Satellite observations from open ground stations.
SatNOGS API is a free Science & Math API that requires no API key, is served over HTTPS and has unconfirmed CORS support.

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3D structures of proteins and nucleic acids
Yes. The SatNOGS API is listed in the Science & Math category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The SatNOGS API requires no authentication, so you can start making requests without signing up.
CORS support for the SatNOGS API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The SatNOGS API is served over HTTPS, so it's safe to use in production.