The best-known scientific instrument in history was dying. After nearly 20 years in space and hundreds of thousands of spectacular images, the Hubble Space Telescope's gyroscopes and sensors were failing, its batteries running down, and some of its instruments were already dead. The only hope to save Hubble was a mission so dangerous that in 2004 NASA cancelled it because it was considered too risky.

The episode was rated 7.20 from 10 votes.

Orange-and-black wings fill the sky as NOVA charts one of nature's most remarkable phenomena: the epic migration of monarch butterflies across North America. To capture a butterfly's point of view, NOVA’s filmmakers used a helicopter, ultralight, and hot-air balloon for aerial views along the transcontinental route. This wondrous annual migration, which scientists are just beginning to fathom, is an endangered phenomenon that could dwindle to insignificance if the giant firs that the butterflies cling to during the winter disappear.

The episode was rated 7.33 from 12 votes.

What killed the mammoths? Near the end of the last ice age 13,000 years ago, these mighty beasts disappeared from North America together with some 35 other groups of mammals, including giant ground sloths, saber-tooth cats, and American lions. For four decades, debate has raged over the cause of their abrupt demise. In Last Extinction, NOVA presents an exclusive in-depth investigation of a bold and controversial new hypothesis, which suggests that a massive impact from space could be the culprit. According to the theory, an incoming comet broke up over North America in a devastating series of explosions, causing widespread forest fires and wiping out plants, animals, and prehistoric people. The evidence comes from a mysterious black mat layer discovered at more that fifty sites across the continent. The black mat contains exotic materials including rare microscopic nanodiamonds that are claimed to be the signature of an extraterrestrial impact. But other scholars question the evidence and argue that ancient hunters armed with a lethal stone weapon the Clovis spear point drove the giant beasts into oblivion. Still others believe the unstable climate at the end of the Ice Age was responsible. NOVA explores the dramatic new evidence of a cosmic explosion and the possibility that all three explanations may be correct.

The episode was rated 7.92 from 13 votes.

Once every 48 years, bamboo forests in parts of northeast India go into exuberant flower. Then, like clockwork, the flowering is invariably followed by a plague of black rats that appear to spring from nowhere to spread destruction and famine in their wake. For the first time on film, NOVA and National Geographic capture this rat population explosion in vivid detail and show how scientists are unraveling the connections between bamboo flowering and rat outbreaks. Ultimately, their research should help local people better cope with the next attack—due in 2056.

The episode was rated 8.00 from 19 votes.

Can California's ambitious plan to cut greenhouse gases actually succeed?

The episode was rated 8.00 from 11 votes.