Weird Asteroid-Comet Hybrid Space Rock Has Huge 450,000 Mile-Long Tail

Astronomers have discovered an object in the solar system that defies normal categorization because it has the traits of both an asteroid and a comet.

The space object 2005 QN173 orbits the sun like any other asteroid and sits in an area of the solar system called the main asteroid belt. Unlike other rocky asteroids found in this region, which is between Mars and Jupiter, it seems to be changing as moves through the solar system, growing a tail long enough to reach from Earth to the moon and back again.

The observation of 2005 QN173, which was discovered in 2005, has revealed the object is active and has a thin, straight tail of dust and gas, a trait usually associated with comets. The tail tells astronomers that icy material is changing from a solid straight to a gas, a process called sublimation.

When seen around an asteroid such as 2005 QN173 the tail feature indicates ice sublimating at its surface and shows that it's a rare object called an active asteroid. What makes 2005 QN173 even more extraordinary is that this asteroid appears to have been active on more than one occasion.

Of half a million known main-belt asteroids, 2005 QN173, which is also designated Asteroid 248370, is only the eighth to be discovered that has been active twice in its history.

"248370 can be thought of as both an asteroid and a comet, or more specifically, a main-belt asteroid that has just recently been recognized to also be a comet," Planetary Science Institute Senior Scientist Henry Hsieh said in a press release. "It fits the physical definitions of a comet, in that it is likely icy and is ejecting dust into space, even though it also has the orbit of an asteroid."

2005 QN173 was discovered to be active in observations made by the Asteroid Terrestrial-Impact Last Alert System (ATLAS) survey between July and August this year.

Hsieh, the lead author of a paper discussing the discovery, and his colleagues measured the asteroid to have a solid nucleus at its head of 2 miles across. They measured the tail as being over 450,000 miles long. This is twice the distance between Earth and the moon.

While impressively long, the tail of 2005 QN173 is so thin that if the whole asteroid was brought down to the length of a football field, its tail would be just 7 inches wide, and its nucleus would be only a millimeter across.

Comet/asteroid hybrid 2005 QN173
An image of 2005 QN173 as it streaks through the solar system. Originally believed to be an asteroid, the object confused astronomers when it developed a tail of dust and gas normally associated with comets. Henry H. Hsieh PSI), Jana Pittichová (NASA/JPL-Caltech/NASA

"This extremely narrow tail tells us that dust particles are barely floating off of the nucleus at extremely slow speeds and that the flow of gas escaping from the comet that normally lifts dust off into space from a comet is extremely weak," Hseih added. "Such slow speeds would normally make it difficult for dust to escape from the gravity of the nucleus itself, so this suggests that something else might be helping the dust to escape."

The researcher added that this helping hand may be provided by the nucleus spinning fast enough to fling dust off into space that was partially lifted by escaping gas.

Comets develop icy tails because they have spent less time close to the sun than asteroids, with most of the former originating from the cold outer solar system around Neptune. Comets' highly elongated orbits only bring them close to the sun occasionally.

Most asteroids, on the other hand, have spent a great deal of time, around 4.6 billion years, in the warm inner solar system near Jupiter, where they are still found currently.

This means that as they are relatively close to the sun any ice in these objects is expected to be long gone. Yet, some asteroids continue to challenge this perception.

Active Asteroids

In 2006, Hsieh and David Jewitt classified a newly discovered type of solar system body called main-belt comets, also known as active asteroids, possessing the orbital characteristics of asteroids with the physical properties of comets.

The objects are a significant area of investigation because some theories about our planet's development suggest that much of Earth's water content was delivered by asteroids from the main asteroid belt.

Studying active asteroids like 2005 QN173 could help scientists discover the role of such objects in this key step in the evolution of our planet and the development of life.

Hsieh concludes: "This duality and blurring of the boundary between what were previously thought to be two completely separate types of objects – asteroids and comets – is a key part of what makes these objects so interesting."

A file photo of a comet streaking through the sky over Earth. Astronomers have discovered a space object with the orbit of an asteroid and the physical qualities of a comet. ClaudioVentrella/Getty

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