Spacecraft with computers with 69 kilobytes of memory and designed for a 4-year mission have done the impossible. For almost 45 years Voyagers have been roaming space. But it looks like their exciting journey is coming to an end.
Two identical spacecraft, Voyager 1 and Voyager 2, were launched into space in the summer of 1977, only 15 days apart. After nearly 45 years of space exploration, both vehicles are still operational, sending data back to Earth from outside the solar system every day.
Both Voyagers have gone further and lasted longer than any other spacecraft in the history of mankind. They were the first man-made objects to cross the border between the Sun's zone of influence and the rest of the galaxy.
This record, it seems, will not be broken for at least another few decades, even despite the achievements of modern science. The result can be called fantastic – considering that the Voyager mission was supposed to last only 4 years.
Even at the beginning of their journey, the Voyagers took the first pictures of the moons of Jupiter and Saturn, which turned out to be covered with ice and active volcanoes. Prior to these images, scientists believed that distant satellites are in many ways similar to our moon. In 1986, Voyager 2 was the first in history to fly past Uranus, and 3 years later passed Neptune. At the moment, this is the only device that has made such flights. Now the Voyager spacecraft are almost 20 billion km from Earth.
But apparently, the Voyager space odyssey is gradually coming to an end. In 2022, NASA plans to turn off some systems of the vehicles so that the energy reserves will last at least until 2030.
“The duration of the main mission was four years. But if the engineers were able to supply a more expensive part, but not too necessary for a four-year mission, they still did it. Even more remarkable is the fact that two ships were built for the mission at once, and both of them are still working,” says Suzanne Dodd, who has led the project since 2010.
Voyagers in space: the pinnacle of technology
During the construction of the Voyagers, only one spacecraft used the gravity of one planet to fly to another. The Mariner 10 probe performed a gravity assist near Venus on its way to Mercury. The Voyagers had to make several such flybys with an error of tens of minutes.
The first stop of the Voyagers was near Jupiter, which is 10 times farther from our planet than Mercury. Also, the vehicles had to pass through the asteroid belt.
“Prior to the launch of Voyagers, there was a fierce debate about whether it would even be possible to pass through the asteroid belt without being torn apart,” says Ralph McNutt, a physicist at the Johns Hopkins University Applied Physics Laboratory (APL), who devoted most of his life to the project Voyager.
In the early 1970s, two spacecraft, Pioneer 10 and Pioneer 11, flew safely through the asteroid belt, which turned out to be rather “empty” space. Thus, these two vehicles paved the way for the Voyagers.
The Voyagers needed some kind of built-in intelligence to deal with all the problems along the way. Devices the size of a Volkswagen Beetle were equipped with computers with 69 kilobytes of memory, which is less than one hundred thousandth of the capacity of a modern smartphone. But the comparison with smartphones does not look very good.
“Voyager computers have less memory than a car key fob,” says JPL planetary scientist Linda Spilker, who began working on the Voyager missions in 1977.
Data from spacecraft is recorded on eight-track cartridges and then sent to Earth using a 23-watt transmitter. Its power is approximately equal to the power of a light bulb in a refrigerator. In order to amplify the signal transmitted to Earth, both Voyagers carry parabolic antennas 3.5 meters wide to send and receive signals.
“During the creation of the Voyagers, we thought that we were at the pinnacle of technology. We were able to complete the ships surprisingly quickly,” says Caltech physicist and Voyager crew member Alan Cummings.
In just four years, the research team completed three spacecraft, of which one was a working test vehicle. A few months before the launch of the mission, the two vehicles were renamed Voyager 1 and Voyager 2.
Already in March 1979, Voyager 1 reached Jupiter, passing 546 days since launch. Voyager 2 arrived at Jupiter in July of that year, but used a different trajectory than its twin brother.
Both Voyagers were built to remain stable at all times for the best camera imagery. The cameras used red, green and blue filters so that color images could be captured on Earth. The devices hardly rotate during the flight, their rotation is almost 15 times slower than the movement of the hand counting the hours.
Voyager discoveries and one pale blue dot
First proof of this that not all satellites in the solar system are like the moon appeared when the Voyagers were about 1.6 million km from Jupiter. The Low Energy Charged Particle Detection System (LECP) has picked up unusual signals.
“We saw a large number of oxygen and sulfur ions enter the detector. The density of these ions jumped three times compared to the levels that the detector recorded before,” says Stamatios Krimigis, who developed the LECP instrument.
During the entire period of their scientific activity, the Voyagers have taken more than 33 thousand photographs of Jupiter and its satellites. Each of these images was a major scientific discovery.
It was thanks to the Voyagers that scientists learned that Jupiter has rings, and Europa is covered with an incredible crust of ice.
When the device left Jupiter, it received a gravitational push from it at a speed of 57,400 km/h. Without this push, the spacecraft would not have been able to overcome the gravity of the Sun and enter interstellar space.
Voyager 1 and Voyager 2 parted ways near Saturn. The first one flew through the rings of Saturn, took the impact of thousands of dust particles, passed Titan and headed out of the solar system.
