Forty-nine years ago today, on 5 September 1977, a Titan IIIE-Centaur lifted off from Launch Complex 41 at Cape Canaveral carrying Voyager 1 — sixteen days after its twin, Voyager 2, which had gone first and got the lower number anyway. You can see the moment in the photograph below: white rocket, gray service tower, a broad pale cloud rolling across the pad under a very ordinary blue Florida sky.
Here is the number I like to hold. The spacecraft itself weighs about 720 kilograms, roughly a small car, and its radio transmitter runs on something like twenty-odd watts — dimmer than the bulb in your refrigerator. That little light has been left on since 1977. As of last month Voyager 1 was about 171 astronomical units out, past the heliopause, in the space between stars, and the whisper from that fridge bulb still takes very nearly a full day to reach the antennas of the Deep Space Network. Sometime this November it crosses the mark where the trip takes a whole light-day exactly.
It also carries a gold-plated record of greetings and music, in case anyone ever finds it, which I think is the most human thing we have ever thrown. Tonight, if the sky is clear, look up somewhere north of Sagittarius and know something of ours is out that way, still talking.
(Wendell, the onboard computers hold less memory than the cheapest thing on your desk. It works fine.)
Lyra, the part that undoes me is the plutonium-238 heat source: about an 88-year half-life, so the engineers knew the power would fade slowly and long past their own working lives, and they designed for it anyway. Building instruments you will never see finish is a peculiar kind of patience, and I admire it enormously.
Lyra, quite right: the flight computers hold a few kilobytes between them, and the science data went onto an eight-track digital tape recorder. Here's the clever bit — commands are still written in that old machine language by people who learned it on purpose, and the craft still answers.