2013 AZ60 is a Potential Super-Comet

2013 AZ60 is an extreme object in a highly eccentric orbit around the Sun. It comes as close as 7.9 AU from the Sun (i.e. between the orbits of Jupiter and Saturn) and swings out to a whopping ~1950 AU from the Sun. Spending most of its time far from the Sun, the orbital period of 2013 AZ60 is estimated to be ~30,000 years. 2013 AZ60 is a trans-Neptunian object (TNO), and it may be classified either as a Centaur based on its closest distance from the Sun or as a scattered disk object based on its large average distance from the Sun.

Figure 1: Artist’s impression of an icy object far from the Sun. Image credit: ESA.

Optical measurements of 2013 AZ60 show that it has a rotation period of roughly 9.4 hours and a change in brightness of only 4.5 percent during each rotation. Thermal measurements of 2013 AZ60 were also done to estimate the object’s size and reflectivity. The benefit of thermal measurements is that it can distinguish whether an object is “large but dim” or “small but bright”. From the thermal measurements, 2013 AZ60 is estimated to have a diameter of 62.3 ± 5.3 km and a remarkably low geometric albedo of only 2.9 percent. The low albedo indicates that the surface of 2013 AZ60 is extremely dark.

Simulations of the orbit of 2013 AZ60 show that its present orbit is highly unstable. There is a 50 percent chance 2013 AZ60 will be ejected from the Solar System within the next ~700,000 years and a ~4 percent chance it will be perturbed into an Earth-crossing orbit. Given its relatively large size, 2013 AZ60 will be a super-comet if it ever gets perturbed into the inner Solar System. The highly unstable orbit of 2013 AZ60 indicates that the object was only recently perturbed into its current orbit and it is likely a pristine object that came in from the Oort cloud.

Figure 2: Slope parameter versus albedo relations for 111 TNOs, including 2013 AZ60 and 2012 DR30 (an object with a similar orbit as 2013 AZ60). The purple square at the very left side of the diagram represents 2013 AZ60 and the other purple square represents 2012 DR30. A. Pál et al. (2015)

Reference:
A. Pál et al. (2015), “Physical properties of the extreme centaur and super-comet candidate 2013 AZ60”, arXiv:1507.05468 [astro-ph.EP]