If Venus's pass across the sun earlier this week yields a bounty of information for hunters of transiting worlds in other planetary systems, it's because Venus is a known entity. Studying the June 5 Venus transit as if it were a faraway exoplanet "gives us a reality check," says planetary physicist Colin Wilson of the University of Oxford. "We can check on all those exoplanet techniques to see how accurate they really are." Such data may enhance NASA's Kepler mission as well as the many ground-based campaigns using planetary transits to identify distant worlds, a method that has led to the discovery or characterization of more than 200 exoplanets.

That reality check would not be possible without the data planetary scientists already have about Venus. And humanity's up-close exploration of Earth's cloud-shrouded closest neighbor began in earnest 45 years ago, when the Soviet probe Venera 4 launched on June 12, 1967.

The first two Veneras had failed after launch. Venera 3, launched in 1965, is thought to have crashed on Venus but returned no data.

At last, on October 18, 1967, Venera 4 became the first man-made object to enter another planet's atmosphere and send back data. "It was before any Mars landers or anything," Wilson says. And it changed our view of Earth's sister planet forever.

Not much was known at the time about Venus, according to Roald Sagdeev, former director of the Space Research Institute of the U.S.S.R. [now Russian] Academy of Sciences. What was known, he says, "was that it was covered by very dense clouds...and it was established that the atmosphere was in a state of super-rotation," meaning that the winds whip around Venus up to 60 times faster than the planet rotates. The unknowns were many, Sagdeev adds: "the temperature, the thickness of the atmosphere, and the atmospheric pressure. And nobody knew at the time that carbon dioxide was a major constituent of the atmosphere."

Venera 4 made it deep enough into Venus's atmosphere to reach pressures 22 times that at sea level on Earth (one atmosphere), sending back data all the way. Venera's instruments showed the atmosphere was mostly carbon dioxide and detected no oxygen or water.

From a technical perspective, too, the probe was an achievement, says Wilson, who works with the European Space Agency's Venus Express, currently orbiting the second planet. "It was the first planetary entry system that worked using the atmosphere to decelerate. They had to decelerate 300 g's. Building things to sustain that kind of shock is incredible."

The mission was not perfect, though. Partway through the dense atmosphere, Venera 4 conked out. "The immediate interpretation was that the spacecraft managed to hit the surface of the planet and terminated its work," says Sagdeev, who managed the Venera program starting with probe number 9. It was not until a few years later that the scientists realized that the spacecraft had been crushed by the pressure. "At least it helped them to design the next generation of Veneras," he says. And in fact, Venera 8, which Sagdeev called "very successful," was a beefed-up version of Venera 4. That lander survived on the surface for almost an hour, withstanding crushing pressure 92 times stronger than on Earth's surface.

After Venera 4, the Soviets sent 12 more Venera spacecraft to Venus—all successful—over the next decade and a half whereas the U.S. only flew one mission, Pioneer Venus, launched in 1978. Yet since the Venera program ended, there have been few Venus missions from any nation, with the notable exceptions of NASA's Magellan and the ESA's Venus Express, along with two Soviet Vega probes whose dual mission included a Halley's Comet flyby. There is still plenty more to learn about the planet.

Venus Express, for instance, has been seeking signs of active volcanism on the planet. "It's been looking for volcanically generated gases, volcanic plumes in the lower atmosphere," says Wilson, adding that the craft's findings have import beyond the present state of the cloudy, scorching planet. "We can start to understand the wider [question] of how Venus and Mars and the Earth evolved over their lifetimes. They were all created at the same time out of the same material, so how is it that these initially similar planets have evolved so differently?"

Unfortunately, sending another lander to Venus to build on the Venera tradition would be pricey. "The main concept that's been on the drawing table for a long-lived lander is you have to take an air conditioner to keep your electronics cool," Wilson says. "And you don't have all that much sunlight, so then you take a nuclear power source to run your cooler." A cheaper way to answer some big questions, he says, would be to deploy a balloon in Venus's clouds, as was done during the Vega missions of the 1980s. At the right altitude temperatures would be a balmy 20 degrees Celsius. "Not too hot, not too cold, and atmospheric pressure of half of one Earth atmosphere." A hypothetical astronaut, Wilson ventures, "could probably get away without even wearing a pressure suit. It's comfortable—except you're surrounded by poisonous sulfuric clouds."

Studying the surface from a perch in the clouds would be challenging, Wilson acknowledges, but a balloon mission could at least investigate how Venus's cloud chemistry works and taste the atmosphere to directly determine its composition. Perhaps by the next transit of Venus, in 2117, planetary scientists will have the answers to these questions and more.