To the world at large, this warbling tone was a signal that the United States had moved ahead—reached out to the planets. Mariner was exploring the future, seeking answers to some of the unsolved questions about the solar system. THE DOUBLE STAR OF THE ANCIENT WORLD Venus, the glittering beacon of our solar system, has intrigued man for at least 4,000 years. The Babylonians first mentioned the brilliant planet on clay tablets as early as 2,000 years before Christ. The Egyptians, the Greeks, and the Chinese had thought of Venus as two stars because it was visible first in the morning and then in the evening sky. The Greeks had called the morning star Phosphorus and the evening star Hesperos. By 500 B.C. Pythagoras, the Greek philosopher, had realized that the two were identical. Galileo discovered the phases of Venus in 1610. Because of the planet’s high reflectivity, Copernicus falsely concluded that Venus was either self-luminous or else transparent to the rays of the Sun. Venus was tracked across the face of the Sun in 1761, from which event the presence of an atmosphere about the planet was deduced because of the fuzzy edges of the image visible in the telescope. Throughout the eighteenth and nineteenth centuries, Venus continued to excite growing scientific curiosity in Europe and America. [Illustration: _Venus’ orbit is almost circular. At inferior conjunction, the planet is between the Earth and the Sun, approximately 26,000,000 miles away; at superior conjunction, Venus is on the other side of the Sun. The elongations are the farthest points to the east and the west of the Earth._] Even the development of giant telescopes and the refinement of spectroscopic and radar astronomy techniques in recent times had yielded few indisputable facts about Venus. Until radar studies, made from Goldstone, California, in 1962, neither the rate nor the angle of axial spin could be determined with any degree of accuracy. The ever-shifting atmosphere continued to shield the Venusian surface from visual observation on Earth, and the nature of its atmosphere became an especially controversial mystery. THE CONSENSUS PRIOR TO MARINER II Venus is a virtual twin of the Earth; it approaches our planet closer than any celestial body except the Moon, a few vagrant comets, and other such galactic wanderers. Long fabled in song and legend as the most beautiful object in the sky, Venus has an albedo, or reflectivity factor, of 59% (the Moon has one of 7%). In its brightest or crescent phase, Venus glows like a torch, even casting a distinct shadow—the only body other than the Sun and the Moon yielding such light. Venus’ diameter is approximately 7,700 miles, compared with Earth’s 7,900. Also as compared with 1.0 for the Earth, Venus’ mean density is 0.91, the mass 0.81, and the volume 0.92.