 Which planet wasn't well-known according to the text?
“I want to make a Crossing at perihelion and I want to cross on the surface. That makes the Brightside of Mercury at perihelion the hottest place in the Solar System, with one single exception: the surface of the Sun itself. The Twilight Lab, near the northern pole of Mercury, was the obvious jumping-off place. The Major was counting on Sanderson knowing something about Mercury as well as the Sun when we camped at the Lab to make final preparations. But the only way to avoid it is to travel over the Pole, which would lose us days and offer us no guarantee of less activity to the west. There was an atmosphere and a constant atmospheric flow from Brightside to Darkside.
 Mercury
 Jupiter  Venus  Mars  Mercury