A celestial body that has been known for a long time suddenly does something unexpected – that raises questions: Is (248370) 2005 QN173 not an asteroid at all, but a comet?
Tucson / Frankfurt – Asteroids and comets are actually two completely different classes of celestial bodies, which primarily have one thing in common: Both are leftovers from the time when the solar system was formed. Apart from that, comets are designed like “dirty snowballs”, they consist of dust and loose rock that is held together by ice. When a comet gets closer to the sun, part of the ice sublimes and becomes directly gaseous – the tail characteristic of comets is created. Asteroids, on the other hand, are usually boulders that orbit the sun without developing a tail.
Actually, asteroids and comets are completely different types of celestial bodies in astronomy. Occasionally, however, researchers discover an object that does not seem to fit into either of the two categories – or into both. The latter applies to the celestial body with the cryptic designation (248370) 2005 QN173. The celestial body, which was discovered back in 2005, has long been mistaken for an asteroid. It is located in the asteroid belt (also called the main belt) between Mars and Jupiter and was inconspicuous for a long time. But now researchers at the Planetary Science Institute (PSI) in Tucson (Arizona) have announced that the asteroid is active, that is, it is outgassing and has developed a tail. The corresponding study was accepted for publication by the specialist journal Astrophysical Journal Letters.
Comet or asteroid? Celestial Body (248370) 2005 QN173 seems to be both
“Its behavior strongly suggests that its activity can be traced back to the sublimation of icy material,” explains the lead author of the study, Henry Hsieh from PSI. This makes the asteroid a so-called “main belt comet”, explained Hsieh when presenting his study at a virtual meeting of the American Astronomical Society. “It is one of only about 20 objects that are currently confirmed or suspected main belt comets,” said Hsieh. Some of the 20 objects were also observed to be active only once, while activity had already been observed several times for the celestial body (248370) 2005 QN137.
“247370 can be viewed as an asteroid and a comet, or more precisely as a main belt asteroid, which was only recently recognized to be a comet,” Hsieh tries to define the celestial body. “It meets the physical definitions of a comet because it is very likely icy and blows dust into space, even if it has the orbit of an asteroid,” continues Hsieh. “This duality and the blurring of the lines between what were previously thought to be two completely different types of objects – asteroids and comets – is an essential part of what makes these objects so interesting,” says Hsieh happily.
Main belt comet and asteroid in one: (248370) 2005 QN173 is exciting for research
Hsieh’s research team found that the comet’s core, which is surrounded by a cloud of dust, is 3.2 kilometers in diameter. In July 2021, the comet’s tail was longer than 720,000 kilometers and around 1,400 kilometers wide. The very narrow tail shows that the dust particles are moving away from the core at very low speeds and that the comet’s gas flow is very weak. This in turn suggests that something else is helping the dust to escape from the comet’s nucleus – the nucleus may be rotating quickly and thus hurling the dust out into space. However, more observations are needed to confirm this, says Hsieh.
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PSI researcher Hsieh and his colleague David Jewitt were the first to discover the new class of main belt comets in 2006. The rare comets in the main belt are particularly exciting for research: there is the theory that asteroid impacts once brought a large part of the water to earth. Since outgassing comets in the main belt are likely to mean that they still contain water, they could be worthwhile targets for future exploration. (tab)
