 Why was Robert the only choice for returning to Earh?
I do not think it was what Peter called an emotion; and yet it had nothing to do with logic, either. I did not even wish that Peter were not dead, for reason had told me that he had to die. Then there would remain only the task of blasting the great, shining ship out into the carefully-calculated orbit that would be its home. "Since the Invaders first appeared in Wood River, Oregon, 24 hours ago, they have not once acknowledged our attempts to communicate, or in any way taken notice of our existence as reasoning beings. They have examined us, dissected us, driven us mad, killed us with no discernable provocation; and this is more intolerable than any normal invasion. "I have no fear that the people of Earth will fail to meet this challenge, for there is no alternative. It was only two stories down the moving ramp to Lorelei Cooper's laboratory. Peter took it in fifteen seconds, running, and stumbled to a halt in front of the door marked "Radiation." She had set her door mechanism to "Etaoin Shrdlu," principally because he hated double-talk. There were left only two blood-red, malignant monstrosities somehow defiling the air they floated in; and below them, a pitiful huddle of flesh no longer recognizable as human beings. There aren't going to be any noncombatants, this time. Forty-five ... forty-six ... forty-seven ... forty-eight.
 He was the only changeling-child who had not been destroyed
 He was the only changeling-child who grew to have no fear.  He was the only changeling-child who had not been destroyed  He was the only one will the powerfully strong talons that could defeat the Invaders  He was the strongest of the group