midterm essay question as answered by Steven Mockford

26 February 2004


Question: Many people are surprised when they learn that an asteroid explodes when it hits the Earth. Why does this happen?  Explain by describing the physics of explosions.  Give examples of the wide range of phenomena that lead to explosions.  Include numbers, when relevant.


Answer: An explosion happens when an object is turned into a gas and expands very rapidly.  For example, when TNT is "exploded," the resulting gas is heated to a point where it becomes a hot gas.  At this high temperature, the molecules have a higher average velocity; thus, the volume of the gas increases by 1000 times its previous size (10 times greater in every direction).  When the gas expands, it breaks the bomb open and pushes in every direction with a strong force.


With this in mind, it is easier to understand why an asteroid of substantial mass (approximately 10 km wide) explodes on impact.  An asteroid is moving at a high velocity (approximately 30 km/sec) and in the equation for kinetic energy, the velocity is squared.  That means that the energy in the rock is extremely high.  Upon impact, that energy is converted into heat, vaporizing the rock.  The rock is turned into hot gas and expands rapidly, pushing everything out of its way (i.e. dust particles and ground).


There are several ways to create an explosion.  One is by using kinetic energy, such as with asteroids or "smart rocks."  Another is combusting a substance chemically by burning it with oxygen.  This is how gasoline bombs and car engines work.  Lastly, on an atomic level, one can explode atoms in fission to create enough energy to make an atomic bomb.