2I/Borisov: The First Interstellar Comet

Updated: 2019 September 25

The IAU has announced that observations are now conclusive that C/2019 Q4 (Borisov) originated from outside of our solar system. As a result, it has now been officially designated 2I/Borisov (the second interstellar object observed). 

Discovery and Early Observations

On the morning of September 10, A. Novichonok alerted the online comet community to the possibility of an interstellar comet. This comet had been discovered on search images obtained by Gennady Borisov on August 30. It was distinguished from an asteroid by a small fuzzy coma. When a comet has been discovered, its orbit is not yet apparent. It must be followed up by new observations. With each new observed position the orbit begins to become clear. Discussion continued throughout the day, with new observations added, and new analyses of the orbit. Over the next few hours it became clear that the orbit had a large eccentricity (around 3), indicating that it had very likely come from interstellar space. A few hours later the Minor Planet Center (MPC) issued a Minor Planet Electronic Circular (MPEC) officially recognizing the large (hyperbolic) eccentricity of the comet. The MPC serves as the clearing house for tracking small bodies in the solar system. They stopped short of declaring the comet as being interstellar awaiting more observations, but by then many of the leading observers and analysts were publicly calling it interstellar.

For my part, I manually added the comet to the "Current Comets" target list so that users of my SkyTools 4 Imaging software could plan their observations. Using my software I discovered that the moon was approaching full, but the comet could still be observed for about 30 minutes in the pre-dawn sky as seen from the northern hemisphere. I use iTelescope.net for remote imaging, and quickly checked telescope availability for the next morning. My first choice was already going to be in use, but another telescope (T11) at the nearby New Mexico Skies observatory was available. I scheduled 30 minutes on it and also on a larger telescope in California (T24) as a backup. A few minutes later I had plans generated for both telescopes that were uploaded and set to be run the next morning. 

When I reached my desk the next day an email with the results was waiting. The telescope in Califorina had returned an error because the comet was too close to the horizon, but the one in New Mexico said it had completed my three planned 10-minute exposures! The preview images in the email looked fantastic, although it would take some work to find the comet among the stars.

The consensus was that a deep exposure would be most useful at this time, so rather than expose to make an astrometric measurement of the position, I chose three 10-minute exposures while tracking the comet. As a result the comet appeared fixed on each image, but the stars made small streaks, or trails, as the telescope followed the comet during the exposure. 

The star streaks made processing the images difficult. Normally I would use Astrometrica to match the stars (or plate solve) to determine the exact position of everything on the image. I would locate the comet near where it was predicted. Then I would use the software to create a "moving object stack" that would accurately combine the images into one, such that the comet would be stacked in one spot. This would be the rough equivalent of one 30-minute exposure. But the streaks proved too much for the software to handle. Eventually I simply added the images together assuming that the telescope had tracked accurately over the entire 30 minutes. The result is above on the right.

To make a movie showing the comet move, it was a simple matter to use Photoshop to manually align each image such that the star streaks were on top of each other. I made the simple movie (above left) from the three aligned images, flipping between them at 0.5 second intervals. 

Science results (ongoing)

As I write this I am certain that observations are being planned that will reveal the nature of this object. Unlike the interstellar asteroid, 'Oumuamua, which passed us very quickly in 2017, our access will be for up to a year. In particular, spectroscopy will tell us what it is made of and long-term photometry will accurately reveal its rotation rate, inferring its size and shape.

September 14: The Instituto de Astrofísica de Canarias has published spectroscopy. Their preliminary result is that "this object has a surface composition not unlike that found in Solar System comets."

Be sure to check back here regularly. I will update the science results as papers are published and will interpret these results with an eye toward how the measurements are made and the limits of their interpretation rather than simply accept what the press is reporting. In my view, it is still rather unfortunate that a highly disseminated result promoted by ESA/NASA, that reported an extreme shape for 'Oumuamua (10:1), along with an artists rendering, has been popularly accepted when another more plausible tumbling model puts it closer to a 5:1 elongation, which eliminates the need for an extreme composition to explain it.

Links:

MPEC 2019-R106 : COMET C/2019 Q4 (Borisov)

MPEC 2019-R113 : COMET C/2019 Q4 (Borisov)

The Gran Telescopio Canarias (GTC) obtains the visible spectrum of C/2019 Q4 (Borisov)

Naming of New Interstellar Visitor: 2I/Borisov (IAU)

New Interstellar Visitor: 2I/Borisov imaged with Gemini

Interested in imaging this comet and others yourself? It is hard to beat the combination of iTelescope.net and SkyTools 4 Imaging!