Three weeks from perihelion, ISON still puzzling

With perihelion fast approaching, Comet ISON is brightening steadily as it makes its first and only trip towards the inner solar system. The comet, once dubbed “Comet of the Century” by amateur astronomers after it was discovered at an alarmingly bright magnitude over a year ago, has fallen short of expectations since then. But studies by various scientists have only left confusing results and differing opinions. Some say the comet is on its way to disintegration, others are expecting an outburst of brightness within the next two weeks. ISON, however, will do what it wants — as it continues its trek towards the sun, and then outward into space once again.

Comet ISON, imaged November 3rd 2013 by Rolando Ligustri.

Comet ISON, imaged November 3rd 2013 by Rolando Ligustri.

Discovered last year by two amateur astronomers, and named after the International Scientific Optic Network, Comet ISON immediately was given high hopes by its brightness upon discovery. But the premise of such expectations is a bit unfair. ISON was given quite a high bar to reach, with some indicating it could be as bright as the full moon if it continued to brighten as it was at the time.  It did not, and the disappointment and disintegration theories began to surface and multiply. However, the comets answers may lie somewhere in between, and within the information we know and are continuing to collect today.

ISON is traveling along a path that no comet has before — not along the typical belt which comets (or great comets) do.  The typical belt is more common for orbiting comets, and is believed to be the remnants of a large comet which broke up millions of years ago. Still, ISON is on a puzzling path, thrust out of the Oort cloud a long (very, very long) time ago. The comet will slingshot through our inner solar system, graze near the surface of the sun, and then be thrown back out — never to be seen again. This leaves scientists with an interesting dilemma. ISON is the first of its kind, and no studies have been done on comets similar to ISON before.

It was only natural that, when ISON showed signs of departure from its forecast magnitude and secular light curve, speculation began to arrive. The “Comet of the Century” was not performing. Multiple studies published compared ISON to previous comets which had exhibited similar brightening “stalls” — all of which eventually disintegrated. But since then, ISON has continued to brighten quite admirably, although not at an overly exciting pace. The confusion among scientists lies in this very simple fact: We haven’t seen a comet like ISON in the past 50 years, and haven’t been able to study it adequately to be able to predict its behavior.

Secular Light Curve for Comet C/2021 S1 ISON by CIOC. Magnitude measurements courtesy MPC.

Secular Light Curve for Comet C/2021 S1 ISON by CIOC. Magnitude measurements courtesy MPC.

ISON, coming into our inner solar system for the first time, still has all of its “volatiles”, or dust/gasses/materials that are on and near the comet. These haven’t been burned off by the sun before. So, the behavior of the comet is inherently unpredictable. Some studies have concluded that ISON is on a mono-polar rotation, so that half of its surface isn’t feeling the effects of the solar wind — and won’t until it gets closer to the sun. And finally, additional information being collected suggests ISON’s nucleus may be smaller that once anticipated.

This leaves us around three weeks from perihelion, the date when ISON will rendezvous with our sun. ISON will feel intense gravitational pull, incredible heat, and will battle with incredible solar winds. It will undoubtedly brighten. But will it survive? Some say yes, some say no. The lack of much increased brightening to this point is somewhat concerning, but not cause for alarm. The truth of the matter is, we may not know until perihelion. In the next two weeks, we will begin to accumulate much more valuable information, from which we can begin to make more conclusions about ISON’s future. Until then, the comet will continue to puzzle scientists and amateur astronomers alike — all of whom are hoping for its survival, and a celestial show in late November and December.

Looking to get an early view of Comet ISON? The comet is currently hovering just underneath magnitude 8 (depending on who you ask, or which source you use). With a pair of good binoculars or a telescope, you can find ISON in the constellation Virgo during the early morning hours. In fact, it just crossed into Virgo from Leo earlier today. The comet won’t blow you away just yet — it will appear somewhat faint…dim, even. Over the next few weeks it should become easier to see. With some luck, it will become a naked-eye object in the night sky.