Venus is the second planet from the sun and the sixth largest. Venus' orbit is the most circular of any planet, with an eccentric y of less than 1%. Venus, perhaps because it is the brightest of planets known to the ancients, Is named after the Greek goddess of love and beauty. The planet of Venus has been known since prehistoric times and is the brightest object in the sky with the exception of the sun and the moon. Venus' rotation is somewhat unusual in that it is both very slow (243 Earth days per Venus day) 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.

The pressure of the planet's atmosphere at the surface is 90 atmospheres and is composed mostly of carbon dioxide. There are several layers of clouds which are many kilometers thick and composed of sulfuric acid. This dense atmosphere produces a run-away greenhouse effect that raises Venus's urface temperature by about 400 degrees to over 740 K. The planet of Venus is often regarded as Earth's sister planet, in some ways they are very similar. For example, 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 also similar. Because of these similarities, it was once thought that below it's dense clouds, Venus might be very earth like, perhaps to the point of containing life. However, a more detailed study of Venus revealed that many aspects of Venus' atmosphere was much different from that of Earth. There are no small craters on Venus. It appears that small meteoroids burn up in Venus' dense atmosphere before reaching the surface.

Craters on Venus seem to come in bunches indicating that the 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 volcanisms at the time wiped out the earlier surface including any large craters from early Venus' history. The interior of Venus is probably very similar to that of Earth. It consists of an iron core about 3000 km in radius, a molten rocky mantle comprising the majority of the planet. Venus probably once had large amounts of water like Earth but it all boiled away, leaving Venus quite dry.

If Earth had been just a little closer to the sun, it would have had a similar fate. Much of the planets's urface is covered with lava flows and there are several large shield volcanoes. Recently announced findings show 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 despite their presence. Studying Venus The brightness of Venus and it's periodic proximity to the Earth have made Venus an easy target for advances in astronomical observations. Venus has been the object of telescopic observation from the beginning.

The appearance of the phases of Venus by Galileo was a milestone in the modern understanding of the solar system. Early measurements of the speed of light were derived from observations of the transit of Venus across the solar disk as seen from the Earth. Venus was also the first solar system object from which radar signals were first bounced off in the late 1960's. The first radar observations of another planet from an orbiting platform were made from an orbiting platform were made at Venus by the Veneta 15 spacecraft and were followed by Pioneer Venus Orbiter and subsequently by Magellan. The launch of the Mariner 2 spacecraft in 1962 started the modern era of Venus exploration by space-craft. Since then, Venus has been observed by spacecrafts Mariner 5, Mariner 10, and the Soviet.

While these space missions have greatly changed our view of Venus as well as furthered our knowledge, many important questions about Venus and it's atmosphere remain unanswered. Why is the rotation of Venus' atmosphere many times faster than the underlying planet? How does Venus lose it's heat? We may never know.