US Patent 9033274 Balloon altitude control using density adjustment and/or volume adjustment

A balloon having an envelope and a payload positioned beneath the envelope. The envelope comprises a first portion and a second portion, wherein the first portion allows more solar energy to be transferred to gas within the envelope than the second portion. The balloon may operate in a first mode in which altitudinal movement of the balloon is caused, at least in part, by rotating the envelope to change an amount of the first portion that faces the sun and an amount of the second portion that faces the sun, and wherein the control system is further configured to cause the balloon to operate in a second mode in which altitudinal movement of the balloon is caused, at least in part, by moving a lifting gas or air into or out of the envelope.