Back-of-the-Envelope Calculations: Velocity of Asteroids
Asteroids zip through space at truly astounding velocities. Let's try to put that into perspective. It took the Apollo astronauts about 3 days to travel from the Earth to the Moon.
a) If you could drive the distance in your car at 100 km/hour (60 mph), how long would it take you to drive to the Moon?
b) Earth-crossing asteroids typically zoom along at 25 km/second (yes! per second! not per hour!). How long would it take a typical asteroid to travel the distance from the Moon to the Earth?
a) about half a year, and that's driving 24 hours a day without taking breaks. If you stopped for meals and slept at night, it would take about a year. At 100 km/hour, it takes 3840 hours to go the 384,000 km to the Moon. 3840 hours is about 160 days.
b) a little over 4 hours. 25 km/second is 90,000 km/hour. At that rate, it takes 4.3 hours to travel the 384,000 km between the Moon and the Earth. Pretty staggering...
References and Resources
This SERC page describes the use of Back of the Envelope Calculations
The Back of the Envelope : This page outlines one of the essays in the book "Programming Pearls" (ISBN 0-201-65788-0). The book is written for computer science faculty and students, but this portion speaks very well to back of the envelope calculations in general.