Listed below are older updates on Comet ISON, which go back to April 2013. For recent updates, please point your browser to http://www.nightskyinfo.com/ison.

November 15 – Spot Comet ISON with the naked eye
November 14 – Sudden outburst, ISON is now visible with the naked eye!
November 11 – Comet ISON has now two tails
October 31 – Still only magnitude +9.5
October 5 – Comet ISON from the Mars Reconnaissance Orbiter
September 26 – ISON’s future brightness
September 5 – Visual observations and recent magnitude estimates
August 13 – C/2012 S1 ISON finally recovered
August 10 – Comet ISON in mid-August
July 5 – C/2012 S1 ISON remains behind the Sun throughout July
June 10 – John Bortle’s and David Seargent’s latest updates on Comet ISON
June 5 – Time-sequence images from Gemini North Observatory
April 25 – Comet ISON from Hubble
April 5 – NASA’s Swift spacecraft observes Comet C/2012 S1 ISON

November 15 – Spot Comet ISON with the naked eye

Since its major outburst two days ago, Comet ISON has continued to brighten, admittedly at a slower pace. It is now magnitude +5.5 and can be spotted with the unaided eye under a dark sky, low in the morning twilight, some 8° west-northwest of 1st-magnitude Spica.

Observers report that the comet looks stellar-like with the naked eye, glowing with a distinctly greenish hue. Through binoculars and small telescopes, ISON’s coma appears roughly 3 arcminutes in diameter, with a faint tail about 0.5° long.

ISON should continue to brighten as the week progresses, but starting around November 20 it will locate extremely low in the morning twilight. For tips on observing the comet in the following days, complete with finder maps, please see the Viewing Guide for November.

November 14 – Sudden outburst, ISON is now visible with the naked eye!

In the last 24 hours, Comet ISON experienced a major outburst of activity. Astronomers using the TRAPPIST telescope observed a two times increase of gas production rates and the comet brightened by one magnitude (almost three times), literally overnight.

TRAPPIST announcement (External link to the Comets-ml mailing list)

Just minutes ago, we had the first report of a naked eye sighting of ISON. Nicolas Biver from Spain observed the comet at around magnitude +6 (estimated from naked eye and binoculars). This is great news; if you have access to a location with a clear eastern horizon, now is the time to observe ISON!

Observation report (External link to the Comets-ml mailing list)

November 11 – Comet ISON has now two tails

Starting around November 6, amateur astrophotographers noticed a new prominent ion (or gas) tail, which appears as a fork just below the main dust tail. Ion tails develop because of the pressure of sunlight, which drives very small particles out of the head of comets, while dust tails are due to the pressure of the solar wind; in general an ion tail is straight, while a dust tail is curved.

The new tail is a clear sign that ISON is fortunately still in one piece and is experiencing increased activity, as it gets closer to the Sun, with only 17 days left to perihelion. Although the ion tail is evident on photographs, there are still no reports of visual observations.

Comet ISON and its New Ion Tail
Comet ISON on November 6. The bifurcated ion tail is clearly visible just below the main dust tail. Damian Peach

More images from Michael Jager (External link to Spaceweathergallery.com)
More images from Mike Hankey (External link to Mikesastrophotos.com)

October 31 – Still only magnitude +9.5

Comet ISON started October at magnitude +10.5 and throughout the month it brightened by only one magnitude. It is currently magnitude +9.5, and can be spotted with a 6-inch telescope fairly low in the east just before dawn. Based on the sluggish rate at which the comet has brightened so far, it is now clear that by late November (just before perihelion) ISON will be magnitude +7 at best.

When it comes to brightness predictions for closest approach to the Sun on November 28 and afterwards, nobody knows how Comet ISON will fare. Chances remain that ISON will reach magnitude -5 (a little brighter than Venus at its best) for just a few hours on perihelion day, very close to the Sun in broad daylight. After November 28, the comet will begin to fade rapidly, and will remain lost in the Sun’s glare until about December 5.

If ISON will emerge intact from its passage through the solar corona, it will likely be only 4th or 5th magnitude after December 5. This means that it will never reach naked eye visibility and will be observable only with binoculars low in the bright twilight.

In the likely scenario that ISON will break apart just after perihelion, we can truly hope for a Great Comet. Disintegration at the right moment would expose more icy surface to the solar heat, meaning that a huge amount of gas and dust will be released (this, in turn, will make the comet appear much larger and brighter).

For now, ISON continues to brighten slowly but steadily and is looking reasonably healthy but far from spectacular.

Hubble Space Telescope View of ISON
This image of Comet ISON was taken with the Hubble Space Telescope on October 9. At the time, the comet was still 138 million miles (223 million kilometers) from the Sun. NASA/ESA/STScI/AURA

October 5 – Comet ISON from the Mars Reconnaissance Orbiter

The Mars Reconnaissance Orbiter, in orbit around Mars since March 2006, pointed its HiRISE camera on Comet ISON a few days ago, September 29. The spacecraft imaged a small spot at the comet’s position, and although far from spectacular, it is not everyday that you see a picture of a comet taken from a Mars orbiter.

