Novel with a lot of jumping between timelines. It has aged badly and is probably only enjoyable for the dedicated Pohl fan.
In this classic, a peculiar asteroid is found orbiting the sun. It contains an ancient spaceport filled with ships. Volunteers come to travel on the ships. These cannot be controlled, but they can take the passengers on incredible adventures. A trip can yield nothing, immense riches, or death. And there is no way of knowing beforehand. This gripping tale mixing psychology and adventure.