The Bizarre Case Of A Main Belt Comet
By Judith E Braffman-Miller
Some of the smaller denizens of our Solar System are playful little objects that refuse to be neatly categorized as one thing or another. Like small children with a wicked, teasing grin, some asteroids and comets seem to take a perverse pleasure in their obstinate game of hide and seek with astronomers who seek to understand them. Asteroids and comets are the relics left over from the ancient era of planet formation, that occurred in our Solar System 4.6 billion years ago, with the collapse of a relatively small, extremely dense blob (pre-solar nebula), that was tucked gently within the blanketing folds of a giant, cold, dark, and very beautiful molecular cloud. However, because some asteroids and comets possess characteristics of both of these separate groups of primordial objects, their mysterious origins have sometimes been somewhat elusive. In September 2017, astronomers using the Hubble Space Telescope (HST) announced that they had successfully determined the true identity of one such perplexing object when it photographed a duo of playful asteroids orbiting each other, while displaying a characteristic comet-like tail of dust. This binary asteroid, sometimes categorized as a main-belt comet, probably broke into two pieces as a result of its dangerously fast rotation.
Astronomers categorize minor Solar System objects according to where they are located and their physical composition. Comets are a fragile, loose collection of dust and ice that migrate towards the golden light and melting heat of our Sun from their distant, frozen home beyond the orbits of the quartet of gaseous giant planets inhabiting our Solar System’s outer limits. In contrast, asteroids are rocky or metallic bodies that are primarily relegated to what is called the Main Asteroid Belt between Mars and Jupiter. However, Mother Nature avoids such simple, tidy categorizations.The odd object, dubbed 2006 VW139/288P, a binary asteroid sporting a comet-like tail, is thought to have broken apart about 5,000 years ago.
NASA’s HST helped an international team of astronomers discover that this bizarre object, inhabiting the Main Asteroid Belt is, in fact, not one, but two asteroids with comet-like attributes. In addition to the tattle-tale tail, these attributes include a bright halo of material, called a coma–which is a cloud that normally envelopes a comet’s icy nucleus.
HST was used to image the strange asteroid, designated 300163 (2006 VW139), in September 2016. This was shortly before the asteroid made its closest approach to the Sun. HST’s clear images reveal the separate objects composing the duo. The two asteroids proved to be almost the same mass and size, orbiting each other at a close distance of 60 miles.
Games For A Masquerade Ball
Main Belt Comets (MBCs) are playful little bodies that dwell in the Main Asteroid Belt, and they are puzzling small worldlets that display both the physical characteristics of comets and the orbital characteristics of asteroids. Because they show the characteristics of two separate and distinct groups of Solar System inhabitants, their mysterious origins have proved to be difficult for astronomers to determine. Some astronomers propose that the members of this strange family are really fragments left by larger Main Belt Asteroids when they destroyed themselves in violent and catastrophic collisions with one another.
Both comets and asteroids are really the lingering relics of an immense population of dancing primordial bodies that were born when our Solar System was very young. These frozen visitors from far away are tiny icy bodies that travel through our Solar System in long elliptical orbits. They are showy little denizens of our Sun’s family, and are famous for their thrashing, sparkling tails that stream out behind them whenever they manage to journey too close to our fiery, searing-hot, roiling Star.
When our Solar System was first emerging out of its collapsing natal cloud of gas and dust, most of the small, dense blob congealed at the center of this pre-solar nebula, and ultimately ignited as a result of the process of nuclear fusion–giving birth to our Sun. What was left of the lingering mass flattened out into what is called a protoplanetary accretion disk out of which the major planets, moons, comets, and asteroids eventually formed.
Protoplanetary accretion disks form at about the same time the baby star is born, and they serve the important function of feeding nourishing gas and dust to the hungry new stellar object. In the beginning, the accretion disk is both extremely massive and searing-hot, and it can hang around a young star for as long as 10 million years.
By the time a young Sun-like star reaches what is called the T Tauri stage of its existence, the nourishing accretion disk has cooled off and thinned out. A T Tauri is a stellar toddler–a very active variable star that is less than 10 million years old, which makes it very young in star-years. T Tauris sport diameters that are several times larger than that of our Sun today, but they are still in the process of shrinking. Unlike human babies, T Tauris shrink as they grow older. By the time the T Tauri tot has reached this stage of its young life, less volatile materials have started to condense close to the center of the protoplanetary accretion disk, and these create very sticky motes of dust that are so tiny that they look more like smoke than the dust that we sweep off our tables on Earth. These very fine dust particles also contain crystalline silicates.
