Comets that take more than 200 years to make one revolution around the Sun are notoriously difficult to study. Because they spend most of their time far from our area of the solar system, many "long-period comets" will never approach the Sun in a person's lifetime. In fact, those that travel inward from the Oort Cloud -- a group of icy bodies beginning roughly 186 billion miles (300 billion kilometers) 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 these distant wanderers.
Scientists found that there are about seven times more long-period
comets measuring at least 0.6 miles (1 kilometer) 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 findings are published in the Astronomical Journal.
"The number of comets speaks to the amount of material left over from
the solar system's formation," said James Bauer, lead author of the
study and now 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."
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, study co-author based at NASA's Jet Propulsion
Laboratory, Pasadena, California, and principal investigator of the
NEOWISE mission. "They are the most pristine examples of what the solar
system was like when it formed."
Astronomers already had broader estimates of how many long-period and
Jupiter family comets are in our solar system, but had no good way of
measuring the sizes of long-period comets. That is because a comet has a
"coma," a cloud of gas and dust that appears hazy in images and
obscures the cometary nucleus. But by using the WISE data showing the
infrared glow of this coma, scientists were able to "subtract" the coma
from the overall comet and estimate the nucleus sizes of these comets.
The data came from 2010 WISE observations of 95 Jupiter family comets
and 56 long-period comets.
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.
"Our results mean there's an evolutionary difference between Jupiter family and long-period comets," Bauer said.
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.
Researchers also found clustering in the orbits of the long-period
comets they studied, suggesting there could have been larger bodies that
broke apart to form these groups.
The results will be important for assessing the likelihood of comets impacting our solar system's planets, including Earth.
"Comets travel much faster than asteroids, and some of them are very
big," Mainzer said. "Studies like this will help us define what kind of
hazard long-period comets may pose."
NASA's Jet Propulsion Laboratory in Pasadena, California, managed and
operated WISE for NASA's Science Mission Directorate in Washington. The
NEOWISE project is funded by the Near Earth Object Observation Program,
now part of NASA's Planetary Defense Coordination Office. The
spacecraft was put into hibernation mode in 2011 after twice scanned the
entire sky, thereby completing its main objectives. In September 2013,
WISE was reactivated, renamed NEOWISE and assigned a new mission to
assist NASA's efforts to identify potentially hazardous near-Earth
objects.
For more information on WISE, visit: https://www.nasa.gov/wise
News Media Contact
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov
Source: JPL-Caltech/News
