Main Belt Comets and LSST
One of the main ideas of Comet Hunters is to see what activity in the asteroid belt looks like on a whole when we have the sensitivity of a 8-m class telescope. The Large Synoptic Survey Telescope (LSST) expected to start full science operations in 2022 will be surveying the sky nightly providing a new inventory of the Solar System. Michael Knight,a research scientist at the University of Maryland, has written a nicely blog post talking about cometary activity in the era of LSST. You can read it here.
Update on Comet Hunters
I know it’s been a long time since we’ve posted on the blog. As most astronomers and planetary scientists, the science team is juggling multiple projects and other support and service duties. It’s a new year, and some of us have have more time to devote back to Comet Hunters. Many thanks to our Talk moderators who have been pointing out are tirelessly pointing out questions and helping out with new members of the Comet Hunters community.
We’ve still been having issues getting new HSC subject images ready for the site, for now I’ve paused that workflow to focus on the Archival search, which is planned to be the project’s first paper. Thanks to your help we’ve moved through of the search, and we’ve uploaded the new batch of images. This set will basically finish off our sample of asteroids we wanted to search for the first paper. That’s why we decided to focus on this right now, rather than the HSC search.
This batch of Archival images includes some of the asteroids observed at launch but has improved positional accuracy and has sources identified in the images that were popping up as blank. It will important to have these classifications so that all the asteroid observations were produced the same way .
Thanks for your help with Comet Hunters. More news soon.
Making Simulated Main-belt Comets
Hello everyone!
I am Ishan, a summer student at ASIAA, Taiwan. I started working with the Comet Hunters team, about 3 weeks ago, on creating simulated Main-Belt Comet (MBC) images. Using appropriate mathematical functions, we are trying to create asteroid images with variable attributes like the direction of motion, brightness of tail and coma, etc. When ready, these images will be fed into the Comet Hunters website intermixed with the real images. How the project as a whole performs on these images will help us gain better insight into how well Comet Hunters can find different strengths of cometary activity and thus the true number of main-belt comets. For example, we can figure out up to what minimum brightness level of the coma (with respect to the nucleus) of the asteroid do the volunteers generally detect it.
For my present work, I am considering the nucleus to be just one pixel wide. For modelling the coma around it, we are using a 1/r profile centered around the nucleus. A sample coma is shown below. Note that the actual coma will be much fainter than the nucleus.
As we can see, there is ‘cross’ visible at the center. This is due to the fact that we are plotting a circular function in square pixel-grid. Now when we trail this image in a randomly chosen direction, we get a weird output.
As you can see, there is a skewed ‘X’ at the center of the trail. To check whether the trailing function is faulty, I fed it with a simple 2D gaussian coma. The resultant image looks pretty decent!
We are currently trying to figure out the issue with the 1/r profile. Maybe using polar coordinates will resolve this. I will get back to you with the developments!
Cheers!
Blends, Blends Blends
The science team is working on incorporating data from the Hyper Suprime-Cam (HSC) survey into Comet Hunters. We started with the archival Suprime-Cam data first to get a better understanding of what are the false positives and challenges for identifying Main-Belt Comets (MBCs) in data from 8-10-m class telescopes. We’ll continue with both datasets as there’s more Suprime-Cam asteroids, but when we have the chance we’ll move to reviewing the new HSC observations hopefully a few days after they’re taken.Most previous asteroid detection surveys are using 1-3-m class telescopes, so there are bound to be surprises that we wanted to know about before we developed the decision tree for the HSC snapshots on to the site. So we launched Comet Hunters with the archival Suprime-Cam images first. Now that things are going smoothly, we can turn our attention to the HSC data.
We combined your classifications from the first batch of Suprime-Cam images and had 125 candidates in need of further vetting. Thanks to volunteer Tadeáš Cernohous who on Talk went through our list comparing repeat images of the asteroid at slightly different positions in the same batch of subjects. What we learned that all of the candidates are unfortunately blends with stationary background sources. There are lots of faint background blobs that the asteroid moves on top of overlapping in the images creating very tail-like features. All of these images the science team would have had said has a tail.
