Some say the world will end in fire, Some say in ice. From what I've tasted of desire I hold with those who favor fire. But if I had to perish twice, I think I know enough of hate To say that for destruction ice Is also great And would suffice. Robert Frost, Fire and Ice

Assignment: Sign up for Observing Lab #3 Problem Set #10 is due next Thursday, 1 December.

In Class: Meteors Bright transient flashes Space rocks and debris buring up in the upper atmosphere (30-50 miles up) Typical size: quarter Meteorites Larger objects that actually make it to the ground. Produce a very bright streak across the sky. 90% of meteorites are stones, ~10% irons -- consistent with composition of the asteroid belt objects. Larger Impact Hazards Bigger objects have the potential to do substantial harm. Luckily, there aren't as many big objects. Maybe a million earth-crossing objects larger than 50 m in size Maybe only a thousand or so larger than 2 km in size. Effects range from local catastrophe to global devastation. For big impactors, the real danger comes from substantial climate change. "Impact winter" comes from the ejection of dust and soot into the global atmospheric circulation, thereby blocking sunlight and cooling the Earth. The Torino Scale Impacts in History Tunguska, Siberia -- 30 June 1908. Approximately 40 Megaton impact (Hiroshima = 0.02 MT) Blast knocked down trees over a region half the size of Rhode Island. Forest fires burned 1,000 square kilometers. If this impact had occurerd over a populated area, it would have probably produced 500,000 to a million casualties. Impacts like these probably happen once every 1000 years. The "Dino-killer" --- 65 million years ago. Probably a 10 km asteroid. Probably impacted the Yucatan Peninsula in Mexico. Probably produced "impact winter" and killed all large species (mostly dinosaurs and reptiles -- some mammals apparently survived). This type of impact probably occurs once every 10-100 million years.