arXiv:1807.08728 -- P/2017 S5: Another Active Asteroid Associated with the Theobalda Family

Paper: P/2017 S5: Another Active Asteroid Associated with the Theobalda Family
Authors: Bojan Novakovic

Abstract: In this note we have shown that a newly discovered comet P/2017 S5 (ATLAS), that moves around the Sun in an asteroid-like orbit, is a member of the Theobalda asteroid family.

My Comment: Super short letter, but packed full of references giving a clear "how you do this science" outline. Love it. One of my favorite experiences with an undergrad research student was when she serendipitously found a candidate active asteroid while working with me on a comet project. Turns out it (most likely) wasn't but working out that puzzle involved her getting time on a 3.5m telescope(!), and having her work praised by Jocelyn Bell Burnell(!!).

My Scrawling Notes:

Further investigation of changes in cometary rotation: arXiv:1806.11158

Paper: Further investigation of changes in cometary rotation
Authors: Beatrice E. A. Mueller, Nalin H. Samarasinha

Abstract: Samarasinha & Mueller (2013) related changes of cometary rotation to other physical parameters for four Jupiter family comets defining a parameter X  , which is approximately constant within a factor of two irrespective of the active fraction of a comet. Two additional comets are added to this sample in this paper and the claim of a nearly constant parameter X  for these six comets is confirmed, albeit with a larger scatter. Taking the geometric mean of X  for all the comets above excluding 2P/Encke (as X  for each comet was determined with respect to that of 2P/Encke), the expected changes in the rotation periods for a sample of 24 periodic comets are derived. We identify comets from this sample that are most likely to show observationally detectable changes in their rotation periods. Using this sample and including the six comets used to determine X  , we find a correlation between the parameter ζ  (i.e. the total water production per unit surface area per orbit approximated by that inside of 4 au) and the perihelion distance q  ; specifically we derive ζ  ∝  q −0.8   and provide a theoretical basis for this in Appendix A. This relationship between ζ  and q  enables ready comparisons of activity due to insolation between comets. Additionally, a relationship between the nuclear radius R  and the rotation period P  is found. Specifically, we find that on average smaller nuclei have smaller rotation periods compared to the rotation periods of larger nuclei. This is consistent with expectations for rotational evolution and spin-up of comet nuclei, providing strong observational evidence for sublimation-driven rotational changes in comets.

My Comment: The empirical law given seems to work on a factor of a few, which is better than the order of magnitude spanning effects being modeled. This paper does not justify its creation, with the details being presented in an earlier work from the authors in 2013.

My Scrawling Notes:

Believing in comet dust

Well, I have a few things to add since my Comet Kerfuffle post a few months back.  Since then I've had the chance to take some images of comet C/2012 S1, aka Comet ISON through a fairly decent sized telescope (3.5 meter).  I'm starting to believe that we are in for quite a show this fall.  Why?  Well here's my reasons to be optimistic:


1.  At about 5 AU, where Jupiter orbits, the comet already has a very pronounced coma and tail.  This is still far enough out that it isn't getting warmed all that much by the Sun, and in fact not all of the gas species that drive the coma and tail have even reached sublimation temperature yet.  As one would would expect from an Oort Cloud object making a fresh return to the inner solar system, it is a very, very active comet.

Comet C/2012 S1 @4.9AU (Hammergren, Solontoi, Gyuk)

2. Its going to pass close to the Sun.  Really close.  Close enough that the tidal and thermal forces associated with the comet's passage by the Sun may cause it to fragment.  If that happens it will be quite a show for sure.  Unfortunately from the look of the orbit that event would most likely happen with the comet behind the Sun viewed from Earth.

3.  Folks who have been tracking the observed magnitudes of this thing are saying that it is fairly odd.  Most comets brighten suddenly and then "level off" at a certain point (if you're squinting at them in the right logarithmic axis!).  This one looks like it may have already gone through this change due to the way it is increasing in magnitude.  If that's the case it isn't a wild prediction to say that C/2012 S1 is trending toward "lunar" magnitudes - potentially as bright as the Moon!

Now I'm not saying that I am predicting that this is going to be a day-time comet - it will be hidden by the Sun when it is at perihelion for instance.  Many, many things can and will change in this comet's life between now and November of this year, but the way things are shaping up I am starting to really think that this comet will be spectacular in one way or another.

A Comet Kerfuffle

On September 24th a very faint comet was discovered in the constellation of Cancer at a distance from Earth of about one billion kilometers.  Now named C/2012 S1, or colloquially “Comet ISON” it has picked up quite a bit of press in the past weeks.  Looking at the orbit of this comet it certainly looks like it has the potential to be “spectacular” as seen from Earth.  It will pass close (within 1.2 million kilometers) to the Sun in late November 2013, and then pass by the Earth at about 40% of the Earth-Sun distance (~ 65 million kilometers).  The comet already seems to be active, that is showing some fuzziness as a result of gas and dust being liberated from its surface, even though it is still out beyond the orbit of Jupiter.  These two things have a whole lot of folks talking about this being a great comet, one that will put on a heck of a visible show in the night sky.  The idea being that this comet will produce a tremendous cloud and tail of gas and dust as it passes by the Sun, and will shine brightly in the night sky.  Some have even proclaimed that Comet ISON could put on a show similar to that of the Great Comet of 1680. Or, you know, it could be a total dud, viewable by almost none <cough Kohoutek cough>

The Great Comet of 1680 by Lieve Verschuier

Now, how can people on one had be comparing this comet to very visible, spectacular looking comets like the Great Comet of 1680, or even Comet Hale-Bopp, yet at almost the same time caution that we may end up seeing nothing?  It all has to do with how a comet behaves, and how unpredictable that behavior actually is.

What we see of the comet in the sky is not the object itself: The actual comet is a small, dark, potato-shaped object made up of rocks, carbon, and ices.  In 1950 Fred Whipple described comets as “Dirty Snowballs,” and that’s how we still see them, although recent observations, and fly-bys of several comets have me leaning toward calling them“Icy-Dirtballs” to better describe them.  As these objects (the comet nucleus) nears the sun, many of the ices near the surface will vaporize, producing a “coma” or cloud of gas and dust around the comet.  This is what we see when we look at the head of a comet.  This gas and dust interacts with the solar wind and gravity and eventually produces tails for the comet, completing our mental image of what a comet is.

That said, the exact processes and amounts of gas and dust to be liberated are unknown quantities on a comet-by-comet basis.  How much ice is left near the surface from previous passages through the solar system?  How deep can the Sun actually warm the comet?  Will the comet crack, or break apart under the gravitational and thermal stresses as it passes by the Sun?  Will a large amount of dust be liberated with the vaporizing gasses?  All of these questions play a vital role in determining how a comet will look in the sky.  A very gassy, dusty comet that dredges up material from deep within could put on quite a show.  At the same time, one that only vaporizes a thin surface layer of ice could certainly be a “dud” for the eyes (although it could still have quite a bit of scientific value!).

So what will happen in late November 2013 when Comet C/2012 S1 comes by?  No one has any clue, but we may get lucky and have a really nice comet hanging in the sky at the end of that year.  Or not.