Spirit and Opportunity were two small rovers that were solar powered.
Spirit and Opportunity reached Mars in 2004, using airbags and parachutes to soften the landing. The small rovers using solar energy to power their equipment. As the Martian dust blew around the planet, the dust settled on the panels reducing the amount of charge the panels could provided the batteries.
Spirit became stuck in 2010 and was discontinued in 2011.
Opportunity is still sending back data today and reached a crater in 2011 having travelled a total of 35km on the Martian surface.
Both rovers moved around the surface of Mars so they needed six wheels, independently driven by electric motors.
On each side of the rover, two wheels were on a rocker-bogie system.
This meant that the rover could adapt to the changing terrain.
Each wheel was 26cm in diameter and had cleats to grip the soil.