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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
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