This document discusses the potential for using airships as science platforms for Earth and space science. It proposes the 20-20-20 Airships Challenge through NASA's Centennial Challenges program to encourage development of stratospheric airships. The challenge would award prizes for airships that can carry payloads of 20kg to 20km altitude for 20 hours (Tier 1) or 200kg to 20km altitude for 200 hours (Tier 2). Airships could enable new types of long duration observations for Earth science, atmospheric science, and astrophysics at lower costs than current space missions. The document reviews some example science instruments and investigations that could be performed from airship platforms.