A spectacular cosmic sky show could be coming in the form of a mountain-size comet. Hurtling toward us from billions of miles away, it will reach its fiery peak on Nov. 28, when it nears the sun. Here's a tool that can tell you exactly the path it's following — as well as the path of other comets that have come before

(Update: Thanksgiving Day, 2:18 PM ET: Astronomers working with NASA’s Comet ISON Observing Campaign (CIOC) continue to monitor ISON’s progress as it swings around the sun. No word yet on its condition, though the CIOC website’s latest blog entry puts things succinctly: “Hanging on by its fingernails” is all it says. The comet’s break-up will occur—if at all—at some point in its perihelion, or closest approach to the sun, today. If it makes it through that firestorm, it should be visible sometime before tomorrow. It will then seem to brighten over the next couple of weeks as it approaches, and then passes, Earth.)

(Update: Showtime for Comet ISON comes Thanksgiving night, when it makes its closest approach to the sun and will flare brightly—or not. It all depends on how well the comet has survived its inward trip through the solar system, and a slight dimming in its light yesterday has astronomers worried that it might have already begun to break up. ISON is moving fast—in excess of 150,000 mph [240,000 k/h]—but these matters still can’t be rushed. Tonight, we’ll know for sure.)

Every comet that’s ever been spotted is a fugitive on the lam. Comets really have no business visiting the inner solar system at all. They don’t remotely respect the orderly wheels of the orbiting planets — coming in at crazy inclinations and extended ellipses, colliding with any world that gets in their way and sometimes making suicide plunges into the fires of the sun. Where they really belong is out in the Kuiper Belt, a ring of icy, rocky objects that circle the solar system at a maximum distance of 4.6 billion miles (7.4 billion km), or in the vastly larger Oort Cloud, which stretches more than 5 trillion miles (8 trillion km) into deep space.

But now and again a comet gets jostled or gravitationally perturbed, breaks free and begins the long dive inward. Sometimes it embarks on a permanent, predictable path, sometimes it’s on a one-way trip and sometimes we just can’t tell — which is one of the things putting a recently discovered comet named ISON in the news.

It was only last year that Russian astronomers Vitali Nevski and Artyom Novichonok spotted the incoming rock, though they were denied the privilege of giving it their name because they were working as part of the International Scientific Optical Network, and the astronomical rules give the name — or acronym — of an umbrella organization precedence. Still, that makes Comet ISON no less real, and made it no less sensational when astronomers announced that it could be what some were describing as “the comet of the century.” What started that kind of buzz is the fact that ISON is a so-called sungrazer, in this case making a close approach to the solar fires of just 1.15 million miles (1.85 million km) on Nov. 28, 2013, and — if it survives that — swinging back out and passing earth at about 40 million miles (64 million km) on Dec. 26.

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Of course, comet of the century has been bandied about before. Remember Comet Kohoutek, which flew by in 1973? Probably not, mostly because it was a dud. Even the fabled Halley’s failed to live up to its rep in 1986, though that’s not saying it won’t make up for that underachieving when it comes back around this way in 2061. Already astronomers are walking back the predictions of the kind of sky show ISON is going to put on — or even retracting them altogether.

“It’s not going to be the comet of the century,” says Carey Lisse, senior scientist in NASA’s Comet ISON Observing Campaign, which is seeking to enlist as many telescopes and other instruments as possible to observe, record and study ISON’s ride through the solar system. “[But] going straight from the frozen Oort Cloud to next to the surface of the sun, it’s going to be the comet of the half-century — something closer to Kohoutek. Still, that makes it special.”

And something well worth paying attention to. ISON, like all comets, is ancient, made of material that coalesced when the primal cloud that was here long before we were was just forming into the sun, planets and moons. “Think of these things as the fossils of the early solar system,” says Lisse. ISON’s size — about 1 km to 2 km in diameter (0.6 miles to 1.2 miles) — is not uncommon for comets, but — as with other comets too — that makes it a bit of a puzzle. “Modeling-wise, we don’t know how to make a comet,” says Lisse. “We’re able to build planets from comets, but we don’t know how to build an object the size of a small mountain from primordial cosmic dust.”

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That dust is surprisingly tenuous stuff. Scientists measure density in units of grams per centimeter cubed. The more grams stuffed into that small space, the denser the object is. “Water ice is about 1 g/cm³ and rock is 3 to 5,” says Lisse. “Like other comets, we expect ISON is about 0.3 to 0.5, and that makes it very fluffy and porous.”

Such a loosely packed object, especially when it’s flying a sungrazer route, can come apart easily, and ISON — which has made trips only through the outer, colder solar system before, most recently a few million years ago — may not survive its current visit. “We see three possible outcomes: it’ll break up on the way in, it’ll die in the sun or it will survive and come back around to pass earth,” says Lisse.

But that same fragility is also what makes ISON such a good specimen for study. The comet has already begun fluorescing green as it begins feeling the effects of the sun, and color can reveal clues to composition. Hale-Bopp, which flew by the sun in 1997, bled off a lot of carbon monoxide, which gave a bluish cast to its gas. ISON’s gas is dominated more by cyanogen (CN, or a combination of carbon and nitrogen) and two different carbon radical molecules (C2 and C3), which likely accounts for its color. If the solar system were a biological organism, that would be like learning the chemical makeup of its most ancient cells.

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To maximize the number of eyeballs — both mechanical and human — monitoring ISON, Lisse and his group have helped gather more than a dozen major ground-based observatories, as well as 18 different spacecraft, which will observe the comet’s passage from their various outposts around the solar system. Among the spacecraft doing their share are the Curiosity rover on Mars, the Lunar Reconnaissance Orbiter circling the moon, the European Space Agency’s orbiting Venus Express and the Swift spacecraft, which studies gamma-ray bursts — not to mention NASA’s Hubble, Spitzer and Chandra telescopes, which will watch the comet in the optical, infrared and X-ray frequencies respectively. Amateurs everywhere are welcome — indeed encouraged — to pitch in themselves.

“We’d like to get every telescope in the solar system looking at ISON,” says Lisse.

He may or may not succeed at that, and ISON may or may not disappoint whatever telescopes do pay attention. But a half-century comet that last passed this way at the dawn of humanity is nonetheless arriving when 21st century science makes it possible to give it a going-over like no other. If nothing else, this particular refugee from cosmic history will reveal some secrets before it goes.

(PHOTOS: Window on Infinity: Pictures From Space)

Methodology

The orbital calculations for the interactive above were derived from Kepler’s laws of motion with ample help from the Jet Propulsion Laboratory, the source code of the OrbitViewer application and William Thompson of the Goddard Space Flight Center. The positions of the planets and comets are slight approximations and should not be used for the purpose of planning shuttle launches. Our unrefined source code is available on GitHub.

Code by Chris Wilson. Design by Alexander Ho.