Last November, the antennas that followed the course of the Voyager 1 They began to receive indecipherable gibberish. It’s not that the connection was interrupted; It is simply that that string of ones and zeros, computer language, made no sense. The probe was more than 24 billion kilometers away from us. Under these conditions, it is fantastic that the last fault, the one that prevented us from understanding the information it was sending, could be resolved. Since the end of April, the Voyager 1 calls home again, and one of the main telephones here on Earth is on the outskirts of Madrid, in Robledo de Chavela, which for the first time oriented its six antennas towards the damaged probe.
After weeks of analysis, the problem could be traced back to a faulty memory chip. Like some large dinosaurs, probes Voyager They have three brains: one to decode the orders sent to it, another to manage the navigation and orientation of the antenna and the third to format and transmit the data to Earth. The latter was the one affected by the ruling. Of the three, it is the most complex, with a total of 69 kilobytes of memory. Not megabytes or gigabytes. Kilobytes. Less than a floppy disk of the time.
The solution consisted of repositioning some program routines, using electronic surgery, to avoid using the damaged chip. The patch did not fit in a single free memory space, so it had to be divided into smaller segments, distributed throughout all the memory banks, taking care, however, not to affect other functions. For now, the deep space network antennas—in California (USA), Canberra (Australia) and Robledo—maintain occasional contact with the two spacecraft, either to download information from their detectors or to carry out maintenance work. No one knows for sure how much longer they will be able to do this.
The rest of the instruments continue to function and send information, except for a plasma sensor, which failed some time ago. The ship and its twin, the Voyager 2, are embarked on an extended mission to study space, not only interplanetary but interstellar. It is increasingly difficult to communicate with them. Its very weak signal (which weakens with the square of the distance) is clouded by background noise coming from space. Since they were launched, the size of the antennas has been expanded and the sensitivity of the receivers has been pushed to the limit to be able to capture their very faint murmurs.
For this reason, for the first time in history, the six radio frequency antennas of NASA’s Madrid Deep Space complex carried out a test to simultaneously receive data from the spacecraft. Voyager 1 On April 20. Combining the receiving power of several antennas makes it possible to collect very weak signals from distant spacecraft: at the moment, five antennas are needed to transmit scientific data and, as the Voyager 1 moves away, all six antennas will be needed.
The signals arrive encrypted along with an error correction system, a series of additional bits that are interspersed with the data itself to guarantee its integrity. But that extra bit is only for detecting errors; More bits also allow them to be rectified automatically. At first, Voyagers used a repair system that required as many additional bits as the data itself. That was almost equivalent to transmitting them in duplicate. The new algorithms have reduced that burden to just 20%: one bit of verification for every five bits of information.
The problem is that at the distance they are in, the transmission speed is very slow. Information arrives at a rate of only a few hundred bits per second. Sending an order to these ships requires 22 and a half hours and the antennas have to radiate with many kilowatts of power to ensure that Voyager’s antenna (a dish barely three meters in diameter) will be able to hear anything.
Four decades of travel
It had been more than 40 years since it had visited its last target, Saturn’s moon Titan, and since then there was not much else to see in space. Only in the early 1990s had flight controllers activated their camera to record a family photo of all the planets in the solar system, seen from a distance. Afterwards, they disconnected the power outlet system to save energy.
The Voyagers are the only two ships that, until now, have left the Sun’s zone of influence to enter a never-explored environment. Two other probes, Pioneer 10 and eleven, they launched earlier, but on a slower trajectory that allowed Voyager to overtake them; the New Horizonswhich Pluto explored, is also on an escape route, but there is still a long way to go.
At the end of 2004, the Voyager 1 It passed through the shock wave that occurs when the solar wind (the jet of subatomic particles ejected by the Sun) collides with the interstellar wind. It is not a clear border, but it is a limit that the onboard instruments easily detected. The Voyager 2 He did it in 2007.
Eight years later, the magnetometer of the Voyager 1 detected that the galactic magnetic field was beginning to prevail over that of the Sun. It thus escaped from the “bubble” of our star to definitively enter interstellar space. It is still going very fast: about 550 million kilometers per year, or almost four times the distance from the Earth to the Sun.
At the time, Titan was a prime target, so the trajectory of the Voyager 1 It was adjusted so that it flew over it at a short distance. This made it impossible to direct it towards more distant planets and it was thrown in the same direction that the Sun moves with respect to nearby stars.
Instead, the Voyager 2 (at a range of https://twitter.com/NSFVoyager2/status/1785096806650581071 at the speed of light) was not subject to that commitment and after Jupiter and Saturn he was able to head towards Uranus and Neptune. All of the nearby photographs we have of those planets and their family of satellites were obtained by that single probe. Now, its trajectory is heading more or less in the opposite direction to its companion, sinking into the southern hemisphere, so that only antennas located in Australia can follow it. The Voyager 1 (to https://twitter.com/NSFVoyager2/status/1784915827659100495 at the speed of light) is visible from all stations.
The probe computers were designed at a time when microprocessors as we know them today did not exist. The documentation and programming instructions from then are not digitized. They are thick manuals or simple data sheets stored for forty-odd years in some archive at JPL. Time has made them yellow, but the worst thing is that those who wrote them—and understood them—have retired or disappeared. Very few technicians today are familiar with those programming techniques. Old machine code or, at best, assembly code. No high-level languages like Python or Java. Just ones and zeros.
At its peak, at the end of the 1980s, the team in charge of taking care of the ships reached 300 people; Today there are only a few dozen left, who have achieved the miracle of reestablishing contact with the venerable device. The same ones who also saved the Voyager 2 when it lost its link with Earth for weeks in 2023.
The nuclear reactors that power both probes have fuel for perhaps a couple more years. Its hydrazine reserves – which allow it to point its antenna towards the Earth – will probably last longer, since they are only consumed in short occasional bursts of a few milliseconds. And then? Only the immense emptiness of space opens before them. Within 40,000 years, the Voyager 1 It will pass less than 2 light years from the anonymous star AC+79 3888, in the constellation of Camelopardalis; In about 3,000 more centuries, the Voyager 2 It will do so near Sirius, the brightest in our sky.
Afterwards, its course is already unpredictable. It is possible that they will be trapped in an orbit within the Milky Way itself, carrying with them those two golden disks that Carl Sagan designed with photos and sounds of Earth and the remote hope that someone will be able to collect them and interpret them in the future, when our civilization no longer exists.
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