Introductory Astronomy 9精选.ppt

*ets&,2010*Asteroids1Asteroidsarerockyobjects(esimals)orbitingtheSun,,.*Asteroids2Thereareprobablybillionsofasteroidslessthan1km(1000m)across,millionslargerthan1km,withabout200largerthan100km(60miles),,s,,morethan3300asteroids,withorbitsintersectingtheEarth’sorbit,,,theaveragedistancebetweenasteroidsisabouttenmillionkm,sothatthepossibilityofacollisionwithapassingspacecraftisnegligible.*AsteroidOrbits*s,’simmensegravity,whichalsocreatedgapsintheasteroidbelt,,Ceres,,isalmostspherical,withadiameterof930km(580miles),makingitalittlesmallerthanPluto’ssatellite,,PallasandVesta,havediametersofroughly600kmand530km(330miles)respectively.*,levelwithJupiter’sorbitalpath.*DawnAsteroidMission2007-20151TheDawnasteroidmissionwaslaunchedinJuly2007,andisscheduledtouseMarsforaslingshotin2009,toreachtheasteroidVestain2011,,the4thlargestasteroid,washotandvolcanicearlyinitslife,’ssurfaceisdominatedbyagiantcrateratitssouthpole,,(andpossiblyliquidwater)40to80milesthick,whichhaskeptitssurfacecool,andeliminatedvolcanicactivity.*DawnAsteroidMission2007-20152Unlikethechemicalrocketsofp