Satgen 470 Ancient Comet Hits ? by GM4IHJ (BID SGEN470) 1998-03-28 The spectacular multiple impacts, as the debris stream from Comet Shoemaker-Levy hit giant planet Jupiter in 1994, caused scientists to wonder whether anything similar had happened to the Earth. The Earth is a much less effective capture agent than Jupiter. So events of this kind have probably been rare. None the less, a recent paper in the science journal Nature (Vol 292 of 12 March 98 at p171 ) , suggests that a suitable set of candidate craters may have been found. Three of the craters ( in Western Canada, Eastern Canada and France), lie on the same Latitude ( corrected for plate tectonics shift of the continents ) and are evenly separated in Longitude, and the dates of crater formation, cluster around a time 214 million years ago. Less obviously connected are two other crater. One in Western USA and the other in the Ukraine, which lie on great circle arcs inclined at 37.2 degrees with reference to the western Canadian and the French crater respectively. Satellite user familiar with orbital calculations will appreciate that these great circle offsets may be the result of some material completing more than one orbit of the earth before impacting. Indeed, two more small craters in Tennessee and North Dakota USA also lie on convenient 37.2 degree inclination great circle tracks extended through the other craters. In has yet to be proved that these smaller craters are of the same age as the big ones. Most of these craters are now partially or wholely buried in later sediment, so dating is difficult, but the dates of the 3 big craters are consistent with a single comet or asteroid break up and subsequent impact. Only one of the craters is clearly visible on satellite pictures. But it is over 70 kms diameter and it fortunately impacted on very old rock in the Quebec province of Canada, where any subsequent sediments or glacial till cover, have been swept away . The exact location of this crater in the Manicouagan Lakes region of the province of Quebec, is at 68 deg 37 min West , 51 deg 28 min North. Both NOAA14 and Meteor 3-5 weather satellites ( 137.62 and 137.85 MHz respectively) can give good images of this crater if the skies above it are clear of mist and clouds. From Western Europe, Manicouagan is near to the zero elevation limit on NOAA passes but can come up to 8 degrees of elevation on some Meteor passes, from Scotland. Stations in Eastern and Central USA and Canada should have no difficulty in getting good images on NOAA or Meteor, but the following rules apply - a. Try to pick a orbit where the Sun is not directly overhead the crater, so that you get some shadow detail b. Remember the crater is at 51N so solar illumination is poor in winter. c. Being Sun synchonous NOAA14 gives excellent results any clear skies afternoon for North American operators as it goes south to north. d. Meteor is not Sun synchronous so its gradual change of RAAN means you may have to wait several weeks for orbits at the right time of day. Meteor also switches off when solar illumination is poor. e. This location is often near the polar auroral front, so signal scintillation can be a menace, particularly for Europeans. Pray for clear skies