Archive | January 2016

Meet the Team: Henry Hsieh

Today we have the next post in our Meet the Comet Hunters Team series. This time we’re focusing on principal investigator (PI) of Comet Hunters, Henry Hsieh.



Name:  Henry Hsieh

What is your current position and where/institution?

Research Scientist with the Planetary Science Institute, living in Honolulu, Hawaii

Where are you originally from/where did you grow up?

New Jersey, USA

What is your role in Comet Hunters?


Beyond Comet Hunters, what else do you work on?

Besides Comet Hunters, I also work on other main-belt comet and disrupted asteroid research including targeted observational analysis to understand their physical properties, dynamical analyses to understand their orbital evolution, and exploring different ways to discover more.

In 3 lines explain your PhD thesis?

I did the first in-depth observational analysis of the first discovered main-belt comet, 133P/Elst-Pizarro (although the term “main-belt comet” did not yet exist at the time).  I then performed a targeted observational search for more “Elst-Pizarros”, the success of which led to the recognition of main-belt comets as a new class of comets.

Why are you interested in main-belt comets?

Besides being the topic of my PhD dissertation, main-belt comet research is extremely new and so has many opportunities to make new discoveries.  It also has very interesting implications for understanding the formation of our solar system and maybe even the origin of water, and therefore life, on Earth itself.

Name one hobby of yours?

Free diving

What is the most recent tv show you have watched?

Mr. Robot

What is your favorite movie?


What’s one thing most people don’t know about you?

I have used telescopes on every continent (including Antarctica) except for Australia.

Favorite cocktail or beverage?


Meet the Team: Kiwi Zhang​

Today we have our next in our Meet the Comet Hunters Team series.

Name:  Kiwi Zhang​

What is your current position and where/institution?

​Project Support Engineer/Scientist at Academia Sinica Institute of Astronomy and Astrophysics (ASIAA)

Where are you originally from/where did you grow up?

​Taipei, Taiwan​

What is your role in Comet Hunters?

Develop the Subaru image analysis pipeline​

Beyond Comet Hunters, what else do you work on?

Work on the Trans-Neptunian Automated Occultation Survey (TAOS) project for developing control software

In 3 lines explain your PhD thesis?

Developed an analysis pipeline to process the image data and to detect the candidate events produced by the occultation by Trans-Neptunian Objects (TNOs)​

Why are you interested in main-belt comets?

They are a mystery group in the Main Belt.

Name one hobby of yours?


What is the most recent tv show you have watched?

The Wired​

What is the latest book you have read?

The Devotion of Suspect X by Keigo Higashino
Divergent trilogy by Veronica Roth

What is your favorite band/music artist?


Favorite cocktail or beverage?

Beer in general except STOUT​

What is an overlap and how can we tell?

Many of the comet candidates that have been flagged on Comet Hunters so far have turned out to be cases where an asteroid is “overlapping” a background star or galaxy, and the question has come up as to how do we identify such cases.  First, just to explain, an “overlap” is what we call a situation where an asteroid passes very close to, or even over, a background star or galaxy, as viewed in the sky (they are of course very far away from each other in physical distance!).  If the asteroid and background object are positioned just right relative to one another, it can make the asteroid look like it has cometary activity, when in fact it does not.

In terms of identifying overlap situation, we first note that in most cases, it is actually often not possible to say anything definitive from the displayed image alone, such as the one shown here, which at first glance, certainly looks like an excellent comet candidate:

overlap1Subject 1295819

What we do in these cases is go back to the original image and search for other images of the same field, ideally from the same night and same telescope (since this means the images will look similar, having been taken under similar observing conditions and probably with similar exposure times), to compare them.  Astronomers often take multiple images of the same field within the same night because they are looking to see how a target changes over short periods of time, or because they want to be able to reduce the effects of image artifacts (adding together multiple images can aid the removal of cosmic ray hits, or by moving the telescope slightly between exposures, they can remove the effects of bad pixels on the detector), so in many cases, we can take advantage of this fact to allow us to check for overlaps.

If we can identify observations from the same night, we can look for a consistent appearance of the comet candidate from one exposure to another.  If a “tail” appears to switch sides between two different exposures, this is a good sign that it is actually due to the asteroid moving across a background object (real comet tails do occasionally appear to switch directions as seen from Earth, but not in the time span of a single night), as in the example below where we have identified another image of the same asteroid:

Subject 1295819 and Subject 1295854

or if the tail appears to simply disappear from one image to the next:

Subject 1295819 and Subject 1295842

Looking for a consistent appearance between exposures also helps us distinguish real comets from asteroids that only look like they have tails because they are faint, meaning that background noise can randomly appear to mimic activity.

Finally, if we can find many images of the same field/asteroid from the same night, we can sometimes actually see the asteroid moving across a background object and perhaps even see the background object emerge completely from behind the asteroid in question:

overlapSubjects 129581912958421295854, and 1295866

Of course, if multiple images are not available of the same field/asteroid from the same night, this task becomes somewhat more complicated, where we might need to retrieve data taken on different nights or even by different telescopes besides Subaru to see if we can identify a background source at the location of the suspected activity.  This is more difficult and may be less conclusive than the method described above because smaller telescopes or shorter exposures or images taken under worse observing conditions may not be sensitive enough to detect the suspected background object, making the check inconclusive.

Another check that can be done is to search for other images of the same asteroid taken at a similar time (e.g., using the SSOIS web tool, which we will discuss in more detail in a future blog post), or if that is not possible, then perhaps at least at a similar point in its orbit.  If similar activity is seen, then there is a good chance that the activity is real.  If not though, then this raises the chances that the original suspected activity was due to an overlapping source, or perhaps just noise or poor observing conditions.

It is of course possible that none of these checks will reveal anything conclusive.  This is one of the limitations of using archival data, that we don’t have control over the way the data is/was obtained, and so sometimes we will simply lack enough information to directly confirm or reject overlap cases.  In these cases, we have to rely on our intuition.  Is the star field crowded or not?  If it is, it suggests that the likelihood of an overlap is higher since there are more potential background objects to overlap.  Is the image quality good or not?  If not, it means that it is easier for a background star to be fuzzy and indistinct enough that it could reasonably be mistaken for a tail.

Finally, we might also ask if the object’s orbit and orbit position at the time of the observations consistent with those of other known main-belt comets.  We want to try very hard to avoid resorting to this particular line of reasoning because it creates a bias against discovering main-belt comets in unusual orbits or at unusual orbit positions, but in the event that no other information is available, we may use this to make a judgement call…i.e., if the orbit and orbit position are consistent with previously known main-belt comets, the object might be considered a stronger candidate than if those are not consistent.  We will still not be able to absolutely confirm that the activity is real, but we can put the object higher on our list of candidates to observe in the future when they again reach a point in their orbits at which activity might be expected.

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:

images_133p133P/Elst-Pizarro in 2002 and 2007 (from Hsieh et al. 2010, Monthly Notices of the Royal Astronomical Society, Vol. 403, p. 363-377)

images_176p176P/LINEAR in 2005 (from Hsieh et al. 2011, Astronomical Journal, Vol. 142, article 29)

images_238p_2005238P/Read in 2005 (a-c) and 2007 (d) (from Hsieh et al. 2009, Astronomical Journal, Vol. 137, p. 157-168)

images_238p_2010238P/Read in 2010 (from Hsieh et al. 2011, Astrophysical Journal Letters, Vol. 736, article L18)

images_324p324P/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)

images_p2012t1P/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.

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