The Jupiter Theft
Del Rey 
Synopsis: In the latter half of the 21st century, the two super-powers, China and the USA, are preparing a joint crewed mission to Jupiter. Already hampered by attempts by the communist government of China and the post ACWII authoritarian government of the US to spy on each other, the mission and all of humanity are apparently threatened with extinction as a powerful x‑ray source is detected headed towards the solar system at near light speed. The x‑ray source slows and goes into orbit around Jupiter. A probe gets a brief glimpse of the visitor before being fried by an energy beam: Jupiter has a new Earth-sized moon which is orbited by obvious artifacts.
The Jupiter mission is altered to take the potential threat into account, being armed with nuclear devices. These do not get used as the occupants of what turn out to be city-sized ships near Jupiter swarm the human ship on arrival, crippling it and taking those humans who are not killed prisoner.
Communication with the Cygnans, as they are called, is difficult, since their language requires perfect pitch to understand and electronic equipment for humans to reproduce it. One crewmember, Jameson, is lucky enough to have the requirements and to be interesting to the two zookeepers who run the sections where the biological trophies of the Cygnans are kept. He discovers that the Cygnans are from Cygnus X‑1, a double star [a blue giant orbiting a black hole] which is an x‑ray source [which may have masked their x‑ray output en route to the solar system]. The Cygnans evolved on a planet orbiting the pair before the black hole was formed and fled their system to avoid the super-nova which would have formed the black hole. They don’t plan on staying in our system long and Jameson is relieved to find that while the aliens will be stealing Jupiter for fuel, as long as they leave soon, as is planned, they won’t cook Earth or tug it out of its orbit on the way through the inner system.
Jameson manages to irritate his captors and they place him in captivity with the other humans. A faction composed of Red Menace Chinese Party members and Snidely Whiplash Reliability Board members plus some lackies and victims decide to hurt the Cygnans by escaping using a smuggled gun and using the nuclear weapons from the ship on the Cygnan homeships. Jameson and company believe this will at best delay the Cygnans, who may then accidentally destroy Earth as they leave. The attack faction starts first, leaving a trail of dead Cygnans. Jameson’s group follows, pausing only long enough to release two humanoid aliens from a nearby enclosure. Luckily, these aliens are friendly and armed with powerful and flexible chemical abilities and the attack group is foiled, with heavy casualties on both sides. The survivors of the two groups get to the human ship and head for home, with samples of Cygnan tech.
Since the Cygnans are decadent, odds are the gadgets will be easily copied and improved on by humans and we see this is true in a brief epilogue ten years later, as Jameson and his friends head to 61 Cygni to return their alien allies to their home system.
This is an amazing book: it gets worse every time I read it. The science is awful: Moffitt has fixated on 3×108 m/s / 9.8 m/s/s = ~1 year and thinks you can get arbitrarily close to light speed in a year at one gee, limited only by available energy. He thinks the relativist equations mean the gravitational mass of objects increases as they get close to the speed of light. He has a complex ecosystem evolving on a planet of a super-giant when those stars have briefer lives than most species on Earth. He also sees to think 9.8 m/s/s is some kind of magic acceleration value everyone will use: the Cygnans do, even though their native gravity is much less.
Ignoring the science, the characterization is mostly a collection of stereotypes. The actions people and aliens take make no sense: at one point, armed with photon drives able to accelerate people at gee, they close to hand to hand range and duke it out. The photon drives are more than able to slice humans up, having a truly absurd power output but picking the other side off one by one wouldn’t have been as dramatic. The characters also make amazing leaps of logic, aided by the author’s knowledge of where the leaps of logic must end up. Bah. This book makes my head hurt.
Moffitt wrote four more books, a pair about an Islamic culture apparently researched by reading the Big Boys Extremely Edited Arabian Knights [also dependent on relativistic effects increasing gravity] and a less flawed pair of books about humans raised in another galaxy by aliens who received our genetic codes via radio and the humans’ attempt to find out what happened to the original humans. He has apparently stopped writing or getting published at any rate.