Philip Grossman finds firefighting gear under a hospital in Chernobyl, sending his Geiger counter climbing. He believes burying the gear was the Soviet’s attempt to hide the scope of the disaster.

Philip Grossman goes inside the plant near the massive radio tower he finds maps of the U.S and pictures of mushroom clouds. Was the Duga for protecting Chernobyl from incoming missiles or part of a nuclear offensive?

Survivors who worked at nearby factory report production areas they weren’t allowed to visit. Philip visits and finds the basement full of lab after lab of gas masks, radioactive testing materials and sand.

Since the 1960s, NASA has been keeping an eye on man-made objects in space. Here, engineers are replicating the devastating impact of space debris. 'Alien Encounters' airs Tuesdays at 10/9c.

Author Nick Sagan hasn't seen his godfather, Frank Drake, in over 30 years. We capture their reunion - and find out how Drake became friends with Nick's famous father, Carl Sagan, in this exclusive clip.

What would receiving a message from an alien civilization tell us about human evolution, and about surviving the most serious problems facing our planet?

Since launching in 2009, the Kepler program has detected hundreds of previously unknown planets. How many of them are Earth-like, and what does that mean for the probability of intelligent life beyond our solar system?

Will ETs look like us? Will they resemble giant insects? Scientists debate how various environments could have created life forms beyond our wildest imagination.

If the chirality of aliens is different than ours, it couldn't interact with us, and therefore would be non threatening.

An alien spacecraft might not rely solely on sunlight to power its trajectory. Find out what other energy technology aliens could use to travel through the universe.

It's possible that life on earth began off-planet and spread here.

It's likely that an advanced civilization would combine the advantages of a machine with the advantages of biological systems.

It's 1967, the Cold War is on and there's a glowing orange light hovering over a U.S. Air Force base.

John Smith and his cousin Billy have a mysterious encounter while driving on a remote highway in the Yukon. Was it a UFO?

Reports of a downed aircraft bring a volunteer firefighter to a remote wooded area.

The Hale - Bopp comet was overhead but a strange formation of lights in the sky didn't appear to be natural.