Giant Asteroid Headed Your Way?: How We Can Detect and Deflect Them

We get many sensational news stories, saying a giant asteroid will hit Earth, or will nearly miss us. Several so far this year. Many of us worry about these things. So what is the truth behind them? Actually astronomers have this well in hand for the largest asteroids. They have found 90% of the largest asteroids likely to hit Earth. This is mainly due to Pan-STARRS, which takes a 1.4 gigapixel image of a three degrees span of the night sky several times a minute. It continues to find a new large asteroid about once a month, and will find most of the remaining 10% by the 2020s. The focus has started to turn to smaller asteroids, ones large enough to destroy a city. These are harder to spot, especially if they approach Earth from the direction of the sun. Our asteroid search telescopes on the ground can't operate in daylight. We hope to find 99% of the asteroids larger than 300 meters in the near future. Below that, however, the B612 foundation's Sentinel space telescope or similar may be needed to complete the survey. Sentinel can get down to 20 meters diameter, and surveys the hard to spot asteroids between us and the Sun. They hope to launch it in 2018-9, if they can raise the funding. But progress is slow, and the main problem is funding. They have only raised around 5% of their annual funding goal of 30 - 40 million dollars. Without its help, the search for the smaller asteroids may take several decades. We also know how to deflect asteroids, if it is needed, especially if they are discovered well in advance. This is one of the few natural disasters we can actually prevent. With enough warning, the tiniest change in the speed, sometimes by as little as a millionth of a meter per second, can prevent an asteroid from hitting Earth decades later. Many people must worry about this, as we get asked questions over and over on Quora, in the Asteroid Scares topic. I thought it might help to make some of these answers into a booklet. Some of the questions I answer here are: * What is the truth behind these stories? * How likely is it that we will be hit by a giant impact? * How do we detect them, and why do the probabilities keep changing after the first detection? * Can we destroy them or deflect them before they hit? * What is the largest size of asteroid that could hit Earth? * Could it hit us by surprise, with just a few weeks notice or a few months notice? Includes material from my Science20 blog and several of my Quora answers