Some say the world will end in fire, some say ice, and others say it'll end when carbon-dioxide levels fall to the point where photosynthesis is no longer possible and all plant species die.

The end of the world is a favorite topic of just about every major religion, whether heralded by the lion-scorpion monsters and Four Horsemen of the Apocalpse described in the Christian Book of Revelation or the battle of the gods foretold by Norse mythology. It's also fodder for the new movie Seeking a Friend for the End of the World, in which leading man Steve Carell embarks on a fateful journey with an eccentric neighbor, played by Keira Knightley, while the apocalypse looms.

While 2011 had a dud apocalypse prediction from preacher Harold Camping -- who joins preacher William Miller, televangelist Pat Robertson and a 19th-century English chicken in the fellowship of failed doomsday prophets -- the subject is still on many minds, thanks to the supposedly ominous end of the Mayan calendar this year.

Regardless of whether you give credence to ancient Mayan or Christian prophecies, there are some slightly more credible scenarios in which the world will end -- either with a bang or with a whimper.

Stop This Asteroid, I Want To Get Off

As in the movie, the most likely candidate for humanity's demise -- leaving out more obvious self-inflicted methods involving nuclear weapons or biological warfare -- is a rogue asteroid. Such an impact was thought to have wiped out the dinosaurs about 65 million years ago. It seems surprising that humanity hasn't had to face such a disastrous reckoning just yet.

NASA's Near-Earth Object Program keeps an eye on all comets and asteroids that are passing through Earth's vicinity, most of which are leftover bits and pieces from the birth of the solar system. About 100 tons of material from space makes impact with our planet each day, but much of that is in the form of tiny particles, according to NASA.

Last November, however, Earth had a bit of a close shave when the asteroid YU55, about the size of an aircraft carrier, passed within about 202,000 miles of us.

Jay Melosh, a Purdue University distinguished professor and expert in impact cratering, took the liberty of imagining what would happen if YU55 had not missed Earth. Had the asteroid impacted at 11 miles per second, he calculated it would blast a crater in the ground four miles wide and 1,700 feet deep, enough to obliterate a large city such as Chicago. People 60 miles away from the point of impact would get first-degree burns.

Impacts from asteroids of this size are very rare, Melosh said in a statement last year. They occur about once every 100,000 years, so the chances of an actual collision with an asteroid like YU55 is about 1 percent in the next thousand years.

Melosh also created an online impact calculator that lets you enter data on hypothetical asteroid strikes to see the resulting cataclysm.

The next most credibly threatening asteroid is Apophis, which is about one-third of a mile in diameter. There is a miniscule chance -- about one in 250,000 -- that it will strike Earth in 2036.

According to Melosh's calculator, the impact of Apophis, though a bona fide disaster, wouldn't exactly mean certain doom for all of humanity: It would create a crater a little more than two miles wide and 1,500 feet deep.

Another kind of end-of-days scenario doesn't so much posit destruction of the entire world -- just a radical change in its composition that makes it impossible for us to live. And we're probably likely to cause this change ourselves, albeit unintentionally.

One version of this kind of scenario is found in Kurt Vonnegut's novel Cat's Cradle, whose plot centers on a fictional substance called ice-nine, a special form of ice that converts all liquid water into more Ice-nine. When a sample of ice-nine is dropped into the ocean, it locks up all of Earth's water and brings about the end of humanity.

While there is a real kind of crystal called ice-nine, it doesn't have quite the catastrophic effects as Vonnegut's version. But there are other hypothetical ways in which a runaway chain reaction could transform much of the matter on Earth into something less habitable for life as we know it.

You Got Gray Goo In My Ice-Nine!

One possible scenario involves nanobots. Microscopic robots have a wide array of uses in medical and communication technologies, but manufacturing them might be difficult for us. Some nanobots, though, might be designed to be self-replicating, using materials around them to build more of themselves.

But how do you tell a nanobot what's acceptable building material or when to stop making copies of itself? If such checks aren't in place, the robots could take material from anything -- a car, a dog, a person -- to keep churning out more nanobots. And since those copies would also make copies, the process could continue exponentially until all the atoms on Earth are used as raw materials for nanobots, a seething planet of robotic grey goo.

The Earth could also be converted into strange mush thanks to an even more unlikely group: physicists. A current theory exists that particle accelerators like the Relativistic Heavy Ion Collider, or RHIC, at the Brookhaven National Laboratory in Upton, N.Y., could create strangelets, a particular kind of matter not yet observed on Earth. If certain hypotheses are correct, once a strangelet comes into contact with regular matter, it would set off a chain reaction that ends much in the way that Cat's Cradle did, with the planet Earth and all its inhabitants converted into a ball of strange matter.

But physicists say there's little chance of any of this happening. No particle accelerator has yet produced a glimpse of a strangelet, and scientists haven't yet found any strange matter on the surfaces of neutron stars, where it should be if the theory holds up.

A writer for New Scientist magazine pointed out in 2000 that the kinds of collisions that occur at the RHIC occur naturally when cosmic rays smash into heavy nuclei on the Moon. Yet the Moon has existed for 5 billion years without being devoured by a ravenous strangelet.

You Don't Have To Colonize Other Worlds, But You Can't Stay Here

Even if humanity manages to not blow up, liquefy, or otherwise destroy itself -- and the asteroids whizz harmlessly by -- Earth's days are still numbered, thanks to the aging of the sun.

In the Monthly Notices of the Royal Astronomical Society, researchers describe how 1 billion years from now, the sun will have grown so bright that it will bake Earth's surface to an average of 117 degrees Fahrenheit (47 degrees Celsius). If any life survives, it'll be in small pockets of water at the North and South poles. Over the next few hundreds of millions of years, the sun will continue to grow more luminous, until Earth is an overcooked wasteland like Mercury.

Our sun doesn't have enough mass to go supernova, but it will grow much bigger, possibly destroying Earth as it expands to 256 times its size 7.9 billion years from now.

Some researchers think Mars might be the only planet in the inner solar system to survive this cataclysm, so it might be best to start planning for a future summer home there -- at least until the universe ends.

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