She liked to make the planets stare, and wished no better mirth
Than just to see the telescopes aimed to her from the Earth–“A Naughty Little Comet,” Ella Wheeler Wilcox
There are certain small objects that frolic around playfully in our Sun’s family that obstinately refuse to be easily categorized. Main Belt Comets (MBCs) are such objects! They are a recently recognized new group, twirling around in our Solar System within the Main Asteroid Belt, that lies between the orbits of Mars and Jupiter. MBCs are strange and perplexing little worldlets that exhibit both the physical characteristics of comets and the orbital characteristics of asteroids. Because they possess characteristics of two distinct groups of Solar System denizens, their origins have proven somewhat difficult to determine. Some astronomers now suggest that the members of this weird family are the shattered fragments of larger Main Belt asteroids that were destroyed in catastrophic smash-ups with other asteroids.
Comets are small, icy bodies that make a long, treacherous journey through the Solar System in long elliptical orbits. They are famous for their glowing, thrashing tails that stream out whenever they travel sufficiently close to the Sun. This is because the heat and solar winds join forces to melt the surface of their frozen cores.
Comets and asteroids are the leftovers of a myriad of tumbling bodies that existed when our Solar System was very young. Our Solar System was born about 4.6 billion years ago with the collapse of a relatively small dense pocket embedded within a gigantic dark, cold molecular cloud. The lion’s share of the collapsing pocket congealed at the center, and eventually caught fire due to nuclear fusion, giving birth to our Star, the Sun. The remaining mass flattened out and became what is termed a protoplanetary accretion disk from which the eight major planets, their numerous bewitching moons, comets, asteroids, and other small Solar System objects, formed.
Protoplanetary accretion disks have been observed circling numerous stars dwelling in youthful stellar clusters. They develop at the same time a baby star is born, but the earliest stages cannot be seen because they are veiled by an opaque envelope of dust and gas. The accretion disk feeds material to the hungry baby star, or protostar, and it is both very hot and extremely massive. These disks can hang around their youthful stars for about 10 million years.
By the time the young star reaches what is termed the T Tauri stage, the nourishing disk has grown both more slender and much cooler. A T Tauri star is a very young bouncing baby of an energetic variable star that is less than 10 million years old. Such stars possess a mass that is similar to, or perhaps a bit less, than that of our Sun–which is a relatively small star. T Tauri stars have diameters that are several times greater than that of our Sun, but they are still in the process of shrinking. Baby Sun-like stars shrink as they grow older. By the time the young star has reached this stage, less volatile materials have begun to condense near the center of the accretion disk, forming very, very sticky dust grains that are so fine that they resemble smoke, rather than the household dust that must be swept away. These sticky, smoke-like grains of very fine dust contain crystalline silicates.
The very fine particles of smoke-like dust stick together to form ever larger and larger objects within the dense disk environment in which they dwell. The little dust grains form objects up to several centimeters in size, and these further aggregate to give rise to planetesimals. Planetesimals are the building blocks of planets,and they can reach sizes of 1 kilometer across–or larger. Planetesimals are extremely abundant, and spread throughout the protoplanetary accretion disk–and some survive long enough to remain as relics billions of years after the formation of the mature planetary system. Asteroids, such as those inhabiting are own Solar System, are left-over rocky planetesimals. Comets, on the other hand, are relic icy planetesimals from the outer limits of our Solar System. The asteroids are the remnant building blocks of the rocky inner planets–Mercury, Venus, our Earth, and Mars. The comets are the leftover building blocks of the outer gas- and ice-giant planets–Jupiter, Saturn, Uranus, and Neptune.
The asteroids are primarily found in the Main Asteroid Belt, although there are various, differing populations of these rocky objects that reside outside of this Belt, as well. Comets are fragile and ephemeral bodies, sometimes dismissively called “dirty snowballs” or “icy mudballs”, depending on the observer’s point of view. They are strange, beautiful travelers from afar.
The icy, dirty comets streak into Earth’s inner, warm region of the Solar System from two dark and frozen domains in the outer limits. The first is called the Kuiper Belt. The Kuiper Belt twirls around our Star beyond the orbit of Neptune–the furthest of the eight major planets from the Sun. The second domain of comets is the Oort Cloud, which is an enormous sphere of icy objects that is thought to encircle our entire Solar System.
Every time a wandering comet makes its dangerous journey into the inner Solar System, it loses a bit of its mass by way of sublimation of its surface ices and gas–the icy surface of the nucleus converts to gas, and creates a cloud termed a coma. Solar radiation forces dust particles away from the coma, and this forms the famous dusty thrashing, flashing tails of comets.
