The astronomy community was abuzz with news about a comet discovery on Tuesday, leading to speculation that it could make a spectacular flyby of the sun, giving Earth a ringside seat of an extremely bright celestial event. It could become so bright that it even outshines the moon.
On the other hand, it might not. Confused? Well, that's comets for you.
The only thing that is certain is that a "big" cometary body has been discovered just beyond the orbit of Jupiter and it has an orbital trajectory that will take it very close to the sun - only 0.012 AU, or 1.8 million kilometers from the solar surface - toward the end of November 2013.
Its discovery is based on observations made by the 16-inch (0.4-meter) Santel reflector of the International Scientific Optical Network (ISON) in Russia. As pointed out by my colleague Alan Boyle over at Cosmic Log, since its discovery, "astronomers have gone back through their files to find ‘pre-discovery' images and calculate the comet's orbit." The comet has been designated as C/2012 S1, but it is being referred to as Comet ISON, unsurprisingly.
"In the best case, the comet is big, bright, and skirts the sun next November. It would be extremely bright - negative magnitudes maybe - and naked-eye visible for observers in the Northern Hemisphere for at least a couple of months," Karl Battams, of the NASA-supported Sungrazer Comet Project, told Spaceweather.com.
However, Battams concedes that this outcome is far from certain. "Alternately, comets can and often do fizzle out! Comet Elenin springs to mind as a recent example, but there are more famous examples of comets that got the astronomy community seriously worked up, only to fizzle," he said.
In a guest blog for the Planetary Society, astronomer Bill Gray agrees, pointing out that Comet ISON's orbit has been very well constrained, but just how bright the comet will become is anyone's guess.
"...estimating comet brightnesses a year ahead of time is about like asking who's going to win the World Series next year," writes Gray. "It could be astonishingly bright, or it could fizzle. I think it was David Levy (co-discoverer of Comet Shoemaker–Levy 9) who said that comets are like cats: they have tails, and do whatever they want to do."
So why all the uncertainty over Comet ISON's brightness as it careens through the inner solar system?
Comets originate from the outermost reaches of the solar system and are composed of icy volatiles such as water, carbon monoxide, carbon dioxide, methane and ammonia, plus dust, rocks and any other debris that happened to be floating around as our sun evolved. In the case of Comet ISON, it appears to originate from a hypothetical cloud of frozen comets surrounding the solar system.
The Oort Cloud - located approximately one light-year from the sun - is thought to contain billions of cometary nuclei that formed during the early evolution of the solar system.
"This is quite possibly a ‘new' comet coming in from the Oort cloud, meaning this could be its first-ever encounter with the sun," added Battams. "If so, with all those icy volatiles intact and never having been truly stressed (thermally and gravitationally), the comet could well disrupt and dissipate weeks or months before reaching the sun."
As comets approach the sun, the increase in solar energy causes frozen volatiles to sublime - i.e., turn from a solid ice to vapor, without passing through a liquid phase. This sublimation causes an eruption of gas and dust that gets swept back by the solar wind, forming a tail. The solid cometary nucleus continues its journey past the sun and, depending on its constituents, can create a very impressive tail that scatters sunlight, producing an impressively bright show.
But it all depends on what material the comet contains and how it formed in deep space. The comet may erupt early, fracture and break apart long before close approach, or it may remain solid long after it has swung past the sun, releasing very little material.
The uncertainty in Comet ISON's brightness shouldn't dent your confidence in its orbit, however. Although doomsayers get very turned on by new comet discoveries (take Comet Elenin for example), don't let them fool you into thinking Comet ISON is on a collision course with Earth - it doesn't come remotely close to us. According to the ace comet-hunters at Remanzacco Observatory, Italy, ISON will make closest approach with Earth around the beginning of January 2014 - at a distance of 0.4 AU (that's 40 percent the Earth-sun distance, or 60 million kilometers).
So, for now, sit back, relax and take note from the professionals: "I'd give it about a 30 percent chance of being exciting, with a 60 percent chance that I'm wrong. In other words, it'll certainly bear keeping an eye on, but I don't think anyone can say for sure right now," concludes Gray.
Image: False color rendition of Comet ISON, observed remotely from RAS Observatory. Credit: E. Guido, G. Sostero and N. Howes