Introduction to the Solar System

6 months after this class, it will be mid-winter
The mid-winter sky is spectacular, with more bright stars and bright constellations than the summer
We've concentrated on the sky we see in the summer, but if you happen to think about the sky 6 months from now, when this course is long past, I'd like to show you what you might be looking at

North:
North star, little dipper upside down
Cassiopeia
Big Dipper opposite Cassiopeia
Cepheus below
Pegasus and Andromeda West
Perseus above
Auriga E of Perseus
Gemini E of Perseus
Leo setting

South:
Orion
Lepus
Canis major
Canis minor
Taurus (El Noth upper horn)
pleiades
Cetus, Pisces, Pegasus (due W of Orion)

Seminal paper by Clark Chapman and David Morrison 1994 Nature
Chapman currently researching in Boulder
1 in 10,000 chance that a large 2 km asteroid or comet will hit the Earth in the next century
This would disrupt the ecosphere and kill a large fraction of the population of the Earth
But this is an asteroid 1/5 as big as what killed off the dinosaurs
Your chance of being killed by an impact is 1/20,000, same as airplane accident
Problem is, nobody has ever been killed by an impact!
But we ride in airplanes all the time
There's a perceived difference between common accidents that kill small numbers of people all the time (car crashes) and massive events that kill enormous number of people
The statistics may be the same, but we tend to think the big ones won't happen
Small impacts happen a lot, big ones rarely
We can make a size-frequency plot, just like for craters on the Moon

Here we see a plot of the size of an impacting object from space, vs. how often they happen
A 3 m object hits the Earth every month, but burns up in the atmosphere
It takes a 50 m object --half a football stadium size, to make it all the way through the atmosphere without breaking up
Every year, an atomic bomb sized object, about 10 m in diameter (house)
Every thousand years, we get a 50 m size object that explodes near the ground --Tunguska in Siberia
If this had exploded above a city, it would have caused 10's of thousands of deaths
Every 10,000 years (just slightly longer than recorded history) a 1 km size object could hit the Earth
A 1 km size object is enough to cause global damage, with 10% or more of the Earth's population being killed
A 10 km object hits every 100 m years or so (massive comet or mid-size asteroid)
That means there have been about 40 of these incredibly catastrophic events during the history of the Earth
We are just now beginning to understand the significance of this -- may have shaped the evolution of life to a great degree, by killing off most species, and allowing proliferation of new species on the rebound

1/100 chance of automobile accident
Murder is next (in the USA, highest murder rate for a country not at war)
Impact death chance may be as high as 1/3000
Asteroid comet impact about the same as airplane crash, but more likely than flood, tornado, snake bite
Drinking water because of organic contamination 1/10 million
Authors cite huge governmental expenditures to try and reduce the chances of all these risks, but don't consider impacts
Conclusion -- we should be investing resources in saving the planet from a huge impact
This may be a policy issue that faces us in the future
But it will no doubt be a debate with lots of facts thrown around
Critical thinking and a careful consideration of the facts is necessary
Perhaps we should think about the ways we use our resources to put off risks -- health care crisis, pouring tons of money into saving the lives of people who are terminally ill, for example
We are in a position with our technology to do almost anything to preserve life, while at the same time, it is very likely that the quality of the life that we preserve goes down, because of the unhindered growth of science and technology.
Should we pour money into an ambitious program to stave off death from space?

6/25/96