“What keeps me up at night is the asteroids we don’t know about," said NASA's planetary defense chief.

In the whole history of Earth's climate, few events are as extreme as those that geologists call "Snowball Earth." ...

"We don’t know where they are,” said NASA’s planetary defense officer Kelly Fast at the AAAS conference in Arizona this week.

Key climate tipping points may be closer than expected, raising the risk of accelerating warming, sea-level rise, and ...