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From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

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And I'm Audie Cornish.

These days, you can go online or pick up almost any book about our solar system, and you'll see vivid detailed photographs of rocks on Mars or the moons of Saturn. It's easy to forget that not too long ago, the planets were alien and mysterious. That only began to change when NASA launched the first probe to visit another world. NPR's Nell Greenfieldboyce reports that 50 years ago today, a little space probe called Mariner 2 flew past Venus.

NELL GREENFIELDBOYCE, BYLINE: Back in the 1950s, scientists knew that all the other planets existed. They'd been seen through telescopes.

ERIK CONWAY: But very little is known about them, and much of what was believed to be true about them was - turned out to be wrong.

GREENFIELDBOYCE: Erik Conway is a historian at NASA's Jet Propulsion Laboratory, which built Mariner 2. He says, consider Venus, which is entirely covered by clouds.

CONWAY: Venus was generally believed, especially in the public, to be kind of a warmer Earth. It would be something like a Mesozoic jungle age - maybe even with dinosaurs.

GREENFIELDBOYCE: But all that was about to change. In 1957, the Soviets launched the satellite Sputnik, and the space race was on. The Soviets weren't only aiming for the moon. They were trying to send a spacecraft to Venus. So NASA had to do the same, even though a lot of people thought the idea was crazy. Venus was 36 million miles away. Conway says the trip would take over three months.

CONWAY: There was considerable skepticism in places like NASA headquarters itself that you could actually make a spacecraft that would live long enough to get to another planet.

GREENFIELDBOYCE: Nevertheless, on July 22nd, 1962, NASA tried to send a hastily built space probe caller Mariner 1 to Venus. The rocket veered off course. It was ordered to self-destruct. Luckily, the engineers had built a spare probe, Mariner 2. One month later, its launch came close to failing. But the rocket somehow righted itself, and Mariner 2 was on its way. Fifty years ago today, reporters were gathered at NASA headquarters waiting to hear some truly unearthly sounds. On this scratchy recording, laboratory director William Pickering tells them everyone is about to hear live radio signals coming from Mariner 2.

(SOUNDBITE OF ARCHIVED BROADCAST)

WILLIAM PICKERING: So Pasadena can you turn on the signal again, please?

GREENFIELDBOYCE: This was Mariner 2 making its closest approach to Venus, passing within 22,000 miles. Later, Pickering said that even he had been struck by the seemingly casual way that they'd communicated back and forth with something so far out in space.

PICKERING: To sit outside and look at the star Venus in the sky and realize that a piece of our equipment was just then going past the planet was really a very surprising thing to do. And one had to, as it were, pull oneself together now and again and realize that this was indeed real, and this was happening.

GREENFIELDBOYCE: The public loved it. This was the first time NASA had beat the Soviets. It was also a scientific success. Mariner 2 showed that the surface of Venus was way too hot for life. This mission set the stage for all the exploration of our solar system that was to come. Planets are becoming almost familiar places. They've all been visited in some way, except for the dwarf planet Pluto, and NASA's New Horizons probe is headed there now. James Green is director of planetary science at NASA.

JAMES GREEN: We have really quite a wonderful armada of missions that are currently exploring.

GREENFIELDBOYCE: He ticked off a bunch: Cassini is orbiting Saturn, Messenger is at Mercury, Curiosity is on Mars, Juno is flying towards Jupiter. And Green says, 50 years from now, NASA could be doing all kinds of new missions.

GREEN: We're going to be bringing back a sample from Europa.

GREENFIELDBOYCE: That's an ice-covered moon of Jupiter.

GREEN: Or we're going to be wandering around the lakes of Titan.

GREENFIELDBOYCE: That's a moon of Saturn. Its lakes are made of liquid methane.

GREEN: So 50 years, you know, is an enormous leap. And all you have to do is sort of look back at the last 50 years, where we've come, and you'd recognize that even what I've just said is probably going to be eclipsed by what we'll really end up doing in that 50-year timeframe.

GREENFIELDBOYCE: And we'll be learning more about the planets beyond our solar system, the ones circling other stars. Meanwhile, good old Mariner 2 is still orbiting our sun, silently going around somewhere between Venus and Mercury. Nell Greenfieldboyce, NPR News.

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