Voyager probes beam back universe’s puzzle pieces, NASA scientist says

The twin Voyager spacecraft, mankind’s only working interstellar probes, help scientists unravel the Solar System’s birth and pave the way for future deep space missions. All of that using late ‘70s tech.

Voyager-1, launched in 1977, is the farthest human-made object away from the Earth. Just saying that it’s 15.1 billion miles away from our planet hardly encapsulates the unimaginably vast distance. For example, if flying at cruising speed, it would take a Boeing 737 around three thousand years to reach the point where Voyager 1 is right now.

While it only takes eight minutes for the Sun’s beams to reach our planet, it takes 22 hours for light, or radio signals, to travel from Earth to Voyager 1. Scientists at NASA’s Jet Propulsion Laboratory (JPL) have to take this into account, as communicating with objects so far away presents its own challenges, Linda Spilker, Senior Research Scientist serving as the Voyager Project Scientist, says.

“We send commands up at the speed of light. It takes a little over 22 hours to reach Voyager. And then to respond back, another 22 hours. If you think about it that way, almost a light day in each direction,” Spilker told Cybernews.

• The Voyager program was launched during a once-in-175-year alignment of Jupiter, Saturn, Uranus, and Neptune.
• Voyager probes weigh 1,592 pounds (721.9 kg) each.
• The Golden Record was prepared by a team headed by Carl Sagan.

What’s the Voyager’s data speed?

Researching interstellar space was not the Voyager program‘s primary goal. Initially, Voyager 1 was supposed to visit Jupiter and Saturn, while Voyager 2 was destined to additionally explore Uranus and Neptune. Since the two spacecraft successfully accomplished their goals, the mission was extended for the probes to leave our stars’ grip.

“By the time we flew by Neptune (1989), Voyager was already thought to be an old mission. And here we are, having flown through the heliosphere and crossed the heliopause, the boundary where the Sun's influence diminishes and interstellar space occurs,” Spilker explained.

The invaluable data that the spacecraft collect is transmitted back to NASA via radio telecommunications. Scientists beam out commands using S-band radio frequency while the two Voyagers respond via X-band radio. While the S-band provides a reliable way to send commands through Earth’s atmosphere, the X-band frequency is better suited to transfer scientific data from the craft moving at over 38,000 miles per hour.

Predictably, data transfer rates from the edge of our Solar System are very slow. Typical downlink speed hovers around 160 bits per second, which includes science and engineering data. If scientists want to send only engineering data, the transfer rate can drop as low as 40 bps.

For comparison, an average photo snapped with a later-generation iPhone roughly takes up 25 megabytes. It would take anywhere between 43 and 173 hours to download the photo using data transfer speeds NASA employs to communicate with humanity’s most distant creation.

The spacecraft‘s uplink and downlink speeds were not always so low. When Voyager 1 flew by Jupiter, its data transfer was up to 115,200 bps, and 44,800 bps at Saturn. However, by now, Voyager is approximately 14 billion miles away from Saturn. Meanwhile, Earth is only 0.8 billion miles from the ringed crown jewel of the Solar system.

• Voyager 1 has four active scientific instruments, while Voyager 2 has five.
• Voyager 2 was launched two weeks earlier than Voyager 1.

Interstellar 8-track tape

When they were launched, Voyager probes were state-of-the-art machines. So much so, NASA separated the twin spacecraft from the Mariner program which they initially were a part of. The spacecraft were just too advanced and different from those of the Mariner.

For example, both probes were the first extensively protected against radiation, setting the standard for radiation design still in use for space missions today. Another innovation was the craft’s eight-track digital tape recorder, tech contemporary twenty-something-olds would hardly recognize.

“Voyager 1 has a digital tape recorder onboard. Tape is literally running across the heads of the tape recorder, and then, just like your digital eight-track tape recorder, we play back the tracks of the tape recorder with the data on it,” Spilker explained.

What that means is that 22 hours after someone at NASA presses a (likely virtual) button, a 328-meter-long (1,075 ft.) tape starts running inside a bus-sized interstellar spacecraft on the edge of the Solar System.

While eight-track tech may seem redundant now, it enabled spacecraft to store around 100 full-resolution photos according to late ‘70s standards, or 67 megabytes of data. Most of Voyager's predecessors could store several hundred words at best, transferring data in real-time, which was not an option for craft bound to visit the gas giants.

• Voyager 1 is the first human-made object to venture into interstellar space.
• Voyager 1 snapped the famous “Pale blue dot” photo.
• Voyager 1 was the first to measure density of interstellar medium – material ejected by ancient supernovae.

Ultra long-distance repair

In mid-November 2023, the mission team noticed that Voyager 1 started sending a repeating pattern of ones and zeroes as if the craft was trapped in a loop. While scientists could still command the probe, it could not transfer back any meaningful data.

