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The asterisk (er, asteriod) on comets.

LIKE MASKED entertainers at a 17th-century ball, the true identity of certain bewitching objects that perform a weird, mysterious pageant upon our solar system's festive stage cannot be determined easily--until their masks have fallen off.

Comets and asteroids are small remnant bodies left over from our primordial solar system, which was born about 4,456,000,000 years ago. Although they represent two distinct populations of relatively small objects that dance around the sun, certain members of the two groups have masked their identifies very well and cannot easily be categorized as either one or the other.

Comets are small, icy, dusty bodies that travel a long and treacherous path in long elliptical orbits. They are famous for their flashing, thrashing tails that stream out whenever their orbits take them sufficiently close to the sun. These alien visitors from the outer limits streak into Earth's inner, warm region from two remote, dark, and frozen realms. The first is the Kuiper Belt, which circles around the sun beyond the orbit of Neptune. The second is the Oort cloud, an enormous sphere of icy objects thought to encircle our entire solar system. Comets are fragile and ephemeral objects, sometimes dismissively referred to as "dirty snowballs" or "icy mudballs," depending on the observer's point of view.

Every time a wandering comet makes its hazardous, long journey into the warm inner solar system, it loses some of its mass by way of sublimation of its surface ices and gas--the frozen surface of the nucleus turns to gas and then forms a cloud called a coma. Radiation emanating from the sun pushes grains of dust away from the coma, and this creates the famous bright, dusty, flashing tails that comets are so famous for.

Asteroids, meanwhile, primarily are found in the Main Asteroid Belt between Mars and Jupiter, although there are other large populations of these rocky objects that dwell elsewhere. Rocky, metallic asteroids traveling in the Main Belt are not supposed to exhibit comet-like behavior. Asteroids should not develop comae or flashing, thrashing tails, but some do.

Comets and asteroids are planetesimals: the leftover remains of a vast population of primordial objects that were the building blocks of the major planets of the solar system, which came into being when a relatively small dense blob--embedded within a gigantic, frigid, cold molecular cloud---collapsed under its own weight. Most of the material that composed the collapsing, dense pocket congealed at the center, and eventually caught fire due to the process of nuclear fusion, giving birth to our star, or sun. The remaining mass flattened out into a pancake-like protoplanetary accretion disk from which the eight major planets and their numerous moons, comets, asteroids, and other small solar system bodies emerged.

Protoplanetary accretion disks have been spotted circling a number of stars inhabiting youthful stellar clusters. They form at about the same time that a bright new star is born, and they proceed to feed a rich formula composed of gas and dust to the hungry baby star, or protostar. The accretion disk is searingly hot and quite massive, and can linger around a young star for 10,000,000 years or so.

By the time the bouncing baby of a star reaches what is called the T Tauri stage of its young life, the nurturing disk of gas and dust has become cooler and thinner. T Tauri stars are extremely active young variable stars of the tender age of 10,000,000 years or less. By the time the stellar toddler has reached this phase, less volatile materials have started to condense near the center of the surrounding protoplanetary accretion disk, and these form very sticky, fine grains of dust.

The dust grains "glue" themselves to one another, creating ever larger and larger bodies. The tiny, sticky dust grains ultimately form bodies up to several centimeters in size--and these further aggregate together to create the planetesimals. Planetesimals were extremely abundant in our primordial solar system--and some of them have survived long enough to tell the long-lost story of this vast population of primordial objects. Asteroids are the relic building blocks of the rocky, terrestrial inner planets--Mercury, Venus, our Earth, and Mars. Comets, on the other hand, are the tattletale relics of the building blocks of the outer gas and ice giant planets: Jupiter, Saturn, Uranus, and Neptune.

However, line identifies can be well-hidden in this celestial masquerade. Masked players that still hang around to tell the story of our solar system's origins have kept some of their secrets very well. The "faces" that lurk behind their masks are not always the ones that astronomers expect to see.

Asteroids are not supposed to look like comets. They should not sport shimmering kite-like tails, nor should they develop glowing comae, but some do. There is a newly recognized class of little objects that exist within the Main Belt between Mars and Jupiter where only asteroids are supposed to frolic--and they look very much like comets. This new class, termed Main Belt comets (MBCs), have thrashing tails that glow and comae that glisten--and, for many, they have been extremely difficult objects for astronomers to unmask.

The very first member of this enigmatic group to be spotted was "asteroid" 1979 OW7 (Comet 133P/Elst-Pizarro). This denizen of the Main Asteroid Belt was "rediscovered" and renamed in 1996 as 1996 N2, and then designated a comet by Erik Elst and Guido Pizzaro. Comet 133p/Elst-Pizarro is considered to be the prototype of MBCs. For years, determining the true identity of this bizarre, flashy little worldlet presented a problem for astronomers. It originally was thought to be the result of a crash between two Main Belt asteroids. However, 133P showed the comet-like hallmarks of sublimation, and it formed a brilliant, thrashing tail on a trio of successive perihelion orbits. This behavior made it look just like a real comet-which it is.

