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Matt Quandt
Assistant editor
Astronomy magazine
(t) 262.796.8776 x419
(e) mquandt@astronomy.com
October 14, 2005
WAUKESHA, WI — A 15-year dry spell in Venus exploration ends this month with the European Space Agency’s (ESA) launch of Venus Express. The mission is scheduled to launch from Kazakhstan’s Baikonur Cosmodrome on a Russian Soyuz-Fregat rocket October 26. The launch window will remain open between October 26 and November 25.
Venus outshines everything in the sky except the Sun and Moon. It’s Earth’s closest planetary neighbor, orbiting the Sun about 30-percent closer. When Venus passes between Earth and the Sun, the planet lies only about 100 times farther away than our own Moon.
Veiled planet
A thick mantle of clouds shrouds Venus and hid its true character until the 1950s. Until then, astronomers thought of Venus as Earth’s twin based on similarities like size, mass, and distance from the Sun.
But Venus turns out to be a grotesque parody of Earth. Its perpetual clouds are made not of water, but droplets of sulfuric acid. Carbon dioxide makes up most of the atmosphere, which is so dense the pressure it exerts on the planet’s surface is more than 90 times what we experience on Earth. This is the same pressure a submarine would experience nearly 3,000 feet (900 meters) beneath the sea’s surface. Spacecraft have landed on the surface — the Russian Venera spacecraft of the 1970s and 1980s — but they didn’t survive long.
Venus has no oceans, lakes, rivers, or rainfall. The planet’s high surface temperature — a constant 870° Fahrenheit (465° Celsius), or nearly twice as hot as an oven’s broiler — prevents any liquid water. In fact, Venus is 100,000 times drier than the driest place on Earth. Call it global warming gone wild. The thick carbon-dioxide atmosphere lowers the planet’s ability to radiate heat into space, so the temperature rises.
Yet scientists think Venus may once have held oceans, just like Earth, and they’d like to understand why the two worlds are so dissimilar now.
Venus or bust
Venus Express is the first probe dedicated to studying our neighbor planet since NASA’s radar-mapping Magellan mission launched in 1989. Magellan peered through the clouds, using radio waves to map 98 percent of the planet at high resolution, revealing impact craters, rift valleys, and volcanoes. It ceased operation in October 1994.
Venus Express will focus on the planet’s dense, poorly understood atmosphere. Existing meteorological models fail to reproduce the venusian atmosphere’s behavior. For example, clouds at the equator circle the planet in just 4 days. Several spacecraft have seen hints of lightning on Venus, but definitive evidence has yet to be found.
Although scientists have identified sulfuric acid droplets in the upper cloud deck, spacecraft passing through lower clouds suggest the presence of crystalline particles, not droplets. If so, their chemical make-up must be a more exotic chemical than sulfuric acid.
The spacecraft’s Venus Mapping Camera will be the first to study the planet at wavelengths all across the spectrum, from the infrared to the ultraviolet. It will even be able to look for hot spots on the surface possibly caused by volcanic eruptions in progress. Although the Venus Mapping Camera was developed for Venus Express, most of the instruments were built from flight-qualified spares from two previous ESA missions — Mars Express and Rosetta.
Once launched, Venus Express will cruise through space for 153 days. When captured by Venus’ gravity, the spacecraft will adjust its orbit over a period of 5 days, eventually looping around the planet’s poles. At its closest, it will reach an altitude of 155 miles (250 km) above the planet’s high northern latitudes. At its farthest, it will be 41,000 miles (66,000 km) above the planet’s southern hemisphere.
Mission planners expect the mapping mission to last 500 Earth days, but Venus Express carries enough fuel to maintain its polar orbit twice this long.
Astronomy magazine and Astronomy.com will provide extensive coverage of the Venus Express mission.
Fast facts about Venus Express
Launch window: October 26 to November 25, 2005
Launch vehicle: Russian Soyuz-Fregat
Dimensions: 4.9 feet by 5.9 feet by 4.6 feet (1.5m by 1.8m by 1.4m), excluding the extended 26-foot (8m) solar panels
Weight: 2,730 pounds at launch; 1,250 pounds (570 kilograms) of maneuvering fuel
Spacecraft: Modified from Mars Express
Venus at a glance
Diameter: 7,521 miles (12,104 km) — 95 percent Earth’s size
Mass: 10.7 x 1024 pounds (4.9 x 1024 kilograms) — 82 percent of Earth’s
Surface temperature: 870° F (465° C)
Surface atmospheric pressure: 92 times Earth’s
Atmospheric composition: 96.5 percent carbon dioxide
3.5 percent nitrogen
Rotation period: 243 days
Solar day (noon to noon): 117 Earth days
Axial tilt: 177.4°
Sidereal orbital period: 224.7 days
Synodic orbital period: 583.9 days
Mean distance from Sun: 67.2 million miles (108.2 million km) — 72 percent of Earth’s
Orbit inclined to Earth’s: 3.4°
