In its journey beyond the boundary of the solar wind’s bubble, the probe observed some notable differences from its twin, Voyager 1.
The Voyager 2 spacecraft burst out of the bubble of gases expanding from the sun and into the wild of the Milky Way a year ago. It was the second spacecraft to cross that boundary and directly observe the interstellar medium. Its faster-moving twin, Voyager 1, made the crossing six years earlier, in August 2012.
Launched 42 years ago, when Jimmy Carter was president, the twin spacecraft have persisted far longer than envisioned, as has their ability to send scientific findings home to Earth.
In a series of papers published on Monday in Nature Astronomy, scientists report what Voyager 2 observed at the boundary of the solar wind’s bubble and beyond.
“We’re certainly surprised,” Edward C. Stone, the mission’s project scientist, said of the probe’s longevity during a news conference on Thursday. “We’re also wonderfully excited by the fact that they do. When the two Voyagers were launched, the space age was only 20 years old. It was hard to know at that time that anything could last over 40 years.”
In many ways, the measurements echoed Voyager 1’s: a jump in the density of particles accompanied by a sharp decrease in their speed, a shift in the magnetic fields.
Voyager 2 also noted some differences, which could give clues about the complicated dynamics in that region of the solar system.
The sun spews in all directions a continuous stream of particles called the solar wind traveling at a speed of a million miles per hour. The particles are mostly hydrogen, but, heated to some 3 million degrees Fahrenheit, the atoms are ripped apart into protons and electrons.
At a distance of more than 11 billion miles from the sun, the solar wind, thinning out, is increasingly buffeted by the flow of particles in the interstellar wind and a galactic magnetic field generated by the long ago explosions of distant stars. The interstellar wind is much cooler — just tens of thousands of degrees — and denser.
Voyager 2 is heading in a different direction than Voyager 1, which could explain some of the differences. The sun was also more active in 2012, near the maximum phase of its cycle of activity. The sun is now near its lull, known as the solar minimum.
With Voyager 1, the outward velocity of the solar wind dropped to zero long before the boundary; it was pushed sideways. With Voyager 2, the outward velocities fluctuated, sometimes dipping to zero but then rising again.
Curiously, the distances from the sun for the two crossings out of the solar system were similar. Scientists had expected that the bubble would be pushed outward during the solar maximum and collapse inward during the solar minimum.
“A lot of the models leave a lot to be desired,” Stamatios Krimigis, a scientists at the Johns Hopkins Applied Physics Laboratory in Laurel, Md., and the principal investigator of one of the Voyager instruments, said in an interview.
Voyager 2 also measured what scientists describe as a magnetic barrier, “like the pile-up of slowly moving cars on a major highway, a few miles ahead of the scene of an accident,” Leonard F. Burlaga, a scientist working with the spacecraft’s magnetometer, wrote in an email.
When solar wind slows, the density of particles increases and the magnetic field strengthens.
“Again, it’s like the cars, which turn away from the lanes of the accident and move slowly along the available lanes,” Dr. Burlaga said. “The cars are more densely spaced, the drivers are heated, but they eventually move along.”
The missions were originally designed to last four years to fly by Jupiter and Saturn. Voyager 2 also visited Uranus and Neptune. Voyager 2 still has five functioning instruments for measuring the void; Voyager 1 has four.
Both Voyagers are expected to last another five years or so until their batteries die out. Both are powered by electricity generated by the heat of radioactive plutonium. As the plutonium diminishes, the spacecraft receive less and less energy.
Once the Voyagers shut down, there will be no more data from beyond our solar system for years. Only one other spacecraft, the New Horizons probe that flew by Pluto in 2015 and visited another object in the distant Kuiper belt in January this year, is headed that way. But it is moving more slowly and its plutonium power will run out before it reaches interstellar space.
“Right now, when the Voyagers go offline, that’s kind of it unless we do something else,” said Ralph McNutt, a physicist at the Johns Hopkins Applied Physics Laboratory.
Dr. McNutt is leading a study to look at what it might take to build an ultrafast spacecraft that could leave the solar system in a hurry. The development of NASA’s long-delayed giant rocket, the Space Launch System, makes that mission more plausible.
The mission could also perform what is known as an Oberth maneuver, named after Hermann Oberth, a German physicist who came up with the idea in 1927. A probe would first head to Jupiter, using the giant planet’s gravity to accelerate toward the sun. As it then swings around the sun, the spacecraft would fire a rocket engine, accelerating to a speed where it could cover close to a couple of billion miles a year. That would be more than five times the speed of the Voyagers.
That would be tricky to pull off, however. For such a powered flyby to be effective, the probe would have to travel within a million miles or so from the sun.
“Well, it’s not as easy as it sounds,” said Dr. Krimigis, who is taking part in the study which should be completed in a couple of years. “At that distance, every metal we know melts.”