Great setting and a classic writing, I could understand why this could be called a classic hard science fiction, having gotten released in 1972. The story revolves around a mysterious space object that was previously thought out as an asteroid, is actually an alien space-craft. With a rapid rotation period of 4 minutes and incredibly big, the spacecraft presents as being a boring plain object to the cameras of Sita, a space probe launched from Phobos, until the manned vessel Endeavor reaches it for exploration.
Grand ideas, perfect setting for the series, although the text is philosophical and subtle looking into the nature of Humans, this is one book that should not be missed by a fan of fiction much less by a fan of sci-fi.