ISON passed Earth in early November on its way toward the sun, and was due for another fly-by in late December, with astronomers hoping for a particularly good view in the first days of the month.

Now, that's looking less likely.

While it's too soon to say exactly what ISON's fate will be, the comet seems to have been mostly destroyed by the heat and gravitational pull of the sun. Striking videos from NASA's SOHO Spacecraft show the comet dimming, then flaring, then dimming again as it shoots away from the Sun.

"ISON really is not looking at all healthy now... fading fast, no nucleus..." tweeted Karl Battams, a comet scientist for the Naval Research Laboratory, on Saturday, November 30th.

Still, says Andrew Fraknoi, ISON has given astronomers plenty to be excited about, and provided some spectacular imagery from telescopes on land and in space.

What makes a really great comet?

What people look for in a comet is both that the evaporated material from this chunk of dirty ice should make a beautiful cloud, and that there should be a long tail. A really magnificent comet will have a tail that goes across a considerable part of the sky, and is just a beautiful display in the night sky.

When is the last time we had a comet like that?

The best northern hemisphere comet was Comet Hale Bopp, which happened in the late 1990s. We are now some time away from the last good Northern hemisphere comet, although the southern hemisphere has been graced by a comet or two since then.

In a way, we’re getting a glimpse into the material that formed the solar system five billion years ago.

Let’s talk about what comet ISON is doing. Where is it coming from? Where is it going?

What we think is that it’s coming from the deep freeze of something called the Oort Cloud, a vast reservoir of comets way, way beyond the orbit of the outermost planet or dwarf planet.

It’s falling in toward the sun and then will speed around the sun and back out to the depths of space.

Sort of like a boomerang?

Yes, like a boomerang, except a boomerang will go back to where it started; this comet will go back to the depths beyond the planets.

Tell me more abut the Oort Cloud. What’s it like out there?

The Oort Cloud is roughly 50,000 times further from the sun than the Earth is. And it is a reservoir of icy leftover chunks, a couple of miles across, typically, which mostly stay out there minding their own business.

But occasionally a couple collide or something comes by and launches one of them toward the inner solar system.

And that’s what we think happened with Comet ISON, that it came from this great reservoir of icy leftover chunks, then something knocked it inward toward the sun and toward our vantage point.

This animation from NASA Goddard shows ISON's trajectory as it enters the inner solar system and boomerangs around the Sun:

http://svs.gsfc.nasa.gov/vis/a000000/a004000/a004017/CometISON2.slate.HD720.mp4

When was comet ISON discovered and by whom?

Comet ISON was discovered in September of 2012 by two young Russian observers, who are part of a network of people who are looking not so much for comets, but for asteroids that are coming near the earth and might represent a danger by crashing into the Earth. Their cameras revealed this new comet for the first time. And in honor of their network, the initials “ISON” became part of the name of the comet.

This is probably a good time to point out that ISON is not going to hit us. But it’s still very interesting to astronomers.

We are going to study the heck out of it. We have not just telescopes on Earth, but we have telescopes in space. There are telescopes near the Sun; we have a telescope orbiting the planet Mercury. So, we are going to study the heck out of this little comet.

And that’s important because it is fresh stuff. It’s never been processed by the Sun, by interactions with material in the inner solar system. So in a way, we’re getting a glimpse into the material that formed the solar system five billion years ago. The virgin stuff out of which everything in the solar system originally was made, and learning something about what the gases are, what the dust is, the composition is of the material.

"About half the water we have was delivered by comets that crashed to Earth in the early history of the solar system."

That’s a clue to our own origins five billion years ago.

Is it pure science fiction, this idea that a comet may have crashed into Earth carrying the seeds of some primordial life form that gave rise to us?

It sounds like science fiction, but it’s actually a very serious part of astronomy these days.

We know for a fact that when the early Earth formed, all that intense heat of forming the Earth probably drove away a lot of the water that was originally part of the material of our planet. So we believe comets that later crashed into Earth were a water delivery system.

About half the water we have was delivered by comets that crashed to Earth in the early history of the solar system. That’s well understood. Whether or not some of the actual building blocks of life, which we do see in other places in the universe, might have hitched a ride on a comet, and arrived on Earth after the Earth cooled, and helped life on Earth to form? That’s more speculative.

But it’s certainly something that gives us an extra interest in comets: The idea that comets could be the frozen delivery system of, not life itself, but some of the chemical building blocks of life.