Voyager 1 is on its way to becoming humanity’s first-ever interstellar spacecraft. Having spent more than 36 years flying through space and, in that time, traveled more than 11 billion miles from Earth, the spacecraft is advancing toward the outer point where the sun’s radiation gives way to the cosmic-ray particles of deep space.
NASA launched Voyager 1 and its sister spacecraft, Voyager 2, in 1977 and sent them en route to Jupiter, Saturn, and the other outer planets. Voyager 1 extensively surveyed Jupiter and Saturn, in the process discovering volcano’s on Jupiter’s moon Io and providing intricate close-up photography of Saturn’s rings. Voyager 2, meanwhile, explored Uranus and Neptune.
Both Voyagers passed the orbit of Pluto in 1989 and are now deep within the heliosheath, an outer zone where outgoing radiation from the sun runs up against interstellar gas and dust, in the process creating massive, turbulent clouds of charged particles. Voyager 1 entered the heliosheath in August of last year, according to NASA’s Jet Propulsion Lab (JPL), which manages both Voyager missions. It took all those years to reach the heliosheath, as its beginning point starts at about twice the distance between the sun and Pluto.
The next threshold to cross will be the heliopause, the heliosheath’s outer boundary. Voyager 1 will get there first, as it is more than 2 billion miles further along on its journey than is Voyager 2—the two spacecraft are, respectively, 11.5 billion miles and 9.4 billion miles distant from Earth.
Both have a very long way to go before reaching the solar system’s exit point, however. First, they will have to make it through a few billion more miles of heliosheath to reach the heliopause. No no one knows for sure just how many, since no one has yet determined the heliopause’s precise location. That whole region of space is far too dark for any telescopes to directly observe it, and there are no large bodies of matter within it for them to view even if they tried.
Scientists gather information on the heliosphere the same way that they gain information on exoplanets and black holes: indirectly, by viewing its effects on other objects. In this case, they use telescopes to monitor the zone’s effects on the cosmic-ray particles that are making their way from deep space into our solar system, and to study the radio waves that are created when gas particles from the sun cross into it.
The Voyagers may shed some light on the situation, however. Being the first and only human machines to actually travel through the heliosphere, they are in a position to measure its exact size. NASA is hopeful that before the Voyagers will give us some conclusive answer on the heliospere’s dimensions will be one of the Voyagers’ final contributions to science before they sign off for good.
And even the heliosheath might not mark the end of the solar system. According to many astronomers, the Voyagers will still technically be within our solar system until they have passed an even more distant threshold known as the Oort Cloud. This hypothetical region, which may exist at nearly a full light-year from the sun, is where comets are thought to originate. This is only an educated guess, as astronomers have not yet proven the Oort Cloud’s existence. Maybe the Voyagers will shed some light on this, too, if they are still functioning by that point.
Many of Voyager’s instruments have fallen out of commission, an expected consequence of the harsh toll that this extended journey through space must take on the spacecraft, compounded with the complete lack of any periodic repair or maintenance. NASA expects Voyager to continue to deliver data for another seven years, or maybe longer, but the wear and tear of space will inevitably claim it at some point after that.
Nevertheless, those systems that are still online are now relaying substantial amounts of data on the heliopause and its particle phenomena. The Voyager team reported that, starting last August, Voyager instruments had started indicating a mix of high-energy and low-energy interstellar particles. They also saw a dramatic and fast decline in the presence of solar-originating particles, as well as a decrease in the remaining solar-originating particles’ intensity. JPL researchers deduced the existence of a “magnetic highway,” in which magnetic fields inside the heliosphere and outside it connect and in so doing allow exchanges of particles.
This sudden drawdown in solar particles and their intensity are important signs of the spacecraft’s approach to interstellar space, according to NASA scientists, who are looking closely for a third sign: the appearance of an abrupt change in the direction of the magnetic field. This will indicate the crossing of the heliopause and Voyager going fully interstellar. As distant as it is, Voyager is still within the bounds where the sun’s magnetic field is a dominating force.