What Are Active Asteroids, or Main Belt Comets?
Categories are critical to any science, but especially so in astronomy. Planets and stars are clearly different things. One shouldn’t confuse a pulsar from a black hole. Asteroids and comets were like this, with one being rocky and the other icy, but new objects seen in the night sky call the old distinctions into question. Maybe they are not so different after all…
First Signs of Trouble
Scienitsts have known for years that no perfect definition distinguishing asteroids and comets has been found. Some consider chemical properties as the guideline while others feel orbital distances are key. Even how they interact with Jupiter can be the guiding principle for some. But fuzzy areas exist on the boundaries of the generally accepted parameters. No one fully agrees on the ice/rock content to differentiate the two, for example. And other physics can change orbital positions like radiation and mass loss, so some objects will be in places they otherwise normally wouldn’t be (Jewitt).
Discovery of an Active Asteroid
So when did we find the first one of these troublemakers? That would be in 1996, when a previously identified asteroid 7968 Elst-Pizarro started to exhibit a tail like a comet and continued to do so for 2 months. Now dubbed 133P/Elst-Pizzaro, it presented astronomers with a big issue: which object is it? It was located in the main asteroid belt but at perihelion it displayed a tail. Maybe it was a short-term event, like a collision (which have bene spotted), but then upon reentering that same portion of its orbit once again displayed a tail, according to December 2002 observations by Hsieh and Jewitt. Then by fall of 2003, the tail was gone again. Initially called a main-belt comet, more were found (despite their faintness and lack of proximity to the sun) but new and different types involving possible collisions were also spotted in 2010, and they were far from the sun at those times. P/2010 A2 and 596 Scheila were the first examples of the so called disrupted asteroids, and models indicated that a 98 foot wide object impacting the 71 mile long Scheila could have resulted in the observations seen. For P/2010 A2, a 3.3 to 6.6 foot long object impacting the 62 mile long object would also result in the observations seen for it. So to incorporate all of this data a new term was coined: active asteroids. This covers main-belt comets and disrupted asteroids, since the distinction between them is murky at best (Hsieh, Redd 30-1).
The Active Asteroids
Several candidates have been spotted, including:
-300163 (2006 VX139)
-P/2010 R2 (La Sagra)
-107P/(1949 W1) Wilson-Harrington
Notice how some of those asteroids have comet designations. This goes to show how scientists initially felt the observations pointed to comets because of coma and mass-losing events, and how some are still considered main-belt comets (Jewitt).
How Are They Losing Mass?
Several theories are in play for what may be causing these objects to be active. One is sublimation, which is what drives comets. Why would that be a candidate here, then? Turns out, a thin layer of regolith as shallow as 1 meter in depth can cause ice to be trapped for almost a billion years, only becoming exposed when a collision occurs. Perhaps small pockets of ice formed in shadowed regions of asteroids and were not melted away by the radiation from the closeness of the sun. Maybe instead we are witnessing some projectiles coming from a recent collision with another space object, or maybe an object spinning apart because of a large torque. The problem is, the asteroid belt is not like how it looks in the movies. It is mainly empty space with the average distance between objects clocking in at 600,000 miles. With 800,000 asteroids in the belt, that translates to a lot of real estate available. Therefore, collisions should be quite rare (Jewitt, Redd 31).
Electrostatic forces may also be at play. Turns out, solar radiation involves a bombardment of not only photons but also electrons and protons. As an object spins in space, surfaces are hit with the radiation and electrons, being of a smaller mass, travel away faster than protons. This causes a net charge to develop as the objects spins and the surface falls into the dark side. But as it spins towards the light again, protons come into play again and electrostatic forces can cause particles to rise. If enough of a charge is developed, then the dust can achieve escape velocity and away they go. But the math shows it may only work for smaller asteroids, plus the moon models this is based off of may be incomplete (Jewitt).
Thermal properties can also be at hand. Fracturing caused by extreme temperature changes as an object approaches the sun can cause particles to escape. Another possibility is liquid water escaping the surface (as opposed to sublimation, where it goes directly from a solid to a gas), taking particles with it, whether that water loss be driven by heat differences or by shock compressions from collisions (Ibid).
All that being said, some odd details remain. For example, take Body 288P. Found by Hubble in 2011, it was clearly an active asteroid but would take 5 years until the object was close enough to reveal it is also a binary asteroid. Any their masses are pretty close, plus they are about 100 kilometers apart. This hints at a possible torque breakup within 5,000 years ago, with the gasses released furthering the breakup. It is so far a class of one, a unique object. Maybe. P/2016 J1 may also be a possible binary active asteroid as well, with hints of 2 components separating apart in 2010. It becomes active when near the sun, hinting at interior material being heated and released as a gas dust mix (Irving, Koberlein).
Main-belt comets can provide scientists with a potential new angle into water studies of the early solar system. At that time, water was found closer to the Sun and as it expanded, the region where liquid water could exist migrated outward. But these main-belt comets could be potential reservoirs of this early water, giving us a clue as to the amount present, what ions existed, and maybe other chemical clues unknown to us at this time. These may even be the leftovers of the water-delivery system to early Earth. Deuterium/hydrogen levels will be needed if a meaningful study on this is to be done. Meanwhile, disrupted asteroids can give us an interior look and see how asteroids formed as well as provide data to better model the formation of the early solar system. They can also give us a better feel for impact rates and the distribution of asteroids in the belt (Hsieh, Redd 31-2).
The line between these objects is now not quite as distinct, but we have gained much because of this. Who knows what new lines of though and discoveries await us as we continue to probe into he mysteries of the solar system.
Hsieh, Henry. “Active Asteroids: Main-Melt Comets and Disrupted Asteroids.” arXiv: 1511.01917v1.
Irving, Michael. “Hubble Spots a Strange New Type of Celestial Object.” Newatlas.com. Gizmag, 20 Sept. 2017. Web. 16 Jan. 2018.
Jewitt, David. “The Active Asteroids.” arXiv: 1112.5220v1
Koberlein, Brian. “A Newly Discovered Asteroid Has Started to Look Like a Comet.” Forbes.com. Forbes, 03 Mar. 2017. Web. 17 Jan. 2018.
Redd, Taylor. “Imposters in the Asteroid Belt.” Astronomy Apr. 2017. Print. 30-32.
© 2018 Leonard Kelley