Close to critical

From back cover Ballantine paperback July 1975: CRISIS ON TENEBRA Shrouded in eternal gloom by its own thick atmosphere, Tenebra was a hostile planet... a place of crushing gravity, 370-degree temperatures, a constantly shifting crust and giant drifting raindrops. Unpromising -- yet there was life, intelligent life on Tenebra. For more than 20 years, Earth scientists had studied the nati...