The tiny motes of smoke-like dust bump into one another and form progressively larger and larger objects within the dense disk. The dust motes form objects that can grow to be several centimeters in size, and these go on to aggregate further to form planetesimals. Planetesimals are the building blocks of planets, and they can attain sizes of 1 kilometer across–or even larger. Planetesimals were extremely abundant within the primordial accretion disk. Indeed, some of the planetesimals managed to survive long enough to remain as relics billions of years after the formation of our Solar System–and this is also apparently true for alien stellar systems beyond our Sun. Asteroids are left-over rocky and metallic planetesimals, while comets are what is left of the vast primordial population of icy planetesimals. The asteroids are similar to the planetesimals that built up the quartet of relatively small, solid planets inhabiting the inner Solar System: Mercury, Venus, Earth, and Mars. Comets, in contrast, are the relic building blocks of the four giant, gaseous planetary denizens of the outer Solar System: Jupiter, Saturn, Uranus, and Neptune.
Comets soar into the inner Solar System from three different reservoirs of cometary nuclei: the Kuiper Belt, Scattered Disc, and Oort Cloud. Every time a migrating comet is sent on its dangerous journey into the inner Solar System–usually by gravitational perturbations caused by either passing stars or other sibling comet nuclei—it loses some of its mass as a result of sublimation of its surface ices to gas. The icy surface of the wandering comet creates a cloud (coma) as it comes ever closer and closer to our Sun’s melting warmth and blazing fires. Radiation pouring out from our Star pushes tiny grains of the comet’s dust away from the coma, and this creates the fabulous dusty, sparkling tail that comets are so famous for.
Rocky and metallic asteroids inhabiting the Main Asteroid Belt are not supposed to show this sort of typical comet-like behavior–an asteroid is not supposed to form a coma or a bright tail. However, what sometimes occurs in Nature has taught scientists to expect the unexpected. The newly recognized class of Main-Belt Comets display both a coma and tail, but live with the rocky and metallic asteroids in the Belt between Mars and Jupiter–rather than with their comet kin, in our Solar System’s distant deep freeze, far from our roiling, glaring Star.
Main Belt Asteroids are mysterious. They look like comets with their tails and comae, but they dwell in orbits inside that of the planet Jupiter–just like asteroids!
The Bizarre Case Of A Main-Belt Comet
The first member of this bizarre group of Main Belt Comets to be discovered was originally designated asteroid 1979 OW7, only to be renamed 1996 N2 when it was re-discovered, and then re-categorized as a comet, by Dr. Erik Elst and Dr. Guido Pizarro in 1996. It currently bears a comet designation as 133P Elst-Pizarro. For years this puzzling little object presented a problem. Originally, astronomers thought that it was the tragic outcome of a devastating collision between two asteroids. However, 133P developed a very comet-like tail as a result of sublimation. This occurred on three separate perihelion passages and, as a result, the enigmatic object looked just like a real comet. This, of course, muddied the issue of its identity. The behavior of 133P, at this time, suggested to puzzled astronomers that the asteroid impact model was unlikely.
Some astronomers are currently proposing that Main Belt Comets are, indeed, the shattered relics left by larger bodies that blasted into each other during recent catastrophic collisions. However, these fragments retained their progenitor asteroidal ices, which caused them to develop a comet-like appearance.
Asteroid 300163 (2006 VW139) was discovered by Spacewatch in November 2006 and the possible comet-like activity was first detected in November 2011 by Pan-STARRS. Both Spacewatch and Pan-STARRS are asteroid survey projects of NASA’s Near Earth Object Observations Program. After the Pan-STARRS observations it was also bestowed with the comet designation of 288P. This makes this puzzling small denizen of our Solar System the first known binary asteroid that is also simultaneously classified as a Main Belt Comet.
More recent HST observations found ongoing activity in this binary asteroid system. “We detected strong indications for the sublimation of water ice due to the increased solar heating–similar to how the tail of a comet is created,” explained team leader Dr. Jessica Agarwal in a September 20, 2017 Hubblesite Press Release. Dr. Agarwal is of the Max Planck Institute for Solar System Research, in Germany.
The combined attributes of this bizarre binary asteroid also make it one-of-a-kind among the few known binary asteriods that also have a wide separation. 288P is unique because, in addition to the two component asteroids possessing a wide separation, they are almost equal in size, and also display comet-like activity. Gaining a new understanding of the mysterious origin and evolution of this strange, small binary system may provide astronomers with valuable insights into the well-kept secrets of the ancient Solar System. Main Belt Comets may provide the answer to how water traveled to the primordial Earth billions of years ago.
The most likely scenario for 288P’s formation 5,000 years ago is the breakup of a single object as the result of a fast rotation. After, the object split into two chunks, they may have moved further apart as an effect of ice sublimation, which would provide a small nudge to an asteroid in one direction as water molecules are sprayed in the other direction.
But the fact that 288P is so mysteriously different from all the other known binary asteroids raises still more questions about how common such systems are in the Main Asteroid Belt. “We need more theoretical and observational work, as well as more objects similar to this object, to find an answer to this question,” Dr. Agarwal noted in the September 20, 2017 Hubblesite Press Release.
The paper describing this research is published in the September 21, 2017 issue of the journal Nature.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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