Adam Hadhazy

Far out beyond the reach of human exploration sits the mysterious Oort Cloud.

The Kuiper Belt sits at the edge of our solar system, and as such holds a number of unsolved mysteries.

Star Trek turns 45 this week. Why does it still get fans so excited?

The Apollo program was cut a few missions short. If it had continued, though, we can't help but wonder if we'd have a base on the moon today.

Everything we know about Pluto can be scrawled on a few 3x5-inch index cards, so mysteries abound about the dwarf planet.

Neptune is nearly 3 billion miles away, which makes it pretty difficult to study, but it's packed full of mysteries.

The only sideways planet in the solar system holds several mysteries.

Saturn is one of the coolest planets in our solar system, so it's no wonder that its moons are equally compelling

Saturn's rings are the coolest thing in the solar system, but no one knows how they got there.

Europa, Io, Callisto and Ganymede harbor some of the most mysterious environments in the solar system, from surprise magnetospheres to oozing molten cores.

Jupiter is the largest planet in our solar system, and so it harbors some equally giant mysteries.

One of the most mysterious planets in our solar system is the one that never actually formed.

Whether there's life on the Red Planet is just one of the Mars biggest secrets.

Scientists still aren't sure how the moon came to be, but they believe it is one of the keys to life evolving on Earth.

Venus is our closest neighbor, but we know very little about how it became the hellish planet it is today.

Each week we investigate lingering questions about cosmic objects. Now batting: Mercury.

What if dinosaurs and Neanderthals had not gone extinct? What if the sun was twice as large? How 10 key moments in history could change the present.

What if dinosaurs and Neanderthals had not gone extinct? What if the sun was twice as large? How 10 key moments in history could change the present.

What if dinosaurs and Neanderthals had not gone extinct? What if the sun was half as large? How 10 key moments in history could change the present.

If doomsday happens tomorrow, will humans be able to survive in space?

What if dinosaurs had not gone extinct? What if the Earth was twice as large? How 10 key moments in history could change the present.

Judgment Day comes on May 21. Here's the gear for getting through the end times.