The six-wheeled bots are being designed to carry a significant scientific payload, oriented around the search for life, but won't slouch in the maneuverability department, with six independently rotating wheels.

ENGADGET: ESA's six-wheeled Mars bot prototypes ready to kick some martian ass

The rollers move passively as the wheels turn, or the wheels can lock in place with only the rollers rotating.

FORBES: Airtrax topples the tyranny of steering.