U.N. ASTEROID DEFENSE SYSTEM. NEAR-EARTH ASTEROIDS.

THE UNITED NATIONS IS BEGINNING STEPS TO MOUNT AN ASTEROID DEFLECTION SYSTEM. 
FOR MANY YEARS WE HAVE BEEN WELL AWARE OF THE POSSIBILITY OF A CATACLYSMIC ASTEROID STRIKE ON EARTH. THE ASTEROID THAT DAMAGED RUSSIA WAS SMALL IN COMPARISON TO SOME OF THE MORE THAN 10,000 KNOWN NEAR-EARTH OBJECTS (NEOs) THAT HAVE CLOSE APPROACH, AND ARE OVER .6 OF A MILE WIDE. 
One of the most dangerous asteroids on record zipped close by Earth last month.  
2013 TV135 was discovered on October 8, while NASA was closed during the government shutdown.
IT GOT THAT CLOSE BEFORE ANYONE DISCOVERED IT!

WE HAD A MUCH CLOSER BRUSH WITH AN ASTEROID THIS YEAR, ANOTHER ONE NOT DISCOVERED UNTIL IT WOULD HAVE BEEN TOO LATE FOR DEFLECTION.

That pales by comparison to the closest shave the Earth got from an asteroid of considerable size in recorded history.

On February 15 this year, asteroid 2012 DA14, which measured 150 feet wide, slipped in below the moon's orbit and squeaked by our planet just 17,200 miles from its surface. [THERE ARE VERY GOOD VIDEOS OF THIS ON THAT 2nd LINKED SITE.]

Scientists spotted it only ONE YEAR AGO from an observatory in the south of Spain.
An asteroid this size passes this close to Earth only every 40 years and collides with it only once every 1,200 years.

U.N. AGREES TO TAKE MEASURES TO PROTECT EARTH
The U.N. General Assembly last week gave the green light to an asteroid defense plan drawn up by the former astronauts at the Association of Space Explorers (ASE) , Scientific American reports.
The plan calls for member nations to join forces in an "International Asteroid Warning Group" committed to sounding the alarm if a dangerous rock is spotted.
A spacecraft would then be launched to knock the asteroid off course.
The ASE also wants the U.N. to run a practice deflection mission, to make sure the method actually works. 

Right now, no nation has an explicit responsibility to defend against an asteroid, and one former astronaut explained why the task should fall to the U.N., reports Space.com: "If something goes wrong in the middle of the deflection, you have now caused havoc in some other nation that was not at risk. Therefore, this decision of what to do, how to do it, and what systems to use have to be coordinated internationally." 

ANY INCOMING ASTEROID WOULD HAVE TO BE DEFLECTED 5 YEARS BEFORE IMPACT.
Further, early detection is essential, because the deflecting spacecraft will likely have to hit the rock five years before its rendezvous with Earth in order to be successful.

"If we don't find it until a year out, make yourself a nice cocktail and go out and watch," one former astronaut says.

How Many Near-Earth Objects Have Been Discovered So Far?

As of October 25, 2013, 10,359 Near-Earth objects have been discovered. Some 862 of these NEOs are asteroids with a diameter of approximately 1 kilometer or larger. Also, 1,431 of these NEOs have been classified as Potentially Hazardous Asteroids (PHAs). 

BELOW IS A GRAPH OF THOSE WE KNOW OF OVER 1 KILOMETER WIDE.
THE PROBLEM IS, FEW ARE SPOTTED BEFORE THEY COME TOO CLOSE TO EARTH TO BE DEFLECTED.

THERE IS NOW A PROGRAM THAT LETS CITIZENS JOIN THE SEARCH FOR NEOs.
FINALLY, NASA IS SCRAMBLING TO LOCATE THEM.

What Is A Potentially Hazardous Asteroid (PHA)?
Potentially Hazardous Asteroids (PHAs) are currently defined based on parameters that measure the asteroid's potential to make threatening close approaches to the Earth. Specifically, all asteroids with a minimum orbit intersection distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or less are considered PHAs. 
Particles from space bombard our planet every minute -- at a rate of 100 tons a day, NASA says.  

TWO ASTEROIDS HAVE RECENTLY BEEN SPOTLIGHTED AS HAVING THE BEST CHANCES OF IMPACTING EARTH.
2013 TV135 
If it struck the earth, the impact from 2013 TV135 would cause damage to an area of approximately 100,000 square miles with an equivalent force to about 2,500 megatons of TNT, or 2,500 standard nuclear warhead-bearing US Minute Man II missiles.
AND
2011 AG5
It had a 1 in 500 chance of impacting the Earth on 5 February 2040. In September 2013, there was an opportunity to make additional observations of 2011 AG5 when it came within 0.98 AU (147,000,000 km; 91,000,000 mi) of Earth. The 2013 observations allowed a further refinement to the known trajectory. The asteroid will also pass 0.0121 AU (1,810,000 km; 1,120,000 miles) from the Earth on 3 February 2023  ^[7][10


THE LARGEST ASTEROIDS WE KNOW OF
THERE ARE MANY  WITH INCREDIBLE MASS, BUT WE CONCENTRATE ON NEAR-EARTH LARGE ASTEROIDS.

