PEOPLE OFTEN MISTAKE VENUS for a star. After the Moon, it is the brightest object in our night sky.Because it is so close in size to Earth, until the 20th century astronomers assumed that it might be in some ways like Earth. The probes sent to investigate have shown that this is not so. The dense cloudy atmosphere ofVenus hides its surface from even the most powerful telescope. Only radar can penetrate to map the planet's features. Until it became possible to determine the surface features-largely flat, volcanic plains-scientists could not tell how long the Venusian day was. The atmosphere would be deadly to humans. It is made up of a mixture of carbon dioxide and sulfuric acid that causes an extreme greenhouse effect," in which heat is trapped by the atmosphere. The ancients, however, saw only a beautifully bright planet, and so they named it after their goddess of love. Nearly all the features mapped on the surface ofVenus have been named after women, such as Pavlova, Sappho, and Phoebe.

LOOKING AT VENUS: In 1978 the United States launched the Pioneer Orbiter, designed to map the surface ofVenus by using radar to penetrate its densely clouded atmosphere. It was followed in 1989 by Magellan, which circled Venus every 3 hours and 9 minutes and had a 12-ft (3.7-m) radar dish that beamed radar images back to Earth for analysis. Computers were used to build up pictures of the surface-mainly volcanic plains.

LANDING ON VENUS: Venera 13 landed on Venus in 1982. The landscape appeared barren, made up of volcanic rocks. There was plenty of light for photography, but the spacecraft succumbed to the ovenlike conditions after only an hour.