'Dark comets' may be a much bigger threat to Earth than we thought, new study warns

Mysterious, nearly invisible objects known as "dark comets" may pose a bigger threat to Earth than scientists thought, new research suggests. 

These small, rapidly spinning objects wander near Earth, likely after migrating from more distant reaches of the solar system. They might be a source of water and other volatile elements — and also a potent source of danger.

Usually, comets are very distinct from asteroids. Comets come from the outermost region of the solar system, where temperatures are low enough to allow molecules like water to freeze. While comets typically have stable orbits, occasionally they can be disturbed by gravitational interactions with the giant planets, sending some of the icy rocks spiraling toward the inner solar system. When they do, the heat from the sun causes them to disintegrate — a process that also gives comets their signature tails.

Asteroids, on the other hand, typically live in the inner solar system, usually between Mars and Jupiter. They are much rockier than their cometary cousins and, therefore, can survive much longer in the glare of the sun. But they, too, occasionally tumble into unstable orbits that bring them dangerously close to Earth.

But there's a strange, third type of space rock that astronomers have only recently begun to identify: dark comets, which behave like both asteroids and comets. Now, in a paper accepted for publication in the journal Icarus, a team of astronomers has tried to identify the Mysterious origins of dark comets.

Dark comets are small — only tens of kilometers across. They show no visible outgassing or evaporation of volatile elements like water. But they don't move in perfect orbits, either. Instead, they show evidence for "nongravitational" acceleration, implying that there are some other forces capable of gently nudging their orbits. 

All small objects in the solar system, including asteroids, have some amount of nongravitational acceleration, but astronomers can usually identify the cause. For example, asteroids are unevenly heated by the sun, which causes a tiny-but-measurable shift in their orbits.