# Back-of-the-Envelope Calculations: Velocity of Asteroids

## Summary

*Question*

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?

## Assessment

*Answer*

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

A View from the Back of the Envelope (more info) : This site has a good number of easy simulations and visualizations 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.