The ancient Greeks looked into the night and saw that some of the brightest stars were moving on a regular basis. They called them “wanderers”—planets. A few thousand years later, humanity itself has become the planetary wanderer, by sending robots to explore them. Rovers crawl on the surface of Mars; a lander clings precariously to a comet; a spacecraft swooshes past Pluto.
The questions posed by these probes are not all that different from those of our forebears: How did the solar system come to be? Is Earth the only place where the ingredients for life were catalyzed into the real thing? Robotic planetary exploration began a half century ago with sobering reality checks: our nearest neighbors—the moon, Mars and Venus—were fairly horrible places for life. More recently, scientists have been intrigued by not-so-inhospitable conditions in the icy worlds of the outer solar system—moons such as Europa, Titan, and Enceladus.
Now, the race is on to look past the edge of the solar system and into the planetary gardens of our galactic neighbors. New astronomical tools have brought thousands of exoplanets into view; scientists are on the cusp of discovering a true Earth twin. Strange new planetary species abound, such as “hot Jupiters”, giant planets that hug their parent stars in an all-too-warm orbital embrace—evidence that, at least in other star systems, planets really do wander. The menagerie of exoplanets has provided new natural laboratories for scientists to test out models of planetary formation and evolution. As always, however, the questions remain roughly the same: Is the solar system a freakish accident, and Earth even more special? Or could there be fellow travelers on our fellow wanderers?
The Sun
The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the Solar System (Jupiter contains most of the rest). It is often said that the Sun is an "ordinary" star. That's true in the sense that there are many others similar to it. But there are many more smaller stars than larger ones; the Sun is in the top 10% by mass. The median size of stars in our galaxy is probably less than half the mass of the Sun.
Jupiter
Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus). It has been known since prehistoric times as a bright "wandering star". But in 1610 when Galileo first pointed a telescope at the sky he discovered Jupiter's four large moons ., Europa, Ganymedeand Callisto (now known as the Galilean moons) and recorded their motions back and forth around Jupiter.
Mercury
In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman counterpart of the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it moves so quickly across the sky. Mercury has been known since at least the time of the Sumerians (3rd millennium BC).
Venus Facts
Venus is the second planet from the Sun and the sixth largest. Venus' orbit is the most nearly circular of that of any planet, with an eccentricity of less than 1%.
Planet Profile
orbit: 108,200,000 km (0.72 AU) from Sun
diameter: 12,103.6 km
mass: 4.869e24 kg
History of Venus
Venus (Greek: Aphrodite; Babylonian: Ishtar) is the goddess of love and beauty. The planet is so named probably because it is the brightest of the planets known to the ancients. (With a few exceptions, the surface features on Venus are named for female figures.)
Venus has been known since prehistoric times. It is the brightest object in the sky except for the Sun and the Moon. Like Mercury, it was popularly thought to be two separate bodies: Eosphorus as the morning star and Hesperus as the evening star, but the Greek astronomers knew better. (Venus's apparition as the morning star is also sometimes called Lucifer.)
Since Venus is an inferior planet, it shows phases when viewed with a telescope from the perspective of Earth. Galileo's observation of this phenomenon was important evidence in favor of Copernicus'sheliocentric theory of the solar system.
The first spacecraft to visit Venus was Mariner 2 in 1962. It was subsequently visited by many others (more than 20 in all so far), including Pioneer Venus and the Soviet Venera 7 the first spacecraft to land on another planet, and Venera 9 which returned the first photographs of the surface. The first orbiter, the US spacecraft Magellan produced detailed maps of Venus' surface using radar. ESA's Venus Express launched in November of 2005 and arrived at Venus in April 2006. The Venus Express is conducting atmospheric studies, mapping the Venusian surface temperatures and the plasma environment.
Venus' rotation is somewhat unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus' year) and retrograde. In addition, the periods of Venus' rotation and of its orbit are synchronized such that it always presents the same face toward Earth when the two planets are at their closest approach. Whether this is a resonance effect or merely a coincidence is not known.
Venus is sometimes regarded as Earth's sister planet. In some ways they are very similar:
Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass).
Both have few craters indicating relatively young surfaces.
Their densities and chemical compositions are similar.
