 What isn't an obstacle on Mercury?
Across to the right was a group that Baron knew vaguely—Andean climbers, or at least two of them were. Over near the door he recognized old Balmer, who had mapped the first passage to the core of Vulcan Crater on Venus. I first met him on Venus; we spent five years together up there doing some of the nastiest exploring since the Matto Grasso. Then he made the attempt on Vulcan Crater that paved the way for Balmer a few years later. “I want to make a Crossing at perihelion and I want to cross on the surface.
 zero gravity
 zero gravity  rough terrain  volcanoes  extreme temperatures