Robert Silverberg 
Starborne 

It will be the greatest voyage of exploration in human history. Fifty men and women are chosen to crew the Wotan. Their mission: to travel deep into the unknown galaxy in search of habitable worlds, to rekindle the dying human spirit. Their only contact with Earth is the telepathic link between one of the crew members and her sister back home. But when the mind-link with Earth is abruptly broken the Wotan is lost in the pearl-gray twilight of nospace. Then just as all seems lost, the Wotan encounters a massive alien presence. Suddenly the crew is forced to realise that their every assumption about life and death, humanity and the universe, may be dead wrong.