Rare asteroid comet 248370 captured in July. Credit: Henry H. Hsieh (PSI), Jana Pittichová (NASA/JPL-Caltech)

Rare Object That’s Both an Asteroid and a Comet Imaged by Experts

Out of more than half a million known objects in the Main Belt, only two dozen asteroids have more than once exhibited activity typical of comets.


The recently discovered object of the Main Belt called (248370) 2005 QN173 has now been recognized as a rare “asteroid comet” with features characteristic to both types of objects. According to scientists, the object is probably made of ice but has an asteroid’s orbit. 

What is this rare asteroid comet and how do such objects even exist?

What is a comet?

Common comets, such as Halley’s comet, are composed mostly of ice, methane, and other volatile compounds and spend most of their time in the cold outer solar system beyond the orbit of Neptune. Their elongated eccentric orbits allow them only occasionally and briefly to approach the Sun and Earth.


The cometary activity of asteroid 248370

The cometary activity of asteroid 248370, previously designated as 2005 QN137, was detected on July 7, 2021, by the ATLAS tracking system for dangerous asteroids. After further observations in the following days, astronomers confirmed the appearance of a tail.

Dust tail length and width

The dust tail of the object stretches 720 thousand km in length, which is three times the distance from the Earth to the Moon. At the same time, the tail width is only 1400 km. According to scientists, this is extremely small in comparison to the actual length of the tail.

Diameter of the nucleus

The diameter of the solid nucleus in the “head” of the comet, surrounded by a dust cloud, or coma, is a little more than 3 km.

What does the size of the tail mean?

The researchers believe that the final velocity of particles flying off the surface of the comet’s nucleus is extremely low (one meter per second for micrometer-sized particles), which suggests that the dust emissions may be facilitated by the rapid rotation of the nucleus or other processes.


A rare “asteroid comet”

Asteroid 248370 is one of a very small group of objects with dual characteristics: out of more than half a million known objects in the Main Belt, only two dozen asteroids have more than once exhibited activity typical of comets.

Such objects should not exist

Based on our current knowledge of the Solar System, objects like the rare asteroid comet that have been officially confirmed should not exist.

Main Belt objects

Astronomers believe that most of the inhabitants of the Main Belt have been there for at least 4.6 billion years, and during this time any ice contained in them should have already evaporated. Astronomers do not understand how and why such comet-like asteroids in the Main Belt have retained the necessary amount of ice to support cometary activity.

Perihelion point of the rare asteroid comet

The next approach to the perihelion passage of asteroid 248370 will be on September 3, 2026. This means that astronomers will focus their attention on monitoring the object around this period as this is when it will reveal most about itself.

Earth’s past

Astronomers also noted that studying the object could reveal valuable information about Earth’s past too. Interest in objects like the rare asteroid comet is also caused by the fact that they are considered as possible suppliers of water to the young Earth in the distant past. If scientists can confirm that comet-like asteroids like 248370 do, in fact, have water despite their place of existence, this theory will gain significant weight.

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Avery, D. (2021, October 4). Astronomers say newly discovered asteroid is also a comet because it contains ‘icy material’. Daily Mail Online.
Fischer. (2021, October 5). Is new finding an asteroid or a comet? it’s both. Planetary Science Institute.
Hsieh, H. H., Chandler, C. O., Denneau, L., Fitzsimmons, A., Erasmus, N., Kelley, M. S. P., Knight, M. M., Lister, T. A., Pittichova, J., Sheppard, S. S., Thirouin, A., Trujillo, C. A., Usher, H., Gomez, E., Chatelain, J., Greenstreet, S., Angel, T., Miles, R., Roche, P., & Wooding, B. (2021, September 30). Physical characterization of Main-Belt Comet (248370) 2005 QN173. arXiv.org.
Phys.org. (2021, October 4). Is new finding an asteroid or a comet? it’s both.
Starr, M. (n.d.). Weird space rock confirmed as super-rare hybrid of comet and asteroid. ScienceAlert.

Written by Vladislav Tchakarov

Hello, my name is Vladislav and I am glad to have you here on Curiosmos. As a history student, I have a strong passion for history and science, and the opportunity to research and write in this field on a daily basis is a dream come true.

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