... The craters were caused by the impacts of comets and asteroids. ... In fact, up to a diameter of about 10 meters (33 feet), most stony meteoroids are destroyed in the atmosphere in a terminal explosion. ... The famous meteor crater in northern Arizona, some 1.2 kilometers (4,000 feet) in diameter and 183 meters (600 feet) deep, was created 50,000 years ago by a nickel-iron meteorite perhaps 60 meters (197 feet) in diameter. ... These range in diameter from a few meters to about 8 km. ...

... B efore the Galileo encounters with Ganymede it was thought that Ganymede and Callisto were composed of a rocky core surrounded by a large mantle of water or water ice with an ice surface (and that Titan and Triton were similar). ... G anymede's surface is a roughly equal mix of two types of terrain: very old , highly cratered dark regions (left), and somewhat younger (but still ancient) lighter regions marked with an extensive array of grooves and ridges (right). ... Callisto .. ...

... Future seminars . P a s t seminars . ... Deep Impact: Excaving Comet Tempel 1 " . ... On 4 July 2005 Deep Impact collided with comet Tempel 1, excaving a crater. Deep Impact consisted of two fully functional spacecraft: an impacting spacecraft weighting 364 kg and a flyby spacecraft for observing the impact and relaying data from the impactor. ... The impact was also observed by over 80 ground-based observatories and by several spacecraft (Rosetta, Hubble Space Telescope, Chandra, Spitzer). ...