This artist’s impression shows the binary asteroid 288P, located in the main asteroid belt between the planets Mars and Jupiter. The object is unique as it is a binary asteroid that also behaves like a comet. The cometlike properties are the result of water sublimation, caused by the heat of the sun. The orbit of the asteroids is marked by a blue ellipse. Photo: ESA/Hubble, L. Calçada

With the help of the NASA/ESA Hubble Space Telescope, astronomers have observed the intriguing characteristics of an unusual type of object in the asteroid belt between Mars and Jupiter: two asteroids orbiting each other and exhibiting cometlike features, including a bright coma and a long tail. This is the first known binary asteroid also classified as a comet. The research is published in the journal Nature.

Asteroid 288P was discovered by the University of Arizona's Spacewatch program in 2006, according to Stephen Larson, a senior staff scientist at the UA's Lunar and Planetary Laboratory and a co-author on the paper. Cometary activity was noticed in Hawaii Pan-Starrs data in 2011 as it approached perihelion  — close approach to the sun — and is the first known "main belt comet" as it orbits in the main asteroid belt between Mars and Jupiter.

In September 2016, just before the asteroid 288P made its closest approach to the sun, it was close enough to Earth to allow astronomers a detailed look at it using the NASA/ESA Hubble Space Telescope. Like any object orbiting the sun, 288P travels along an elliptical path, bringing it closer to and farther away from the sun during the course of one orbit.

The images of 288P revealed that it actually was not a single object but two asteroids of almost the same mass and size, orbiting each other at a distance of about 100 kilometers (62 miles). That discovery was in itself an important find; because they orbit each other, the masses of the objects in such systems can be measured.

But the observations also revealed ongoing activity in the binary system. The team of authors of this paper has been working for several years trying to identify the mechanism for the observed activity using the Hubble Space Telescope, Larson said. 

"These so-called active asteroids are likely comets that have lost most of their volatile ices and may provide the link between comets and some asteroids," he said. "There are about 20 known active asteroids that result from collisions, volatile gas outgassing and splitting due to rotational spin-up."

"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," explained Jessica Agarwal of the Max Planck Institute for Solar System Research in Germany, the team leader and main author of the research paper. This makes 288P the first known binary asteroid that also is classified as a main-belt comet.

Understanding the origin and evolution of main-belt comets — asteroids orbiting between Mars and Jupiter that show comet-like activity — 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. Current research indicates that water came to Earth not via comets, as long thought, but via icy asteroids. Since only a few objects of this type are known, 288P presents itself as an extremely important system for future studies.

"The high resolution of the Hubble Space Telescope provides unique data constraining the various processes at work in active asteroids," Larson said. "A few of these have been discovered by the UA Catalina Sky Survey, which also provides 14 years of data on historical activity of these active asteroids."

The various features of 288P — wide separation of the two components, near-equal component size, high eccentricity and cometlike activity — also make it unique among the few known wide asteroid binaries in the solar system. The observed activity of 288P also reveals information about its past, Agarwal noted: "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."

From this, the team concluded that 288P has existed as a binary system for only about 5,000 years.

"The most probable formation scenario of 288P is a breakup due to fast rotation. After that, the two fragments may have been moved further apart by sublimation torques," said Agarwal.

The fact that 288P 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. As finding 288P included a lot of luck, it is likely to remain the only example of its kind for a long time.

"We need more theoretical and observational work, as well as more objects similar to 288P, to find an answer to this question," Agarwal concluded.

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.