Rockets in Ursa Major

the long version

"The British spaceship DSP 15 returns from the stars of the Ursa Major stream, thirty years late, crewless and carrying its captain's last message: ""If this ship returns to Earth, then mankind is in deadly peril,"" specifically an alien invasion, as Dr. Warboys discovers with his new radar transmitter. Earth is caught in a galactic guerrilla war between the Yelas' and the friendly fleet of Belegeuse, whose species is also humanoid. It's too late to explain the mixup to the Yelas so humanoids ally in a sun-burst bomb retaliation. This is accelerated propulsion again, as in the Hoyles' first collaboration Fifth Planet (1967), fueled with more scientific theory, not a little satire and first rate fantasy."--Kirkus Review.