Comet ISON approaches the Sun

Hubble images Comet Ison April 2013

In this Hubble Space Telescope composite image the icy sun-approaching Comet ISON floats against a seemingly infinite backdrop of numerous galaxies and a handful of foreground stars. Taken with Hubble’s Wide Field Camera 3 on April 30, 2013. / Credit: NASA/ESA Hubble Heritage Team (STScI/AURA)

Laura Nielsen for Frontier Scientists

Our patch of universe has a visitor from afar. Comet C/2012 S1 (ISON), or Comet ISON, has astronomers across the world training telescopes and eager eyes on outer space.

Don Yeomans, manager of NASA’s Near-Earth Object Program Office, watches for objects whose orbits come close to Earth. He says ISON is “…Coming from the very edge of our solar system so it stills retains the primordial ices from which it formed four-and-a-half billion years ago. It’s been traveling from the outer edge of the solar system for about five-and-a-half million years to reach us in the inner solar system, and it’s going to make an extremely close approach” to the Sun on November 28th 2013, Thanksgiving Day. As the Comet is warmed by the Sun’s heat it brightens, releasing gasses long trapped in ice.

The anatomy of a comet

Comets consist of rock, dust, and ice bound together in a dense ball. We theorize that such space material could have provided the raw material for planets in our early solar system.

The center or core of a comet formed of rock and ice is called the nucleus. Surrounding the nucleus is a coma, a cloud of vapor, carbon dioxide, and dust particles that have been heated by the comet’s approach to a star. Behind the comet a tail or two can form as loose particles are pushed by the solar wind that emits from active stars. Comets might develop two tails: one made of dust that appears reddish as it reflects sunlight, and one made of bluish glowing gasses escaping the nucleus’ core.

Comet ISON’s coma cloud is greenish, which suggests that the comet’s core ices might hold an unusual amount of carbon dioxide. As the ice heats up, the carbon dioxide is released as gas (outgassed). That outgassing causes the comet’s core to spin.

Whatever materials create comets might have served as early chemical building blocks to life.

NASA Stereo A images Comet Ison and Encke near Earth Nov 2013

NASA’s STEREO-A spacecraft continues to observe Comet ISON as it approaches the sun. This still from the spacecraft’s Heliospheric Imager shows Comet ISON with long tail, the smaller Comet Encke and Earth (with bright vertical line) on Nov. 25, 2013. The sun sits right of the field of view. This is a cropped and enlarged view. / Credit: NRL/NASA STEREO/CIOC

ISON’s pathway

Comet ISON will be making a near approach to the Sun. Its perihelion, or closest point of approach, will pass about 683,508 miles [1,100,000 kilometers] from the Sun’s surface. 1 AU (astronomical unit) is the distance from the Earth to the Sun, and ISON will be a mere 0.0124 AU from our star.

ISON’s approach is coinciding closely with the approach of smaller Comet Encke, which orbits our Sun once every 3.3 years. If the Sun’s activity caused a coronal mass ejection to strike both comets during their trip around the Sun, we would get to see the CME’s result on celestial bodies of different compositions.

ISON comes from the outer reaches of our solar system, and –unlike Encke–  has never experienced a near-pass with the Sun’s radiation or immense gravitational pull. On Thursday we will get to see fairly pristine matter from the solar system’s creation interacting with the Sun for the very first time.

The three possible outcomes

On November 28th Comet ISON will make its closest approach to the Sun, circling our star before shooting away again back into the dark reaches of space.

Perhaps, ISON will survive its near-pass with the Sun fairly whole and blaze away. We’ll be treated to a bright show after Thanksgiving as ISON once again heads to the outer reaches of our solar system.

Alternately, the Sun’s gravitational pull might wreak havoc on the comet. ‘Tidal forces’, or gravitational forces that pull more strongly on the side of the comet nearer the Sun than they do on the side further away, could elongate and dismantle the comet’s nucleus, turning the comet into multiple fragments. Even if that doesn’t happen, the outgassing from melting ice might be so extreme that the comet’s spin accelerates and rotational forces cause ISON to fall to pieces. This result would also make a lovely sight: a string-of-pearls formation crossing the sky as many fragments of the comet continue along ISON’s original trajectory, leaving multiple bright spots scattered across the sky.

If skywatchers are particularly unlucky, the comet might not be dense enough to survive its near-Sun pathway. ISON could simply disintegrate as it follows its path around the sun, leaving no fragments large enough to light up Earth’s night sky.