At the same time, Voyager 2 went to visit Uranus and Neptune. In 1986, Voyager 2 discovered 10 new satellites of Uranus and added it to the list of planets with rings. Three years after that, Voyager 2 passed over the atmosphere of Neptune, where it recorded the highest wind speed among the planets of the solar system, which reached 600 m/s. The device also found that the largest satellite of Neptune, Triton, is one of the coldest places in the solar system. The temperature on the surface of the satellite reaches -235 degrees Celsius.
Photos of Neptune and its moons could be the last pictures taken by Voyager 2. But the famous astronomer Carl Sagan, who was also a member of the imaging team, offered to take one last picture.
The fact is that after the completion of the main mission, NASA planned to turn off the cameras of both devices. In interstellar space, where the Voyagers went, the cameras of the vehicles would not be able to take pictures due to lack of light.
But Sagan suggested to NASA that Voyager 1 take a final series of images. On Valentine's Day in 1990, the probe turned towards the inner solar system and took the last salvo of 60 photographs. The most famous image was the image of the Earth from a distance of 5.9 billion km. Sagan described the Earth in the photo as a “pale blue dot”. To this day, this photo remains the most distant image of the Earth. On it, our planet really looks like a barely visible blue dot that does not even take up a full pixel.
Voyager probes and access to interstellar space
Now both probes are so far from Earth that a radio signal that travels at the speed of light takes almost 22 hours to reach Voyager 1 and more than 18 hours to Voyager 2. Every day, devices move away from the Earth for a few more light seconds.
Voyagers are still in contact with Earth thanks to NASA's system, which consists of three tracking complexes scattered around the planet. The system provides uninterrupted communication with vehicles, despite the rotation of the Earth. But as the probes move away, their signals become weaker and weaker.
“Earth is a very noisy place. Radios, televisions, mobile phones – everything makes noise. Therefore, it becomes more and more difficult for us to hear the quiet whisper of two incredibly distant spacecraft,” says Glen Nagle, a member of the NASA Deep Space Communications Network.
Despite all the problems, the Voyagers were still able to enter interstellar space. In other words, the probes have overcome the boundary that separates the interstellar medium from the region where the solar wind reaches. Scientists call this boundary the heliopause, the place where the final deceleration of the solar wind occurs. Outside the heliopase, interstellar plasma already “rules the ball”.
Scientists ask not to confuse the heliosphere with the edge of the solar system, which includes the distant Oort cloud. The Voyagers will have to fly another 300 years to this point.
The vehicles spent almost 20 years on their way to the interstellar medium. Voyager 1 crossed the heliopause on August 25, 2012. But the probe's data were so inconsistent that the announcement of the discovery had to be delayed.
Voyager 1 did indeed detect the expected jump in plasma density, its detector showed an 80-fold increase. But there was no indication of changes in the surrounding magnetic field in the data. Note that the heliopause is also considered the boundary of the Sun's magnetic field.
In November 2018, Voyager 2 also saw no difference in the magnetic field. Moreover, the device threw scientists another riddle. Voyager 2 crossed the heliopause at a distance of 120 astronomical units from Earth, in essentially the same place as Voyager 1. Such data is inconsistent with theoretical models, which say that the Sun's sphere of influence should expand and contract in accordance with the 11-year solar cycle.
Voyager 2 arrived at the site when the solar wind was at its peak, and according to the models, the heliopause boundary should have moved further than 120 AU. p>Voyager 2 currently has five instruments operational, while Voyager 1 has four. As you know, all of these instruments are powered by a device aboard the probes that converts heat from the decay of plutonium into electricity. But power is declining by about 4 watts a year, and NASA had to choose.
About two years ago, the space agency turned off the heating of the cosmic ray detector, which played a decisive role in the study of the heliopause. NASA believed that after turning off the heating, the instrument would fail.
“The temperature dropped by about 60-70 degrees Celsius, which significantly exceeded all possible limits, but the device continued to work,” emphasizes Linda Spilker .
Finally, the last two Voyager instruments to be turned off are most likely the magnetometer and the plasma science instrument. The fact is that both instruments are located inside the probes, where they are heated by the heat of the computer itself. The rest of the spacecraft's instruments are suspended from a 12-meter boom.
“When we turn off the heaters, these external instruments get very cold. If everything goes according to plan, we may be able to extend the mission until the 2030s in by shutting down scientific instruments,” adds Spilker.
But even after the Voyagers have to be turned off completely, their incredible journey will continue. In about 16,700 years, Voyager 1 will visit our neighboring star Proxima Centauri. In another 3,600 years, Voyager 2 will repeat this path. Afterwards, the probes will continue to roam the galaxy for millions of years.
Most likely, the probes will outlive the Sun, which will explode, and will find a time when there will no longer be the heliosphere and that same “pale blue dot” . At the end of their journey, the Voyagers will transmit the last message, but its final destination will no longer be people.
This message was written on copper plates, covered with gold and sealed in an aluminum shell. The well-known “gold plates” encoded images, sounds and other data that can give an idea of the place where the Voyagers came from – our planet.
Based on Scientific American.