Deadman Switch

From the back cover of the Baen second printing: "Blood offering - Solitaire: a unique ring of heavy-metal moons. For decades after its discovery and unexplained "effect sphere" kept all hyperships away from a metal-hungry galaxy's most tantalizing prize. Until a research ship penetrated the Cloud entirely by accident - when the man at the helm died of a heart attack just before entering the sphere. For the first time, a hypership's drive did not fail as it approached Solitaire. For the researchers the difficult part came when it was time to leave... Now, every ship heading for the Ring Mines carries two death-row felons - one for entry, the other for exit. It's as bizarre a method as the effect it counters, but it works... until the hero of Deadman Switch discovers that one of his condemned criminals isn't guilty..."