A surprise happened over the skies of Chelyabinsk in Russia on February 15, 2013, where a stony meteor/asteroid exploded in the air producing a shock wave that shattered windows for miles around. The airburst explosion was equivalent to about 300 kilotons of TNT, though it may have been as large as 500 kilotons of TNT. The Russia asteroid was not related to the other near miss on that day: asteroid 2012 DA14. The Russia asteroid was probably about one-third the size of DA14 and it was definitely traveling in a different direction and speed than DA14. The Russia asteroid originated in the asteroid belt between Mars and Jupiter while DA14 is a "Near Earth Object (NEO)" with an orbit around the Sun that is similar to the Earth's.
[I use the definition of "asteroid" used by Don Yeomans of the NEO program, that asteroids are small, inactive, (usually) rocky bodies orbiting the Sun a meter or larger in size, while a meteoroid is less than a meter in size, therefore the "Russia meteor" is really a "Russia asteroid".]
Infographic of the Russia asteroid airburst from The Telegraph (UK). Click on the image to view a larger version.
Want to find out what we are doing to make sure we don't get surprised again? Go to the B612 Foundation.
Something like the 150-foot diameter asteroid DA14 passes near the Earth every 40 years on average and an impact happens roughly once every 1200 years. DA14 has no chance of hitting Earth for at least the next century. Something like DA14 hit the plains of northern Arizona about 50,000 years ago to make the kilometer-wide crater now called "Meteor Crater". However, the Meteor Crater asteroid was made of iron, so it packed even more of a punch than would a typical asteroid like DA14 that is made primarily of stone. More recently, something the size of DA14 is thought to have created the airburst over Tunguska, Sibera that leveled hundreds of square miles of forest in 1908. An impact of something as small as DA14 would be equivalent to 2.4 megatons of TNT---enough to wipe out a city the size of Bakersfield but not have a global effect. Something a kilometer or more in size hitting the Earth would have a global effect, threatening the survival of our civilization.
In April 2029, the asteroid 99942 Apophis will make a near-flyby of just 19,820 miles above the Earth's surface---slightly farther away than asteroid DA14 did in February 2013. The Earth's gravity will deflect Apophis' path by a such large amount that there was initially a possibility the asteroid could impact the Earth in 2036 because of the uncertainties in the measurements of Apophis' position and motions. Recent very careful observations with telescopes and radar have greatly fine-tuned our knowledge of Apophis' orbit and its rotational motion, so that there is essentially NO chance of a 2036 impact. A possible impact a few decades later in 2068 is the most likely now but that chance is just 2.3 in a million---still a very small chance. At 1070 feet in size, Apophis is about seven times bigger than DA14, so if it is made out of the same stuff as DA14 (primarily stone), Apophis would hit with about 360 times bigger punch than DA14 or about 860 megatons of TNT, assuming the speeds are the same. To see the effects of such a hit, go to the Killer Asteroids website and find out the effects from its impact calculator.
Why is there still some uncertainty in the impact chances despite all of the telescope and radar observations? An effect called "Yarkovsky effect" can slowly nudge an asteroid toward or away from the Earth. In the Yarkovsky effect, there is a slight mis-alignment of the energy emitted by the asteroid and the energy it receives from the Sun. Because any material takes some time to heat up, the asteroid's afternoon side emits more infrared energy than the morning side. The afternoon emission of infrared energy from solar heating is not pointed right at the Sun, so the thermal radiation from the asteroid is not exactly balanced by the solar photons.
This results in a pushing that can move the asteroid inward toward the Sun or away from the Sun. If the asteroid is rotating in the same direction that it moves in its orbit around the Sun ("prograde rotation"), the asteroid will be pushed away from the Sun; if the asteroid is rotating in the opposite direction from its orbital motion ("retrograde rotation"), the asteroid will be pushed toward the Sun. The effect is very small but it is continually acting on the asteroid so over many years it can have a measurable influence on the asteroid's motion.
Very recently (mid-April 2013), analysis of Apophis' rotation showed that it is tumbling about two axes so the infrared radiation it emits from solar heating is randomly directed instead of being in one consistent direction. Without a consistent direction of the infrared emission, the Yarkovsky effect is eliminated.
This reduction in the Yarkovsky effect acting on Apophis has essentially eliminated the chance of a 2036 from the original January 2013 published value of less than 1 in a million. A related effect called the "YORP effect" arises from the non-uniform shape and reflectivity of various parts of the asteroid that can cause one side of the asteroid to be a better emitter of its infrared energy than another part.
The YORP effect can speed up or slow down the asteroid's spin rate and also change the spin axis direction leading to a possible increase in the Yarkovsky effect.
See the "Deflecting and Using Asteroids" section in Astronomy Notes for more about how we would deflect an Earth-bound asteroid or comet and what mineral resources we can extract from the near-Earth asteroids.
Here are the things I do to fact-check any media news item of a possible asteroid/comet impact. I check out the news headlines on NASA's Near Earth Object Program website and the IAU's Minor Planet Center at www.minorplanetcenter.net. I will also look at Sky and Telescope's news section at www.skyandtelescope.com for something that hasn't been reported yet in the weekly news email they send out. Take a look at the NEO and MPC websites when evaluating any scary story of an upcoming collision. There are a lot of "junk science" websites and TV shows out there, most designed to capture your attention by scaring you in order to sell the products advertised on the website or show. To find out more about the effects of an impact on Earth, see the Earth Impacts webpage on my Astronomy Notes site at www.astronomynotes.com