An ambitious work, long and complex in its plotting, alternating between time periods before and after the expedition to gradually reveal the events that occurred on the alien world, though it is so slow to get going that I nearly gave up; the first third develops so slowly that I felt mired in mud as I tried to get to the meat of the story. Although spirituality and religion are key components, the long and detailed expositions of Sandoz's state of mind become repetitive and tedious.
The characters are well-drawn both physically and in personality. Their back stories are interesting and detailed, so that we can really engage with them.
There are aspects of the plot that are implausible. I winced at the description of how interstellar travel was achieved --- anyone with a passing familiarity with the rocket equation will understand why. More serious is the sheer thoughtlessness of a group of supposedly intelligent and educated protagonists in their dealings with the alien species. The harm they do is well-drawn, but the lack of awareness of the characters is not believable.
I was not convinced either by the climax which, for me, failed to deliver on the promise of the rest of the story. The sudden reveal of the relationship between the two alien species was obvious from early on, and the rationale for Sandoz's behaviour after his return was unconvincing, as was his so easy redemption.
