Onabright,coldmorningon15thFebruary2013,ameteoriterippedacrosstheskiesabovetheUralmountainsinRussia,distintegratingintothreepiecesandexplodingwiththeforceof20Hiroshimas.ItwasastarkreminderthattheEarth'sjourneythroughspaceisfraughtwithdanger.Adaylateranothermuchlarger14,3000tonneasteroid,passedwithinjust17,000milesof...