Let's begin at the beginning. This is a site dedicated to highlighting and discussing space opera in animation. Why do this, and what is it? Well, as to why, there is a base and simple reason: I think outer space is cool. I can't remember a time I wasn't fascinated by it. The cosmos are a limitless expanse and a wonderous work of creation that impose all manner of questions about our place in existence, how it all came to be, and how much is left to discover. In a world where we humans so often try to give themselves a sense that we are the masters of our environment and in control, outer space is at once both a thrilling and intimidating existence. To seriously consider the universe is to allow profound wonder and sobering, humanizing humility into our hearts.
For the purposes of fiction, then, Star Trek had the beginning of an accurate conception of outer space. It is the final frontier, the greatest frontier, the most mysterious frontier. The sheer scale of it offers endless opportunity for tales of discovery and of adventure, for countless ideas about other forms of life. Different worlds can offer a multitude of diverse settings.
Space Opera is a genuine subgenre of science fiction, although for much of the 20th century it existed as a pejorative meant to denigrate supposedly second rate pulp fiction. But as a practical matter, space opera as we know it in the modern era was redefined, revitalized and expanded upon in the 1970s when a certain little movie called Star Wars Episode IV: A New Hope struck popular culture like lightning in 1977.
Its impact and influence led to a conception of adventure set in space as legitimate space opera, and it's further arguable that Star Warsran the gamut of what space opera had been and would later be. Exotic settings on alien worlds that some would associate with a distinct "planetary romance" subgenre were a part of Star Wars. Space travel? Star Wars didn't fixate on this as Star Trek did, and yet much of the original trilogy's plot centered around the voyages of the Millennium Falcon. It took place in the context of an Interstellar War, and yet its key story was fundamentally the personal one for Luke Skywalker. Said war involved no shortage of spaceships and other military hardware, yet the franchise has always also had one foot in the realm of mysticisim by virtue of the Jedi order and the nature of The Force. Few to none would deny that Star Wars is essentially synonymous with space opera in our collective consciousness, and so the fact that it is an amalgamation of many distinct and even dissonant qualities is a testament to the subgenre's flexibility.
All this isn't to say space opera in animation didn't exist until after 1977, though. Star Wars was simply what sparked a reevaluation of the subgenre, it was not its origin. The true beginning of it for cartoons can be traced to the period of 1973 - 1974. In America 1973 meant the advent of Star Trek Animated from Filmation, while 1974 brought about a true landmark achievement in Japan when Space Battleship Yamato hit the airwaves. Star Trek Animated essentially proceeded as if it were a new season of the live action show from the 60s, while Yamato represented an original piece of science fiction that featured a serialized, plot-driven story and a serious voyage very different from the Enterprise's mission of exploring for its own sake.
Yamato is an extremely important work for space opera in animation both for Japan and in general. Star Trek Animated was a worthy spinoff, but Yamato was a substantial hit on television and forged a market for this sort of fare in Japan that begat other memorable works within the subgenre. With Mobile Suit Gundam the space odyssey of Yamato and was minged with military hardware and giant robots, resulting in a narrative that amounted to a war story for kids rather than a bold voyage to a new frontier. In contrast the Galaxy Express 999 television series ran from 1978 to 1981 parallel to Gundam and the derivative works it left in its wake, where the journey through space and discovery was what drove the program. Pulp adventure was also not forgotten during this time, thanks to the arrival of Space Pirate Captain Harlock the same year as Galaxy Express.
So in brief, I do consider the period of 1973 to 1981 to be the first period of space opera in animation simply because the representative works created during this time show the breadth of what the subgenre can offer. The limit of what it space opera can include is really limited only by the imagination of creators.