Whether the bright spot seen moving away from the sun was simply debris or a small nucleus of the original ball of ice was unknown, although all that remains by now is likely only dust, the scientists said Monday.
As it neared the sun Thursday, the comet was obscured and became invisible to observing instruments, leading many scientists to surmise the comet had disintegrated completely. However, something did reappear some time later -- though it was significantly less bright.
While it seems likely that as of Sunday there was no nucleus left, the best chance of knowing for sure will be if the Hubble Space Telescope makes observations later in December, NASA said.
Regardless of its fate, Comet ISON did not disappoint researchers; the number of space-based, ground-based and amateur observations were unprecedented, with 12 NASA space-based assets observing over the past year, yielding data that will be analyzed for years to come, the space agency said.
The comet was named after Russia's International Scientific Optical Network, where it was discovered.