Paging Roland Emmerich: The biggest threat to our little blue planet may turn out to be something that we don't see coming... literally. "Dark comets" are invisible to detection... and that may be the problem.
Ignore any Marvel Comics influence inferred from the melodramatic name; "dark comets" are so-called because they have avoided detection from our puny human technology only, according to New Scientist:
Hazardous comets and asteroids are monitored by various space agencies under an umbrella effort known as Spaceguard. The vast majority of objects found so far are rocky asteroids. Yet UK-based astronomers Bill Napier at Cardiff University and David Asher at Armagh Observatory in Northern Ireland claim that many comets could be going undetected. "There is a case to be made that dark, dormant comets are a significant but largely unseen hazard," says Napier... In 1983, Comet IRAS-Araki-Alcock passed by Earth at a distance of 5 million kilometres, the closest known pass by any known comet for 200 years. It was spotted only two weeks ahead of its closest approach. "It had only 1 per cent of its surface active," says Napier. Comet Borrelly, visited by NASA's Deep Space 1 probe in 2001, was found to have extremely dark patches over much of its surface.
Choosing to look on the bright side of this, there's something comforting in knowing that we're almost guaranteed death from above, whether it's comets we can't see or various pieces of space debris falling on top of us.
Image from NASA's Deep Space 1.
'Dark' comets may pose threat to Earth [New Scientist]