 Why doesn't the Major want McIvers to scout ahead?
Brightside Crossing by Alan E. Nourse JAMES BARON was not pleased to hear that he had had a visitor when he reached the Red Lion that evening. I’d always liked the Major—he was big and quiet and cool, the sort of guy who always had things figured a little further ahead than anyone else and always knew what to do in a tight place. Four thousand miles of butcherous heat, just to have some joker come along, use your data and drum you out of the glory by crossing at perihelion forty-four days later? “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. The setup there wasn’t very extensive—a rocket landing, the labs and quarters for Sanderson’s crew sunk deep into the crust, and the tower that housed the Solar ’scope that Sanderson had built up there ten years before. 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 Solar ’scope could take that much change and they’d get good clear observation of the Sun for about seventy out of the eighty-eight days it takes the planet to wheel around. 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.
 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.