Comet ISON from the Mars Reconnaissance Orbiter
Comet ISON from the Mars Reconnaissance Orbiter. The image shows a 256×256-pixel patch of sky and has a scale of 8 miles (13.3 kilometers) per pixel. NASA/JPL/University of Arizona

More on this story (External link to the HiRISE website)

September 26 – ISON’s future brightness

ISON currently glows a little brighter than magnitude +11 and can be spotted with 6-inch telescopes in the constellation Leo the Lion. It lies very close to golden orange Mars in the predawn sky, about one-third of the way up from the eastern horizon. On October 1 the comet will make its closest approach to the Red Plant (see FAQ #6) and hopefully the scientists at NASA will use the cameras onboard the Curiosity and Opportunity rovers to take pictures.

When it comes to ISON’s future brightness, there is really no saying. The editorial team at Skyandtelescope.com (which includes comet expert John Bortle) suggests that ISON will never reach naked eye visibility, unless the nucleus will break up at or just after perihelion on November 28.

If the nucleus breaks at the right time, ISON will likely become a spectacular sight in mid-December. There is a precedent for this – Comet Lovejoy of 2011 had a nucleus about ten times smaller than that of ISON and was still magnitude +13 just three weeks before perihelion. The comet fragmented during the coronal passage and became a long-tailed spectacle for Southern Hemisphere observers.

Comet ISON on September 24
Comet ISON on the morning of September 24. Damian Peach

September 5 – Visual observations and recent magnitude estimates

A handful of experienced amateur astronomers have reported visual observations of C/2012 S1 ISON, even though the comet is still faint and very low in the morning twilight. ISON seems to be developing quite nicely and has certainly brightened since its recovery in mid-August. The comet is now around magnitude +12, with a coma one arcminute across.

Visual observations in chronological order:

Date: August 31
Magnitude: +13.2
Diameter: 0.8 arcminutes
Instrument used: 13 inch (33 cm) reflector
Magnification: 182x
Observer: Piotr Guzik

Date: September 1
Magnitude: +11.2
Diameter: 1.8 arcminutes
Instrument used: 8 inch (20 cm) reflector
Magnification: 133x
Observer: J. J. Gonzalez

Date: September 1
Magnitude: +13.1
Diameter: 0.6 arcminutes
Instrument used: 16 inch (41 cm) reflector
Magnification: –
Observer: Alan Hale

Date: September 3
Magnitude: +12.0
Diameter: 1.0 arcminutes
Instrument used: 14.5 inch (37 cm) reflector
Magnification: 257x
Observer: Bob King

Date: September 4
Magnitude: +13.0
Diameter: 0.8 arcminutes
Instrument used: 13 inch (33 cm) reflector
Magnification: 182x
Observer: Piotr Guzik

It appears that Comet ISON is still too close to the Sun for large ground-based observatories or the Hubble Space Telescope to risk their optics in the process of imaging it, so no professional images are available at this time. The Deep Impact spacecraft, which was supposed to begin observations of ISON in August, encountered an error and NASA lost contact with it…

In the meantime, amateur astronomers are taking amazing images of the comet. Below, a false-color image obtained on September 3 by Bruce Gary. The field of view is about 9×9 arcminutes, with north at the top and east to the left.

Comet ISON on September 3

August 13 – C/2012 S1 ISON finally recovered

Amateur astronomer Bruce Gary from Arizona recovered Comet ISON under challenging conditions, on the morning of August 12. He used an 11-inch telescope equipped with a SBIG ST-10XME CCD camera to image the comet, which at the time was only 6° above the horizon in the bright morning twilight.

It appears that ISON glows around magnitude +14, a lot fainter than many expected, but still five times brighter than in mid-May when it was becoming difficult to observe. Congratulations Bruce!

Recovery Image of Comet ISON
The first recovery image of Comet ISON (the small smudge at the center of the white rectangle is the comet). Bruce Gary

More on this story (External link to Bruce Gary’s announcement)

August 10 – Comet ISON in mid-August

Around mid-August, Comet ISON is expected to cross the “frost line” (also called the “ice line” or “snow line”). At this boundary, located roughly between 230 million miles (370 million km) and 280 million miles (450 million km) from the Sun, water ice within the nucleus will begin to sublimate more quickly and the comet should appear brighter.

In the meantime, on August 8, the Deep Impact probe on the other side of the Sun from the Earth has resumed observations of ISON. No data is available yet, as the downlink back to Earth is planned for today, August 10.

Amateur astronomers are currently trying to recover the comet, which is located very low in the glow of dawn. No luck yet, but hopefully in a week of two we will have some magnitude estimates.

On a closing note, something to read until some “real” news on ISON will emerge. It is a great article posted on August 8 on the NASA Comet ISON Observing Campaign website, which deals with all the negative speculation surrounding ISON recently.

Why I’m not particularly worried about ISON’s brightness (External link to the NASA Comet ISON Observing Campaign website)

July 5 – C/2012 S1 ISON remains behind the Sun throughout July

C/2012 S1 ISON remains behind the Sun throughout July, out of observation range. At mid-month, it lies 363 million miles (584 million km) from the Earth and 269 million miles (433 million km) from the Sun, between the orbits of Mars and Jupiter, in the asteroid belt. When astronomers will first spot it again, sometime in early to mid-August, we will be back with updates.

