Why I’m excited about what LSST will do for comet science

Today we have a blog from Matthew Knight.  Matthew Knight is a research scientist at the University of Maryland. He has observed comets on hundreds of nights at UV (ultraviolet), optical, and IR (infrared) wavelengths from the ground and with space-based observatories.

For a field that dates back more than 2000 years and has featured successful spacecraft visits to five comets since 2001, there is still quite a bit to learn in comet science.

Much of what we know about comets comes from observations of the occasional bright and spectacular ones – like Hale-Bopp and Halley – that keep astronomers at telescopes for months on end and percolate into the realm of pop culture. Arriving about once a decade, such “great comets” are often bright enough to be studied over a much longer range of time and distance than other comets, and they often permit observations using new and unusual techniques.

Much of the rest of our knowledge comes from the so-called “Jupiter Family Comets” (JFCs), comets with short orbital periods, typically less than 10 years. There are a few hundred known JFCs, and every year or so one approaches Earth favorably enough to be studied in detail. This year’s close approacher is 46P/Wirtanen, while last year featured two, 41P/Tuttle-Giacobini-Kresak and 45P/Honda-Mrkos-Pajdusakova.

Where our knowledge is most fuzzy – and where LSST will excel – is the faintest, and hardest to see comets. These may be faint for a variety of reasons: being far from the Sun, being very small, or having already lost their frozen gases, to name of few. They are difficult or impossible for many telescopes to observe, and they certainly are not studied as regularly or in the level of detail as their brighter, better known brethren.

LSST will allow us, for the first time, to survey huge numbers of the faintest comets. What is more, LSST will reimage these comets every few days, over and over. This will revolutionize our knowledge of the comet population as well as our understanding of how they (and the solar system) evolve.

New discoveries – LSST is predicted to discover ~10,000 comets over its 10-year lifetime. By comparison, about ~5000 comets have been discovered to date, and of these, more than 3000 are tiny fragments of broken up comets seen only by the SOHO spacecraft as they are destroyed close to the Sun. These new discoveries will yield far more robust statistics about the number and variety of comets currently in the Solar System. After a lot of computer modeling, this will greatly improve our understanding of the population of “cometesimals” in the disc of material out of which the planets formed around our proto-Sun 4.6 billion years ago.

LSST will quickly balloon the numbers of all types of comets, from the traditional classes (short period JFCs and long period comets arriving from the Kuiper Belt or the Oort Cloud) to more exotic and confounding recent discoveries like “active asteroids” (apparently cometary bodies on asteroidal orbits), “dead comets” (weakly active or inactive bodies on cometary orbits), and “centaurs” (comet-like objects thought to be migrating in from the Kuiper Belt and currently on nearly-circular orbits between Jupiter and Neptune). Hopefully, it will also detect more interstellar objects like 1I/‘Oumuamua and maybe even things we never imagined exist!

Evolution – I’m also excited about what we’ll learn about how our solar system continues to evolve. As I just noted, LSST will find many objects transitioning into (centaurs) or out of (dead comets) the active comet population. It will also allow us detect spectacular short duration events like fragmentation, outbursts of activity, or even impacts. Often, these phenomena are not discovered until much later, and it is unknown how frequently they are simply missed entirely. LSST’s high cadence will ensure such events are observed soon after they happen, and the rapid data reduction pipeline will make it possible for them to be detected in the data almost immediately. With proper algorithmic triggers in place, astronomers will be able to follow up on unusual developments from other observatories within minutes or hours.

LSST will also give new insight into how comets behave when they are far away from the Sun, a regime where we currently know very little. A hallmark of the modern era of comet science has been the understanding that cometary activity is driven by the sublimation of water ice. Water ice does not sublimate appreciably beyond the so called “snow line” around 3 AU, yet many comets are active at larger distances. This indicates that other frozen gases, likely CO and CO2 but possibly others, are important in at least some comets. Surveying advances in the last decade have pushed detections of comets substantially beyond the snow line, with objects now routinely being discovered beyond 10 AU, and occasionally before they have even started displaying cometary activity. LSST will systematically survey large number of such objects, which will (hopefully) allow us to find patterns and better understand the inner workings of comets.