 Which planet wasn't well-known according to the text?
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. It was the planet that whipped us, that and the Sun . He was with Armstrong on Mars during his Service days, did a good deal of the original mapping and surveying for the Colony there. 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.
 Mercury
 Jupiter  Venus  Mars  Mercury