A few examples are below (all blends with faint background sources):
Looks like a candidate tail, but a blend with a background source. We’d want this marked as ‘has a tail’ in the classification interface.
Looks like a candidate tail, but a blend with a background source. We’d want this marked as ‘has a tail’ in the classification interface.
Looks like a candidate tail, but a blend with a background source. We’d want this marked as ‘has a tail’ in the classification interface.
Looks like a candidate tail, but a blend with a background source. We’d want this marked as ‘has a tail’ in the classification interface.
There’s a lot more blends than we had anticipated given some of the team’s past experience with 2-m asteroid survey data. It’s still very much worth digging into the rest of the Suprime-Cam archive to look for MBCs. There might be many blends, but there could still be undiscovered MBCs too! Knowing that the background blends rate is much higher because of the increase in the photon collecting bucket is extremely useful. From the candidates, we could see the blends are faint blobby structures that would be likely hard to get a source extractor to pick up in all cases. Because of the quality of the HSC data and the repeat observation cadence we can try and take this into account possibly by checking the image of the asteroid and the repeat image of the same position take later on in the same night (not all Suprime-Cam images will have that and are taken in all types of sky conditions).
Now the Comet Hunters team is thinking about how best to develop a classification interface for the HSC data to include this. In the meantime, there are new Suprime-Cam images in need of review at http://www.comethunters.org if you have a minute or two to spare.
First Set of Images Complete but More Coming Soon
You might have noticed the blue banner currently on the Comet Hunters website. That’s because thanks to your help, we’ve completed the classifications needed to retire all the images that were live on the site. The team has been working to process a new batch of asteroid images. We’ve taken our time to improve on some of the data reduction issues you might have noticed in the launch images (streaked asteroids, more off center asteroids images, and some bad quality images). By having people spot and comment on these features in the images, we’ve been able to refine the data processing pipeline for this next batch of images. We will have those images live ASAP. Stay tuned to this space.
Most of the Comet Hunters science team chatted today, and we’ve decided to put on Talk our top comet candidates based on your classifications. As we’ve found thanks to your classifications and Talk comments, overlaps with background sources are a huge source of false positives for 8-m class telescope images of asteroids when you’re searching for comet-like tails. If you’re interested, we could use your help to review other images to see if the potential tail is a background galaxy or star when you view the same area after the asteroid has moved. More details here.
Suggested Comet Hunters Talk Hashtags
After classifying the asteroid image in the main interface, you’re presented with an option to discuss the image you’ve seen in Comet Hunters Talk, if you hit the ‘Talk’ button.
Comet Hunters Talk is a place where you can discuss the images with other volunteers and with the science team. You can also label then image you’ve classified with descriptive hashtags like #tail.
Thanks to our Talk moderators, we now have a list of preferred hashtags (see below) we’d like suggest you use on Talk to help flag images in ways above and beyond what we can learn from the classification interface and the questions we ask you there.
We aim to also do a search using these preferred hashtags later on in the year to search for comet candidates and identify false positives.
#tail – see a very clear and definite tail. Currently many people use this for any sign of a tail, but we’d like you to use this for anything you’re very sure of it. If you think there’s any chance it might a faint background star or galaxy then use the #possible tag. (example)
#possible – maybe has a tail, but not sure if it’s real or if it could be an overlap with a found background star or galaxy (example)
#overlap – asteroid is overlapping another star or galaxy (example)
#elongated – asteroid appears to be elongated/elliptical compared to the reference stars (example)
#offcentercandidate – you see a tail but it’s on a source not in the center of the crosshairs
#nearbyobject – where there is an asteroid visible in the center of the crosshairs, but there is a nearby detached faint object that’s not a tail (example)
#badrefstars – one or both of the reference stars are bad (looks like a galaxy or isn’t visible) or it’s hard to see one or both of the reference stars (example, example)
#poorimage – the asteroid image is of bad image (example)
These are suggestions. Talk enables flexible labeling, so if you don’t find any hashtags from the list above that matches what you see, definitely create a new one!
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.
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