If you're referring to the surface of the Earth, coasting after reaching speed and ignoring drag, the highest speed is orbital velocity at zero altitude, about 7907 m/s. At any higher speed you're in a higher-energy orbit and are off the surface some of the time.
There is obviously no human health limit for an unmanned launch vehicle (as there is for the Space Shuttle, 3 g), but if you are referring to large payload carriers up to Ariane size, 4-8 g is a practical limit, based on fuel costs for too-low acceleration (more delta-V needed the closer you get to simply hovering at 1 g) and added structural mass needed for high acceleration. Pegasus, an air-launched small-payload carrier, has a peak acceleration of 13 g. If what is being launched is a warhead and the vehicle is some sort of interceptor, up to 50 g (and possibly more) has been used.What is the highest speed an object can reach without burning airborn but close to ground?
Theoretically, light speed. Realistically, we've had objects travel up to Mach 15 and above, objects such as rockets.
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