Jupiter Takes a Hit for the System
Comet Shoemaker-Levy 9 Impacts
16-22 July 1994

Science-fiction readers long have been aware of the extremely-low-probability but extremely-high-damage risks presented to humanity simply by our living upon our planet Earth in the Solar System. For this is a dynamic arena, with rocks and miscellaneous debris of all sizes moving through it along a wide spread of paths. Fortunately for the development and survival of life on Earth, most of these rocks and debris were swept up by the planets billions of years ago. The impact craters on Luna and the Jovian moons are scars of very long ago; Meteor Crater in Arizona was made not nearly so far back.

We tend to be complacent about dangers from space, thinking that the early eons of the Solar System may have seen a lot of impacts, but nothing too serious has hit the Earth since dinosaur times. Well, not quite: the Tunguska strike in Siberia of 1908 is just yesterday, astronomically speaking: almost within living memory. If that mysterious object had arrived a few hours later, the Earth would have rotated major cities of Europe into its blast zone.

The small Inner Planets (Mercury, Venus, Earth, Mars) are fortunate that the gas-giant Outer Planets (Jupiter, Saturn, Neptune, Uranus) — especially Jupiter — with their large gravitational fields, sweep up a lot of the ice, rock, and miscellaneous debris that still wanders in from the Oort Cloud.
  

This collision was not in dinosaur times; we watched it happen. On the night of 24 March 1993, the astronomical team of Carolyn and Eugene Shoemaker and David Levy discovered a comet on a photograph taken by the Palomar Observatory.

When the comet was observed on May 17 [1994], its train of 21 icy fragments stretched across 710 thousand miles.

The impact storms created on Jupiter, shown on the observatory photo below, are each larger than Earth.

Here are some relevant pages or index-pages from the National Aeronautics and Space Administration / Jet Propulsion Laboratory, for Comet Shoemaker-Levy 9 and other Near Earth Objects:

NASA's home page for
Comet Shoemaker-Levy 9's collision with Jupiter

Images from the NASA / JPL
Near Earth Object Program

Results from NASA's NEOWISE survey
Edge-on View of Near-Earth Asteroids
  

As of 2002, Carolyn Shoemaker is credited with discovering 32 comets and more than 800 asteroids. Some folks joke about the President of the United States getting some kind of emergency phone call in the wee dark hours of the morning; others think such nightmares are too scary to be funny. Since 1994 I've felt that one of the very worst of possible calls could go something like this (an entirely fictional example):

Mr. President, this is Carolyn Shoemaker. I'm afraid I have some very bad news. I've been studying the motion of near-Earth objects against the starfield, looking for possible patterns. I have found a new object — call it a narrow-profile comet, or a strung-out massive asteroid group of ice, rock, and nickel-iron — which is much closer to Earth than we thought any such undiscovered objects could be. It appears to be moving on a navigationally constant bearing: a collision course. If it does strike our planet, the effect may resemble that of the asteroid which ended the Cretaceous Period, wiping out much of life on Earth.

Mr. President, please take what steps you can to prepare the American people. This object is very close.
  

© 2012 Robert Wilfred Franson