On November 8th, 2011, while many of us were sipping our coffees, eating our breakfast, and going about our everyday lives, scientists in Pasadena, California where casually recording an asteroid flying over our heads named Asteroid 2005 YU55. The asteroid was as close as 201,700 miles (324,600 kilometers) to Earth, that’s a distance closer than the Earth is to the moon.  Asteroid 2005 YU55, like thousand others, is categorized as a “Potentially Hazardous Asteroid” or PHA because it orbits such that it has the potential to make close approaches to the Earth and is a size large enough to cause significant regional damage on impact.  The diameter of this asteroid is 1,300 feet (400 meters) making it almost four times longer than the length NFL football field.
Though there are vast arrays of celestial bodies in our solar system, asteroids capture people’s imagination outside the scientific community because of the potentially destructive threat they pose to life on Earth. Science fiction and blockbuster Hollywood movies have further popularized this idea by making asteroids into a mortal enemy to our civilization. But probably the greatest reason for their popularity lies with the widely accepted, but highly debated theory that a massive asteroid about 6 miles (10 kilometers) in diameter, hit the Earth 65 million years ago, starting a catastrophic global event that resulted in the mass-extinction of dinosaurs. An event closer to our present day, in 1908, a “smaller” asteroid measuring 330 feet (100 meters) in diameter exploded over Tunguska in Siberia. The asteroid devastated more than half a million acres of forest. 
So what are these agents of doom and where did they come from? Asteroids are inactive, metallic, icy, and rocky bodies without atmosphere that orbit the sun, but are too small to be considered as planets. They range in size from 600 miles (1,000 kilometers) across to small baseball-sized particles. They are found throughout our solar system, but many of them are found in the asteroid belt, between the orbits of Mars and Jupiter. The theory is that they are material left over when the planets were forming billions of years ago, never fulfilling their potential to become planets because Jupiter’s strong gravity prevented this from happening.
With thousands of asteroids throughout our solar system, it would be pretty easy to lose track of what’s happening in our busy neighborhood. Luckily, we have NASA operating a celestial observatory named the Near-Earth Asteroid Tracking (NEAT) system at NASA’s Jet Propulsion Laboratory (NPL). Here they are spending their time cataloging and tracking NEO (Near-Earth Objects) and grade their threat to the Earth.
So let us say a really big asteroid is menacingly hurling towards our small planet Earth. What can we realistically do with the technology we have? Because, there are vastly different asteroids with different paths, scientists must discover and categorize the approaching asteroid as soon as possible in order recommend the best solution to deflect the asteroid.
In a report, to the U.S. congress in 2007, NASA suggested the following strategies to deflect an asteroid:
- A nuclear standoff explosion that will nudge or break-up the asteroid.
- Non-nuclear kinetic impactor, a fancy way of saying to slingshot a big object at the asteroid.
- Slow-push mitigation techniques, moving the asteroid slowly over time with tiny constant thrusts from a spacecraft hovering over it to pull the asteroid into a new non-threatening orbit.
The last suggestion seems the least fascinating because it is painfully tedious and expensive without any climatic explosion or fireworks.
Let’s hope we will never actually be forced to execute one of these solutions, but it’s generally accepted that a PHA will hit the Earth since impact from one occurs on average around once per 10,000 years. Regardless of the solution, the successful deflection of an asteroid will be a great feat for the human civilization.