June 10 – John Bortle’s and David Seargent’s latest updates on Comet ISON

On the Comets-ml mailing list, a group for comet observers, long time comet researcher John Bortle posted an updated prognostication for Comet ISON. Things are not looking good…

Because from January through May Comet ISON remained stuck at magnitude +16, Bortle forecasts that it will brighten a lot slowly than initially expected. ISON will not reach naked eye visibility until mid-November, just a week or two before closest approach to the Sun. On November 28, at perihelion, Bortle expects the comet to briefly reach magnitude -6, making it visible to the naked eye in the daytime sky, close to the Sun.

After perihelion, Comet ISON will fade rapidly, and will keep brighter than magnitude +2 or +3 for just a few days. This is about as bright as the stars in Orion’s belt. Although not very bright, Comet ISON will display a spectacular tail, 30° or even 45° in length. The tail will remain impressive for just a week to ten days, after which it will rapidly wane.

This summarizes John Bortle’s group post, as he anticipates that any further critical predictions of the comet’s future behavior will have to wait until at least early September.

In a reply to Bortle’s message, coming one day later, David Seargent (comet researcher and author) agrees that John Bortle’s predictions will likely turn out to be close to reality. He adds that observing Comet ISON with the naked eye prior to perihelion seems rather doubtful and that the post-perihelion performance will make ISON a great comet or an overhyped cosmic dud.

John Bortle’s group post (External link to the Comets-ml mailing list)

David Seargent’s group post (External link to the Comets-ml mailing list)

June 5 – Time-sequence images from Gemini North Observatory

Astronomers from the Gemini North Observatory, located on Hawaii’s Mauna Kea volcano, released a set of time-sequence images of Comet C/2012 S1 ISON. The four images, spanning from early February through early May, show the comet’s remarkable activity, even at a distance of roughly 400 million miles (645 million km).

ISON’s growing coma and the short tail pointing directly away from the Sun are clearly visible. The coma forms when dust and gas trapped within the nucleus escapes, and surrounds the rocky surface to form a relatively extensive atmosphere. Solar wind and radiation pressure push the coma’s material away from the Sun to form the comet’s tail, which we see here at a slight angle (thus its stubby appearance).

Time-Sequence Images of Comet ISON
Images of Comet ISON obtained from the Gemini North Observatory on February 4, March 4, April 3, and May 4 (left to right, respectively; Comet ISON at center in all images). Gemini Observatory/AURA

More on this story (External link to Gemini Observatory website)

April 25 – Comet ISON from Hubble

The Hubble Space Telescope, responsible for some of the most dramatic discoveries in the history of astronomy, photographed Comet ISON on April 10. At that time, the comet was 386 million miles (621 million km) from the Sun, slightly closer than Jupiter’s orbit.

Even at this great distance, the comet showed good activity. Preliminary measurements from NASA scientists suggest that the comet’s nucleus is 3 miles (5 km) in diameter, and the coma extends more than 3,100 miles (5,000 km) across. ISON’s dust tail stretches for 57,000 miles (92,000 km), far beyond Hubble’s field of view.

Comet ISON from Hubble
This image of Comet ISON was taken with the Hubble Space Telescope on April 10. At that time, the comet was well inside Jupiter’s orbit, at a distance of 386 million miles (621 million km) from the Sun. NASA/ESA/J.-Y. Li/Hubble Comet ISON Imaging Science Team

The image was taken in visible light, using the Wide Field Camera 3. This 16 megapixel, high sensitivity, low noise array, is Hubble’s most technologically advanced instrument for taking images in the visible spectrum. The blue false color was added to enhance detail in the comet’s structure.

More on this story (External link to NASA website)

April 5 – NASA’s Swift spacecraft observes Comet C/2012 S1 ISON

NASA’s Swift space observatory, used primarily for the study of gamma-ray bursts, observed Comet ISON on January 30. At that time, the comet was 460 million miles (740 million km) from the Sun and glowed at magnitude +15.7. This is about 5,000 times fainter that the threshold of human vision.

The observations revealed that the comet was shedding about 100,000 pounds (50 tones) of dust per minute and smaller quantities of water, estimated at only 135 pounds (60 kg) per minute. These two figures have enabled scientists to estimate the diameter of ISON’s nucleus, at about 3 miles (5 km) across.

Comet ISON Imaged by Swift
NASA’s Swift multi-wavelength observatory imaged Comet ISON (the smudge at the center of the photograph) on January 30. NASA/D. Bodewits/UMCP

This is a typical size for a comet, but ten times larger than the nucleus of comet C/2011 W3 Lovejoy, which became a fine naked eye sight for Southern Hemisphere observers in late 2011.

Dennis Bodewits, astronomer at the University of Maryland College Park, notes: “Comet ISON has the potential to be among the brightest comets of the last 50 years, which gives us a rare opportunity to observe its changes in great detail and over an extended period”.

More on this story (External link to NASA website)