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.
No comments:
Post a Comment