[meteorite-list] Fire vs. Ice: The Science of ISON at Perihelion

[Ron Baalke]

http://www.nasa.gov/content/goddard/fire-vs-ice-the-science-of-ison-at-perihelion/

Fire vs. Ice: The Science of ISON at Perihelion
Karen C. Fox
NASA's Goddard Space Flight Center, Greenbelt, Md.
Dec. 10, 2013

After a year of observations, scientists waited with bated breath on Nov. 
28, 2013, as Comet ISON made its closest approach to the sun, known as 
perihelion. Would the comet disintegrate in the fierce heat and gravity 
of the sun? Or survive intact to appear as a bright comet in the pre-dawn 
sky?

Some remnant of ISON did indeed make it around the sun, but it quickly 
dimmed and fizzled as seen with NASA's solar observatories. This does 
not mean scientists were disappointed, however. A worldwide collaboration 
ensured that observatories around the globe and in space, as well as keen 
amateur astronomers, gathered one of the largest sets of comet observations 
of all time, which will provide fodder for study for years to come.

On Dec. 10, 2013, researchers presented science results from the comet's 
last days at the 2013 Fall American Geophysical Union meeting in San Francisco, 
Calif. They described how this unique comet lost mass in advance of reaching 
perihelion and most likely broke up during its closest approach, as well, 
as summarized what this means for determining what the comet was made 
of.

"The comet's story begins with the very formation of the solar system," 
said Karl Battams, an astrophysicist at the Naval Research Lab in Washington, 
D.C. "The dirty snowball that we came to call Comet ISON was created at 
the same time as the planets."

ISON circled the solar system in the Oort cloud, more than 4.5 trillion 
miles away from the sun. At some point a few million years ago, something 
occurred - perhaps the passage of a nearby star - to knock ISON out of 
its orbit and send it hurtling along a path for its first trip into the 
inner solar system.

The comet was first spotted 585 million miles away in September 2012 by 
two Russian astronomers: Vitali Nevski and Artyom Novichonok. The comet 
was named after the project that discovered it, the International Scientific 
Optical Network, or ISON, and given an official designation of C/2012 
S1 (ISON). When comet scientists mapped out Comet ISON's orbit they learned 
that the comet would swing within 1.1 million miles of the sun's surface, 
making it what's known as a sungrazing comet, providing opportunities 
to study this pristine bit of the early solar system as it lost material 
while approaching the higher temperatures of the sun. With this knowledge, 
an international campaign to observe the comet was born. The number of 
space-based, ground-based, and amateur observations was unprecedented, 
including 12 NASA space-based assets observing Comet ISON over the past 
year.

Near the beginning of October, 2013, two months before perihelion, NASA's 
Mars Reconnaissance Observer, or MRO, turned its HiRISE instrument to 
view the comet during its closest approach to Mars in October 2013.

"The size of ISON's nucleus could be a little over half a mile across 
--- at the most.  Very likely it could have been as small as several hundred 
yards," said Alfred McEwen, the principal investigator for the HiRISE 
instrument at Arizona State University, in Tucson. 

In other words, Comet ISON might have been the length of five or six football 
fields. This small size was near the borderline of how big ISON needed 
to be to survive its trip around the sun.

During that trip around the sun, Geraint Jones, a comet scientist at University 
College London's Mullard Space Science Laboratory in the UK studied the 
comet's dust tails to better understand what happened as it rounded the 
sun. By fitting models of the dust tail to the actual observations from 
NASA's Solar Terrestrial Observatory, or STEREO, and the joint European 
Space Agency/NASA Solar and Heliospheric Observatory, or SOHO, Jones showed 
that very little dust was produced after perihelion, which may suggest 
that the comet's nucleus had already broken up by that time.

While the comet was visible in STEREO and SOHO images going into perihelion, 
it was not visible in the data from NASA's Solar Dynamics Observatory, 
or SDO, or from ground based solar observatories during its closest approach 
to the sun. Dean Pesnell, project scientist for SDO at NASA's Goddard 
Space Flight Center in Greenbelt, Md., explained why Comet ISON wasn't 
visible in SDO and what could be learned from that: SDO is tuned to see 
wavelengths of light that would indicate the presence of oxygen, which 
is very common in comets.

"The fact that ISON did not show oxygen despite how close it came to the 
sun provides information about how high was the evaporation temperature 
of ISON's material," said Pesnell. "This limits what it could have been 
made of."

When Comet ISON was first spotted in September 2012, it was relatively 
bright for a comet at such a great distance from the sun. Consequently, 
many people had high hopes it would provide a beautiful light show visible 
in the night sky throughout December 2013. That potential ended when Comet 
ISON disrupted during perihelion. However, the legacy of the comet will 
go on for years as scientists analyze the tremendous data set collected 
during ISON's journey.