Passage to Pluto

Three astronauts travel to Pluto and explore it. Walters pays attention to science, regarding fuel usage, orbits, orbital mechanics, etc, and doesn't get boring while doing so.At Pluto, the astronauts discover a small natural(?) satellite. It's tidally locked to Pluto, is in a decaying orbit, and has an outrageous gravity gradient. This and other problems affect their mission and return, and the story is quite gripping.My only complaint would be that he leaves us totally hanging, not even having the characters speculate as to the nature of this satellite. A chunk of neutron star? An artificial construct? Either is plausible for the story, but we're left not knowing. Still, for the educated juvenile SF fan, it's a great book.