A comet looping behind the sun right now could emerge this fall as a once-in-a-lifetime spectacle that shines in the sky so brightly, it is visible with the naked eye around the world — if it survives.
Comet C/2012 S1, dubbed Comet ISON in honor of the network of observatories responsible for spotting it, is expected to pass about 40 million miles from Earth in December. As it grazes the sun, it could glow on the early morning and evening horizons from November into January if it survives that close-up encounter.
Some astrophysicists expect the sun's heat to break the icy ball into two or more chunks, meaning less of a show for the rest of us. But for scientists in Baltimore and around the world, there will be plenty to learn from the comet no matter what — about the sun, other solar systems, and potentially Earth's history.
"We prefer to wait before calling it the comet of the century," said Max Mutchler, a scientist at the Space Telescope Science Institute whose research projects are based on Hubble images and data. "You don't have to oversell this comet."
Some have predicted the comet could shine as brightly as a full moon, but more recent data suggests that won't be the case. Still, comets visible with the naked eye are relatively rare, with recent examples including Comet McNaught in 2007 (largely visible only in the Southern Hemisphere), Comet West in 1976 and Comet Ikeya-Seki in 1965.
No one knows how Comet ISON will compare because it has been hidden behind the sun for the past few months.
It had been on a journey likely tens of trillions of miles long when scientists from Belarus and Russia spotted its faint impression last September on images captured by a telescope near Kislovodsk, Russia. The telescope is part of a scientific collaboration across 10 countries called the International Scientific Optical Network.
Since then, telescopes and other instruments around the world and in space have tracked it as it hurtled from its origins in the Oort Cloud, a cluster of billions of comets that surrounds our solar system.
They include the Hubble, managed at the institute on the Johns Hopkins University's Homewood campus; a robotic spacecraft called Swift that is managed at the NASA Goddard Space Flight Center in Greenbelt; and EPOXI, a mission led by the University of Maryland, College Park. It also includes the Mars Reconnaissance Orbiter and solar observation satellites, as well as NASA's land-based telescopes and those of amateur astronomers.
What they have seen is a comet that, so far, is not quite as bright as some other so-called comets of the century. Beyond that, scientists haven't confirmed many details.
The comet, like others, is a "dirty snowball" made of ice and other particles. But it's not known just how those materials are structured. Too much ice at its core, and it's more likely to break up, for example. Scientists also haven't agreed on just how large the comet's nucleus is, with estimates ranging from three to four miles, or on what sorts of rocks, minerals or other compounds it contains.
Once they figure out all those details, it could shed new light on the formation of planets and other heavenly bodies.
"It's what gets kicked out when a solar system forms," said Dean Hines, a scientist at the space telescope institute whose projects focus on the evolution of stars and planetary systems.
Relative to frequently observed comets in our solar system, Comet ISON is pristine. Others with periodic orbits like Halley's Comet, which passes Earth once every 76 years, get repeatedly heated and cooled as they pass by the sun and back into outer space, making them appear black and scorched on the outside.
But scientists say Comet ISON hasn't made such a trip before, coming into our solar system in what is likely the same form in which it has spent billions of years circling the Oort Cloud. All the way out there, the comets are closer to our sun than any other star.
"One of the things we like to try to find out is, what was the composition of the original solar nebula? What were comets made of 4.5 billion years ago when the sun formed?" said Frank Summers, an astrophysicist with Space Telescope institute located at Johns Hopkins University. "ISON will tell us more about that than a comet like Halley."
The comet's makeup and density also could provide clues into the characteristics of other solar systems and the creation of planets like Earth. One theory suggests that ice-loaded comets helped fill Earth's oceans, but other comets in our solar system typically carry water with different types of molecules than those in our oceans, Hines said. Scientists will be eager to see if the ice on Comet ISON is more of a match, he said.
"It's like a fossil," Mutchler said of the comet.
And for any chance this comet will hit Earth, he offered "an emphatic no" — it will be 40 million miles away at its closest, on Dec. 26. Some time around Jan. 12, the planet will pass through Comet Ison's trail of dust and debris, but the particles are likely to be too small for a meteor shower, according to NASA.
The comet will serve as a "test particle" for solar physicists studying the sun's corona. At its Nov. 28 perihelion, when it is closest to the sun, the comet will be a mere 700,000 miles above the sun's surface. That means it will graze the layer of plasma that surrounds the sun, known as the corona, which is hundreds of times hotter than the sun's surface.
Observing how the comet reacts to the plasma could tell scientists more about the structure of the corona, assuming the comet stays in one piece, Summers said.
Answers to all of the questions should become clear in the coming months. Comet ISON became visible to some telescopes in late August, but won't be within Hubble's field of view until October.
Indications are so far that the comet isn't shining as brightly as it could have been based on earlier forecasts, which would make predictions that it will appear as bright as a full moon in our sky unlikely, Hines said. But it still could be as bright as the brightest stars in the sky, and with its wispy trail of gas and dust.
"There's a clamoring sense of excitement," said Summers, who's been invited on a cruise to the Canary Islands late this fall to talk about the comet. "We don't want to get too excited too early, but it's very intriguing to see whether it will develop."