During the millennia of humanity staring at the stars and the decades of launching probes to explore our universe, only two spacecraft carrying working tools have ever managed to escape. the bubble of space ruled by our sun.
The Voyager twin spacecraft Launched in 1977 on an epic tour of the outer planets; both flew past Jupiter and Saturn Voyager 2 added Uranus and Neptune to its orbit. Both spacecraft have since departed, and several of their instruments have continued to make observations, despite the challenges. aging technology and dwindling power supplies. And on December 16, 2004, Voyager 1 reached its termination shock, the beginning of its year-long transition. to interstellar space. Voyager 2 passed the same threshold in 2007. Since then, the spacecraft has been giving humans their only direct taste of what lies outside and beyond the bubble of the sun’s influence on space, the area scientists call the heliosphere.
“Now we know how little we know about the heliosphere,” says Boston University space physicist Merav Opher. “It’s much more complex, more dynamic than we thought.”
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Here’s what scientists do know: We everyday Earthlings might think of the sun as a dense ball of light in the distance, in part because our atmosphere protects us from the worst dangers of our star. But in reality, the sun is an enormous mass of plasma and magnetism, radiating billions of kilometers in the form of the solar wind, a constant stream of charged plasma pouring out of our star. The Sun’s magnetic field travels with the solar wind and also affects interplanetary space. The heliosphere expands and contracts in response to changes in the sun’s activity levels. 11 year cycle.
“Throughout the heliosphere you see these 11-year highs and lows and dramatic peaks and troughs,” says Jamie Rankin, a Princeton University space physicist and deputy project scientist for the Voyager mission. And, he notes, astronomers of all stripes are caught up in this chaotic background in ways that may or may not affect their data and interpretations. “All the measurements so far, until the Voyagers cross the heliopause, have been filtered through different layers of the sun,” says Rankin.
Voyager 1 crossed the heliopause, or edge of the heliosphere, in August 2012. Heading in a different direction, Voyager 2 crossed another part of the heliopause in November 2018.
On their journey into interstellar space, the Voyagers had to cross a series of boundaries: first, an termination shock seven to eight billion kilometers from the sun, where the solar wind suddenly begins to slow down, then the heliopause, where the outer pressure. equals the pressure within the interstellar medium from the solar wind. Between these two hard boundaries is the heliosheath, the region where solar material continues to slow down and even reverse direction. Voyager 1, the fastest of the twin probes, took almost eight years to travel across these boundaries; such is the vastness of the scale at stake.
Beyond the heliopause is interstellar space, which Voyager 1 entered in 2012 and Voyager 2 reached in 2018. The environment inside our heliosphere is very different, quieter but almost calmer. “The solar system is a relic of the environment from which it was born,” Rankin says of the interstellar medium. It contains energetic atomic fragments called galactic cosmic rays, as well as dust ejected by dying stars over the eons of the universe, among other components.
The interstellar medium varies throughout the galaxy, alternating denser and fainter regions the milky way spiral arms Our sun and the bubble it creates plows through this interstellar medium, and the interaction between the dynamics of the sun and the interstellar medium influences the shape of the heliosphere.
What this form actually is, however, scientists do not yet know. The shape of the heliosphere can resemble that of a comet, with a long tail of a dense nose pushed by the sun into interstellar space. Or perhaps the interaction between the sun’s magnetic field and the interstellar medium shapes the bubble into a croissant-like shape, two lobes behind our star. The shape of the heliosphere could take another shape that scientists have not yet considered; certainty is difficult from our limited perspective on Earth. “Goldfish look like we’re trying to measure our fishbowl from the inside, and we can’t even reach the edges,” says Sarah Spitzer, a space physicist at the Weizmann Institute of Science in Rehovot, Israel.
The Voyager probes are unexpected exceptions to this challenge. The twin spacecraft were designed as scouts for the outer planets, and the program provided humanity’s first—and so far only—up-close view. Uranus and Neptune By 1989, these observations were complete, but the probes were still in good health. So NASA continued, even as it turned off instruments that would not produce interesting data without planets to observe the planets. Years passed and the Voyagers drifted outward, swimming toward the walls of our golden cosmic vessel.
“Voyagers are like biopsies of the heliosphere. … We know nothing about the global three-dimensional structure of the outer heliosphere, just from these two sets of points.”
But the goldfish weren’t sitting still. In 2008 NASA launched the Interstellar Boundary Explorer (IBEX), which orbits the Earth and samples particles called energetic neutral atoms that enter the edge of the heliosphere. Scientists can use IBEX measurements of the properties of these particles to reconstruct what is happening far away, billions of kilometers away.
Among the key contributions of IBEX has been the discovery of a ribbon of energetic neutral atoms covering the heliosque. Scientists believe that the ribbon may be caused by particles entering and exiting the heliosphere. But in an example of cosmic bad luck, the Voyager spacecraft were unable to directly analyze the IBEX ribbon: they passed on both sides of it. “Among them is the largest and most striking thing in the outer heliosphere,” says David McComas, a Princeton University space physicist and IBEX principal investigator.
It’s a situation that shows the limits of relying on local observations of something as vast as our star’s influence bubble. “Voyagers are like biopsies of the heliosphere,” says McComas. “We know nothing about the global three-dimensional structure of the outer heliosphere from just these two sets of points.”
IBEX is still observing, having lasted much longer than originally planned, and the spacecraft has managed to collect data over a full 11-year solar cycle to see how the heliosphere responds to solar activity. But McComas is also working on another mission he leads, ready to launch next year. He describes the Interstellar Mapping and Acceleration Probe (IMAP) mission as “IBEX on steroids,” with the same basic capabilities but with sharper resolutions and the addition of additional measurements, such as analysis of interstellar dust grains—the debris of dead stars. which enter the solar system.
Meanwhile, other scientists plan to collect more direct observations of the region. One more spacecraft is already on its way to leave the Voyagers heliosphere: NASA’s New Horizons missionWhich passed by Pluto in 2015. After studying the dwarf planet (and, in 2019, a even further rocky objects Named Arrokoth), the spacecraft is on its way to crossing the heliopause in perhaps another decade or so. And scientists hope his tools will continue to workready for humanity’s third expedition beyond the influence of the sun.
Scientists have also designed a mission, duplicated Interstellar probeUnlike Voyagers and New Horizons, it is designed to illuminate the outer heliosphere and beyond. It would use a massive rocket to take a fast path through the solar system, carrying instruments designed to study plasma and magnetic fields instead of rocky bodies, and ideally travel far enough to look back and glimpse the elusive shape of our heliosphere from afar. But that mission was not recommended as a priority the recently released Decadal Survey It slated U.S. heliophysics for the next decade, hurting the nation’s scientists’ chances of sampling the interstellar medium soon. (Chinese researchers may be more fortunate because the country continues own interstellar mission.)
For now, scientists are still looking for signals coming back from the Voyagers. Somehow, it’s a ton of information: about two decades of data from two ships in two different locations at the border of interstellar space and beyond. And the returns are rich in oddities, with one spacecraft apparently crossing the termination shock five times, perhaps as it entered and exited the heliosphere in sync with the changing forces of the solar wind. But the Voyagers’ distant observations are also mere breadcrumbs, tantalizing glimpses of a region where lies lie. almost out of our hands, precisely data that raises more questions than answers.
One thing is for sure: no matter when their mission ends, the Voyager spacecraft will leave scientists wanting more data from interstellar space. “The tools will shut down before you get the whole picture,” Opher says. “But having the Voyagers spread out as far as we can is priceless.”
