How I Discovered Comet Vorobjov

By Tomas Vorobjov

October 15, 2012. It was a bit of an unusual Sunday. It had been some time since ARI (H21) was clouded out on Sunday which meant no NEOCP follow-ups to measure. I kept refreshing the view of my SSON FTP folder almost every minute for an hour or so, waiting for files from 8 jobs to be uploaded. The same eight jobs we originally scheduled for the Saturday night but the Mt. Lemmon SkyCenter telescope was used for local operations by visiting astronomers. Therefore, I had to move the jobs to Sunday. If the Schulman Telescope (G84) had been running for SSON on Saturday I would most likely not be writing this article right now.

The purpose of the scheduled observations was to gather data for minor planet search in one of the school campaigns organized by the International Astronomical Search Collaboration (IASC). The IASC is an education outreach program started by Dr. Patrick Miller in October 2006 at Hardin-Simmons University (Abilene, Texas).

Through this program high school and college students make original discoveries of Main Belt asteroids. IASC collaborators include Pan-STARRS (University of Hawaii – F51), Astronomical Research Institute (Westfield, IL – H21), the Xinglong Station (National Astronomical Observatories of China – 327), and since September 2012, we’ve been also using the Mt. Lemmon Sky Center (G84) through SSON. We get survey image sets from these observatories and the IASC Data Reduction Team (IDaRT), of which I’m the director, handles the image analysis. I compile and send a report to Minor Planet Center (MPC) with our discoveries (so that we get measurements in as soon as possible). We then distribute the images to schools via the Internet. When students identify objects in these images, the discovery credit is shared with them and they are offered to suggest names for objects once a permanent designation (number) is assigned. Over the years, students in IASC asteroid search campaigns have made 500+ provisional discoveries reported to the Minor Planet Center, including 15 numbered asteroids being named by their student discoverers. There have been two NEO discoveries including one potentially hazardous asteroid (PHA) along with one Jupiter Trojan discovery.

In September 2012, IASC started using the Schulman Telescope at the Mt. Lemmon Sky Center (G84) to gather survey images for the All-China Asteroid Search Campaign. During six nights (Sept. 18, 19, 21, 23, 24, and 26) we gathered survey images from 68 fields (each field is approximately 20’x20′), measured and reported 142 objects and discovered 27 new minor planets which now have received a provisional designation from the MPC.

Plot of the sky coverage by G84 between September 18 and 26, 2012

After a short break due to the brighter and brighter Moon and a transient cloud cover the plan was to restart the survey during the weekend of 14/15 Oct. The area of the sky to scan is not chosen randomly; there is a bit of reasoning behind the selection process. The best location to scan for new main belt asteroids is close to the ecliptic and up to about 2 hours in the right ascension (RA) ahead of the opposition point. Due to the opposition effect, undiscovered objects may brighten enough to be observable with a relatively short (60-120s) exposure. The constraints to stay close to the ecliptic and ahead of the opposition point reduces the suitable “real estate” for our survey to about 600 square degrees. The second important aspect to consider is the area of the sky observed by other observers. Ideally, the survey should scan an area not visited by another observer at least during the current lunation. As our survey aimed to reach objects down to magnitude 21, of which G84 is capable with about 120s single exposure, the only other observers needed to be considered are the major sky surveys: Catalina Sky Survey (703), Mt. Lemmon (G96), Pan-STARRS (F51) and Spacewatch (691, 291). The Sky Coverage service  provided by the MPC comes quite handy here. All these surveys submit their sky coverage data to the MPC usually at the end of an observing night so anyone can see which areas of the sky have been surveyed (and when).

Sky coverage plot displaying areas of the sky surveyed by the major sky surveys between Oct. 8 and 15, 2012

Examining the sky coverage plots to find the best area to survey, there didn’t seem to be any suitable “real estate” within 2 RA hours of the opposition point. There was a plenty of unobserved sky about 1 RA hour past the opposition point but observing there would be of low value. First of all, objects in that area of the sky are already becoming fainter and fainter (leaving the opposition effect) as the phase angle is increasing. Secondly, a vast majority of these objects have most likely been already observed at least during one night in the last 2-3 months when they were brighter (closer to the opposition point).

