It’s a good prime number.  The Greeks considered it the perfect number as it was the number of intervals in a musical octave.  The number of days in a week, what’s not to like about the number?   Expressed in “Miles per Second,” it is the escape velocity of the planet earth.  For any object to leave our gravitational field, it must be travelling radially (vertically) at that speed or in more familiar units, 25,200 miles per hour.  That’s fast.  Most of us have a hard time imagining moving as fast as our astronauts must.  But I want to take your thoughts in a different direction on this.  Not going up but coming down. That figure is also the minimum speed of any astronomical body slamming into the earth.  Typically, meteorites are going much faster because they are already moving on a trajectory long before the earth’s gravitational field does its work.  The entry speed of these objects is the sum of their orbital velocity and the escape velocity.  That’s really really fast.   A grain of sand at those speeds packs the same momentum punch as a loaded 18-wheeler travelling 70 mph.  Now imagine an iron-based asteroid weighing billions of tons a half mile in diameter smacking into our lonely planet.  It’s not an event as big as the carbonaceous chondrite meteor that slammed into the Yucatan peninsula of Mexico 65 million years ago likely causing the extinction of the dinosaurs.  But researchers may have discovered the landing point of this mini-behemoth.   As part of an investigation of our planet’s ice sheets, scientists in 2015 discovered what appears to be an impact crater.  It took them three more years of painstaking investigation before they made the announcement earlier this year.  (That’s how careful scientists are to get things right.)  But they still don’t have a conclusive date for the crater.  It lies under 3,000 feet of ice on a Greenland glacier.  Later this year, the ice will be drilled, and core samples taken from the crater.  These will be dated to provide more definitive proof.    Nonetheless, the earth has endured six mass extinctions since life began.  The extinction of the dinosaurs likely caused by a meteor impact millions of years ago is the most dramatic of these, but there have been others.  Ever wondered what happened to the North American wooly mammoth and sabre-toothed tiger?  There was a smaller and more local (to N. America) extinction event about 13,000 years ago called the “Younger Dryas Period.”  The ancient Clovis Indians of North America also went the way of the large animals at that time.  No one really has a good theory for their disappearance.  You can find “Clovis (arrowhead) points” today throughout Missouri and Oklahoma.   This period lasted about 1,000 years.  The planet’s temperatures plunged giving us a 1,000-year long ice age.  Many large animal species in North America became extinct, but not in the southern hemisphere and a much lesser extinction rate in Europe.  This ice age left North America covered in glaciers up to a mile thick.  You can find the rocky remains of glacial rivers deposited up and down hills in the northern US and Canada.  These are called “eskers.”  The present north-south course of the Mississippi River was the ancient drainage channel for glacial melt from the eastern Rockies to the Ohio Valley.  At the glacial terminus around the present bootheel of Missouri, the mile-high waterfall was so powerful that it cut through the granite crustal continental plate, weakening it and leaving us vulnerable to killer earthquakes such as the 7.9 New Madrid earthquake in 1811-1812.   Right now, we have some excited speculation that the Greenland impact crater may, in fact, be the impact that led to the Younger Dryas era.  Since the impact of natives across the planet was minimal (but not zero), we can truly call this a “natural variation.”  It will take several more years to link the Greenland crater to the Younger Dryas era, but there are more sobering lessons here.   Other natural variations (often from volcanic eruptions) in the past have caused the planet to turn into a giant hot house.  Fossilized tropical palms and other tropical plants have been found above the arctic circles.  There have been periods of 100,000s of years where the earth was cloudy, incredibly humid and hot from the equator to the poles.  Ice age or hot house, we need to pay attention to any natural variations headed our way.  Avoidance is not a good strategy for survival because some things are just too big to ignore. ————————————