Rocky, metallic asteroids in the Main Belt normally do not exhibit this sort of comet-like behavior. Asteroids in the Main Belt are not supposed to develop comae or flashing, thrashing tails. Yet, the newly recognized class of Main Belt Comets exhibit precisely this type of behavior, while orbiting the Sun in a way that is characteristic of asteroids–not comets!
Naughty Little Comets
MBCs are extremely perplexing. They look like comets with their tails and comae, but they twirl around in orbits inside that of the planet Jupiter–just like asteroids!
The first of this puzzling group to be discovered was asteroid 1979 OW7, which was re-discovered and re-named 1996 N2, and then designated a comet by Dr. Erik Elst and Dr. Guido Pizarro in 1996. It now bears a cometary designation as 133P/Elst-Pizarro. For many years, determining the nature of this enigmatic little object was a problem. It was originally conceived to be the tragic result of a collision between two asteroids. However, 133P sublimated and developed a tail just like a comet on three successive perihelion passages, and this muddied the issue. The perihelion of an object refers to that point in its orbit when it is closest to the Sun–its aphelion is when it is furthest away. This behavior, at the time, indicated to bewildered astronomers that the asteroid impact model was unlikely.
Some of the currently known MBCs, in addition to 133P, are 176P/LINEAR, 238P/Read, P2008 R1 (Garradd), P/2010 R2 (La Sagra), P/2010 A2 (LINEAR), 596 Scheila, 300163, and P/2012 F5 (Gibbs). Some astronomers are now suggesting that these objects are, indeed, the remnants of larger bodies smashed to pieces in recent collisions, and that they inherited their parent asteroid’s ices, causing them to exhibit comet-like behaviorl To add credibility to this theory, however, it was necessary to first determine if the MBCs are members of a distinct family of asteroids–groups of asteroids bearing similar orbital characteristics, such as orbital inclination and eccentricity. The members of a particular asteroid-family are all believed to be chunks blasted off a larger asteroid that was pulverized in a collision.
Serbian astronomer Dr. Bojan Novakovic, from the University of Belgrade’s Department of Astronomy, is currently investigating the mysterious origins of MBCs, along with colleagues Dr. Henry Hsieh and Dr. Alberto Cellino. The team of astronomers focused on one primary example–the P/2006 VW 139 MBC, and their goal was to see if it could be assigned to a family of asteroids. P/2006 VW 139 was originally classified as an asteroid, but was ultimately re-classified as an MBC in 2011, when astronomers in Hawaii discovered that it had a tail.
Dr. Novakovic began his study at the AstDys database, which is a catalogue of 398,841 asteroids. There are less than a dozen verified examples of MBCs, and so finding other members of this family of asteroids among hundreds of thousands of potential candidates was not exactly easy work. The team of astronomers used what is termed the hierarchical clustering method and the cut-off distance parameter to weed their way through the candidates. They successfully narrowed down the possibilities to a mere 24 asteroids. The next step was to determine the orbital characteristics possessed by these 24 asteroids. For this task, Dr. Novakovic used a mathematical model called backward integration, “which can be used to distinguish real family members from interlopers,” he commented in the November 7, 2012 International Science Grid This Week (isgtw).
In order to investigate the orbital characteristics of candidate family members, Dr. Novakovic needed to sift through a large number of calculations that were “computationally expensive and would take months to be done on a typical PC,” he added. Ultimately, Dr. Novakovic and his colleagues were able to reduce the P/2006 VW139 family down to just 11 members.
The most valuable conclusion of Dr. Novakovic’s study, that was published in the Monthly Notices of the Royal Astronomical Society, is that P/2006 VW139 and its 11 siblings are part of a youthful asteroid family that probably was born about 7.5 million years ago. “[Asteroid] families are considered young if they are less than about 10 million years old,” Dr. Novakovic continued to explain. This indicates that about 7.5 million years ago, a large asteroid smashed into another asteroid and broke up into 11 smaller chunks–P/2006 VW139 and its 10 sisters.
P/2006 VW139 became a MBC because it inherited the deeply buried ice belonging to its catastrophically pulverized parent asteroid. This ice ultimately vaporized, producing a thrashing tail that is characteristic of comets. “The fact that ice still exists close to the surface of P/2006 VW 139 means that its surface is relatively young,” Dr. Novakovic noted in the November 7, 2012 isgtw. This means that P/2006 VW 139 and its 10 sisters were born long after the Main Asteroid Belt.
This finding is important because it supports the theory that MBCs are born in asteroid smash-ups within the Main Asteroid Belt, and not through some other means.