Comet ISON
Comet of the Century
Or Another Kohoutek?
Probably won’t be the comet of the century,
but it should be very interesting anyway
Matthew Knight
4/26/13
Matthew
Knight 4/26/13
What Are Comets?
• Dirty Snowballs??
• Icy Dirtballs??
“Model” made in class using dry ice (frozen CO2), water, dirt, Windex (ammonia),
chocolate syrup (CHx). White areas are frost that forms because of the cold dryice freezing out the humidity in the air.
Why study comets?
• Comets are remnants of
the formation of the solar
system
• Stored in the outer solar
system
– Largely unchanged since
formation
• Contain ice, dust, and
organics
– Building blocks of the
planets
– Ingredients for life
• Hazardous to life on Earth
Images from Google image search
Matthew Knight 4/26/13
Comets
Comets are Highly Diverse And Very Unpredictable!
Some Great Comets of the Past
C/2012 S1 (ISON) Historical
• discovered on September 21, 2012
• Artyom Novichonok and Vitali Nevski
• International Scientific Optical Network
(“ISON”)
• http://www.isoncampaign.org/History
Why ISON?
• Likely its first passage through the inner solar system
– Discovered at a large enough distance to study in more detail
than most “dynamically new” comets
• On a “sungrazing” orbit that will bring it very close to the
Sun
– Discovered much earlier than any previous sungrazer
– Perihelion Nov 28, 2013 at a distance of 2.7 solar radii
• Projected to get very bright near perihelion
• Favorable viewing geometry for observing from Earth
post-perihelion
Dec 25, 2013
Image credit: NASA
GSFC Scientific
Visualization Studio
Matthew Knight 4/26/13
Why are sungrazers special?
• Different temperature and stress
regime than typical comets
– Reveals least volatile components of
solar system
– Fragmentation is common
• High phase angles that can’t be
seen elsewhere in the solar system
– Yields unique information about dust
properties
C/2011 W3 Lovejoy
Image credit:
helioviewer.org
• “Solar probes” that can inform
studies of the solar wind, magnetic
field, corona, etc.
Matthew Knight 4/26/13
Sungrazing comet basics
• Perihelion distance less than a
few solar radii
– 1 solar radius = 0.0046524 AU ≈
700,000 km
• A handful of historically bright
comets in history
– Most are dynamically related to
each other as members of the
Kreutz group
• Coronagraphic observations over
last ~30 years have revealed a
steady stream of small fragments
– Most do not survive perihelion
– Typical observations span hours to
days
– Includes several new groups of
“sunskirting” comets
Top: Ikeya-Seki in 1965, Bottom: Great Comet of 1680
from Google Image Search
Matthew Knight 4/26/13
Orbit
• Perihelion is 28 Nov 2013
• 1,100,000 miles from the sun
• 6,700,000 miles from Mars on 01 October 2013
• 39,000,000 miles from Earth on 26 December 2013
e
1.000003965769344
a
-3151.448801895306
AU
q
.01249791904782889
AU
i
61.89330313591056
deg
node
295.7315226419997
deg
peri
345.5130824347451
deg
M
359.9982853205539
deg
tp
2456625.282733283460
(2013-Nov-28.78273328)
JED
period
na
na
D
yr
Orbit
•
•
•
http://svs.gsfc.nasa.gov/vis/a010000/a011200/a011222/index.html
Has some details about the path of ISON, but hidden on the page is a link to
http://svs.gsfc.nasa.gov/vis/a010000/a011200/a011222/Paper_Model_of_C
omet_ISONs_Orbit.pdf
Observing Comet ISON
Observing Comet ISON
These charts are from
http://www.mattastro.com/ison/charts.html,
but there are other sites like Stuart Atkinson’s
http://isonatlas.wordpress.com/
What can we expect to see?
• Potential to be visible
during daytime/twilight
• Visible from northern
hemisphere
• Lovejoy is a good
analog although it was
probably ~2-5x smaller
Matthew Knight 4/26/13
Missions Asked to Observe Comet ISON (to date)
• Deep Impact – no joy during planned summer observing window
• Stereo
• SDO
• Juno
• LRO
• ISS
• MSL/Curiosity
• MRO
• BRRISON
• MESSENGER
• SOHO
• Hubble
• Chandra
• SWIFT
• Rosetta
• Ground-based Observatories: Keck,
Comet Lovejoy as seen from the
NASA/IRTF, Kitt Peak, Sac Peak, Big Bear
International Space Station in late 2011.
• Amateur astronomy clubs
17
Comet ISON Potential Observations
Outbound
Inbound
M
M
V
E
V
E
M
M
• Inbound: Mars - Comet closest approach ~0.08 AU (Oct 4,2013)
– Late Sept: Earth - 8-10” telescopes will be able to observe
– Late Oct-Nov: Earth – Naked Eye Brightness
• Perihelion: Sun – Comet closest approach Nov 28, 2013, ~0.008 AU!
• Outbound: Earth - Comet closest approach ~0.44 AU (Jan 2, 2014)
Other NASA assets
• BRRISON balloon experiment (October)
– 120,000 ft altitude allows some UV/IR not possible
from ground

• Deep Impact Flyby spacecraft (ongoing)
– Unique ability to measure CO/CO2/H2O
– Continuous monitoring over many days possible potentially yields lightcurve and rotation period
– Observes ISON when not possible from ground
• Various Mars missions (October ~2)
– Close approach (0.07 AU) so high spatial resolution
• MESSENGER at Mercury (November ~19)
– More favorable viewing geometry than Earth at the
same time
Matthew Knight 4/26/13
Space telescopes
• Hubble
– Already observed in April, additional observations planned in
May, October, post-perihelion
– Key science: nucleus size estimate, composition, fragment
search
• Spitzer
– Solar elongation constraints: May-June 2013, Jan-Feb 2014
– Key science: dust properties, constrain nucleus size
• Swift
– Already observed in Jan/Feb, monitoring throughout apparition
– Key science: production rates
• Herschel
– Already observed in March
– Key science: composition, nucleus size
• X-ray telescopes (Chandra, XMM, Suzaku, etc.)
– Minimal pointing constraints
– Key science: Solar wind diagnostics, possible comet/corona
interaction
Matthew Knight 4/26/13
Will ISON live up to expectations?
• There is always a chance it will
underperform (e.g. Kohoutek in 1973) or
disintegrate completely (e.g. Elenin in
2011)
• Even if it massively underperforms it
should still be very impressive in
SOHO/STEREO images
• Recovered by amateurs ~12 Aug… still
difficult to observe since it is in the
morning twilight… not quite as bright as
one would hope…?
Kohoutek 1/12/74 at Palomar
Matthew Knight 4/26/13
Links
• www.isoncampaign.org/
• www.cometisonnews.com/
• waitingforison.wordpress.com/
– isonatlas.wordpress.com/
• solarsystem.nasa.gov/smallworlds/cometison.cfm
• www.astronomy.com/Events/Year%20Of%20The%20Co
met/Ison.aspx
• www.skyandtelescope.com/community/skyblog/observin
gblog/Comet-ISON-Updates-193909261.html
• svs.gsfc.nasa.gov/vis/a010000/a011200/a011222/