Large, distant comets more common than previously thought

Large, distant comets more common than previously thought:

An artist’s rendering of the NASA’s WISE mission, renamed NEOWISE in 2013, observing comets and other deep space objects. Image Credit: NASA

Data from NASA’s Wide-field Infrared Survey Explorer (WISE) mission has shown that large, distant comets are more common than previously thought. This is according to research published in the Astronomical Journal. These “long-period” comets originate from the distant Oort Cloud, and the information provided by the NASA’s spacecraft is contributing to a better understanding of how common these icy worldlets might be.

While most people are likely familiar with icy objects such famous comets as Halley and Shoemaker-Levy 9, the latter of which broke up and impacted the gas giant Jupiter in July 1994. These, along with nearly all of those most of us have heard about (or seen) are from the family of “short-period” comets. Short-period refers to the length and distance of the period, or the time it takes to make one full orbit, of the object.

Short-period comets take less than 200 years to make a full orbit around the Sun. These are generally separated into two families: Jupiter family and inclined-period comets. Jupiter family comets, of which Shoemaker-Levy 9 was one, have orbital periods of less than 20 years. Inclined-period comets, like Halley’s Comet, have orbital periods between 20 and 200 years in length.

This illustration shows how scientists used data from NASA’s WISE spacecraft to determine the nucleus sizes of comets. They subtracted a model of how dust and gas behave in comets in order to obtain the core size. Image and Caption Credit: NASA / JPL-Caltech

A short-period comet tends to orbit within the ecliptic – the plane of space where the planets orbit around the Sun. This is likely due to where they originate from, which is suspected to be the Kuiper Belt – the icy band of objects at the edge of the Solar System where Pluto, the majority of dwarf planets, and about a thousand other Kuiper Belt Objects (KBOs) roam. The Kuiper Belt exists at a distance of some 2.7 billion to 5.1 billion miles (4.4 billion to 8.2 billion kilometers).

Unlike short-period comets, long-period comets originate from much further away in the Oort Cloud, an area of the Solar System believed to be a vast a spherical bubble of icy material thought to extend approximately 186 billion miles (300 billion kilometers) out to as far as 4.45 trillion miles (7.5 trillion kilometers). Objects originating from this area have periods greater than 200 years, with some taking thousand or even millions of years to make a single orbit.

In the paper published about long-period comets, researchers looked at data from the WISE mission that did a full sky survey from 2009 to 2011. Data from an eight-month span of time was reviewed and a total of 95 Jupiter family comets along with 56 long-period comets were identified.

“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 in 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.”

The study also found that there were seven times more long-period comets measuring at least 0.6 miles (1.0 kilometer) across than previously predicted, with the average width measuring 1.3 miles (2.1 kilometers), about twice as large as the average diameter of Jupiter family and inclined-period comets. Additionally, over that eight month period, the number of long-period comets that passed by the Sun was 3‒5 times more than previously anticipated.

The suspected reasons for the differences in the size between Jupiter family comets and long-period comets are believed to be due to two main possibilities; the first being that because Jupiter family comets make far more frequent trips nearer to the Sun, they are subjected to more sublimation (ice changing directly to a gas) and thus loss of total mass.

Another possible cause for the size difference is due to evolutionary differences. Because the Oort Cloud is so large, and the objects within it are so widely distributed, the likelihood of objects impacting one another is reduced, giving bodies in this area a better chance of keeping their large sizes rather than suffering impacts that could break them down.

When scientists reviewed the movement of these bodies, they found that there was an inclination (the angle to the ecliptic plane that the planets are aligned on) clustering at 110 degrees with an average perihelion (closest approach to the Sun in its elliptical orbit) of 2.9 astronomical units (270 million miles / 434 million kilometers), putting their closest approach to the Sun at just past the orbit of the dwarf planet Ceres in the main asteroid belt. This could indicate that there were larger bodies that broke up over time leaving behind these icy objects.

As if being big and coming at us from all different angles wasn’t bad enough, comets are fast – really fast.

“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 managed and operated WISE for NASA’s Science Mission Directorate located 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.

Video courtesy of NASA / Jet Propulsion Laboratory

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