  Many rocky exoplanets are heavier and larger than the Earth, and have higher
surface gravity. This makes space-flight on these worlds very challenging,
because the required fuel mass for a given payload is an exponential function
of planetary surface gravity. We find that chemical rockets still allow for
escape velocities on Super-Earths up to 10x Earth mass. More massive rocky
worlds, if they exist, would require other means to leave the planet, such as
nuclear propulsion.