However, many astronomers maintain that some members of the MBC population are, indeed, the shattered remains of larger asteroids that were pulverized in recent smashups, and that they inherited their unlucky progenitors' ices, causing them to behave like comets.

Take the case of P/2010 A2 (LINEAR), for instance. Discovered on Jan. 6, 2010, this small solar system occupant displays traits of both asteroids and comets, and initially was designated a comet. However, its orbit resides within the Main Asteroid Belt, where no self-respecting comet has a right to be--yet it was seen to sport a tail that appeared to be very comet-like to observers. P/2010 A2 (LINEAR) was designated an MBC but, within a month of being discovered, Hubble Space Telescope images suggested that its tail was generated by dusty gravel-not icy comet-stuff--and that it was the result of a head-on smashup between Main Belt asteroids. Therefore, the object's tail was not the result of sublimation of cometary ices. Early studies suggested that the asteroid became active in late March 2009, and peaked about three months later--and then stopped its activity in early December.

Early Hubble observations and the narrow angle camera aboard the Rosetta spacecraft suggested that the comet-like tail was, in fact, a dust trail that likely resulted when a meter-sized object crashed onto the larger asteroid in February or March 2009. P/2010 A2's orbit is consistent with those of the Flora asteroid family. It is considered likely that a Flora family asteroid excavated the famous Chicxulub crater in the Yucatan Peninsula about 65,000,000 years ago ---believed to be a culprit in the extinction of the dinosaurs. The members of a particular asteroid family all are thought to be chunks blasted off of a larger asteroid that was shattered to pieces in a smashing collision.

Serbian astronomer Bojan Novakovic of the University of Belgrade's Department of Astronomy currently is trying to discover the "faces" behind the alluring "masks" of MBCs. Novakovic, along with his colleagues Henry Hsieh and Alberto Cellino, focused on one particular object--P/2006 VW 139 MBC, and their goal was to determine if it could be assigned to a specific family of asteroids.

Novakovic and his team determined that the family of P/2006 VW 139 was composed of 11 members, and that P/2006 VW 139 and its sisters were members of a very young asteroid family that probably emerged a "mere" 7,500,000 years ago. Asteroid families that are less than 10,000,000 years old are considered "young." This young family probably came into existence when a large asteroid crashed into another asteroid and shattered into smaller pieces.

P/2006 VW 139 was the beneficiary of its doomed parent asteroid's deeply buried ices. The ice eventually vaporized, and this created the flashing, thrashing tail that is so 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," Novakovic explained in the Nov. 7, 2012, issue of International Science Grid This Week.

Main Belt comets are far from alone in their game of celestial masquerade. For example, the Centaurs are small, dancing objects that move around the sun between the orbits of Jupiter and Neptune--and their true identity has proven very difficult for astronomers to determine. Are the Centaurs comets or asteroids? A study released in July suggests that they are comets. The study was based on observations derived from NASA's space-borne Wide-field Infrared Survey Explorer (WISE) telescope.

The Centaurs are named after mythical creatures that were part human and part horse, because they display the split personalities of both asteroids and comets. "Just like the mythical creatures, the Centaur objects seem to have a double life. Our data points to a cometary origin for most of the objects, suggesting that they are coming from deeper out in the solar system," explains James Bauer of NASA's Jet Propulsion Laboratory in Pasadena, Calif. He is the lead author of a study published in the July 22 issue of The Astrophysical Journal. According to this paper, the Centaurs are comets following a long, treacherous path towards the sun from the outer limits of the solar system--in marked contrast to asteroids that would be zipping away from our star in the opposite direction.

NEOWISE--the asteroid-hunting arm of the recently revived WISE space mission--was able to determine that the Centaurs were nonreflective and dark like charcoal, rather than brightly shining and highly reflective. Comets sport a dark, sooty coating on their icy surfaces, making them appear darker and less reflective than most asteroids. The study found that approximately two-thirds of the Centaurs are comets.

The study's findings are taken from the largest infrared survey to date of Centaurs and their distant cousins, the scattered disk objects. NEOWISE collected infrared images of 52 Centaurs and scattered disk objects. Fifteen of the 52 proved to be entirely new discoveries, circling the sun in an unstable belt. Ultimately, the gravity of the four outer giant planets tends to kick these small icy bodies out of their dark, frigid home, and into the warm inner solar system--or, alternatively, tosses them out even farther into the remote and frigid blackness far away from the sun. The results of the study suggest that all observed populations of small objects beyond the Main Asteroid Belt are dominated by objects with dark, sooty surfaces, and likely are comets.

The hidden "faces" of some of the most bewildering and perplexing members of the sun's enchanting--and often mysterious--family finally have been seen. Their masks at long last have fallen off.

Judith-Braffman Miller is a freelance journalist.
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Title Annotation:Science & Technology; comets and asteroids that cannot be easily categorized as either one or the other
Author:Miller, Judith-Braffman
Publication:USA Today (Magazine)
Geographic Code:1USA
Date:Jan 1, 2014
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