The Universe in the Classroom

Exploring Venus

Target: Venus
The Soviet VEGA Spacecraft

Two remarkable "busloads" of automated spacecraft were launched toward Venus by the Soviet Union in December of 1984. Each one contains three separate modules: one lander and one balloon-carried atmosphere probe to be dropped off at Venus in June, 1985, and one major package of instruments which will continue on to encounter Halley's Comet in March, 1986.

The two ensembles of instruments are called "VEGA-1" and "VEGA-2", abbreviations of the Russian words for Venus ("Venera") and Halley ("Gallei"). They are now on their way toward their first target, Venus, and reportedly are working well. While the Soviet Union has a relatively long history of sending probes to Venus, these spacecraft are very unusual in two major ways. First, the deployment of the balloon-probes (if successful) will mark the first time any human-made spacecraft will float in another planet's atmosphere, and, second, the craft are truly international: they carry experiments from France — and from the U.S.!

The most fascinating of the VEGAs' experiments at Venus involve thhe two balloon-probes. After being jettisoned from the two main spacecraft, the two shrouded probelets will plummet to the middle layer of Venus's acid clouds. They will arrive on the night side of Venus, when the temperature in the middle cloud deck is expected to be between 60 and 160° Fahrenheit.

When the two spacecraft have fallen to the proper depth, their helium-filled, ten-foot-wide balloons will spring out. Then, tethered some 50 feet below each balloon, a five-foot-tall cylindrical gondola full of instruments will drift with the warm Venusian winds about 30 miles above the planet's superheated surface.

The gondolas carry a number of scientific instruments, including ones you might expect for a "weather balloon" here on Earth: thermometers and barometers, for example. Also, there are devices for monitonng lightning flashes, vertical wind speeds (for quick up- or down-drafts), and the density of the acid droplets in the clouds. (The density-measuring device was designed by NASA scientist Boris Ragent, working with France's National Center for Scientific Research, which is participating with the USSR on the VEGA missions.)

But perhaps the most important information will be simply the gondolas' locations — how they are carried around will disclose much about the unknown circulation patterns of Venus's atmosphere at that level. Radio signals from the floating spacecraft will be picked up on Earth (67 million miles away) by a global netvvork of large radio receivers, the most sensitive of which will be three enormous dish antennas operated by NASA. Computer analysis of the signals received by the network will allow extremely detailed track to be kept of precisely where the probes are and how fast they're moving. In fact, scientists expect to be able to "see" changes in balloon speed as small as 2 m.p.h.!

The batteries which power the balloon-borne instruments can operate for about 60 hours before they run down. but the Soviets expect the mission to end before then: after about 40 hours. the balloons are expected to cross from the night side of Venus to the day side. It's likely that heat from sunlight will cause the balloons to swell — and pop. Even so. they will almost certainly survive far longer than the total time logged so far by Venus landers.

Venus and Earth: A Comparison

  Earth Venus
Average Distance from the Sun
(millions of miles)
92.6 67.2
Orbit period
(in Earth days)
365 225
Spin perioda 23.9 hours 243 Earth days
Spin direction
(as seen from North)
Counterclockwise Clockwise
Mass (times Earth's) 1.00 0.815
Diameterb (miles) 7930 7520
Average density
(times liquid water)
5.5 5.3
Surface gravity
(times Earth's)
1.00 0.905
Number of natural moons 1 O
Number of working artificial satellites hundreds 3c

a — These are the planets' "sidereal" spin periods: the time they take to
rotate with reference to the stars, rather than the Sun.
b — These are the planets diameters across their equators.
cVeneras 15 and 16 and the Pioneer Venus Orbiter.

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