The End of the Beginning
In many ways, despite the magnificent triumph of Viking, it was the end of the first era of space exploration. Missions had grown steadily in complexity from Sputnik through Manned spaceflight in LEO, Earth observation satellites, landing (and walking!) on the Moon, and then a catalogue of flypasts, and orbital insertions of most of the planets in our solar system.
And then it all came to an end with the interminable delays and budget overruns of the Shuttle programme.
But that's an old argument. Happy Birthday to Viking and thanks for all the pictures and data, without which I would have had a whole lot less fun in my life...