QUESTION:
What are Sojourner's wheels made out of?  How big are they?

ANSWER from Cathy Davis on January 2, 1997:
Each tire has a series of steel spikes which look like little spade shovels. 
There are about 200 per wheel.

ANSWER from Matt Wallace on May 24, 1997:
They are made out of aluminum, because aluminum is a fairly light-weight
material. They are about 13 cm high, and have sharp cleats. The cleats
allow the Rover to climb over obstacles that are higher then the wheels
themselves.