A German-led team of Astronomers, with the help of the NASA/ESA Hubble Space Telescope, have observed the fascinating characteristics of a rare type of object in the asteroid belt between Mars and Jupiter: two asteroids orbiting each other and displaying comet-like features, including a long tail and a bright coma.
This is the first recognized binary asteroid also categorized as a comet. The research paper has been published in the journal Nature this week.
The binary asteroid 288P (artist’s impression) (Credit: NASA/ESA)
In September 2016, just prior to the asteroid 288P making its closest approach to the Sun, it was sufficiently close to Earth to allow Astronomers a comprehensive look at it using the NASA/ESA Hubble Space Telescope .
The images of 288P, which is situated in the asteroid belt between Mars and Jupiter, exposed that it was truly not a single object, but two asteroids of virtually the same mass and size, orbiting each other at a distance of almost 100 km. That discovery was in itself a crucial find; because they orbit each other, the masses of the objects in these systems can be measured.
But the observations also exposed current 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,” explains Jessica Agarwal (Max Planck Institute for Solar System Research, Germany), the team leader and the research paper’s main author. This makes 288P the first identified binary asteroid that is also categorized as a main-belt comet.
Having an insight into the origin and evolution of main-belt comets — asteroids orbiting between Mars and Jupiter that exhibit comet-like activity — is a vital element in man’s understanding of the formation and evolution of the full Solar System. Among the queries main-belt comets can help to answer is how water came to Earth . Since only a few objects of this type are identified, 288P presents itself as a highly vital system for future studies.
The numerous features of 288P — wide separation of the two components, high eccentricity, near-equal component size and comet-like activity — also make it unique among the few identified wide asteroid binaries in the Solar System. The observed activity of 288P also exposes 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 metres thick.
Jessica Agarwal, Team Leader and the Research Paper’s Main Author
From this, the team determined that 288P has been prevalent as a binary system for just about 5000 years. Agarwal adds to the formation scenario,
“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.”
The fact that 288P is very different from all other identified binary asteroids raises some queries about whether it is not just a coincidence that it exhibits such novel properties. As discovering 288P included a lot of luck, it is likely to stay as 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.
Jessica Agarwal , T eam L eader and the R esearch P aper’s M ain A uthor
This artist’s impression shows the binary main-belt comet 288P. From a distance the comet-like features of the system can clearly be seen: among them, the bright coma surrounding both components of the system and the long tail of dust and water pointing away from from the Sun. Only a closer look reveals the two components of the system: two asteroids circling each other on an eccentric orbit.
(Credit: ESA/Hubble, L. Calçada, M. Kornmesser)
This time-lapse video, assembled from a set of ESA/NASA Hubble Space Telescope images, reveals two asteroids with comet-like features orbiting each other. The asteroid pair, called 288P, was observed in September 2016, just before the asteroid made its closest approach to the Sun.
These images reveal ongoing activity in the binary system. The apparent movement of the tail is a projection effect due to the relative alignment between the Sun, Earth, and 288P changing between observations. The tail orientation is also affected by a change in the particle size. Initially, the tail was pointing towards the direction where comparatively large dust particles (about 1 millimetre in size) were emitted in late July. However, from 20 September 2016 onwards, the tail began to point in the opposite direction from the Sun where small particles (about 10 microns in size) are blown away from the nucleus by radiation pressure.
(Credit: NASA, ESA, and J. Agarwal (Max Planck Institute for Solar System Research))
 Like any object orbiting the Sun, 288P travels along an elliptical path, bringing it closer and further away to the Sun during the course of one orbit.
 Current research indicates that water came to Earth not via comets, as long thought, but via icy asteroids.