Used https://github.com/mrdoob/three.js/blob/8a6bfc7ce1dbe80e08a51e61c94f0fd7fdbad9ad/utils/converters/obj/convert_obj_three.py to convert from .obj to .js with options:
-i balloon2.obj -o balloon2.js -s smooth -t ascii

Created by Jonathan Olson (PhET) in Blender (no licensing issues).

NOTE (2024-03-19): This python script is really outdated, go see https://github.com/phetsims/buoyancy/issues/101 for a more recent approach.