Astronomers using the NASA/ESA Hubble Space Telescope have spotted an unusual object in the asteroid belt between Mars and Jupiter: two asteroids orbiting each other and exhibiting comet-like features, including a bright coma and a long tail.

The active asteroid 300163 (also known as 2006 VW139) was discovered by Spacewatch in November 2006.

Then the possible cometary activity was seen in November 2011. The object was also given a comet designation of 288P.

Astronomers believe 288P/300163 is part of a young family of at least 11 asteroids that formed from a larger body, about 6 miles (10 km) in diameter, during a shattering collision 7.5 million years ago.

In September 2016, just before 288P/300163 made its closest approach to the Sun, it was close enough to Earth to allow astronomers a detailed look at it using Hubble.

The images revealed that it was actually not a single object, but two asteroids of almost the same mass and size, orbiting each other at a distance of about 60 miles (100 km).

That discovery was in itself an important find; because they orbit each other, the masses of the objects in such systems can be measured.

The Hubble observations also confirmed ongoing activity in the binary system.

“We detected strong indications of the sublimation of water ice due to the increased solar heating — similar to how the tail of a comet is created,” said team member Dr. Jessica Agarwal, of the Max Planck Institute for Solar System Research in Germany.

This makes 288P/300163 the first known binary asteroid that is also classified as a main-belt comet.

Understanding the origin and evolution of main-belt comets is a crucial element in our understanding of the formation and evolution of the whole Solar System.

Among the questions main-belt comets can help to answer is how water came to Earth.

Since only a few objects of this type are known, 288P/300163 presents itself as an extremely important system for future studies.

The various features of this system also make it unique among the few known wide asteroid binaries in the Solar System.

“The observed activity also reveals information about its past. Surface ice cannot survive in the asteroid belt for the age of the Solar System but can be protected for billions of years by a refractory dust mantle, only a few meters thick,” Dr. Agarwal said.

“From this, we concluded that 288P/300163 has existed as a binary system for only about 5,000 years.”

“The most probable formation scenario of 288P/300163 is a breakup due to fast rotation. After that, the two fragments may have been moved further apart by sublimation torques,” she added.

The fact that 288P/300163 is so different from all other known binary asteroids raises some questions about whether it is not just a coincidence that it presents such unique properties.

“We need more theoretical and observational work, as well as more objects similar to 288P/300163, to find an answer to this question,” Dr. Agarwal said.

The research is presented in a paper published in the journal Nature.

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Jessica Agarwal et al. 2017. A binary main-belt comet. Nature 549: 357-359; doi: 10.1038/nature23892