Assertion: hydrogen balloon | /r/ObjectUse | used in early and mid-20th century
Facets:
value: extensively
type: DEGREE
Source sentences:
text: In the early and mid-20th century, hydrogen balloons were used extensively in upper-atmosphere research in such projects as Osoaviakhim-1, the Stratobowl launches, Project Manhigh, and Project Strato-Lab.