The area that most closely met our requirements was a relatively small patch of the night sky, about 4×4 degrees, between a declination of 12 and 16 degrees north past the RA 03:30. As the last step in the sky area selection process I did a relatively fast and simple check:

First, I used the MPChecker service to find a few dozen known main belt objects currently located in the area of the night sky. Then, I entered a list of these designations into the Minor Planet & Comet Ephemeris Service, which apart from the ephemerides also displays the date of the last observation.

Looking at the last observation date for virtually all of the known objects showed that none of them have been observed during the current opposition. Assuming that unknown main belt objects would be moving at approximately the same speed and position angle (PA) as the known ones, and given that none of the previously observed objects have been seen during this opposition, there is a very good chance most of the unknown objects would not have been observed either.

I scheduled 8 jobs to run at the Mt. Lemmon SkyCenter Schulman Telescope (G84) with coordinates RA 03:30:45 (45 RA seconds is a little less than half of the field of view so the edge would start just before RA 03:30) and the declination from +13:10 to +15:30 in 00:20 steps (the field of view is just a little of 20 arc-minutes, leaving a nice little overlap between the images in the series).

The sky covered by the G84 survey jobs on October 15, 2012

After downloading the images from October 15 it was immediately obvious the observing conditions were great. Objects down to magnitude 21.5 were detectable on these 120s exposures; seeing was sub-arc second. Nothing seemed extraordinary throughout the first five sets. There were 20 unknown objects ranging from V19.5 down to V21, all moving with the speed and PA similar to known main belt objects in this portion of the sky. In the sixth set, as I found another moving object, I proceeded with a familiar routine:

  • Determine if there is a known object at these coordinates
  • The object was unknown, so check which observer’s temporary designation was next in line

I marked the object as TOV7DD on all three blinking images and was about to move on when I noticed a tail-like feature happily riding along with what turned out to be a nucleus of the comet.

P/2012 T7 (VOROBJOV) discovery sequence from SSON/Mt. Lemmon telescope data.

As the tail was barely visible I decided I wasn’t 100% sure it wasn’t just a CCD artifact playing a cruel joke on me. Therefore, I reported the object as stellar to MPC and kindly asked Bob Holmes at the ARI observatory (H21) to image the target the following night. When images from H21 arrived, it was immediately obvious that TOV7DD was a comet with an elongated coma and a very nice 20-30″ long tail in PA about 265 degrees. Sergio Foglia who was analyzing and measuring other NEO Confirmation Page (NEOCP) targets observed at H21 that night arrived at the same conclusion about the nature of TOV7DD and reported it as a new comet to both the MPC and CBAT.

Confirmation of comet-like features of TOV7DD taken by R. Holmes (H21)

Because the NEO rating for TOV7DD was only 12, Gareth Williams from the MPC had to manually add it to the NEOCP. Consequently, I informed Gareth about the discovery observations from G84 from the night before and he updated the NEOCP manually again (rather than to wait for the automated routine to make the link after more observations have arrived).

After posting on the NEOCP webpage, other observers have found TOV7DD’s cometary features.

Stack of four 40-second exposures taken by Andrea Boattini at the Mt. Lemmon Sky Survey (G96)

Confirmation image taken by Nick Howes, Giovanni Sostero & Ernesto Guido from Faulkes North (F65)

The discovery of comet P/2012 T7 (VOROBJOV) was announced by the Minor Planet Center on 18 October, three days after the discovery. The preliminary orbit placed the comet into the Jupiter comet family.

Orbital elements:
P/2012 T7 (Vorobjov)
T 2012 June 16.58408 TT                                 MPC
q   3.7859963            (2000.0)            P               Q
n   0.07238300     Peri.  174.76846     +0.88063332     -0.45594956
a   5.7022333      Node   213.34342     +0.42575877     +0.88082526
e   0.3360503      Incl.   13.55433     +0.20788081     +0.12750238
P  13.6
From 46 observations 2012 Oct. 15-18

Preliminary orbit of Comet P/2012 T7 (VOROBJOV) based on observations between Oct. 15 and Oct. 18, 2012. Orbit is generated using the Orbital simulation applet provided by NASA.

 

 

Tomas Vorobjov

 

I was born on March 4, 1984 in Bratislava, Slovakia. I got my high school diploma at the United World College of the Adriatic (Duino,Italy) and a BA in Computer Science and Math at Colby College (Waterville, ME). After graduating from Colby (2006) I worked as a web application developer in New York, London, Amsterdam and back in London until July of this year when I moved back to Slovakia. In 2010 I joined the IASC and later became the director of the Data Reduction Team (IDaRT).

 

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