Are comets born in asteroid collisions?

Comet Halley, seen up close and personal by the European
spacecraft Giotto in 1986. Image courtesy Halley Multicolor
Camera Team, Giotto Project, ESA.

Comets, such as Halley and Hale-Bopp, are small objects travelling through the solar system in long elliptical orbits. Their characteristic tails appear every time they get close to the Sun, when heat and solar winds combine to melt the surface of their frozen cores.

Recently, astronomers have discovered so-called ‘main-belt comets’, a second type of comet within the Main Asteroid Belt. Scientists have suggested that these are actually the remnants of a larger bodies smashed to pieces by recent impacts. To prove this theory, however, you need to show that main-belt comets are part of asteroid families – groups of asteroids with similar orbital characteristics (e.g. eccentricity, orbital inclination). The members of a family are thought to be fragments of larger asteroids destroyed in collisions.

Building an asteroid family tree

Serbian astronomer Bojan Novakovic, from the University of Belgrade's Astronomy Department, is working on the problem with colleagues Henry Hsieh and Alberto Cellino. The team focused on one example - the P/2006 VW₁₃₉ main-belt comet - and their goal was to see if it could be assigned to a family of asteroids.

First described as an asteroid, P/2006 VW₁₃₉ was reclassified as a main-belt comet in 2011 when astronomers working on the Pan-STARRS1 survey telescope in Hawaii discovered its tail. P/2006 VW₁₃₉ is special as it is one of only seven known examples of main-belt comets and finding other members of its family among hundreds of thousands of possible candidates is not an easy task.

Novakovic started his search at the AstDys database, a catalogue of 398,841 asteroids, using the hierarchical clustering method and the cut-off distance parameter to sort through the candidates. This narrowed down the possibilities to 24 asteroids. The next step was to refine the group by looking at the detailed orbital characteristics of these 24 asteroids. For this task, Novakovic used a mathematical method called backward integration, "which can be used to distinguish real family members from interlopers," he says.

To investigate the orbital characteristics of potential family members Novakovic needed to go through a large number of calculations, which were "computationally expensive and would take months to be done on a typical PC," he adds. The solution to this problem was to use grid computing. After looking at the complete orbital histories of 24 objects, using the infrastructure of the Academic and Educational Grid Initiative of Serbia, Novakovic was able to reduce the P/2006 VW₁₃₉ family to just 11 members.

P/2006 VW₁₃₉ and its young family of 11

The most important conclusion of Novakovic's research, published in the Monthly Notices of the Royal Astronomical Society, is that P/2006 VW₁₃₉ and its 11 relatives are part of a young asteroid family formed about 7.5 million years ago. "[Asteroid] families are considered to be young if they are less than about 10 million years old," he explains. This suggests that 7.5 million years ago, a large asteroid crashed with another object and disintegrated into 11 smaller pieces: P/2006 VW₁₃₉ and its 10 family members.

P/2006 VW₁₃₉ became a main-belt comet because it inherited the deeply buried ice of the parent asteroid. It's this ice that vaporizes, producing the characteristic tail of comets. "The fact that ice still exists close to the surface of P/2006 VW₁₃₉ means that its surface is relatively young," says Novakovic. This means P/2006 VW₁₃₉ and its siblings were formed long after the asteroid belt, thus supporting the theory that main-belt comets are born in collision events within the asteroid belt and not through other means.

A version of this story first appeared on the EGI website.