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Message: <blockquote data-quote="Umbran" data-source="post: 5280876" data-attributes="member: 177">In some cases, that might be determined by sampling material around the impact site - our solar system has a particular chemistry, and things that deviate from that might be an indication of another origin.But setting that aside, what they can tell is the density of the interstellar medium, and the average size of the particles therein.  They measure a lot of light passing through space, and that light is affected by the stuff it passes through on the way from there to here.Space is really, really empty.  Items larger than dust grains outside of solar systems are extremely rare.Largely, yes.  To quote wikipedia:"Asteroids with diameters of 5 to 10 m (16 to 33 ft) enter the Earth's atmosphere approximately once per year, with as much energy as Little Boy, the atomic bomb dropped on Hiroshima, approximately 15 kilotonnes of TNT. These ordinarily explode in the upper atmosphere, and most or all of the solids are vaporized.  Objects with diameters over 50 m (164 ft) strike the Earth approximately once every thousand years, producing explosions comparable to the one known to have detonated above Tunguska in 1908.[4] At least one known asteroid with a diameter of over 1 km (0.62 mi), (29075) 1950 DA, has a possibility of colliding with Earth on March 16 2880, with a Torino Scale rating of two."Objects with a 50 m diameter can cause local disasters.  As noted in the quote, the "Tunguska event" was most likely the result of the impact (well, in-air explosion) of such an object, flatting trees within roughly a 10 mile radius.  These things can destroy a metropolitan area.The search for objects of interest (<a href="http://en.wikipedia.org/wiki/Near-Earth_object" target="_blank">"Near Earth Objects"</a>) is ongoing.  <a href="http://neo.jpl.nasa.gov/risk/" target="_blank">Current Impact Risks</a>Hitting an ocean is not necessarily better, as far as humans are concerned.  Most of our cities are on coastlines, and water-strikes can cause tsunamis.  And, for other reasons, once you get bigger than a "city killing" rock, hitting water is very likely worse than hitting land.Yah.  Humans are known for managing risks very badly.</blockquote>

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