 Which isn't a reason why Russell didn't tell Johnson and Alvar directly that he thought Dunbar was crazy?
Russell didn't think they could remain sane in all this blackness much longer. Dunbar, the oldest of the four, an old space-buster with a face wrinkled like a dried prune, burned by cosmic rays and the suns of worlds so far away they were scarcely credible, had taken command. Four men, thought Russell, held together by an invisible string of gravity, plunging through a lost pocket of hell's dark where there had never been any sound or life, with old Dunbar the first in line, taking the lead because he was older and knew where he was and where he was going. Maybe Johnson, second in line, and Alvar who was third, knew too, but were afraid to admit it. But Russell knew it and he'd admitted it from the first—that old Dunbar was as crazy as a Jovian juke-bird. Russell had no idea now how long the four of them had been plunging toward the red-rimmed sun that never seemed to get any nearer. When the four of them had escaped from that prison colony on a sizzling hot asteroid rock in the Ronlwhyn system, that wasn't enough for Dunbar. no—Dunbar had to start talking about a place they could go where they'd never be apprehended, in a system no one else had ever heard of, where they could live like gods on a green soft world like the Earth had been a long time back. "You're still sure it's the sun up ahead ...
 He wanted them to find out for themselves
 All of their communication systems are connected  He had already given up on life  He might have been secretly curious about the stories  He wanted them to find out for themselves