It's primary design: a carrier aircraft for launching smaller spacecraft.

https://www.flightradar24.com/ROCC01/385602fd

How I bumped into this strange aircraft behavior: I've been seeing so many chemtrails lately, so I found the link for watching aircraft. The next time I see chemtrails in my area, and I can get online, I will look and see which aircraft is flying overhead.

I didn't expect to this aircraft that launches spacecraft. I assume this is unrelated to chemtrails.

This same aircraft recently flew loops over the Pacific ocean off the coast of California.



Edit: The license for this Stratolaunch aircraft, N351SL, belongs to Scaled Composites. They have their headquarters in Mojave, California, as does Virgin Galactic. These companies work together and with the US military.