Comets - A Primer
With the recent attention given to comet ISON in the media, many
people wonder where comets come from and why they can produce such spectacular
displays in the night sky.
Like planets and asteroids, comets are members of our solar system. But unlike planets, their orbits are highly elongated which allows them to travel great distances from the Sun. Comets like Siding Spring and ISON originate in a region of space called the Oort cloud. This thick, roughly spherical shell surrounds our Sun at such a huge distance that it stretches nearly halfway to our nearest stellar neighbor. Occasionally some disturbance sends one of these objects on a course towards the Sun in an orbit that may take several million years to complete. Comets that have a much shorter orbital period (less than 200 years) come from a region just outside the orbit of Neptune called the Kuiper belt.
Like planets and asteroids, comets are members of our solar system. But unlike planets, their orbits are highly elongated which allows them to travel great distances from the Sun. Comets like Siding Spring and ISON originate in a region of space called the Oort cloud. This thick, roughly spherical shell surrounds our Sun at such a huge distance that it stretches nearly halfway to our nearest stellar neighbor. Occasionally some disturbance sends one of these objects on a course towards the Sun in an orbit that may take several million years to complete. Comets that have a much shorter orbital period (less than 200 years) come from a region just outside the orbit of Neptune called the Kuiper belt.
Comparing comet orbits -- a Starry Night graphic
Regardless of origin, comets are irregular lumps several kilometers long composed of rock, dust and frozen liquids and gases such as water, carbon dioxide, carbon monoxide, methane and other organic compounds.
As comets approach the Sun, the volatiles begin to sublimate and in the process carry along small bits of dust and rock forming a halo or coma around what has now become the cometΚΌs nucleus. Radiation pressure pushes these materials in a direction away from the Sun in a tail that may stretch for millions of kilometers.
NASA image of Hartley 2
Most comets are faint and require binoculars or a telescope to be seen. But every once in a while a comet has a large and very active nucleus and comes close enough to the Sun and Earth to provide a very bright coma and large dust tail easily seen even in city skies as was the case with comet Hale-Bopp in 1997.
Regardless of origin, comets are irregular lumps several kilometers long composed of rock, dust and frozen liquids and gases such as water, carbon dioxide, carbon monoxide, methane and other organic compounds.
As comets approach the Sun, the volatiles begin to sublimate and in the process carry along small bits of dust and rock forming a halo or coma around what has now become the cometΚΌs nucleus. Radiation pressure pushes these materials in a direction away from the Sun in a tail that may stretch for millions of kilometers.
NASA image of Hartley 2
Most comets are faint and require binoculars or a telescope to be seen. But every once in a while a comet has a large and very active nucleus and comes close enough to the Sun and Earth to provide a very bright coma and large dust tail easily seen even in city skies as was the case with comet Hale-Bopp in 1997.
Comet Hale-Bopp April 1997
Comet ISON was predicted to be such a "Great Comet" but unfortunately it got a little too close to the Sun and was torn apart by the powerful gravitational force of our star. But with an estimated population of over a trillion comets perhaps our next visitor from these icy realms will indeed be the "Comet of the Century".
Comet ISON was predicted to be such a "Great Comet" but unfortunately it got a little too close to the Sun and was torn apart by the powerful gravitational force of our star. But with an estimated population of over a trillion comets perhaps our next visitor from these icy realms will indeed be the "Comet of the Century".
A Comet ISON Retrospective
It was touted by some as "the comet of the century," but
ended up turning into a Thanksgiving turkey. Along the way we had many a side
dish of alphabet soup. It was discovered in September of 2012 by Vitali Nevski
of Belarus and Artyom Novichonok of Russia using a telescope which is part of
the International Scientific Optical Network, or ISON for short. For that
reason, it became known as Comet ISON rather than Comet Nevski-Novichonok, but
its official designation was C/2012 S1.
Early computations of its orbit indicated that it would pass very close to the Sun on November 28, 2013, Thanksgiving Day in the United States. This proximity to the Sun suggested that it might become very bright, even visible in daylight. The media jumped on this prediction almost immediately, and the hype began.
