Asteroids come in all shapes and sizes--and hit our planet in them, too. But what happens if a catastrophically large one approaches earth? By looking on the ground at historical asteroid craters and present-day falls, and up into space for the big ones yet to come, a wide variety of scientists are trying to figure out how to track asteroids--and how to avoid devastating impacts in the future.

"Today, an ancient world is vanishing right before our eyes: the age of giant animals. Over 40,000 years ago, the earth was ruled by megafauna: mammoths and mastodons, saber-toothed tigers and giant sloths. Of course, those creatures no longer exist, and there is only one likely reason for that: the evolution and arrival of the earth's only tool-wielding hunter, the wildly adaptive, comparatively pint-sized human species. Many more of the world's...

When Dominique, Eric, Josephine, and Ajay return to the Smithsonian they see that modern animals are extinct and dinosaurs have been brought back to life, so they time travel to the 1876 Philadelphia Exhibition to stop this wicked plot.