She stumbled out of her little tent, feeling yesterday’s hike in the muscles of her legs. That was surprising. She thought she had trained well. But even in the largest space habitat, you couldn’t go downhill for all that long. On an actual planet, you could go on losing altitude for days. And, as it turned out, those long downhill runs were what really killed your legs. Yesterday she had shed almost two thousand meters, descending from a range of hills toward a blue, water-filled crater thirty kilometers across. She had stopped a few clicks short of its rim, where the ground dropped away toward a swath of grassland between her and the shore. The break in the slope had been subtle, but Kath Two’s throbbing knees had made it obvious enough. She had taken a dozen or so strides down it, gauging its angle in her blistered soles, sensing the air’s currents with her lips, her hair, and the palms of her hands. Then she had turned around and trudged back up to an inflection point that would have been invisible had the low evening sun not been grazing it, casting a sharp terminator on the ground.