One of the possible 2012 doomsday scenarios is the collision of a large meteor with Earth that could cause a mass extinction, and bring about the end of human civilization.
Meteors are the general name astronomers use for small objects in our Solar System that enter our atmosphere. They impact Earth all the time, but most are very small and burn up in the atmosphere before reaching the surface. Big impacts have happened in the past and can definitely happen again in the future.
This artist's impression shows a large planetoid crashing into the primordial Earth. In the last stages of the formation of Earth (4.5 billion years ago), it may have been struck a few times by objects about the size of Pluto (1,500 - 2,000 miles across). Earth has not experienced anything like this impact since that early time. Image Credit: Don Davis/NASA
For evidence of meteor impacts in our part of the Solar System, you need only have a look at the Moon in a small telescope or a pair of binoculars. Its surface is littered with craters that resulted from big impacts in the past. Earth is bigger and more massive, so we’d expect it to have been hit more often than the Moon. We don’t have as many impact craters on the surface of Earth, however. Impacts of smaller objects in the atmosphere and oceans don’t leave behind craters. The craters that are formed by impacts with land get erased over time by weather and tectonic activity.
Smaller meteors impact Earth more often than bigger ones. The reason for this is that there are many more small meteoroids in space than there are big ones. We can look at some examples of known meteor impacts on Earth to get a sense of how frequently they occur.
In 2008, a 3-5 meter-sized asteroid was spotted in space one day before it was predicted to impact Earth in the country of Sudan, Africa. The object impacted on October 8, 2008 just as predicted. Most of the asteroid vaporized in the atmosphere, and the rest soft-landed in the Sudanese desert. About 10 kilograms of the asteroid were recovered. Objects of 1 meter in size impact Earth almost every month, and objects of about 10 meters in size impact about once every decade. These objects pose little threat to people.
In 1908, an asteroid or possibly a comet about 20-30 meters across exploded in the atmosphere above Siberia, Russia. The explosion created a blast wave that flattened trees for a radius of 50 km. Luckily this part of Siberia was completely uninhabited by people. Had the same explosion happened over a major city like New York or Los Angeles millions of people could have died. Objects of this size probably impact Earth every few centuries.
It’s by pure chance that such an explosion has not been directly witnessed in human history. Humans only inhabit a very small fraction of Earth’s surface, and in the past it was an even smaller fraction. The vast majority of Earth’s surface area is oceans and deserts, making them the most likely places for a meteor impact to occur, and therefore go unnoticed by people.
In Arizona, you can visit Meteor Crater, which was created by an impact about 50,000 years ago with a meteor probably 30-50 meters across. The crater is 1.2 kilometers across and the impact event would have caused regional devastation for many years. Events of this magnitude probably occur on Earth every few thousand years.
Meteor Crater (also known as Barringer Crater) in Arizona is only 50,000 years old. Even so, it’s unusually well preserved in the arid climate of the Colorado Plateau. Meteor Crater formed from the impact of an iron-nickel asteroid about 46 meters (150 feet) across. Most of the asteroid melted or vaporized on impact. The collision initially formed a crater over 1,200 meters (4,000) feet across and 210 meters (700 feet) deep. Subsequent erosion has partially filled the crater, which is now only 150 meters (550 feet) deep. Layers of exposed limestone and sandstone are visible just beneath the crater rim, as are large stone blocks excavated by the impact. Image & Caption Credit: USGS
The most famous meteor impact occurred 65 million years ago in the future land of the Maya, Chicxulub, Mexico. The meteor was probably an asteroid 10 kilometers across that smashed a 200 kilometer-wide crater into Earth’s crust. The impact threw up ash and debris into the atmosphere that first heated the planet igniting fires all around the world and then darkened the sky for years, cutting off the bottom of the food chain from its source of energy, the Sun. A majority of animal and plant species went extinct, including the dinosaurs. Events such as these probably occur on Earth every 100 million years.
Big meteor impacts are a very real natural hazard faced by life on Earth. But in any given year, the risks of a major impact are statistically small, because they happen infrequently (hundreds of years between events causing local damage, thousands of years between events causing regional damage, and tens of millions of years between events causing global damage).
Scientists have been searching for potentially hazardous meteoroids for decades now, in an effort to better understand the risks and to predict any future dangers. We estimate that we have found 87% of all nearby objects larger than 1 kilometer (over a thousand objects). None of these pose any risk of impact with Earth for at least several hundred years. So, that means that the estimated chance for a major impact causing global annihilation in the near term is much less than previously thought.
The risks from smaller impacts are less well defined and scientists have been surveying the sky to search for the smaller asteroids that could pose a hazard to Earth. NASA’s WISE space telescope surveyed the whole sky in infrared light in 2010, discovering over 33,000 new asteroids in the Solar System, hundreds of which are near Earth’s orbit around the Sun. The WISE survey was important to understanding impact risks; because it looked in infrared light the survey was not biased against dark asteroids. The WISE survey data are now helping scientists to assess the risks of meteor impacts in greater detail.
The Jet Propulsion Laboratory in Pasadena, California keeps a tally of known asteroids and their risks of impact (http://neo.jpl.nasa.gov/risk/). On this list, there are no objects with a better than 1 in 10,000 chance of impacting Earth for the next few hundred years. And nothing on the list poses any risk in 2012.
So, scientists have found no indication that a significant collision with Earth is coming in December 2012. The chances of such an impact happening on or around the winter solstice are as small as they are at any other time.