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The early Earth had two moons instead of just one — our familiar moon, as well as a smaller companion moon that also rose and set in the sky for tens of millions of years.

That's according to a new theory that says this smaller moon eventually went careening into our moon and is still there, in the form of mountains on its far side.

Scientists have long puzzled over those mountains, and the fact that the two sides of our moon are very different. The near side has flat lowlands, while the far side is high and mountainous.

The final space shuttle mission means that the 30-year-old shuttle program is about to enter the history books alongside the famous Mercury, Gemini, and Apollo programs.

And as the end of the shuttle era looms, NASA leaders say they're about to build a new vehicle, one that will let astronauts go exploring deep into space. But some experts doubt that plan will ever get off the ground.

To understand the big question mark hanging over NASA's future, it helps to first turn the clock back to 2004 — the year after the space shuttle Columbia disaster.

Once space shuttle Atlantis touches down on Earth later this week, workers will start the process of transforming the spaceship into a museum piece.

To see how that mothballing process will unfold, I recently went on a rare tour of Discovery, one of NASA's other shuttles.

Discovery was set up with its landing gear down in a secure hangar at Kennedy Space Center in Florida, where technicians normally do work on the shuttles after each flight.

As space shuttle Atlantis orbits the Earth on NASA's last shuttle mission, it's worth remembering that key parts of this high-tech spaceship were handmade by people back here on Earth.

Five years ago, NPR profiled a few of the workers who make pieces of NASA's shuttles, using everyday tools like sewing needles and X-ACTO knives. With the shuttle program ending, NPR revisited those people to see how their lives are changing now that the shuttles will no longer need them.

If you opened up a copy of the magazine Popular Science back in 1974, you'd see an artist's conception of a blastoff for the new spaceship that NASA was building. The headline: "Reusable Space Shuttle ... Our Biggest Bargain In Out-Of-This-World Research." The era of cheap, routine spaceflight was about to begin.