 Why doesn't the Major want McIvers to scout ahead?
The Twilight Lab, near the northern pole of Mercury, was the obvious jumping-off place. F. No permanent installation with a human crew could survive at either extreme. Sanderson built the Lab up near the pole, where the zone is about five miles wide, so the temperature only varies 50 to 60 degrees with the libration. 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. He spent a week briefing Jack Stone, the third member of our party, who had arrived with the supplies and equipment a few days earlier. 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.
 The Major thinks it's safer if they stay together.
 The Major thinks McIvers is up to something. The Major wants McIvers close, so he can keep an eye on him.  The Major thinks it's safer if they stay together.  The Major doesn't want McIvers to steal the glory by completing the crossing first.  The Major doesn't want to be responsible if McIvers dies.