|Although this is a continuation of The Martian Race, the two books are in fact quite different.
In the former Gregory very carefully described the whole process of extra-terrestrial travel and the details of carrying out space missions, whereas in this book such matters are skipped over; in the former book the science fiction is restrained enough that it is just about believeable, whereas in this book it is very far out.
The worst thing about this book is that the science fiction ideas are told in break-out paragraphs, instead of coming through the telling of the story. The end result is a novel that lurches from once situation to the next -- and those situations are orders of magnitude separated from one another.
The orders really are huge. The book starts with the Mars mission. Then it transpires that there is an on-going mission to Pluto (manned!) and the people on Mars suddenly nip over to Pluto. But when they get there, they decide to carry on travelling to the Oort cloud, even further out than Pluto. Surely Gregory is aware that huge technological advances would need to be made to undertake these journeys, and that they would occur on completely different time-scales, but he chooses to ignore this and writes an old-fashioned zip-around-the-stars-style space opera. And Gregory Benford is not good at writing that stuff. In short, the orders of magnitude are mentioned, but Gregory does not deal with the practicalities.
It also seems to be not terribly imaginative. Gregory has imagined Pluto occupied by aliens which "look like domestic appliances." He describes life-forms which take the form of interstellar electro-magnetic and plasma fields, and communicate with each other via very long wave radio.
In short, I'm finding this very poor science fiction.
The ending, when it finally comes, explores how two races living on different space-scales (but the same time-scales, incongruously), come to discover and learn about each other (although the human side is facilitated by a marvelous machine which does all the hard work -- then again, this is probably realistic).
The very final twist, that one of these alien forms has got itself trapped in Mars' underground caverns and the humans can help another member of the race to reach it using the `shadow' of its nuclear drive, is inspired science fiction, and does ultimately make the two books worth reading. Just.
Still, I find the schizophrenic nature of the science fiction (the tone of the first book is comletely changed by the second) rather unbalancing.