NASA's Voyager 1 spacecraft has gone interstellar.

Launched in 1977, the probe is vintage space hardware. At more than 36 years old, or roughly a millennium in computer years, it's an impressive feat to have the spacecraft drifting through the material between stars. Looking at the machine’s specs is a reminder of how far electronics have come in the intervening years.

The computer room during Voyager's testing days. NASA

“Here on the ground, we keep up with the latest technology,” said engineer Suzanne Dodd, the Voyager project manager at JPL, with most of the science team using Mac power books these days. She recalled when she started working with the mission in 1984, they were using then-state-of-the-art desktop computers with 8-inch floppy drives.

But Voyager 1 and 2 are from an even earlier era and, being 19 billion kilometers away, “you can’t take them into the shop and upgrade them,” said Dodd.

The computers aboard the Voyager probes each have 69.63 kilobytes of memory, total. That’s about enough to store one average internet jpeg file. The probes’ scientific data is encoded on old-fashioned digital 8-track tape machines rather than whatever solid state drive your high-end laptop is currently using. Once it's been transmitted to Earth, the spacecraft have to write over old data in order to have enough room for new observations.

The Voyager machines are capable of executing about 81,000 instructions per second. The smart phone that is likely sitting in your pocket is probably about 7,500 times faster than that. They transmit their data back to Earth at 160 bits per second. A slow dial-up connection can deliver at least 20,000 bits per second.

The Voyager probes are always sending out a signal. Voyager 1 has a 22.4-Watt transmitter – something equivalent to a refrigerator light bulb – but by the time its beacon reaches us, the power has been reduced to roughly 0.1 billion-billionth of a Watt. NASA has to use its largest antenna, a 70-meter dish, or combine two 34-meter antennas, just to hear Voyager.

The science team communicates with Voyager 1 and 2 every day and typically needs at least four hours just to run through all their health and safety checks. On a good day, researchers might spend double that time sending commands and receiving data. The spacecrafts’ original control and analysis software was written in Fortran 5 (later ported to Fortran 77). Some of the software is still in Fortran, though other pieces have now been ported to the somewhat more modern C.

Voyager's internal components during assembly. NASA

Part of the Voyager probes’ longevity comes from the fact that they were robustly built and included plenty of redundant components. Even having two machines to accomplish the same task is something that NASA doesn’t do much of these days (Imagine if we had two Curiosity rovers on Mars).

Practically all of Voyager's redundancy is gone now, either because something broke along the way or it was turned off to conserve power. Of the 11 original instruments on Voyager 1, only five remain operational: the UV spectrometer, the magnetometer, a charged particle detector, a cosmic ray detector, and the plasma wave system. But as the hardware ages, more and more things can break.

“We’re always one failure away from losing the mission,” said Dodd.

Still, the Voyager probes have far exceeded expectations. Looking at engineers' testimonials from when it was built, Dodd said the original designers were told not to worry about reaching interstellar space and focus on making sure the Voyagers could observe Jupiter and Saturn.

“Basically they kind of ignored those directions, nodded their heads and did what they wanted to make it capable of getting to interstellar space,” she said.

As long as they are still functioning, both Voyager spacecraft will continue to collect data and send it back to us through at least 2020, and possibly until 2025. Even if something breaks and they are suddenly unable to hear anything from Earth, they will keep sending data back, repeating the same sets of observations until they run out of power.

But the radioisotope batteries on Voyager 1 and 2 are diminishing, putting out four Watts less per year. Mission managers will one day have to prioritize which instruments are most important during the voyage through interstellar space, shutting them off one by one as the years go on. After that, both probes will remain within range of our antennas until perhaps 2036. Though scientific data won’t be returned, engineering data could still come back if there is enough power to send a signal. All this means that the mission isn’t yet done.

“We’ve just stepped into interstellar space,” said Dodd. “We’re just going across the horizon line and there are many more discoveries to come.”