In 1977, a mere 20 years after the launch of Sputnik marked the beginning of the Space Age, the twin Voyager 1 and 2 spacecraft were launched, bound for tours of the gas giant planets of the outer Solar System. And though it's been over two decades since either Voyager made its final planetary fly-by, to this day both vehicles are in good health, still sending their pioneering data back to us via the worldwide Deep Space Network of radio dishes and still making important scientific discoveries and history.
Though launched 4-5 years after the first spacecraft to cross the Asteroid Belt, encounter gas giants and achieve escape velocity from our Solar System (Pioneers 10 and 11), the Voyagers have since outdistanced the Pioneers to become the most distant human-made craft in space. We also lost contact with the Pioneers, in 1995 and 2003—but the Voyagers are still on line!
At nearly three times the mass and power generation of the Pioneers, the Voyagers continue to blaze trails. As of right now, Voyager 2 is nearly 15 billion kilometers from the Sun and Voyager 1 leads the race at over 18 billion kilometers: quadruple the distance to Neptune! Every second carries Voyager 1, the speedier of the pair, 17 kilometers farther into space.
And though it will be at least 40,000 years before Voyager 1 gets anywhere near another star, recent data from the probe indicates that it may have reached a milestone in its journey anyway.
When the Voyagers were launched, even though their primary missions were to observe the gas giants of the outer Solar System, NASA hoped that the spacecraft would stay active and healthy long enough to make measurements until their inevitable passage through the edge of the Solar System: the boundary layer between the influences of our Sun and the environment of interstellar space beyond.
The Sun not only radiates light and other forms of electromagnetic radiation, but also plasma (electrically charged gas, mostly hydrogen) and accompanying magnetic fields: the solar wind. The material expands outward into space and eventually encounters the influences of the interstellar environment. The collision of the two realms forms a "bubble". The material inside the bubble--the heliosphere, essentially the Sun's extended atmosphere--encompasses a volume of space extending far beyond the planets. The tenuous boundary between the heliosphere and the interstellar medium is called the heliopause.
It's a bit like the difference in water current and wave action between the waters of a protected cove and the open ocean. I remember a trip to Angel Island with my friend Bob back in the 1970's. We rowed a small inflatable raft out into the harbor, sculling around the calm, nearly still waters within the projected arc of the cove. Then we rowed out beyond the confines of the cove, suddenly finding ourselves in an environment of high, choppy waves and a powerful current that threatened to carry us away from the island and out toward the Golden Gate Bridge. Ten minutes of furious rowing were required to get us back inside the peaceful bubble of the cove, and safety—and I'll never forget it!
In the past two years, Voyager scientists have noticed an increase in the amount of electrically charged particles detected by Voyager 1 -- particles not of the solar wind, but high velocity interstellar ions probably formed in supernova explosions in our neighborhood in the past: "galactic cosmic rays." The cosmic ray count rose 25% in the past three years and since May of this year, there has been an additional 9% increase. If this is an indication that Voyager 1 is encountering the heliopause, maybe it's analogous to Bob and I suddenly encountering larger and more waves in our inflatable raft at the boundary of the cove.
Another indicator that Voyager 1 may be breaking on through to the other side will be a change in the direction of the magnetic fields around it, as the magnetic field put out by our Sun bends to the will of the Milky Way Galaxy at large. NASA is currently analyzing Voyager's data to determine if this shift is taking place. I guess it could be likened to the sudden change of current that almost swept Bob and I off to the Golden Gate Bridge.
What will become of these veteran robots that are destined to become humanity's first interstellar envoys, carrying with them their various audio recordings and images of our species? Will they drift off into obscurity, becoming lost in space? Will they become objects for target practice by some trigger happy extraterrestrial spacefarer? Will they one day arrive at an alien world with our messages, prompting a response? Only time--probably lots of it--will tell....