using only rover wheels get into orbit around any body, secondary thrusters can be used to keep the craft on the ground longer but not to increase the craft's acceleration directly.
the tiers are:
easy: use rover wheels on a moon to get into orbit around a planet
medium: use rover wheels on a planet or moon with atmosphere to get into orbit
hard: use rover wheels to generate enough speed to get into orbit around the sun
good luck to all, this challenge may be impossible.