In July 2017 What You Haven't Seen launched by publishing the complete surveillance camera footage from the crash of Asiana 214 at San Francisco International Airport. In the time since, I am proud to have published hundreds of other significant exclusives, but despite all that, I never thought that I would find this.
CNN has a copy of their Challenger launch broadcast footage. It's available on CNN's own YouTube channel, and it understandably has millions of views. CNN's upload is a 360p video whose content is similar to what you see here — with three major differences: (1) this version is of substantially higher quality, (2) this version doesn't have any CNN branding, and (3) this version continues NASA's feed longer than CNN showed on air.
This is a direct copy of NASA's broadcast feed. It was saved by NASA, and at some point a copy was transferred to the United States Geological Survey on a VHS tape. This video is a digitization of that tape; as far as I know, this is the first time it is being shown publicly.