Those of us with more experience with comets recalled David Levy's remark that comets are like cats: they have tails and do exactly what they want. It was tempting to believe the impressive sight which Starry Night (a software) predicted for sunset on November 29:
Early computations of its orbit indicated that it would pass very close to the Sun on November 28, 2013, Thanksgiving Day in the United States. This proximity to the Sun suggested that it might become very bright, even visible in daylight. The media jumped on this prediction almost immediately, and the hype began.
Those of us with more experience with comets recalled David Levy's remark that comets are like cats: they have tails and do exactly what they want. It was tempting to believe the impressive sight which Starry Night (a software) predicted for sunset on November 29:
This was a reasonable prediction based on the orbital elements
known at the time, but two events happened along the way.
First of all, the comet did not brighten as quickly or as much as predicted, so we had to scale back its predicted brightness. On the morning of November 14, it suddenly increased several magnitudes in brightness, and we hoped it might be back on track. This was the only morning I observed it with my own eyes: http://geoffsobservingblog.blogspot.ca/2013/11/1544m-20131114.html
But the really scary thing for everyone was what might happen on November 28, Thanksgiving Day. On that day, the comet was predicted to pass through perihelion, its closest approach to the Sun at an altitude of only 1,165,000 km (724,000 mi) above the Sun's surface.
First of all, the comet did not brighten as quickly or as much as predicted, so we had to scale back its predicted brightness. On the morning of November 14, it suddenly increased several magnitudes in brightness, and we hoped it might be back on track. This was the only morning I observed it with my own eyes: http://geoffsobservingblog.blogspot.ca/2013/11/1544m-20131114.html
But the really scary thing for everyone was what might happen on November 28, Thanksgiving Day. On that day, the comet was predicted to pass through perihelion, its closest approach to the Sun at an altitude of only 1,165,000 km (724,000 mi) above the Sun's surface.
By this time the comet was too close to the Sun to be observed
from Earth, so we turned to solar observatories in space to follow its dive
towards the Sun.
A word about SOHO and LASCO. SOHO is short for Solar and Heliographic
Observatory. This satellite was placed at Earth's L1 Lagrangian point in 1995
to record solar activity 24/7, and is still functioning well after 18 years,
returning images many times every day. These appear on the internet within
hours of them being taken:
http://sohowww.nascom.nasa.gov/data/realtime-images.html.
Two of its cameras were of particular interest for observing ISON, Large Angle and Spectrometric Coronagraphs #2 and #3, known as LASCO C2 and C3 for short. C3 has a field of 32 degrees and C2 6 degrees. In each camera, a disk blocks the Sun's direct image. Because SOHO is in space, there is no atmosphere to scatter the Sun's rays, so the background stars are visible in C3 down to about 6th magnitude. Planets and comets near the Sun are also recorded.
ISON first entered C3's field of view on the afternoon of Tuesday November 26. Two days later its two tails were plainly visible:
Two of its cameras were of particular interest for observing ISON, Large Angle and Spectrometric Coronagraphs #2 and #3, known as LASCO C2 and C3 for short. C3 has a field of 32 degrees and C2 6 degrees. In each camera, a disk blocks the Sun's direct image. Because SOHO is in space, there is no atmosphere to scatter the Sun's rays, so the background stars are visible in C3 down to about 6th magnitude. Planets and comets near the Sun are also recorded.
ISON first entered C3's field of view on the afternoon of Tuesday November 26. Two days later its two tails were plainly visible:
Something serious then happened to ISON. It faded several
magnitudes in brightness, suggesting a catastrophe triggered by the heat and
radiation from the Sun. It continued to fade as it disappeared behind the
occulting disk in the middle of the field.
At this point ISON entered the smaller field of the high-resolution C2 camera:
At this point ISON entered the smaller field of the high-resolution C2 camera:
ISON seemed to coalesce briefly into a wedge of debris, but this
had almost dissipated by the time it was leaving the C3 field of view:
This is probably the last view we will ever see of Comet ISON.
R.I.P.
All this, somewhat edited, is taken from an email I cannot link to. The
author is a Geoff Gaherty in Canada. His Company is at http://www.simulationcurriculum.com/index.html.
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