"The much hyped Comet ISON is not evolving in the fashion we had earlier anticipated. Rather than slowly but steadily gaining in brightness it has stagnated at basically near 16th magnitude for a couple of months now. After experiencing an interval where the coma's degree of condensation grew quite strong, the object threw out an unexpected strong but short tail that has persists right down to today. However, following this episode the coma subsequently faded, became less condensed and smaller, all bad signs regarding the "health" of the comet's nucleus. Whether ISON becomes a Great Comet next fall, or just another in a long string of Great Flops, is now much more a question than ever before."

ISON’s average speed throughout May will be 48,916mph. It will continue to accelerate steadily until 28th November when it’ll whip around our Sun at an astonishing 425,000mph!

September 28, 2012

As of now, Comet ISON, as it’s commonly being called, is roughly 625 million miles away from us and is 100,000 times fainter than the dimmest star that can be seen with the naked eye—it’s only visible using professional-grade telescopes.

Hubble photographed ISON on April 10th. At the time, the comet was 386 million miles from the sun (394 million miles from Earth), just inside the orbit of Jupiter. Even at that great distance the comet is already active as sunlight warms the surface and causes frozen gases to vaporize. A detailed analysis of the image reveals a strong jet blasting dust particles off the sunward-facing side of the comet’s nucleus

The team says these observations are in close agreement with features detected by the Hubble Space Telescope in April this year. As the comet comes closer to the Earth the spatial resolution will improve, and the team should get more detailed views on the coma structure.

Ongoing observations seem to reveal this comet is ‘shedding’ quite a bit of mass...

Nalin’s interpretation is that the curvature of this feature is not due to any rotational effects but is due to radiation pressure pushing dust grains towards the tail. Ultimately, this feature merges in with the tail.

When observed by Hubble, the comet's nucleus was about 3 miles (5 km) across and a dusty tail that stretches more than 57,000 miles (92,000 km) long.

Estimates suggest that the nucleus of ISON is no larger than 4-6 km (3-4 miles) across while the comet’s dusty coma 5,000 km (3,100 miles)across. A dust tail extends more than 92,000 km (57,000 miles).

Although the comet is 5.1 AU from the Sun and 5.3 AU from the spacecraft, and required a long exposure, it shows a diffuse nucleus and an anti-sunward tail indicating that it is already active. Although the tail appears short and stubby in this image, it is at least 65,000 km long and, due to projection effects, could be significantly longer.

Thu, Apr 25, 2013

By contrast, however, the comet was producing only about 130 pounds (60 kg) of water every minute, or about four times the amount flowing out of a residential sprinkler system. At the time, however, the comet was nearly half a billion miles from the sun. Typically, a comet's water content remains frozen until it comes within about three times Earth's distance to the sun — about 280 million miles (450 million km) away. ISON won't be this close to the sun until early July at which time the water production rate should markedly increase.

...scientist Jian-Yang Li led a team that imaged comet ISON with the Hubble Space Telescope on April 10 using the Wide Field Camera 3. At this point, the comet was slightly closer than Jupiter at 386 million miles (621 million kilometers) from the sun and 394 million miles (634 million kilometers) from Earth.

The comet's dusty coma, or head of the comet, is currently about 3,100 miles (5,000 kilometers) across, or 1.2 times the width of Australia. A dust tail extends more than 57,000 miles (92,000 kilometers), far beyond Hubble's field of view.

Earth will basically be caught in a double-whammy meteor shower with dust coming from two directions.

The dust will be very fine, with the largest particles expected to be only a few microns across.

Noctilucent clouds are clouds that form about 50 miles above the earth's surface, pretty much at the edge of space.

“As the comet passes Earth’s orbit going into the sun, you’ll have particles trailing behind it. But since it’s passing so close to the sun, you’re also going to have particles pushed away by the pressure of the sunlight. That means we’ll have particles coming outward and also falling inward. We don’t often deal with particles that come both directions,” said Bill Cooke, lead scientist at NASA's Meteoroid Environment Office at the Marshall Space Flight Center in Huntsville, Ala.

The particles are so small that even though they will hit the atmosphere at about 125,000 mph, instead of burning up, triggering so-called “shooting stars,” they will be stopped entirely, predicts astronomer Paul Wiegert, with the University of Western Ontario in Canada.

According to Wiegert's computer models, the debris stream is populated with extremely tiny grains of dust, no more than a few microns wide, pushed toward Earth by the gentle radiation pressure of the sun. They will be hitting at a speed of 56 km/s or 125,000 mph. Because the particles are so small, Earth’s upper atmosphere will rapidly slow them to a stop.

"Instead of burning up in a flash of light, they will drift gently down to the Earth below," he says.

Don’t expect to notice. The invisible rain of comet dust, if it occurs, would be very slow. It can take months or even years for fine dust to settle out of the high atmosphere.

electric blue
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a strong metallic blue colour

Bill Cooke, lead scientist at NASA's Meteoroid Environment Office, says there's little danger to Earth-orbiting spacecraft. "These particles are just too small to penetrate the walls of our satellites, and they don't stand a chance against the heavy shielding of the ISS." However, he adds, mission operators will be alert around January 12th for possible anomalies.

Vitali Nevski (Vitebsk, Belarus) and Artyom Novichonok (Kondopoga, Russia) reported their discovery of a diffuse comet with an 8" coma on four 100-s CCD exposures obtained on Sept. 21.06 UT with a 0.4-m f/3 Santel reflector of the International Scientific Optical Network (ISON) near Kislovodsk, Russia. Nevski and Novichonok first reported the object to the Minor Planet Center (MPC) with no mention of cometary appearance, and it was posted on the MPC's NEOCP webpage as an apparently asteroidal object (their report to the Central Bureau for Astronomical Telegrams about cometary appearance arrived nearly a day after NEOCP posting); this resulted in the comet being named "ISON" by the Central Bureau, as per the IAU comet-naming guidelines.

We propose Warm Spitzer photometric imaging DDT observations of Comet ISON, as part of a worldwide observing campaign led by NASA of the comet. Warm Spitzer's 3 and 4 um near-IR passbands will provide unique scattering and emissivity information for the comet's nucleus and dust, and allow us to detect CO_2 gas emission from the comet. Spitzer holds a unique place in the solar system to observe the comet, right before it enters the region of rapid water ice sublimation after spending millions of years in the deep freeze of the Oort Cloud. It is likely the comet will undergo a rapid outburst soon after the time of the proposed observations in the week of June 7 - 14, 2013, and understanding its baseline behavior is critical to understanding the evolution of its activity, as is Spitzer's unique ability to detect CO_2 sublimation and outgassing activity from ISON. As for our previous Spitzer observations of comets Tempel 1 and Hartley 2, we will bootstrap from HST DDT imaging lightcurve and spectroscopic measurements taken over in April - May 2013. The combination of the two datasets will place strong constraints on the comet's coma morphology, and thus its pattern of outgassing jets and the rotation state of the nucleus, and the amount and kind of dust and gas emitted by the comet.

A photometric monitoring program of sungrazing comet C/2012 S1 ISON is presented. We have detected from photometry the importance of water sublimation at 5.7 AU.

Sublimation starts when comets are closer than about three astronomical units from the Sun

The Roche limit sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction.[1] Inside the Roche limit, orbiting material will tend to disperse and form rings, while outside the limit, material will tend to coalesce. The term is named after Édouard Roche, the French astronomer who first calculated this theoretical limit in 1848.[2]