Near-Earth Objects are asteriods, comets and large meteroroids whose orbits bring them close to earth, the highest known risk to earth is from an asteriod called Apophis which will pass close to earth on April 13th 2036. The energy released would have the equivalent of between 880 and 1480 megatons of TNT. (The 1883 eruption of Krakatooa was the equivalent of about 200 megatons.) 

On the positive side scientists, astronants, and astronomers are working together on trying to save our planet of near earth asteriods.  For this to happen all the Governments from Nation to Nation (Example, Russia, China, Japan, U.S., Canada and so on) need to get together and help Nasa, among other astronomical volunteers, to put forth money to help save our planet!!