and atmospheres. Manned exploration, meanwhile, has only taken human beings as far as the Moon, in the Apollo program. The last manned landing on the Moon took place in 1972, but the recent discovery of ice in deep craters in the polar regions of the Moon has prompted speculation that mankind may return to the Moon in the next decade or so. Manned missions to Mars have been eagerly anticipated by generations of space enthusiasts, and it was hoped that the first manned interplanetary flights would take place in the 1980's, after the successful Apollo program. The United States now plans manned Lunar and Mars missions as part of the new Vision for Space Exploration.

Main Paul1 Paul2 solar1 solar2 solar3 solar4 solar5

membrane filtration water distillers | yacht charter | oil painting reproductions | Safety Glasses | Night Vision | hawaiian music aloha shirts | Health Care | football shirts | oil paintings | Nashville Real Estate free credit report | Dental Care | caribbean yacht charter | south pacific yacht charter | Kit Car