Matthew Knight, an astronomer at the Lowell Observatory in Flagstaff, Ariz., told veteran space reporter William Harwood of CBS News:
"'Initially when something came out (after close approach), we thought this is just the dust trail and there's not much left, it's just going to fade away,' he said. 'And then images keep coming in and ... the last few, it seems pretty definitely like it's getting brighter. So we do not have a good answer as to what's going on.
"'My best guess right now, and it's really only an educated guess, is that there is something left, probably smaller fragments, because it still doesn't look like there's a nuclear condensation. Inbound, the leading edge was brighter. It doesn't look like that. It just looks to me like there are some smaller fragments that may just actually be disintegrating. They just took longer to do it.'
"But it's also possible, he said, there could be 'still a substantial nucleus there and it's actually outgassing. ... But I don't have any explanation for it if there's just nothing left.' "
ISON's closest approach to the sun, called its perihelion, occurred at about 1:45 p.m. ET, when it got 724,000 miles from the sun's surface. (By comparison, Earth is almost 93 million miles from the sun.)
"Once billed as the comet of the century, Comet ISON apparently was no match for the sun.
"Scientists said images from NASA spacecraft showed the comet approaching for a slingshot around the sun on Thursday, but just a trail of dust coming out on the other end.
" 'It does seem like Comet ISON probably hasn't survived this journey,' U.S. Navy solar researcher Karl Battams said in a Google+ hangout."
Phil Plait, an astronomer who runs the "Bad Astronomy" blog, told AP that even in that event, scientists might still be able to gain valuable knowledge from studying the remnants of the broken comet. "This is a time capsule looking back at the birth of the solar system," he said.
Scientists detected the comet last year and had hoped to continue studying it for information to be mined from its "primordial ices."
National Geographic had explained what ISON would experience on Thursday: "The comet has grown more than ten times brighter in recent days. As it plunges through the sun's outer atmosphere, the comet's icy nucleus will begin to experience intense gravitational forces and temperatures that reach as high as 5000 degrees Fahrenheit."
In September, NASA launched a balloon high into the atmosphere to study ISON, and explained its importance in a Q-and-A with Don Yeomans, manager of NASA's Near-Earth Object Program Office:
"It's coming from the very edge of our solar system so it [still] 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 and hence could become very bright and possibly a very easy naked-eye object in early December."