The astronauts in this story are sent out by a non-profit, donation-funded, independent, global space agency. I love that idea. There's a powerful emotional resonance to the notion of space exploration as something that could unify humanity in an entirely wholesome quest, motivated only by our shared curiosity and awe at the wonders of the universe.
But perhaps such motives only seem wholesome because we haven't yet found anything in space that's capable of feeling. When we do, we'll almost certainly inflict suffering on it, intentionally or not. The astronauts in the story take great care to avoid harming the lifeforms they encounter, but accidents are inevitable. So one of the dilemmas Chambers poses is: how do we justify imposing risks on alien life, when we could just stay home and mind our own business?
Sheer curiosity seems like a callous reason, though I'm not entirely sure. Most of us feel it's acceptable to spend some resources on e.g. art and music even though theoretically those resources could be directed toward urgent problems instead. Perhaps the pursuit of knowledge is, similarly, of such great intrinsic value that we feel justified in making space for it, even if that requires bringing some amount of extra suffering into the world.
Regardless, I think the potential long-run benefits of scientific knowledge - benefits not only to ourselves, but to all conscious life within our sphere of influence - can justify taking some significant risks. Our planet is already awash in pain, and that is likely true for any other planet where conscious life has evolved. The more we know about the universe, the better equipped we are to try to reduce that pain.
These astronauts don't have faster-than-light travel, so an interstellar expedition also requires facing a personal question: can scientific inquiry be worth permanent separation from friends and family? The protagonists have evidently answered yes; they have loved ones on Earth, and they leave them behind, knowing they won't meet again. Given that the mission isn't urgent (they're just exploring, not responding to some kind of crisis), I'd expect people willing to do this to be very unusual. Not only do you have to make peace with losing the people you're leaving behind, you also have to live with knowing that you've caused them enormous heartache.
Chambers's astronauts at least get to keep their most cherished humans near them, because the four-person crew seems to form a polyamorous family. My first reaction is that this sounds a bit fraught - if a relationship goes south and the individuals don't handle it well, it could put the whole mission in danger. But of course, not many humans would deal well with forgoing love and sex for decades. Sending out a crew that already has proven, stable intimate relationships among themselves to satisfy their emotional needs seems less risky than sending mere colleagues.