Yet another book I had forgotten (along with its author, whose name rang no bells), this being published in 1977. Moffitt (1931-2014), wrote only a handful of SF novels, but a larger body of shorter fiction as well as adult spy thrillers.
To start with the blurb:
Within hours after the Lunar Observatory picked up a strange new X-ray source in Cygnus, the disastrous picture was clear. An immense object was hurtling towards the Solar System at nearly the speed of light. And its intense radiation would surely wipe out all life on Earth within six months. There was nothing anyone could do. Then, incredibly, the rogue presence that had appeared out of nowhere suddenly changed its trajectory - and stopped in the region of Jupiter. But that was flatly impossible...
The story is set in a future in which humanity is recovering from devastating wars which had left American and China as the two major powers in uneasy co-existence. The inner planets had been explored and, in the case of Mars, settled, and a major expedition with a mixed Chinese/American crew of 100 had been organised to visit Jupiter and potentially establish a base on one of the moons. However, the arrival of the Cygnus Object results in a rapid change of plan, with the mission repurposed to focus on examining the Object.
During the journey to Jupiter, the author provides lots of background information and introduces several key staff members who remain prominent for most of the story, in a variety of shifting relationships. The central character is Commander Tod Jameson and the events which unroll are largely seen from his viewpoint.
The human crew are astonished when they arrive at Jupiter, since the Cygnus Object is not one but five spacecraft, each thirty miles long and with three folding arms in a configuration which can be adjusted to suit the different requirements of acceleration up to near-lightspeed, cruising at that speed for years, and then decelerating when approach the target system. The alien spacecraft are powered by gas stripped from giant gas planets, and are already stripping Jupiter's atmosphere to fuel their next journey.
The Cygnans soon become aware of the arrival of the human Jupiter ship and swarm over it, capturing most of the crew. Many of them end up in a super zoo, where the tensions between the Americans and Chinese are intensified, and also between the democratic and dictatorial elements of the crews. To make matters even more unstable, the humans have nuclear weapons on board.
In contrast with their enormous ships, the Cygnans' standard method of inter-ship transport is by something like a rocket broomstick which the Cygnans sit astride and manoeuvre by shifting their body-weight around and judging direction by eye. Even more remarkably, they use spray-on space-suits which are almost invisible. This allows the humans to observe the truly weird Cygnans who (among other oddities) communicate by musical sounds; a characteristic which gives Jameson (who has perfect pitch) a major advantage.
The Cygnans rely on a network of transparent tubes to move around at high speed within each ship. To avoid collisions, the tubes are directional and wrap around each other, forming a double spiral. To digress for a moment, this reminds me of a Victorian fort at Dover, Kent, in which the designers wanted to achieve rapid transport of large numbers of troops between the accommodation at the top of a cliff and the defended shore at the bottom. They built an ingenious 140-foot staircase with a triple spiral, known as the Grand Shaft - it still exists. The Grand Shaft was never used in anger, but the story goes that in order to maintain social differences in peacetime, the use of the three staircases was separated into "Officers and their Ladies", "Sergeants and their Wives", and "Soldiers and their Women".
Anyway, to cut a fairly long story short, most of the humans try to break out of the zoo and return to their ship, with some of them being aided by another zoo species who are highly intelligent and possess a unique inherent weapon system.
The blurb credits the author with having the "world-juggling sweep of Larry Niven" and the "scientific expertise of Arthur C. Clarke", and for once I would not disagree with this praise. I might add that Moffitt's handling of his characters is superior to both.