1~ 1036 GANYMEDAt about 21 MILES ACROSS  (32–34 km in diameter), it was discovered by Walter Baade on October 23, 1924. It has a very well determined orbit, and its next pass of the Earth will be at a distance of 0.374097 AU (55,964,100 km; 34,774,500 mi) on 13 October 2024. It is an Amor asteroid, and also a Mars-crosser asteroid, and will pass 0.02868 AU (4,290,000 km; 2,666,000 mi) from Mars on 16 December 2176.^[7]  

2~ 433 EROS
433 Eros is an S-type near-Earth asteroid approximately 34.4 × 11.2 × 11.2 kilometres (21.4 × 7.0 × 7.0 MILES) in size, the second-largest near-Earth asteroid after 1036 Ganymed. It was discovered in 1898 and was the first near-Earth asteroid discovered. It was the first asteroid to be orbited by a probe (in 2000). It belongs to the Amor group.
Eros is a Mars-crosser asteroid, the first known to come within the orbit of Mars.
3~ 4179 Toutatis
On December 12, 2012 at 06:40 UT Toutatis passed within about 18 lunar distances of Earth.
Toutatis will approach Earth in 2016, but will not make a notably close approach until 2069.
HOWEVER, TOUTATIS HAS A VERY CHAOTIC ORBIT.
Radar imagery shows that Toutatis is a highly irregular body consisting of two distinct "lobes", with maximum widths of about 4.6 km and 2.4 km respectively, about 3 x 1.5 miles.

4~ MINOR PLANET/ASTEROID 7482 (1994 PC1)
Asteroid 7482 (1994 PC1) makes closest approach TO EARTH at 0.013 au 18 January 2022, coming within bout 5.2 times distance to the moon. It measures roughly 1900 meters or 1.2 miles wide.

TO SEE OTHER, ACTUALLY MANY, NEAR-EARTH ORBITERS, JUST CLICK HERE.
ALSO SHARING OUR SOLAR SYSTEM, CLOSE-BY IN THE ASTEROID BELT ARE SOME VERY LARGE, ALMOST PLANET-SIZED OBJECTS THAT WE CALL ASTEROIDS AS WELL.
~ 1 Ceres, is the largest asteroid and the only dwarf planet in the inner Solar System. It is a rock–ice body 950 km (590 miles) in diameter and the smallest identified dwarf planet. It contains about one-third of all the mass of the asteroid belt
It appears to be differentiated into a rocky core and icy mantle,^[8] and may harbor an ocean of liquid water under its surface.

~ 4 VESTA
Vesta is the brightest asteroid visible from Earth.
Vesta is the second-most-massive body in the asteroid belt, though only 28% as massive as Ceres.
Measures   342 x  287 MILES (550 km × 462 km).
NASA's Dawn spacecraft entered orbit around Vesta on 16 July 2011 for a one-year exploration, and what is known about Vesta will be refined and extended as data from Dawn is analyzed and published. Dawn left Vestan orbit on 5 September 2012 heading for asteroid Ceres.

Distant catastrophe

Though it seems Earth is safe for now, there is such a thing as a doomsday asteroid.

Scientists say it is likely that the impact of an asteroid over six miles wide wiped out dinosaurs along with much of the life on Earth 65 million years ago.

More like it will come, NASA says.

But they only turn up once every "few million years."

That may give humanity some time to find a way of dealing with it.  
If the nations of earth CAN come together to build an asteroid deflection program, we will still need more intense and far better discovery capability and methods of tracking asteroids within that 5-year deadline for our awareness of their near presence to do us any good.

We MUST find them before they cross that "5-year limit" and be able to launch a deflection immediately. 
BUT, BEFORE ALL THAT, WE MUST DETERMINE IF THIS IS EVEN POSSIBLE. 

Learn what would happen if an asteroid hit the Earth with this detailed "Large Asteroid Impact Simulation".
An asteroid with a diameter of 500 km. Destination: The Pacific Ocean. The impact peels the 10 km crust off the surface. The shock-wave travels at hypersonic speeds. Debris is blasted across into low Earth orbit, and returns to destroy the surface of the Earth. The firestorm encircles the Earth, vaporizing all life in its way. Within one day, the surface of the Earth is uninhabitable.
The evidence shows that this has happened at least six times in Earth's history. 

The video below shows the possibility, all merely theory, of course, of a smaller impact, calling the event a meteorite impact, not an asteroid.