Interstellar 8-Track: The Not-So-Low-Tech Data Recorders of Voyager

On the outside chance that we ever encounter a space probe from an alien civilization, the degree to which the world will change cannot be overestimated. Not only will it prove that we’re not alone, or more likely weren’t, depending on how long said probe has been traveling through space, but we’ll have a bonanza of super-cool new technology to analyze. Just think of the fancy alloys, the advanced biomimetic thingamajigs, the poly-godknowswhat composites. We’ll take a huge leap forward by mimicking the alien technology; the mind boggles.

Sadly, we won’t be returning the favor. If aliens ever snag one of our interstellar envoys, like one of the Voyager spacecraft, they’ll see that we sent them some really old school stuff. While one team of alien researchers will be puzzling over why we’d encode images on a phonograph record, another team will be tearing apart – an 8-track tape recorder?

Old School is Best School

Of course, there are plenty of reasons to send what would be counted as obsolete systems on Earth into space. Mitigating risk is a big part of the job of space exploration, and with the price for failure so high, only proven technologies are generally sent upstairs. That has been true for pretty much the entire history of space exploration and doubly so for manned missions; the Apollo Guidance Computers of the late 1960s were built around resistor-transistor logic (RTL) chips for this reason, even after transistor-transistor (TTL) logic chips had been available for most of the decade.

By the time the Voyager mission’s Grand Tour of the Solar System was being planned in the 1970s, NASA had a fair bit of experience building space probes. The Pioneer missions were the basis for much of what NASA learned, culminating with Pioneer 10 and Pioneer 11 probes being sent to explore beyond the asteroid belt for the first time. Those probes were extremely stripped down, at least as interplanetary spacecraft go, with no onboard data storage to speak of. Commands from the ground and data back down had to be sent in real time.

Voyager 1 Data Tape Recorder. Source: Jet Propulsion Lab

That would never cut it for Voyager, a much more extensive mission with a huge suite of scientific payloads, each of which generated far more data than could be instantaneously streamed. Voyager needed to save data and send it slowly back to earth. A NASA backgrounder on the Voyager missions shows that a data storage was specified that could buffer about 536 Mb, or the equivalent of 100 full-resolution photographs from the spacecraft’s camera.

The data tape recorder (DTR) system was subcontracted to Lockheed and manufactured by Odetics Corp. The specs show that the machine was a belt driven recorder that used a 1,076′ (328 m) long reel of 1/2″ (12.5 mm) wide magnetic tape which recorded data on eight separate tracks. The DTR could record at two different speeds – 115.2 kbps and 7.2 kbps. Playback topped out at a much slower 57.6 kbps, with 33.6, 21.6, and 7.2 kbps being options as well.

It appears that none of the non-flown DTRs exist in any museum collections anymore, and all we have is one picture of the mechanism. It’s clearly much more sophisticated than the standard 8-track cartridge transport for consumer use at the time.

This isn’t your grandfather’s 8-track. First, the tape does not appear to be in a continuous loop as it would in an 8-track cartridge. It appears to come off the lower reel and loop around several idlers and pinch rollers before passing through the read-write heads in the center of the unit. It then routes around several more idlers, two of which appear to be mounted on a rotating platform, before heading to the upper reel.

Built to Last

Voyager 2 DTR bay assembly. Source: National Air and Space Museum

It’s hard to get an idea of the size of the DTR from this photograph, but another artifact exists that gives us some clues. The DTR was installed in a rack in the spacecraft that mounted inside one of the ten equipment bays located in a ring under the high-gain dish antenna. The Smithsonian has a DTR bay assembly in its collection, and lists the size as 24″ high x 19″ wide x 14″ deep (56 cm x 48 cm x 36 cm). Given the arrangement of the connectors on the backplane, the DTR is probably about 24″ tall and about 10″ wide.

Exactly what the composition of the magnetic tape was, and what secrets were used to prevent it from degrading in the harsh environment of space, are unclear. Odetics, the manufacturer, claimed that the tape would travel through the mechanism a distance of 2,700 mi (4,400 km) before discernible wear.

It seems to have worked. The DTRs in both spacecraft performed flawlessly from their launch in 1977 and through the entire Grand Tour mission, as well as the extended mission that set both vehicles on a course out of the solar system. In 2007, the DTR in Voyager 1 was shut down for good, not due to any issues with the unit, but because of the dwindling supply of power coming from the craft’s radioisotope thermal generators. As of this writing, the DTR in Voyager 2 is still working, but is likely to be shut down as the power wanes in that vehicle.

Soon we’ll lose contact with both of these platforms for good, and they’ll pass silently into the interstellar void. And if anyone does find them someday and learn how to power them back up, maybe those primitive tape recorders will spin back up one last time.

Maybe we did send our best stuff out there after all.

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