I was thinking that humans couldn't have been out there exploring deep space and meeting alien races in 1999, but in the context of TOS history and rightfully discounting the Trek spinoffs, I guess there's nothing to preclude that.
But then the voyage of the Discovery One in the year 2001 shouldn't have been anything special.
Kor
The moon in Space: 1999 was hurtling uncontrollably into deep space. It passed through a black hole and later through a couple of "space warps" which pushed it even further out into the universe. The inhabitants of Moonbase Alpha had no contact with Earth whatsoever. So whatever explorations and first contacts with aliens they had remained unknown to Earth.
As for Discovery One I like the little design connection it has with the spherical primary hull of early warp ships like the Daedalus.