Image: University of Arizona, CaelumObservatory
Thursday is a holiday for most of America, but the Universe doesn’t celebrate Thanksgiving. While you’re eating turkey on November 28, comet ISON is going to make its close pass by the Sun coming within 625,000 miles of its surface. Whether it will live up to all the “comet of the century” hype remains to be seen—experts aren’t even sure if it’s going to survive its close encounter. But whatever happens, it’s going to be awesome, and will likely put on a stunning show.
Comet ISON originated in the Oort cloud, that distant cloud of debris, largely icy bodies, that orbits the Sun between 5,000 to 100,000 astronomical units (1 AU is the distance from the Sun to the Earth, about 93 million miles). The Oort cloud actuarially ends about halfway to our nearest neighboring star. Bodies in this outer region of our Solar System are so far from the Sun that the gravity in that region is incredibly weak. So weak, scientists said in a teleconference today, that a passing star a million years ago could have given the icy body we know as ISON the nudge it needed to begin its current journey into the inner solar system.
And that’s what makes it such an interesting object: this is ISON’s first trip to towards the Sun, and it might be its last. And all the information locked within its icy core are untouched and unweathered by solar exposure. Bodies in the Oort Cloud are the remnants of the early solar system, the bits of material that never coalesced into a planet, and this bit has been perfectly preserved in a deep freeze for the last 4.5 billion years.
Astronomers have been, and will continue, watching ISON’s demise as it’s approached the Sun. As it’s bakes and boils traveling through the inner solar system, astronomers study that evaporates, using this as a starting point to understand its composition. From there, they will be able to draw conclusions about what building blocks were present and necessary for our Solar System to develop into the one we know and love today.
Though ISON is visible from the Earth, most of the useful measurements scientists have taken of this comet have been with space-based telescopes. The Hubble Space Telescope sees ice subliming in the visible light spectrum, Spitzer and the SOFIA telescopes are watching dust come off the comet in infrared light, and the Chandra Observatory can see the solar wind in x-rays. Other spacecraft, too, are lending their eyes and instruments to observing the comet, like NASA’s MESSENGER and Mars Reconnaissance Orbiter spacecraft.
So, is it going to survive perihelion?
It’s tough to say; experts are giving it a 30 or 40 percent chance of coming through its close solar encounter in one piece. But whatever happens, it’s going to put on an impressive show. It all depends on ISON’s composition: If it turns out that the core of the comet is loosely bound ice and dust similar to a dirty snowball, it might come apart easily. From there, the broken pieces will become sun grazers in their own right, falling into solar corona and giving us a gorgeous show. If ISON is a more coherent body, it will still get hot and stressed by solar gravity. But it will come out the other side, and become a fine addition to the dawn and dusk in early December.
Opinions on ISON’s fate are so divided because this comet isn’t acting like others before it have. Sun-grazing comets are common, as are new comets from the Oort cloud, but a new comet from the Oort cloud on a Sun grazing trajectory is something new. It’s been acting strangely since it was discovered in 2012. It has flared up and calmed down again, spewed water then dust, and its behaved like an ordinary comet usually does.
Of course, ISON could also be devoured on its way towards the Sun by a Coronal Mass Ejection, a magnetized cloud of plasma flung away from the Sun. Comet Encke was hit with this type of ferocious solar storm in 2007 and had its tail ripped off. Since ISON is passing closer to the Sun than Encke, it’s likely to see more violent storms. Some astronomers would welcome this kind of assault on ISON. It would be a rare opportunity to study the interaction between the material in a CME and a comet’s tail, which acts like a windsock, giving astronomers giving astronomers information on the solar wind speed.
Whatever happens to ISON on Thursday, it’s going to be interesting. And you can watch it live, without risk of solar disintegration, on NASA’s "Fire v. ISON" Google+ hangout. Enjoy the show.