Because of these similarities, it was thought that below its dense clouds Venus might be very Earthlike and might even have life. But, unfortunately, more detailed study of Venus reveals that in many important ways it is radically different from Earth. It may be the least hospitable place for life in the solar system.
The pressure of Venus' atmosphere at the surface is 90 atmospheres (about the same as the pressure at a depth of 1 km in Earth's oceans). It is composed mostly of carbon dioxide. There are several layers of clouds many kilometers thick composed of sulphuric acid. These clouds completely obscure our view of the surface. This dense atmosphere produces a run-away greenhouse effect that raises Venus' surface temperature by about 400 degrees to over 740 K (hot enough to melt lead). Venus' surface is actually hotter than Mercury's despite being nearly twice as far from the Sun. Venus has a vortex at each pole. These vortices rotate vertically and recycle the atmosphere downwards. The north polar vortex has a peculliar double eye shape surrounded by a collar of cool air; it makes a complete rotation in three Earth days.
There are strong (350 kph) winds at the cloud tops but winds at the surface are very slow, no more than a few kilometers per hour.
Venus probably once had large amounts of water like Earth but it all boiled away. Venus is now quite dry. Earth would have suffered the same fate had it been just a little closer to the Sun. We may learn a lot about Earth by learning why the basically similar Venus turned out so differently.
Most of Venus' surface consists of gently rolling plains with little relief. There are also several broad depressions: Atalanta Planitia, Guinevere Planitia, Lavinia Planitia. There two large highland areas: Ishtar Terra in the northern hemisphere (about the size of Australia) and Aphrodite Terra along the equator (about the size of South America). The interior of Ishtar consists mainly of a high plateau, Lakshmi Planum, which is surrounded by the highest mountains on Venus including the enormous Maxwell Montes.
Data from Magellan's imaging radar shows that much of the surface of Venus is covered by lava flows. There are several large shield volcanoes (similar to Hawaii or Olympus Mons) such as Sif Mons. Recently announced findings indicate that Venus is still volcanically active, but only in a few hot spots; for the most part it has been geologically rather quiet for the past few hundred million years.
There are no small craters on Venus. It seems that small meteoroids burn up in Venus' dense atmosphere before reaching the surface. Craters on Venus seem to come in bunches indicating that large meteoroids that do reach the surface usually break up in the atmosphere.
The oldest terrains on Venus seem to be about 800 million years old. Extensive volcanism at that time wiped out the earlier surface including any large craters from early in Venus' history.
Magellan's images show a wide variety of interesting and unique features including pancake volcanoes (left) which seem to be eruptions of very thick lava and coronae (right) which seem to be collapsed domes over large magma chambers.
The interior of Venus is probably very similar to that of Earth: an iron core about 3000 km in radius, a molten rocky mantle comprising the majority of the planet. Recent results from the Magellan gravity data indicate that Venus' crust is stronger and thicker than had previously been assumed. Like Earth, convection in the mantle produces stress on the surface. However on Venus the stress is relieved in many relatively small regions instead of being concentrated at the boundaries of large plates as is the case on Earth.
Venus has no magnetic field, perhaps because of its slow rotation.
Venus has no satellites, and thereby hangs a tale.
Venus is usually visible with the unaided eye. Sometimes (inaccurately) referred to as the "morning star" or the "evening star", it is by far the brightest "star" in the sky. There are several Web sites that show the current position of Venus (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.
On June 8 2004, Venus passed directly between the Earth and the Sun, appearing as a large black dot travelling across the Sun's disk. This event is known as a "transit of Venus" and is very rare: the last two were in 1882 and 2012, for the next you'll have to wait until 2117. While no longer of great scientific importance as it was in the past, this event was the impetus for a major journey for many amateur astronomers.
Both grew from early man’s need to feed and clothe themselves, and agriculture and the domestication of wild animals made it possible for civilizations to be created. Today, modern agriculture feeds most of the people on the planet and is made easier by developments like irrigation, genetic engineering, and the combine harvester. Diseases that typically infect plants are blight, clubroot, gall, and viroid. People who made important contributions to the advancement of agriculture as a science include Jethro Tull, who invented a mechanical drill for sowing seeds, and Eli Whitney, who invented the cotton gin. 
