 What isn't an obstacle on Mercury?
“I want to make a Crossing at perihelion and I want to cross on the surface. The Twilight Lab, near the northern pole of Mercury, was the obvious jumping-off place. But with Mercury’s wobble, the twilight zone between Brightside and Darkside offers something closer to survival temperatures. 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.
 zero gravity
 zero gravity  rough terrain  volcanoes  extreme temperatures