Weird Space Rock Confirmed as Super-Rare Hybrid of Comet And Asteroid

Although both comets and asteroids can be described as types of rocks that exist in space, their differences are quite noticeable.
Comets are usually from the outer Solar System, and they have long, elliptical orbits. They are filled with ices, which subliminate when they get close to the Sun. This creates a dusty, misty atmosphere, also known as a coma, and the famous cometary tails.

Asteroids are found in the main asteroid Belt, which is located between Mars and Jupiter. Their orbits are closer to those of the planets. Asteroids are also believed to be quite dry and rocky so they don’t engage in the beautiful outgassing that is seen in more exotic cousins.

However, a newly discovered space rock appears to share characteristics with both. It is called (248370 2005 QN 173 and hangs out in the main Asteroid Belt, just like millions of other asteroids.

However, it was seen in July showing signs of outgassing as it approached the Sun (perihelion) and a long, cometary tail. This would make it an unusual hybrid of both, a type we call an active asterroid or main-belt camet.

This is one of only 20 of the rare main-belt objects, which are among over 500,000 that have been confirmed to be main-belt comests. It is also the eighth such object. Astronomers also discovered that the object was active multiple times.

Henry Hsieh, an astronomer at the Planetary Science Institute, stated that "this behavior strongly suggests that it is due to sublimation oficy material."

248370 could be considered both an asteroid or a comet. It is a main-belt and comet asteroid, which has been recently recognized as such. It is likely to be icy and is ejecting space dust, which is consistent with the physical definitions for a comet.

This duality and blurring between two types of objects previously believed to be completely different is what makes these objects so fascinating.

Data from the robotic astronomical survey Asteroid Terrestrial Impact Last Alert System (ATLAS) revealed that 248370's behavior was detected on July 7, 2021. Lowell Discovery Telescope confirmed the presence of a tail. A review of data from Zwicky Transient Facility revealed that the tail was visible as early as June.

New follow-up observations using different telescopes were made between 8 July and 14 Aug to confirm earlier data. The asteroid belt had 248370 sporting a very stylin' tail.

Hsieh's team measured the tail and found that it was approximately 3.2 km (2 miles) wide. The tail measured more than 720,000 km (450,000 miles) in length, but was only 1,400 kilometers (900 mi) wide in July. This is insane compared to the tail's length.

Hsieh explained that this narrow tail indicates that dust particles are not floating off the nucleus at very slow speeds, and that the flow gas from the comet that usually lifts dust into space is very weak.

"Slow speeds make it hard for dust to escape the gravity of the nucleus, so this indicates that there might be something else helping dust escape.

"For instance, the nucleus could be spinning fast enough to help fling dust into space that has been partly lifted by escaping gases. To confirm the speed at which the nucleus rotates, further observations will be required.

Additional observations will allow us to better understand the object. Based on our knowledge of the Solar System 248370 and other main-belt comests should not exist. This is because the main asteroid Belt was thought to be there since the formation the Solar System around 4.6 billion years ago.

The Sun is approximately 2.2 to 3.2 astronomical units away from the asteroid belt. At 5 astronomical units, the Solar System's frostline is the temperature at which it is sufficiently cold to allow ice to form in vacuum. It is not clear why these main-belt comests have enough ice to sustain cometary sublimation activity.

They could also help us to understand a bit more about Earth. Impacts from asteroids bearing water might have been one way water got to Earth in the early days. This idea might be explored further if main-belt comets contain water.

The researchers wrote in their paper that 248370 would be well-placed to monitor the approaching perihelion passage of UT 2026 September 3.

Monitoring during this period will prove extremely valuable in further confirming 248370's recurring nature, constraining its orbital range (which has implications for constraining ice thickness on the object), measuring initial dust production rates and comparing the object’s activity levels to other main-belt comests."

The research was presented at The 53rd Annual Meeting, AAS Division for Planetary Sciences. It has been accepted into The Astrophysical Journal Letters. It is currently available at arXiv's preprint site.