Hudson looked round the command room with its cramped balsa-plast desk, single chair and wall bunk. Command room! His mouth twisted, half with amusement, half contempt. All the years of struggling and devoted service, service which had broken up his home and driven his wife to seek comfort elsewhere. All that until, finally, they had given him a command. A command! Not one of the new powerful cruisers or even a fleet of space tugs but a detector post on Ganymede. A hive-shaped dome on an airless satellite with four men and a sergeant to keep him company.

He had given up telling himself that the posting was important. He knew it wasn't. Even with the increased tension and gradual advance of the aliens, the post was nothing more than an expendable speck in the vast perimeter of Earth's defence. Its very position made it of less importance than similar posts elsewhere because the bulk of Jupiter almost wholly nullified the detector instruments with which the dome was equipped. At most, they would have less than three minutes warning if the aliens decided to attack...