After months of traveling through space, in May 2006 the Venus Express settled into orbit around the second planet from the sun. Its mission: to provide scientists with data about Venus' runaway greenhouse effect that makes it the hottest planet in the solar system.

The Russians and Americans have sent probes, including NASA's Magellan radar observatory, to Earth's closest neighbor to map its surface. But the European Space Agency's Express spacecraft is designed to observe Venus' atmosphere and how it interacts with the planet's surface and solar winds to better understand the planet's greenhouse effect.

In general, the greenhouse effect is the rise in temperature caused by certain gases in a planet's atmosphere, including carbon dioxide, nitrous oxide, water vapor and methane, trap the energy from the sun.

The Venus Express is expected to circle and observe Venus for two Venusian days, or a total of 486 Earth days.

"Venus Express is equipped to peer beneath the thick clouds that encircle the planet and probe the mysteries of Venus with a precision never achieved before," project team member Fred Taylor of Oxford University told The New York Times.

The Express spacecraft has seven instruments that weigh only 205 lb. and are designed to collect data on solar winds, magnetic fields, temperature and chemical makeup of the atmosphere.

Venus vs. Earth
"In many ways Venus had the same start in life as Earth," ESA Science Director David Southwood told the New Scientist.

Formed about 4.5 billion years ago, the two are often paired because of their similar size, mass, density and volume. Scientists believe that Venus once had oceans.

Beyond these similarities though, Southwood said, "something went seriously wrong very early on," and Venus ended up a very different planet.

Venus now has no surface water and its atmosphere is made up almost entirely of carbon dioxide (95 percent). The clouds are made of sulfuric acid. At the surface the atmospheric pressure is over 90 times that of Earth at sea level.

Based on previous trips to Venus, scientists know that on the surface there is a gentle breeze but the further from the surface, wind speeds pick up dramatically, according to the ESA.

At the top of the clouds, the atmosphere rotates at formidable velocity, with wind speeds up to 360 km per hour, or 224 mph. Clouds at the equator circle the planet in four days, a phenomenon called "super-rotation."

This toxic atmosphere is perhaps Venus' most striking characteristic. It rules over Venus' climate and is home to the most powerful greenhouse effect in the solar system.

Eighty percent of the incoming sunlight is reflected away from Venus by its giant cumulus-like clouds. Ten percent of the sunlight is absorbed by the atmosphere and the other 10 percent makes it to the surface.

The small amount of sunlight that makes it to Venus' surface is trapped by an atmosphere of greenhouse gasses: water vapor, carbon dioxide and sulfuric acid aerosois. The result is the solar system's hottest surface temperature and a difference of 500 degrees Celsius, or 932 degrees Fahrenheit, between the surface and clouds' top temperatures.

By tracing these results of the greenhouse effect through Venus' history, researchers hope to better understand how that effect developed on a planet that was so similar to Earth.

Although scientists are not saying Earth may end up like Venus someday, studying "the history of planets and the behavior of atmospheres [can] tell us profound things about why the Earth become habitable but Venus did not," said University of Chicago professor Raymond Pierrehumbert.

-- By Bryan Hayes, Online NewsHour