Mission of gravity - Hal Clement

Mesklin is a vast, inhospitable, disc-shaped planet, so cold that its oceans are liquid methane and its snows are frozen ammonia. It is a world spinning dizzyingly, a world where gravity can be a crushing 700 times greater than Earth's, a world too hostile for human explorers.
But the planet holds secrets of inestimable value, and an unmanned probe that has crashed close to one of its poles must be recovered. Only the Mesklinites, the small creatures so bizarrely adapted to their harsh environment, can help.
And so Barlennan, the resourceful and courageous captain of the Mesklinite ship Bree, sets out on an heroic and appalling journey into the terrible unknown. For him and his people, the prize to be gained is as great as that for mankind...
Hal Clement's Mission of Gravity is universally regarded as one of the most important and best loved novels in the genre. The remarkable and sympathetic depiction of an alien species and the plausible and scientifically based realization of the strange world they inhabit make it a major landmark in the history of hard sf.