Tue. May 17th, 2022

In astronomy, comets and asteroids are defined very differently. Comets have a “core” that is usually made of ice and dust, and a tail when they come near the sun, which is the core material that ejects from the comet itself. Asteroids, on the other hand, are small stone balls that orbit the sun. Sometimes, however, some objects meet the criteria to be both an asteroid and a comet – and a team from the Planetary Science Institute (PSI) thinks they have found a new one.

The object, 2005 QN173, is located in the large asteroid belt between Mars and Jupiter. It is one of only eight known head belt objects to be “active” on more than one occasion. Being “active” in this case is defined as changing its orbital pattern due to something specific happening to the object. Sublimation of ice on the surface of the object is the most likely cause of any activity in the case of 2005 QN173.

UT video discussing the asteroid belt.

This activity is what makes this object unique in comparison to other asteroids in the belt. Any ice that can cause sublimation would have been burned off billions of years ago since they have been exposed to the full force of the sun for so long. On the other hand, comets are only exposed to that solar power when they are in the perihelion of their, sometimes very eccentric, orbit. While further away from the sun, solar radiation has little or no effect on the ice the comet contains.

So it’s a little surprising when scientists find an object that gasifies like a comet but is in the same position as billions of other asteroids. In this case, the author of the new paper on 2005 QN173 is also the person who discovered this whole category of unique objects, now called head belt comets.

UT video discussing comet tails

A hallmark of the QN173 itself in 2005, however, is its comet tail. The core of the object is standard enough with a 3.2 km wide cloud around it. However, the tail of the object is strangely shaped. It is more than 720,000 km (450,000 miles) long and only 1,400 km (900miles) wide. As described in a press release from PSI – “if the length of the tail were scaled to the length of a football pitch, it would be only 7 inches wide and the core would be half a millimeter across.”

This elongated, thin tail means that the particles of which it is formed only leave the surface of the object very slowly. However, solar pressure is probably not enough to push the dust particles out of the surface to form a tail. Henry Hsieh, senior researcher at PSI and lead author of the paper, believes that the rotation in QN173 from 2005 may contribute to the energy needed for dust to escape, but warns that more observations are needed before models of the object or the like are completed.

Comparison of the orbits between a standard comet (Halleys) and QN173 from 2005.
Credit – Henry H. Hsieh (PSI)

Long ago, similar objects could have been the source of much of Earth’s water. Models of the early planet suggest that much of the water currently contained on Earth was originally supplied by main-belt asteroids in the early life of the solar system. If this model is accurate, studying everything that still looks like a comet after billions of years in the asteroid belt can help prove or disprove that theory.

Learn more:
PSI – Is there anything new about finding an asteroid or a comet? It’s both
arXiv-Physical characterization of the main belt comet (248370) 2005 QN173
Sciencealert-Strange rum rock confirmed as a super-rare hybrid of comet and asteroid
Space.com – Is it an asteroid or comet? This strange solar system object is actually both.

Lead image:
Composite image of 2005 QN173, whose tail can be seen going from top left to bottom right.
Credit-Henry H. Hsieh (PSI), Jana Pittichová (NASA / JPL-Caltech)

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