Lecture Notes:

• We started by discussing the attention given to asteroid/comet impacts by the press and TV lately, such as the movie "Asteroid" which appeared on Sunday and Monday evenings, and the item in Mondays paper that the site of the impact of 65 million years ago which zapped the dinosaurs has been found by drilling off the coast of Yucatan.
  
  
• Life on the earth has been threatened a number of times in the past, it turns out; we may even have benefited from cosmic debris hitting our planet.
  
  
• The scenario for life starts with the Big Bang some 12-14 billion years ago. Our galaxy was probably formed about 11 billion years ago, and stars began converting hydrogen into helium as sources of energy. Some of those stars ran out of hydrogen and destroyed themselves in supernovae explosions which created new elements such as carbon, oxygen, nitrogen, silicon, and iron and scattered those heavier elements through the neighboring space.
  
  
• Our own sun is a third or fourth generation star that developed out of a collapsing gas cloud. The formation of the solar system probably started with the pulse of a nearby supernova pushing on our gas cloud and causing it to collapse. The collapsing cloud conserved angular momentum and starting spinning faster, with the result that the equator of the cloud was distended. The planets were formed by colliding dirty snowballs and were in place by about 4.6 billion years ago.
  
  
• The first solid particles in the collapsing cloud were probably dirty snow balls of ice and many of these remain in the so-called Oort comet cloud which extends to perhaps 100,000 AU from the sun. This comet cloud is probably a remnant of that very early time before planet formed and is now a source of perhaps a dozen long period comets that enter the inner solar system every year.
  
  
• During the first 800 million years of the life of the solar system, the debris in interplanetary space fell on the planets with such ferocity that their surfaces heated up and magma oceans of molten rock covered them. As a result all water an atmospheric gasses were evaporated. Eventually beginning about 3.8 billion years, the rain of debris decrease as the space began to be cleaned up. Water appeared on the surface of the earth due to volcanic eruptions and within 300-400 million years life appeared in the warm oceans. It may have been generated within those oceans without any outside influenced, or it may have been introduced by rocks or snowballs which already had left-handed amino acid molecules in them. The first life was very simple single cells remained simple and single for nearly 3 billion years. At 550 million years the Cambrian explosion was associated with an amazing proliferation of different forms of life. Almost all major biological phyla were created at that time, except the vertebrates.
  
  
• But life was almost totally destroyed on the young earth by a collision that caused the great dying of the Permian at 250 million years; at that time 96% of all land and sea species became extinct. We nearly lost all life on the earth! Recently shocked quartz has been found in Australia that may have been under that impact. Other extinction events at 210 MY, 140 MY and 65 MY similarly produced by impact. The latter event destroyed the dinosaurs by producing a "nuclear winter" or heavy dust cloud encircling the earth which shut down photosynthesis and eliminated dinosaur food. After that event, there remained no land animals with body weight greater than 25 kg and approximately 75% of the species were rendered extinct.
  
  
• Cosmic collisions are thus expected events; that which occurred 65 MY ago actually may have helped us by clearing the earth of dinosaurs so that mammals could proper. We are constantly bombarded even now by stuff falling form the skies, comets or asteroids; once every million years an object several miles across probably will hit the earth and cause havoc. Now that the Cold War is over, weapons laboratories such as Livermore are proposing sending nuclear bombs to threatening asteroids or comets to divert them from the collision course with the earth. Mostly, I think, they want a job!
  
  
• But we all should worry constantly about getting hit: I have a special deal, for you my friends, on asteroid umbrellas that I can let you have for a really good price!
  
  
• We were spectators to a very spectacular impact several years ago when comet Shoemaker Levy collided with Jupiter. More on this topic in the planetarium on Thursday.