ScienceNasa.gov - 24 November 2013

The Heliospheric Imager on NASA's STEREO-A spacecraft is tracking Comet ISON as it plunges toward the sun. In this movie, which spans a two day period from Nov. 20 to Nov. 22, 2013, the sun is off-screen to the right. Coincidentally, Comet Encke is present too

Nov. 24, 2013:  In 2007, astronomers were amazed when a solar storm hit Comet Encke.  NASA STEREO spacecraft watched as a CME (coronal mass ejection) struck the comet head on and ripped off its tail.

The same thing could be in store for Comet ISON--only worse.

On Nov. 28th, Comet ISON will pass through the sun's atmosphere, flying little more than a million kilometers above the sun's surface. It will be ~30 times closer to the sun than Encke was in 2007 and more likely to encounter a ferocious solar storm.

"For one thing," says Angelos Vourlidas of the Naval Research Lab and a participant in NASA's Comet ISON Observing Campaign (CIOC), "the year 2007 was near solar minimum. Solar activity was low. Now, however, we are near the peak of the solar cycle and eruptions are more frequent."

CHANCE OF FLARES: NOAA forecasters estimate a 25% chance of M-class solar flares today. The most likely blast-sites (sunspots AR1904 and AR1905) are not, however, facing Earth. Any eruptions this weekend will probably miss our planet. Solar flare alerts: text, voice

COMETS IN THE SOLAR WIND: NASA's STEREO-A spacecraft is monitoring Comet ISON as it approaches the sun for a close encounter on Nov. 28th. The latest movie from the spacecraft's Heliospheric Imager captures not only Comet ISON but also Earth, Mercury, and Comet Encke. Click to set the scene in motion:

In the movie, which spans a two+ day period from Nov. 20 to Nov. 22, the sun is to the right, off-screen. "The dark 'clouds' coming from that direction are density enhancements in the solar wind, and these are what are causing the ripples you see in the comet tails," explains Karl Battams of NASA's Comet ISON Observing Campaign.