Rain of Iron and Ice #4

Now back to info about history as it relates to asteroid/meteorite impacts!

The space race is given (rather interestingly) a quick run-through that also manages to capture more info than I knew about it.  Races to get probes to the Moon, Mars, and Venus are described and the successes and failures of both Russians and Americans were numerous.  So it was very exciting when three American probes returned over 17000 images of the moon that depicted enormous craters to those less than a meter in size.  These images also showed that these were all impact craters as evidence for volcanic activity was absent.  However, the plethora of craters told scientists nothing about the frequency of their impacts because no data could yet be collected as to their age.  The crater ages were later measured by Apollo space missions allowing for accurate calculations of cratering rates both on the Moon and the infall rate of asteroids and meteorites on Earth.  The evidence collected by later spacecraft of Mars and Mercury showed that Earth was in prime impact real estate, but scientists were perplexed by an apparent lack of evidence and so a quest to find evidence of impacts on Earth began.  And by 1962 more than 200 Earth craters had been identified and authenticated.  The biggest of these was the 65 million year old, 200+ Km wide Yucatán Peninsula crater which is now believed to be evidence of the asteroid that wiped out the dinosaurs.  Using all this data a variety of frequency estimates were made.  It was approximated that hundred-megaton (about 5 times more powerful than a WWII nuke like the ones from Hiroshima or Nagasaki) impacts occurred every 1500 years, one gigaton impacts every 12000 years, teraton impacts every 800000 years and petaton (globally devastating) impacts every 100 million years (Lewis 74).  Other, smaller impacts are more common but were harder to measure with the devices of the time. 

In 1967 in response to the close approach of the asteroid Icarus (which had no chance of actually hitting Earth but created quite a stir due to the closeness of its approach) an M.I.T. class was tasked with creating a solution if it was discovered that Icarus was on a collision course with Earth.  The end of the year presentation by students was attended by the M.I.T. community, and the press (who ended up circulating widely inflated reports about the circumstances of the asteroid's pass by the Earth).  However, Icarus was nothing new to astronomers who had known about it since 1949 when it became the 13th body to have an orbit that brought it close to Earth. 

I'm glad that I'm finally getting where I want to go with this book.  The information about the relative frequency of asteroid/meteorite impacts on Earth is very intriguing as is the 1967 M.I.T. project which suggested one of the favorite methods in the case of imminent asteroid impact; use a satellite to slowly pull the asteroid out of an Earth-bound trajectory.