What do main-belt comets look like?
Main-belt comets have a wide variety of appearances, or “morphologies”, depending on the strength of their activity, nucleus size, angle at which they are viewed from the Earth, just to name a few of the factors involved. As you look through objects in Comet Hunters, you may wonder what kind of features you should be looking out for. This can be a hard question to answer given the diversity of possible morphologies of main-belt comets though, so in this case, it is perhaps easier to show, rather than tell.
So, first, here is a gallery of representative images of nine of the main-belt comets known to date.
As you can see, main-belt comets can have long, thin tails like 133P, or broad ones like P/2013 R3 or P/2012 T1, or curved ones like 324P or 313P. Some are strongly active, like 238P and P/2013 R3, while others are only weakly active, like 133P, 176P, and P/2012 T1. Some may even have two dust tails, like 288P in the above images, or may be in the process of breaking apart, like P/2013 R3. As you can see, it is not really possible to describe the “typical” appearance of a main-belt comet.
Complicating matters more, the appearance of an individual main-belt comet can vary over time, as its activity strength changes as it approaches and then passes through perihelion (its closest approach to the Sun in its orbit), and/or due to changes in observing conditions between different observations (i.e., on different nights or even different times in the same night, or at different observatories at different locations in the world under different weather conditions). Below are several series of observations of various main-belt comets taken over periods of several weeks, months, or years using a variety of telescopes (ranging in sizes in terms of primary mirror diameter from 1.5m to 10m) to help illustrate this point:
133P/Elst-Pizarro in 2002 and 2007 (from Hsieh et al. 2010, Monthly Notices of the Royal Astronomical Society, Vol. 403, p. 363-377)
176P/LINEAR in 2005 (from Hsieh et al. 2011, Astronomical Journal, Vol. 142, article 29)
238P/Read in 2005 (a-c) and 2007 (d) (from Hsieh et al. 2009, Astronomical Journal, Vol. 137, p. 157-168)
238P/Read in 2010 (from Hsieh et al. 2011, Astrophysical Journal Letters, Vol. 736, article L18)
324P/La Sagra in 2010-2011 (from Hsieh et al. 2012, Astronomical Journal, Vol. 143, article 104)
288P/(300163) 2006 VW139 in 2011 (from Hsieh et al. 2012, Astrophysical Journal Letters, Vol. 748, article L15)
P/2012 T1 (PANSTARRS) in 2012 (from Hsieh et al. 2012, Astrophysical Journal Letters, Vol. 771, article L1)
313P/Gibbs in 2003 (a-c) and 2014 (d-h) (from Hsieh et al. 2015, Astrophysical Journal Letters, Vol. 800, article L16)
Note: Many of these images are created by adding together several individual images to make a composite image equivalent to leaving the telescope shutter open for up to several hours in some cases. During this time, the main-belt comet appears to move relative to the background stars. From the comet’s perspective though, the stars appear to move, and so the series of dotted streaks you see in many of the above images are background stars or galaxies that have been imaged several times (while “moving” between exposures) and then combined together into a single image, keeping the comet at the center of the image at all times.
This rich variety in main-belt comet morphologies is a big reason why we started the Comet Hunters project, given the difficulty of creating computer algorithms capable of identifying several different types of activity. There are still some things that computers can do better than the human eye (such as measure small differences in the profile “widths” of candidate objects as compared to nearby stars), but we hope that the combination of citizen science and modern computing, we will be able to discover many more new main-belt comets.
Comet Hunters Blog 
Very interesting, thanks Henry!
In Talk, there are several known MBCs as Subjects; which, if any, of the images in this blog post were taken with a telescope of comparable size to Subaru, with exposures (integration times) similar to the CH images.
Also, do you have a handle on how ‘detectability’ of MBCs varies with their distance from the Sun and Earth (at the time of observation)? For example, is the data we have heavily biased towards objects that were relatively closer to the Earth?
LikeLike
Hi Jean, the images in this blog post represent a wide range of exposure times on telescopes with a wide range of sizes. To take an example, for the 324P images, the image in panel (a) represents a total exposure time of 80 seconds with the 1.8m PanSTARRS telescope, equivalent to 4 seconds of exposure time on the 8m Subaru telescope. The image in panel (k) represents a total exposure time of 360 seconds with the 10m Keck telescope, equivalent to 560 seconds on Subaru. For reference, the Comet Hunters images are anywhere from 3 to 120 seconds with Subaru meaning that some of the images shown here are deeper (more sensitive to faint object/features) than CH images but some are more shallow, so it’s perfectly possible for you to see activity in CH images that looks stronger, weaker, or similar to the various examples of activity shown here.
And we are always biased towards discovering things that are closer to the Earth and Sun, of course, since they do appear brighter to us at those times. Once we’ve discovered a main-belt comet though, we typically try to follow it as long as possible as it travels away from the Sun to see how long it remains active. This generally requires larger and larger telescopes and/or longer and longer exposure times as the object gets farther and fainter, so it is not clear whether a previously unknown MBC that becomes active far from the Sun would necessarily be discovered by the current surveys (which are mostly conducted with <2m telescopes). By using Subaru data, we may be able to shed light on this issue, since we should be able to identify active objects farther out than, say, PanSTARRS. Right now, all confirmed or suspected MBCs have been seen to be active while close to the Sun (near perihelion) and inactive far from the Sun (confirmed by very long observations by very large telescopes), and thermal calculations indicate this is consistent with what we would expect for water sublimation. However, a discovery by CH of MBCs active while far from the Sun is not out of the realm of possibility, and would present quite an interesting complication. If the pattern of activity close to the Sun continues for CH-discovered MBCs though, then this would represent even stronger evidence that we are dealing with water ice sublimation in these bodies.
Cheers,
Henry
LikeLike