For nearly forty years, NASA's twin Voyager spacecraft, Voyager 1 and Voyager 2, have been traveling through space. Launched in 1977, these iconic spacecraft have forced man to push the limit in what has been explored by him in extraterrestrial space, with neither having yet reached any other object created by man. Because as they reach the outer limits of our solar system, into uncharted interstellar space, their discoveries are changing what we know so far about the universe. What have these space explorers discovered at the edge of the solar system?
The Heliosphere: A Protective Bubble
The heliosphere is one of the more remarkable Voyager discoveries-a very big, protective bubble generated by the Sun. It forms from the solar wind-a stream of charged particles that flows outward from the Sun and extends well beyond the orbit of Pluto-which acts as a shield protecting the solar system from the interstellar medium, the particles, and radiation that exist in the space between stars.
It was only in 2012 that Voyager 1 became the first man-made object to venture out of our heliosphere when it crossed the boundary at a location now known as the heliopause. It took nine more years before Voyager 2 crossed over into interstellar space, revealing that this protective enveloping barrier surrounds the entire solar system. Analysis from both probes at the time showed that the heliopause serves as a defined boundary where the influence of the Sun's solar wind ceases and that of interstellar space begins.
Find the "Cosmic Shoreline
It is more of a cosmic shoreline from where the solar wind gets pushed by interstellar space forces. Voyager's instruments detected the interstellar wind--a stream of charged particles and cosmic rays shot out from the distant stars' explosion, which here collide with the solar wind that leaves the solar system. That collision causes a turbulent region full of magnetic fields and high-energy particles.
At this boundary, called the termination shock, the solar wind is dramatically slowed by the pressure of the interstellar medium. Voyager 2's measurements of this region have brought new insight into how the solar wind decelerates and heats up. The spacecraft saw the solar wind decelerate from supersonic to subsonic speeds marking the beginning of a turbulent area leading to the heliopause.
Cosmic Rays and the Edge of Space
The most critical findings from Voyager at the solar system's boundary with interstellar space concern cosmic rays. These are high-energy particles produced by supernovae and other violent events in the universe. When Voyager crossed the heliopause, it measured a shocking increase in cosmic rays, indicating passage into interstellar space.
The heliopause shielded the heliosphere, effectively reducing cosmic radiation for the spacecraft. When Voyager exited the heliopause, it ventured into a region where deep space cosmic rays pummeled the spacecraft with significant intensity. How cosmic radiation accelerates in the outer expanses of space provides important insight into the environment that is high energy and beyond the bubble of the solar system.
Magnetic Fields: Shocking Discoveries
Another surprise Voyager discovery was the magnetic fields at the edge of the solar system. Researchers were expecting that a magnetic field in interstellar space would certainly be very different compared to the solar magnetic field, but Voyager instruments instead discovered a surprise: the magnetic field in interstellar space seems actually aligned almost identically to the field within the heliosphere.
This raises a mystery for scientists because they postulate that a more profound variation of magnetic field orientation would have been observed when the spacecraft crossed into interstellar space. This magnetic field must be unraveled to understand the heliosphere's structure and the other parts of the galaxy with which it interacts.
"Plasma Waves" and the Sounds of Space
One of the most fascinating aspects of Voyager's journey through the outer regions of the solar system is plasma wave detection. The instruments on board Voyager have recorded vibrations in plasma-the electrically charged gas that fills space. These plasma waves create eerie, otherworldly sounds, scientists converted into audio frequencies.
A much denser and colder plasma region was encountered when Voyager 1 entered interstellar space; the plasma waves observed in the plasma indicated that the spacecraft had moved out of the hot, low-density plasma of the heliosphere.
These plasma waves reveal much about the density of space and the conditions found beyond our solar system. As Voyager presses on in its journey, it will continue to measure these plasma waves, giving scientists new insight into the nature of the interstellar medium.
What's Next for Voyager? The Journey Ahead
Voyager 1 and 2 are now in the interstellar medium but hardly a mission finale. There is still more to the remainder of the journey for these spacecraft as they continue to send information back to Earth, though at a rate much reduced through these immense distances. Once powered by radioisotope thermoelectric generators, probes like these are likely to go on for another few years to reveal that part of space unseen by any human being.
The Voyagers losing ever more control and drifting into the unknown, these spacecraft will continue to collect cosmic ray data, plasma waves, and magnetic fields measurements, which will give humanity its first direct view into the interstellar medium. Currently, though headed toward a trajectory that will eventually bring the spacecraft near other star systems, this journey will take tens of thousands of years.
Even after the instruments stop working, Voyager 1 and 2 will drift silently through space as the representatives of Earth, carrying with them the Golden Record, a message for any other civilizations in space. These records, essentially time capsules, contain sounds and images from Earth, providing a snapshot of human culture and life on the planet.
Conclusion: The Legacy of Voyager
The Voyager mission redefines our concept of the edge of the solar system and of interstellar space beyond it. For starters, these spacecraft have opened a new chapter in space exploration: with the detection of plasma waves and cosmic rays in what had heretofore been unknown territory, they are broadening human knowledge and, at the same time, expanding our view of the cosmos.
Voyager's discovery reminds us that space is not at
all empty, but teeming with tremendous forces of radiation and magnetic fields
that mold the universe in ways we are only beginning to understand. As the
human race looks toward further missions exploring even more into outer space,
the legacy of Voyager will inspire generations of scientists, explorers, and
dreamers.
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