And twice as many as previously believed are in orbits similar to Earth's
                                
                                
                                    (Newser) -
                                    NASA thinks it's got a pretty solid estimate on the number of gigantic asteroids that could plausibly cause regional or global catastrophe by smashing into Earth: 4,700, give or take 1,500. Included in that number are all the rocks that exist, or which they suspect exist, that...