Our first story of the day comes from our very own Planetary Science Institute, and it is fresh off the press release list. For once, we’re going to talk about the science behind a comet and not just whether or not it will become a naked-eye wonder.

Let’s start with some background information. When I was working as an intern, I learned that there were basically two types of comets: long-period comets and Jupiter Family comets. Long-period comets orbit the Sun on a scale of 200 or more years. Jupiter Family comets orbit the Sun from inside the orbit of Jupiter. Then I learned there were also short-period comets that orbit from the Kuiper Belt inward to Jupiter.

There’s another type of object that somewhat falls into the cometary category, however. They’re called Centaurs. The press release explains: Centaurs are icy bodies in unstable orbits between Jupiter and Neptune, and cross the orbits of one or more of the giant planets in their journey around the Sun; the gravity of these planets provide rapid dynamical evolution of the objects, and either eject them from the solar system entirely or cause them to eventually evolve inward of Jupiter, to become Jupiter Family Comets. Prior to this migration, Centaurs began as objects beyond the orbit of Neptune (trans-Neptunian Objects), whose gravitational tugs causes the object to slowly leak into the Centaur population; this entire migration from beyond Neptune to JFC lasts a few million to a few tens of millions of years.

And here is where today’s press release gets interesting: observers using the ATLAS telescopes found Comet 2019 LD2 last year, and it is in the process of transitioning from a Centaur into a Jupiter Family comet, a process we have never witnessed before. Lead author Jordan Steckloff says: We find that 2019 LD2 is currently in the vicinity of a dynamical ‘Gateway’ that facilitates the majority of transitions from the Centaur population into the Jupiter Family of Comets. The dynamical gateway is a region beyond Jupiter, extending to just inside of Saturn’s influence.

Steckloff goes on to state: Most significantly, our work found that LD2 is most likely a pristine comet. Although it has likely lost some supervolatile ices such as carbon dioxide ice in the Outer Solar System beyond Jupiter, it is unlikely to have ever been in the inner Solar System, which is warm enough for water ice to [sublimate]. This means that LD2 is a pristine comet, and presents a unique opportunity to observe how pristine JFCs behave as their water ice begins to sublime for the first time and drive comet activity. Moreover, this transition is likely to finish in only 40 years from now, which is a blink of an eye for astronomy. This means that people alive today will be able to follow this object all the way through its transition into the JFC population.

Forty years! Astronomical events usually happen on astronomical timescales, but I might even still be alive to see that transition finish, which is amazing. This research on this one small body might help answer important questions about solar system evolution, such as why there are active comets and inactive asteroids and how they change definition. Planetary scientists love to categorize objects, and we need to understand the categories fully and the bodies we place within those boxes.

This article was published in Astrophysical Journal Letters and is very interesting to me. We’re hoping to have Dr. Steckloff on the Daily Space next week, so please stay tuned!

More Information

PSI press release 

“P/2019 LD2 (ATLAS): An Active Centaur in Imminent Transition to the Jupiter
Family,” Jordan Steckloff et al., 2020 Nov. 26, Astrophysical Journal
Letters (preprint on arxiv.org)