Scientists have identified a rare solar system object with features of both an asteroid and a comet.
The object, dubbed 2005 QN173, orbit like any other asteroid, but most of these objects are rocks that don’t change much as they travel through the solar system. This is not the case for 2005 QN173, which was first spotted in 2005 (hence the name), according to new research. Instead, it looks like a comet, releasing dust as it moves and sporting a long, slender tail, suggesting it is covered in icy matter vaporizing in space – though comets typically follow elliptical paths that approach and regularly withdraw from Sun.
“This fits the physical definitions of a comet, in that it is probably icy and throws dust into space, even though it also has the orbit of an asteroid,” Henry Hsieh, lead author of the new researcher and planetologist at the Institute of Planetary Sciences, said in a press release. “This duality and blurring of the line between what were previously thought to be two completely distinct types of objects – asteroids and comets – is a key part of what makes these objects so interesting.”
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Despite its characteristics similar to those of a comet, the object’s orbit is that of an asteroid: it revolves silently around the sun in the outer part of the Asteroid belt which falls between Mars and Jupiter, circling once every 5 years or so.
But this summer, astronomers who examined data collected by the Earth’s Asteroid Impact Warning System (ATLAS) investigation in Hawaii on July 7 noticed the object sporting a tail. The feature appeared in additional observations made by a telescope at the Lowell Observatory in Arizona. Scientists then checked previous sightings made by other facilities and again spotted the tail on images collected on June 11 by the Zwicky Transient Facility in California.
In these observations, the object was moving away from the sun, having made its closest approach, or perihelion, on May 14. (Although the close approach of a comet is much more dramatic than that of a typical asteroid in the main belt, the sun moves closer and further away from it during an orbit. The perihelion of the Earth, for example, falls in early January.)
Meanwhile, other scientists looked at 2005 observations QN173 collected by the dark energy camera in July 2016, the last time the object was around perihelion – and voila, here, too, they spotted a tail.
The activity around the perihelion corresponds to the profile of a comet: increasing heat from the sun turns frozen ice into gas, a process called sublimation. Typical comets spend most of their time far enough from the sun that activity is frozen – literally.
“Most comets originate from the cold outer solar system, beyond Neptune’s orbit, and spend most of their time there, their very elongated orbits bringing them closer to the Sun and Earth for only short periods of time. both, ”Hsieh said. “During those times when they are fairly close to the sun, they heat up and release gas and dust as a result of the sublimation of the ice, producing the fuzzy appearance and the often dramatic tails associated with comets.”
Of the half a million objects scientists have examined in the asteroid belt, this is the eighth that scientists have been able to confirm has been active multiple times, and it is one of 20 ” presumed main belt comets.
The new research included old sightings taken from the archives of various instruments originally collected between 2004 and 2020 at times when the comet was not active, in order to better understand the object itself. These observations suggest that the comet’s nucleus or head is about two miles wide, the statement said.
Then, the scientists incorporated new observations of the object made by a multitude of instruments in July and August to better understand the activity of the strange main belt comet. In particular, the researchers were able to measure the object’s tail, which in July was 450,000 miles (72,000 kilometers) long, just under double the size of the object. distance from Earth to Moon.
But despite its massive length, the tail is not that wide, which poses a new puzzle for scientists.
“This extremely narrow tail tells us that dust particles barely float out of the nucleus at extremely slow speeds, and that the flow of gas escaping from the comet that normally lifts dust from a comet into space is extremely low, ”Hsieh said.
“Such slow speeds would normally make it difficult for dust to escape from the gravity of the core itself, so this suggests that something else might be helping the dust to escape,” Hsieh added. One explanation could be that the nucleus spins so fast that it throws extra dust into space, but scientists don’t have enough observations to be sure.
Scientists are marking their calendars for February 2026, when the object can be seen from the southern hemisphere and also reaches the distance from the sun at which it can become active again.
The research is described in an article accepted by The Astrophysical Journal Letters and available for reading as a pre-print on arXiv.org; the research was also presented on Monday (October 4) at the American Astronomical Society’s Division of Planetary Sciences conference being held virtually this week.