“Imagine being on a phone call where you're talking with someone, and all of a sudden, all you get is a dial tone. Now you have to figure out what happened to that person, what's going on, but all you have is a dial tone,” Spilker explained.

After months of investigating, NASA thinks there might be a problem with the crafts’ flight data system computer, which takes the science and engineering data, encodes, processes and packages it to send back to ground control.

“Something might be not right in that chain, within the flight data system. It could be just as simple as a bit flip. That has happened a lot in the past, but now we can't get back what we call telemetry, which is the information that usually comes along with that carrier tone,” Spilker said.

A bit flip error randomly assigns binary values from 1 to 0 and vice versa, potentially corrupting the data. In the past, the NASA team would figure out a way to read Voyager’s memory, compare it to what the scientists think it should look like, and fix the error. However, so far, Voyager’s team has been unable to patch the issue.

As if the distance to the probe was not challenging enough for repairs, the team also has to work blind. Spilker compares the efforts to fixing a computer that has its screen off: while scientists can still type in commands and know the device is receiving them, there‘s no way of knowing what the commands do to the computer.

“I'm cautiously optimistic that we'll figure out how to solve this issue as well. This one is just trickier because we don't have visibility into what exactly is going on within that computer that we think the problem might reside, the flight data system,” Spilker explained.

• Voyager 2 is the only spacecraft to visit Uranus and Neptune.
• Voyager 2 is the only spacecraft to study all four of the Solar System's giant planets at close range.
• Voyager 2 discovered five moons, four rings, and a "Great Dark Spot" on Neptune.

Interstellar lessons

Bar all of the data that the Voyager program delivered via gas giant fly-bys, both spacecraft are providing unprecedented insights into interstellar space. For one, NASA learned that its Deep Space Network (DSN), a global network of spacecraft comms facilities with up to 70 (230 ft.) meter-wide antennas, can only get the agency so far.

Voyager signals are taken up by the DSN, but the farther the probe goes, the bigger the receiver needs to get. With plans to send interstellar probes several hundred times further than Voyager, the DSN will need a serious upgrade.

Thanks to Voyager, engineers working on future missions know that once a craft crosses the heliopause, a boundary where Solar winds are stopped by the interstellar medium, it has to deal with an abundance of very energetic cosmic rays.

“A lot of them are helium, hydrogen, and helium nuclei, and they carry a tremendous amount of energy when they hit a computer component. That's what can cause these bit flips or these errors in the computer,” said Spilker.

Scientists now believe that much like Earth’s magnetic field shields us from Solar radiation, the Sun’s rays protect its entire planet system from highly energetic interstellar radiation.

This means that for us to exist, a multitude of protective layers, from Earth’s atmosphere to the Sun’s rays, need to be present, and life as we know it might only be possible around stars with similar features.

According to Spilker, who has also led a team of 300 scientists during the Saturn-oriented Cassini mission, Voyager probes are directly sampling the interstellar medium in a way that’s possible only by having science instruments on the spot.

“They're kind of like detectives helping us solve the puzzle that might address how the Solar System formed, how the Sun formed from a cloud of gas that collapsed and then that excess gas turned into planets,” Spilker said.

The data Voyager probes send back to Earth grant scientists the opportunity to test new theories and ideas, Spilker explained, about the formation of our resident star system and how its boundaries shaped the evolution of life on our planet.

What’s next?

With the Voyager program extended until the spacecraft’s radioisotope thermoelectric generators (RTGs) stop, ending any data transfer, scientists at JPL hope to know as much as possible about the interstellar medium.

For example, why was a sharp increase in magnetic field and plasma density observed some time after crossing the heliopause? Initial theories suggested that may be caused by shockwaves sent by the Sun, which last a couple of months. However, the increases that Voyager 1 registered outlasted what the initial theories suggested.

“We wonder did we cross some kind of a boundary? Is there some kind of an effect that we don't understand from the Sun with the interstellar medium? And we'd like to get Voyager 1 back and see if it's still there and see whether it does jump up again,” Spilker said.

Even if scientists don’t fix the Voyager 1 probe, its twin is still up and running. At a distance of 12.6 billion miles from Earth, Voyager 2 will continue to inform NASA about what it's like to be at the edge of the Sun’s influence.

Once both probes run out of juice, which is estimated to happen in the late 2020s, they will forever drift further and further from the star system it originated from. Undisturbed, both can continue trekking the universe for hundreds of millions of years, likely outlasting the species that launched them.

Both probes carry so-called “Golden Records.” The phonographic records contain sounds and images of Earth. Even if humanity became extinct, both records will drift in the vastness of space, carrying a message from a distant, and hopefully not extinct, world.

“They will just keep going until perhaps someday some other race might find them and find out information about us. On the cover of the records there's a diagram of how to find our Solar System. So, who knows?” Spilker said.