Life on Earth could be wiped out any number of ways.
Most of the theories don't have scientific backing - but they do have popular followings.
Many doomsday believers expect Dec. 21 to be the date of destruction by a massive asteroid colliding with Earth, a sun storm disabling communication worldwide or other catastrophe.
People have had end-of-the-world astronomical theories for decades, said James McGaha, director of The Grasslands Observatory and a co-founder of Tucson Skeptics Inc. The expected date for doom has changed a few times, but this December is a popular prediction based on the ending of the "long-count" Mayan calendar.
People have multiple theories on each event and its result. The basic concept is earthquakes, tsunamis or other chaos will ensue after one or more astronomical occurrences - such as asteroid impacts or the alignment of certain planets - affect Earth on the same day.
NASA, astronomical organizations and Mayan scholars have tried to set the record straight, but questions persist.
David Morrison, director of the Carl Sagan Center for Study of Life in the Universe, runs NASA's "Ask an Astrobiologist" website. He gets five to 10 questions a day on astronomical apocalypse theories.
He's been answering these types of questions for about four years with the goal of stopping the spread of misinformation, he said.
"I have done that in science for a long time - I believe in education," Morrison said. "But it's been nothing like this. I hope on December 22 it stops."
Apocalyptic prophecies date to the fifth century B.C., said J. Edward Wright, director of the Arizona Center for Judaic Studies, who specializes in early Jewish and Christian apocalyptic texts. Predictions ranged from the demise of kings and empires to worldwide doom.
Wright is teaching a course at the University of Arizona this semester called "The Apocalyptic Imagination," which looks at ancient cultures as well as modern, nonreligious theories. People are fascinated with apocalyptic ideas and "ancient people who seem to know more than we do," Wright said.
"We expect things to end," he said. "And yet science tells us that things have gone on for a long time and will continue to go on for a long time long after we're gone."
Most experts agree that scientists can't predict everything, but there's a good chance we'll wake up on Dec. 22 and the Earth will be the same as when we went to bed.
Here are some of the most popular doomsday theories - and why we'll probably be OK:
The Mayan calendar
It doesn't say the world will end next month, said Mark Van Stone, a professor of art history at Southwestern College in California who wrote "2012: Science and Prophecy of the Ancient Maya."
The long-count calendar runs on a cycle of about 5,000 years, and will end on Dec. 21, 23 or 24, depending on how the dates are correlated to our calendar, Van Stone said. The Mayans said nothing about the world ending or changing on the last day of the cycle, he said.
"They're doing arithmetic, not prophecy."
Scholars speculated about the possible apocalyptic meaning of the date in the first part of the 20th century, and the theories took on lives of their own, Van Stone said. People who knew little about the Maya still held on to prophecies of doom or predictions of utopia even after contradicting theories emerged.
Other Mayan artifacts talk about time beyond 2012 with their kings still performing ceremonies, Van Stone said.
"The calendar will go on and the culture will go on," he said. "There won't be the end to the world. There won't be destruction, and there won't be improvement either."
This theory says the sun and Earth will align with a black hole in the center of the Milky Way galaxy on Dec. 21. The black hole's gravity will cause imbalance and disasters including earthquakes and tsunamis.
There are a few scientific problems with this idea, McGaha said. First, the alignment of the Earth and sun this year isn't unusual.
"The sun never lines up with the center of the galaxy," McGaha said. "It gets close every year. There's no difference this year between last year or next year."
Even if there were an alignment, the Earth would be safe, said Mark Sykes, director and CEO of the Planetary Science Institute in Tucson. The gravitational force exerted on the Earth by a black hole would be minimal because of its distance.
"It isn't going to affect anything," Sykes said. "Essentially, there's no change in force."
Magnetic Pole Reversal
Some believe the Earth's magnetic field will spontaneously reverse, causing chaos.
The phenomenon is real, but the timeline isn't. The Earth's magnetic field reverses periodically, with the last reversal occurring about 780,000 years ago.
The shift does weaken the Earth's magnetic field but takes thousands of years to complete. Scientists can't predict exactly when a reversal will start.
"But the geologic record doesn't show catastrophe as a result," Sykes said.
The idea that a hidden planet - called Nibiru or Planet X - could affect the Earth originated in the 1970s when Zecharia Sitchin wrote "The Twelfth Planet."
Believers say the mystery planet's orbit around the sun is thousands of years long, but the planet could crash into Earth or cause it to flip its poles next month.
If such a planet existed, astronomers would have found it by now, Sykes said. Even if they couldn't see it, its gravitational effects would be noticeable.
"If there was another object, we'd detect its influence on other objects nearby," Sykes said.
Morrison has received more than 5,000 questions about Nibiru on the "Ask an Astrobiologist" website, he said.
"Now I just say, 'Go out and look at the sky. It's not there.' "
If a giant asteroid hit Earth, it would cause destruction - firestorms, earthquakes, tsunamis and sunlight-blocking dust clouds.
But scientists tracking dangerous objects don't see any risks for December.
NASA runs a near-Earth- object search for potentially hazardous comets and asteroids. The program is run from different sites throughout the country, including the UA's Catalina Sky Survey.
NEOs aren't considered potentially hazardous unless they have a diameter of 500 feet or more, but even those smaller objects could cause disturbances that would last for years, said Ed Beshore, former principal investigator for the Catalina Sky Survey, who is now the deputy principal investigator for the asteroid mission OSIRIS-REx.
Scientists quickly assess the orbits of objects they discover and are good at predicting threats up to about 100 years away, Beshore said. The NASA program is also investigating how to mitigate the threat of near-Earth objects.
A spacecraft could be sent to nudge the asteroid and change its orbit. Or an asteroid could be painted white so the effects of solar radiation would change its course.
NASA knows of nearly 1,400 potentially hazardous asteroids, according to its website.
The NEO search "has reduced risk considerably," Beshore said.
Powerful sun storms hitting Earth could severely disrupt communication - or kill us all, doomsday believers say.
Solar flares, or solar storms, are magnetic events on the surface of the sun that spew out high-energy particles, said Tamara Rogers, an assistant professor at the UA Lunar and Planetary Laboratory. The events mainly affect telecommunication satellites and power lines.
The biggest disruption on Earth from a solar flare, called the Carrington Event, occurred in 1859 and put out telegraph systems all over the world. Solar flares have also caused major power outages.
"That's the extent of damage seen on Earth," Rogers said.
The sun goes through solar minimums and maximums during an 11-year-cycle. A solar maximum means more sunspots and solar flares.
Scientists predict the solar maximum within the next year or so, but nothing extraordinary is expected on Dec. 21.
• What: "Apocalypse 2012: The End of the World According to the Mayans, Jews and Others," a lecture by J. Edward Wright
• When: Monday, Dec. 3, at 7 p.m.
• Where: Congregation Anshei Israel, 5550 E. Fifth St.
• Cost: Free
• For more information: web.sbs.arizona.edu/college/events
• What: "2012 Doomsday! World Apocalypse?" lecture by James McGaha
• When: Monday, Dec. 3, at 7:30 p.m.
• Where: Room N210 of Steward Observatory, 933 N. Cherry Ave.
• Cost: Free
• For more information: enterprise.as.arizona.edu/~taf/pubeve/pub_lect.html
• What: "End of the World," a show hosted by the Mount Lemmon SkyCenter
• When: Friday, Dec. 21, at 3 p.m.
• Where: 9800 Ski Run Road, Mount Lemmon
• Cost: $75 adults, $35 children
• For more information: skycenter.arizona.edu/news/events/endoftheworld
Brenna Goth is a NASA Space Grant intern. Contact her at: firstname.lastname@example.org