Comet ISON — Will It or Won't It Survive Close Call With Sun on Thursday?

At about 1:25 pm Eastern Standard Time (EST) on Thanksgiving Day, Comet ISON will pass within 1 million miles of the Sun.  Whether that is its last hurrah or survives to dazzle the people of Earth for another day is literally up in the air.   Scientists are divided on the comet’s likely fate after it encounters the Sun.

During a NASA media teleconference today, Carey Lisse from the Johns Hopkins University Applied Physics Lab (APL) gave the comet just a 30 percent chance of survival, but stressed “That’s one man’s opinion” and he woud be happy to be proved wrong.

Comet ISON was first observed in September 2012 by two Russian astronomers and immediately got the attention of the astrophysics, heliophysics, and planetary science communities.  An analysis of its orbit shows that it is a rare comet coming in towards the Sun for the first time ever in the history of the Solar System, Lisse said.   It is from the Oort cloud at the outer boundary of the Solar System where comets left over from the formation of the Solar System exist in a rather haphazard collection of different orbits.  What makes this one so special is that it is pristine, until now untouched by solar forces that will change it forever.  Karl Battams, an astrophysicist at the Naval Research Lab, said that whatever happens to ISON after it grazes the Sun it “has already been a huge victory for science.”

Comet ISON, November 19, 2013, three minute exposure taken with 14-inch telescope at NASA Marshall Space Flight Center.  Image credit:  NASA/MSFC/Aaron Kingery.

Part of the reason the scientific community has learned so much is because the comet’s trajectory took it past a surprising number of spacecraft that could take a look.  NASA planetary science division director Jim Green listed NASA’s Deep Impact spacecraft (also known as EPOXI), the Mars Reconnaissance Orbiter in orbit around Mars, and MESSENGER, which is orbiting Mercury.  The newly-launched MAVEN spacecraft also may be able to sudy ISON with its ultraviolet (uv) spectrometer once that instrument is activated in a few weeks if ISON survives the solar encounter.  Michael Garcia from NASA’s astrophysics division and  Battams added a list of ground, air and space-based astrophysics and heliophysics assets being used to study it in a variety of wavelengths:  Hubble (optical), Chandra (x-ray), Spitzer (infrared), and SWIFT (gamma ray/x-ray/optical/uv) space telescopes; the NASA/ESA SOHO and NASA’s SDO and STEREO-A heliophysics satellites; sounding rockets; and the airborne Stratospheric Observatory For Intrared Astronomy (SOFIA).   Green called ISON “perhaps the most observed comet from a NASA perspective ever.”

Despite all that observing, scientific opinion is split on ISON’s fate.   The loose collection of dust and ice, which is about 1.5 miles in diameter, “has been in a deep freeze for 4.5 billion years” and will go from that “to the furnace of the Sun” on Thursday, Lisse said.  It has been “behaving oddly” compared to other comets, Battams explained, but it is important that people understand “We’ve never seen a comet like this that is both dynamically new from the Oort cloud and in a Sun-grazing orbit.  So we’ve seen plenty of Oort cloud comets, lots and lots of Sun grazers.  We’ve never seen the combination.”

The close approach to the Sun on Thursday will not be visible by people without special equipment because it is so close to the Sun.  NASA will host a Google+ Hangout that will be broadcast on NASA TV from 1:00-3:30 pm ET as Battams and other scientists follow the encounter.

If it survives, Lisse says it should be visible in the Northern Hemisphere about 10 days – 2 weeks after November 28 just before sunrise or just after sunset in the direction of the Sun.  APL will hold a meeting on December 6, 2013, which will be livestreamed, to discuss what scientists have gleaned to date.  They hope to learn more about how the solar system formed as well as about the Sun as it interacts with the comet, which could be important to understanding space weather.