Ditto the 20 inch wheels. I'd go with Sun CR18 36H. Narrower and lets you use a fatter tire to protect the side wall of the rim. They sell 2.25 inch wide tires. The 36H lets you leverage existing hubs if you choose that route. But there's enough room to easily pump up the tire. Really strong too. Lightness might count, but you'd figure NASA can custom order some titanium spokes on the real deal. You're probably okay looking for 15g or 14/15g double butted spokes.

The challenge you're going to have is the drive train. How would you power the wheels?

Personally, like MichaelW, I'd go with 4x4 drive. But rather than differentials, I'd use 4 wheel independent motors and independent suspension. I'd be looking for tiny reduction gear boxes and small, high torque, high RPM DC motors like those used on today's battle bots. Suspension is going to require some design as you need to clear obstacles. I assume you folks will be using batteries. For testing, standard Sealed Lead Acid works. But assume NASA will leverage Li-ion. I'd budget sufficient power for a small, single board computer, maybe mini-ITX Intel D945GSE-JT Atom N270 32-bit board. Only 11 Watts. Why? Because I'd want to add some USB sensors for inertial guidance, some accelerometers, some level/pitch/yaw sensors and then be able to have compute cycles to calculate how much to drive the wheels. I'd also want to allow drive-by-wire so the riders use a joystick controller, and this would allow someone to use a remote joystick and send commands to drive over the network.

Again, suspension and hubs that make up the drive train might be hard to source. One option I might pursue would be a direct-drive that allows a small motor to mount inline with the hub axle. Take a shimano freehub for disc brakes, and modify the freehub to fix it to the hub body or replace the hub body with a fixed shell, then attach to gear reduction box, and then the motor drive shaft. This assembly has to be small and light and have some type of retractable power/data cord that moves with the suspension.