Comet ISON brightens as it closes in on Thanksgiving's solar climax

For more than a year, Comet ISON has been taking skywatchers on a roller-coaster ride, but the most dramatic thrill is coming up on Thursday — and the prospects look good for a spectacular show.

Over the course of less than a day, the comet's brightness "has increased by at least a factor of four, and indications are it may be closer to a factor of 10," the NASA Comet ISON Observing Campaign reported on Wednesday.

When we last checked in with what was once called the "comet of the century," ISON was heading toward the sun at the same time that a solar storm was pushing outward. Karl Battams, an astrophysicist at the Naval Research Laboratory who's part of the observing campaign, was almost gleeful over the prospect that the cosmic storm cloud would interact with the comet's tail.

However, it turned out that the cloud of electrically charged particles, also known as a coronal mass ejection, had no significant effect. "The source of the cloud is a farside active region, which is not directly facing the comet," SpaceWeather.com's Tony Phillips reported.

Even if the outburst had swept directly over ISON, that alone wouldn't have caused the comet to break up. However, some observers wondered whether the comet's nucleus or tail is being disrupted. So far, the images from the Solar and Heliospheric Observatory's LASCO C3 detector suggest that ISON is keeping it together. Mostly.

Battams reported that the comet had brightened to around magnitude +0.5, which is as bright as the star Betelgeuse in the "shoulder" of the constellation Orion. What's more, ISON appears to be behaving like a classic sungrazing comet. That is, it's behaving like Comet Lovejoy (C/2011 W3), which weathered its whirl around the sun and dazzled the Southern Hemiphere two years ago.

"We cannot comment on whether the nucleus is intact or not, but our analyses indicate that its rate of brightening is directly in line with that we have experienced with other sungrazing comets," Battams wrote. "This has no implications on its chances of survival."

Battams and his colleagues on the observing campaign advised solar observatories to watch for the comet to get even brighter as it rounds the sun. They also advised casual observers not to try looking for ISON in the sky over the next couple of days, due to the risk of eye damage. (But if you're set on trying, Sky & Telescope's Alan McRobert provides an observing guide.)

The best bet is to monitor the comet's passage online.

The climax comes around 1:37 p.m. ET Thursday, when ISON zooms within 730,000 miles (1.2 million kilometers) of the sun at a top speed of 850,000 mph (380 kilometers per second). For expert commentary and views from NASA's Solar Dynamics Observatory, tune into a Google+ Hangout that's scheduled from 1 p.m. to 3:30 p.m. ET Thursday. You'll hear from Battams as well as solar physicists C. Alex Young and W. Dean Pesnel, plus Phil Plait, the astronomer behind the "Bad Astronomy" blog. The Hangout will be streamed via YouTube as well as NASA's Ustream channel.

SDO images and movies will also be posted to NASA's Comet ISON Perihelion website. You can keep tabs on the images from the Solar and Heliospheric Observatory by checking the SOHO website (but don't be surprised if the Web traffic is as frantic as an airport before Thanksgiving). Comet ISON is also popping up in images from NASA's sun-watching STEREO spacecraft. NASA offers loads of information and imagery on its Comet ISON portal page.

Other sources for ISON pictures include this Flickr photo pool and SpaceWeather.com's gallery.

If the comet survives intact — or even if it takes a beating, as Lovejoy did in 2011 — you can expect ISON to make a spectacle of itself starting in early December.

Northern Hemisphere skywatchers would have the best vantage point: The comet would appear in east-southeast skies, just before the sun comes up. As the month progresses, ISON would be visible higher and higher in the sky — and eventually, it could be seen in western skies after sunset as well as eastern skies before sunrise.

But we're getting ahead of ourselves: Let's find out on Thanksgiving Day whether ISON turns into a triumph ... or a turkey.

Update for 5:10 p.m. ET: Battams and his colleagues are at the Kitt Peak National Observatory in Arizona to keep tabs on ISON, and he emailed this dispatch:

Update for 12:35 p.m. ET Nov. 28: The outlook for ISON's survival have dimmed overnight. That's because it looks as if the comet is fading as it nears the sun, suggesting that it has already shed too much of its ice and grit. "This seems to be seriously bad," comet-watcher Jakub Cerny writes on the COMETS-ML forum. Good or bad, stay tuned for the resolution of the cliffhanger.

More about Comet ISON:

• Will Comet ISON beat the odds?
• Observer's guide for Comet ISON
• Flash interactive: Inside a comet
• NBC News archive on Comet ISON

For updates on Comet ISON, keep tabs on Space.com, the Comet ISON Observing Campaign, Hubblesite's ISONBlog, SpaceWeather.com and the websites for Astronomy Magazine and Sky & Telescope. On Twitter, follow @ISONUpdates, @CometISONnews, @CometISON2013, @SungrazerComets and @Cosmos4u.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.