Large, distant comets more common than thought: NASA

Long period comets - that take more than 200 years to make one revolution around the Sun - may be up to seven times more common than previously thought, NASA scientists say.
Since long-period comets spend most of their time far from our area of the solar system, many will never approach the Sun in a person's lifetime.
Those that travel inward from the Oort Cloud - a group of icy bodies beginning roughly 300 billion kilometres away from the Sun - can have periods of thousands or even millions of years.
NASA's WISE spacecraft, scanning the entire sky at infrared wavelengths, has delivered new insights about the distant wanderers that are notoriously difficult to study.

Scientists found that there are about seven times more long-period comets measuring at least one kilometre across than had been predicted previously.
They also found that long-period comets are on average up to twice as large as "Jupiter family comets," whose orbits are shaped by Jupiter's gravity and have periods of less than 20 years.

Researchers also observed that in eight months, three to five times as many long-period comets passed by the Sun than had been predicted.
"The number of comets speaks to the amount of material left over from the solar system's formation," said James Bauer, a research professor at the University of Maryland, College Park.

"We now know that there are more relatively large chunks of ancient material coming from the Oort Cloud than we thought," said Park, lead author of the study published in the Astronomical Journal.
The Oort Cloud is too distant to be seen by current telescopes, but is thought to be a spherical distribution of small icy bodies at the outermost edge of the solar system.
The density of comets within it is low, so the odds of comets colliding within it are rare.
Long-period comets that WISE observed probably got kicked out of the Oort Cloud millions of years ago. The observations were carried out during the spacecraft's primary mission before it was renamed NEOWISE and reactivated to target near- Earth objects (NEOs).

"Our study is a rare look at objects perturbed out of the Oort Cloud," said Amy Mainzer, from NASA's Jet Propulsion Laboratory in the US.
"They are the most pristine examples of what the solar system was like when it formed," said Mainzer, principal investigator of the NEOWISE mission.
The results reinforce the idea that comets that pass by the Sun more often tend to be smaller than those spending much more time away from the Sun.
That is because Jupiter family comets get more heat exposure, which causes volatile substances like water to sublimate and drag away other material from the comet's surface as well.

The existence of so many more long-period comets than predicted suggests that more of them have likely impacted planets, delivering icy materials from the outer reaches of the solar system.
The results will be important for assessing the likelihood of comets impacting our solar system's planets, including Earth.