In August I introduced you to comet ISON. In the last two months we have learned much more about the comet. More positional data has been collected about the comet’s orbit, as well as more observational data on its brightness, so this month I will give you an update.
From the positional data, we believe the comet has a hyperbolic trajectory. This means that this is most likely its first and only trip to the sun.
This data also supports the likelihood that ISON (C/2012 S1) is from the Oort cloud, a hypothesized spherical collection of comets that surround the sun at a distance of one-tenth to one full light-year. If ISON is an Oort cloud comet, it started its journey to the sun hundreds of thousands, perhaps millions, of years ago, meaning that ISON has been traveling toward Earth for all of recorded history.
The hyperbolic trajectory also means that comet ISON is not in orbit around the sun. As it is traveling toward the sun, the sun’s gravity causes the comet to gain speed, which gives the comet more energy. At closest approach it will be traveling at 234 miles per second — 842,400 mph. And at just 724,000 miles above the surface, the sun’s gravity will be so strong that it will cause the comet to swing around the sun and leave with enough energy to escape the sun’s pull and never return.
The comet also passed within 7 million miles of Mars on Oct. 1, which is about six times closer than it will pass by Earth on Dec. 26. This week NASA released images taken of the comet on Sept. 29 by the Mars Reconnaissance Orbiter (MRO), which is in orbit around Mars.
MRO took the images with its High Resolution Imaging Science Experiment camera. The images were taken when ISON was about 8 million miles from Mars. At this distance the resolution of the image is about 8 miles per pixel, so the comet appears as a single dot. The data collected from these images will provide better information about the size of the comet nucleus and its overall brightness.
In February 1,897 observations were used to create a predicted light curve— brightness versus time —for the comet. It predicted that the comet would brighten quickly, with a peak at perihelion on Nov. 28 of magnitude –17, brighter than the full moon. Unfortunately, this is not to be.
Current observations show that the comet has exhibited a “slowdown event,” like many other Oort cloud comets. So the comet is brightening, but at a slower rate than had been predicted.
Based on these latest observations, the new predictions are that the comet will reach a peak brightness of about magnitude –3 to –5, which is still bright for a stellar object. To get an idea of this brightness, look for Venus tonight at sunset low in the southwest sky; it is about magnitude -4.
The biggest unanswered question is, Will the comet survive perihelion on Nov. 28?
At closest approach, the surface temperatures on the comet are predicted to reach about 5,000 degrees Fahrenheit, hot enough to melt iron and vaporize rock and ice.
The gravity of the sun will create tidal forces on the comet that could be strong enough to break it apart even if it survives the heat. The most recent analysis says that it will be close, but predicts that the comet will emerge from its pass around the sun intact. We will not know for sure until after Nov. 28.
But either way, November should be a good month for ISON observing. As it nears the sun, the comet will be moving fast across the night sky. It starts November in the constellation of Leo, then moves to Virgo on about Nov. 6 and into Libra by Nov. 23.
The comet is in the morning sky and best seen in the hours before dawn. At the beginning of the month it is high in the south-southeast sky, moving closer to the horizon and the sun each day. (As the comet gets close to the sun, stop your observation. Looking at the sun for less than a second can do permanent damage to your eyes.) During the week of perihelion, Nov. 28, the safest viewing is at a website like SOHO — at 1.usa.gov/yMPkN — and you don’t have to worry about the weather.
During most of the month you will need a telescope or binoculars to see the comet. It is hoped that by the end of the month it might be visible to the naked eye in darker locations. The comet will pass less than one degree from the star Spica on the night of Nov. 17-18. If the weather is good, this may be the easiest time to find it.
So ISON may not be the comet of the century, but the comet of the decade.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.