ISON’s discovery

Comet ISON was named for the the International Scientific Optical Network. Images from their 16-inch [0.4-meter] Santel reflecting telescope in Russia allowed amateur astronomers to discover the incoming comet in September 2012. After the comet’s discovery, backlogged images were searched for more hints of the comet’s path, and the eyes of the international astronomy community have been trained on Comet ISON’s approach. While earlier estimates pegged the comet as quite large because of its heavy outgassing (releasing gasses into space), newer estimates suggest Comet ISON’s nucleus is no wider than three-quarters of a mile [1.2 kilometers].

Watch NASA’s feature video: STEREO Watches Comet ISON
STEREO A’s Heliospheric Imager shows Comet ISON, Mercury, Comet Encke and Earth over a five-day period from Nov. 20 to Nov. 25, 2013. The sun sits right of the field of view of this camera. Credit: NASA/STEREO
Tracking the comet Many eyes are tracking the comet. Amateur astronomers with simple equipment and professionals with access to huge telescopes are taking advantage of the opportunity. When ISON nears the Sun it gets more difficult to watch. Luckily, fleets of satellites and spacecraft are also tracking Comet ISON’s journey. NASA’s Jet Propulsion Laboratory tracks near-Earth objects like comets, asteroids and space-rocks whose orbits bring them near our planet. NASA’s twin sun-studying spacecraft Solar Terrestrial Relations Observatories (STEREO-A and STEREO-B) are watching, as are the Solar Dynamics Observatory (SDO) and the the Solar and Heliophysics Observatory (SOHO).

SOHO images Comet Ison's aproach to the sun Nov 2013

Comet ISON enters the field of view of the joint ESA/NASA Solar and Heliospheric Observatory, or SOHO, spacecraft’s C3 coronograph, where the brighter view of the sun itself is blocked to focus on the solar atmosphere, or corona. Nov. 26, 2013. / Credit: ESA/NASA SOHO/GSFC

Scientific yield

Whether ISON holds together, breaks apart, or disintegrates, scientists will have gained a great deal of knowledge.

Comets are composed of debris which originated when the solar system formed about 4.6 billion years ago. ISON is from debris that originates particularly far away from the Sun. The mysterious Oort Cloud sits up to 100,000 AU (astronomical units) from the Sun. That’s 93 million miles [150 million kilometers] times 100,000. It’s so far out in our solar system that it’s 1/5 of the way to the nearest star. Scientists have inferred the Oort Cloud must exist because of the nature of other long-range comets approaching our Sun over the years. The Oort Cloud might represent the remains of a far-away failed planet: debris from a protoplanetary disk that formed but never became a complete planet.

ISON has been traveling through cold space, which means that its composition is relatively untouched. It represents what tho solar system was like 4.6 billion years ago. As it approaches the Sun the bundle of ice and rock heats up, and its water boils. We have instruments watching what gasses get boiled off its surface, which lets astronomers and scientists decode what the comet is made of — what raw components were present during the formation of our solar system. With that knowledge, we can work towards understanding what materials helped build our solar system’s planets. ISON’s close interaction with the Sun will also teach us more about how our star’s activity impacts nearby celestial bodies.

Watch it

On Thursday November 28th, NASA will hold a Google+ Hangout from 1 p.m. to 3:30 p.m. EST (10 a.m. to 12:30 p.m. PST) [1800 to 2030 GMT] “FIRE vs. ISON: Watch the epic battle live.” Join the experts to find out the fate of Comet ISON, live

If Comet ISON survives its near-pass with the Sun either intact or in large fragments, it should be visible from the Northern Hemisphere near the Eastern horizon before dawn in early December . Eventually, it will also be visible in the Northwest after sunset. And as ISON continues to rise in the sky, it may become dimly visible during the day. Keep your eyes to the sky, or on the many online resources tracking this visitor from far away.

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  • ‘Comet ISON: What’s Next?’ Dr. Tony Phillips, NASA (Nov16 2013)
  • ‘The first papers about Comet ISON are beginning to appear’ Matthew Knight, NASA Comet ISON Observing Campaign (Sept30 2013)
  • ‘The outer solar system remains mysterious’ Charles Q. Choi for, posted on NBC News (2013)
  • ‘Three Questions About Comet ISON’ DC Agle, Jet Propulsion Laboratory (Nov25 2013)

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