Mariner 10

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Mercury is our second-closest planetary neighbor, on average, yet it hasn't received a lot of attention over the years. In part that's because it's not as interesting as some of the more-distant planets, like Mars and Jupiter.

But largely it's because Mercury is the closest planet to the Sun, so it's hard to study through the Sun's glare. And it's hard to send a spacecraft there, too. The Sun's gravity pulls so hard that the craft is really moving by the time it gets to Mercury, so it takes a lot of energy to slow it down.

The first craft to make the journey arrived at Mercury 35 years ago today.

Mariner 10 didn't land on Mercury, or even go into orbit. Instead, it flew past the little planet. But its orbit brought it back around to Mercury two more times.

Because of the way Mercury rotates, Mariner saw the same side of the planet on each visit. So it snapped hundreds of pictures of one hemisphere, but saw nothing of the other.

Those pictures showed a landscape that resembles the Moon, with countless impact craters and rugged mountains. They also showed cliffs that were close to a mile high and many miles long.

It took three decades for a second probe to visit Mercury. Messenger has flown by twice, with a third pass scheduled for this year. Each pass slows the craft down a little. So when Messenger returns to Mercury in 2011, it'll be able to enter orbit -- and finally give us a long, lingering look at the Sun's closest planet.

Script by Damond Benningfield, Copyright 2009

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