Into the great unknown: Voyager, an epic journey to interstellar space
From Fukushima to the darkest corners of the ocean, robots built for extreme environments and an appetite for discovery continue to enlighten our understanding of places too dangerous to tread. Those launched into deep space may be the most daring examples, continually pushing the limits of human ingenuity and expanding our understanding of the universe. In this series New Atlas profiles space probes, both past and present, tasked with pushing the boundaries of science by leading us into the great unknown. This week: the Voyager mission, humankind’s most epic astronomical adventure to date.
If you are a space agency hoping to visit the planets of our solar system, there may be no better time to do it than when the orbits of Jupiter, Saturn, Uranus, Neptune and Pluto bring them into a tight and neat formation. This rare planetary alignment occurs only once every 175 years, and as NASA embarked on a new chapter of space exploration under President Richard Nixon in the 1970s, it was presented with a unique opportunity.
Occurring for the first time since the year 1800, the rare planetary alignment to take place that decade would enable NASA to visit all of Saturn, Uranus, Neptune and Jupiter with a single spacecraft, by using the planets’ gravity to slingshot between each of them, burning minimal propellant in the process. Or so it hoped. In the end, the budget didn’t allow for this US$1-billion “Planetary Grand Tour,” with the US instead settling on a more humble five-year mission to intensively study Saturn and Jupiter.
But NASA’s engineers, clever and ambitious as they were, went to some lengths to equip the spacecraft to travel much, much further. In addition, they opted for a flight path that would allow the spacecraft to continue onward to Uranus and Neptune, and even towards the edges of the solar system, if required to do so.
Launched in August and September of 1977, the twin Voyager probes carrying identical scientific payloads soon returned their first images from Jupiter, painting us a vivid picture of orbital rings, a volatile Great Red Spot and moons dotted with active volcanoes. Another intensive flyby followed, this time of Saturn and its big moon Titan. This was our first real look at Saturn and its magnificent ring system, with the spacecraft identifying four new moons and revealing an atmosphere made up almost entirely of hydrogen and helium.
Voyager 1 and Voyager 2 went their different ways at this point. Voyager 1 carried out a close flyby of Titan to find a body shrouded in smog and then continued on towards interstellar space. Voyager 2, meanwhile, was given the green light to continue on to Uranus, where it captured detailed photos and insights of its moons, magnetic field and rings.
Neptune was Voyager 2’s next stop, flying past in 1989 to capture stunning images of the cold, dark world and finding icy geysers spewing material from the surface of its frozen moon Triton. Voyager 2’s flybys of Neptune and Uranus remain our only visits to these planets to date.
At this point, with Voyager 2 skimming past Neptune and joining Voyager 1 on an intrepid, albeit different, trajectory towards interstellar space, the mission had far exceeded the official expectations of NASA, but not the hopes and dreams of the people behind it. From here, the intrepid spacecraft took on a new mission, to peer beyond the solar system’s outer planets to investigate the reach of the Sun’s influence.
But not before snapping one of the most iconic space photographs of all time, as instructed by famed astronomer Carl Sagan. As Voyager 1 moved beyond the orbit of Neptune, the probe turned its camera towards the starting point of its journey to photograph our home planet from a distance of around six billion km (3.7 billion mi), an image that came to be known as the Pale Blue Dot.
Then renamed the Voyager Interstellar Mission, this new chapter in space exploration’s longest story focused on the fringes of the bubble encasing our solar system known as the heliosphere. The Sun at the center of this bubble generates magnetic fields and solar winds, with protons and electrons streaming outward from our star until they collide with the crushing pressures of interstellar space.
This meeting point, the boundary of the heliosphere, is known as the heliopause, and nobody knew exactly where it lay until the trailblazing Voyager probes moved through the area. Here they found the solar winds slowing from a million mph (1.6 million km/h) to a gentle 250,000 mph (400,000 km/h) as the Sun’s influence began to weaken and different conditions began to take hold.
In August of 2012, 35 years after it was launched, Voyager 1 passed though the heliopause and entered interstellar space, becoming the first human-made object to do so. As it did, it relayed readings of huge spikes in interstellar cosmic rays and a huge dip in solar rays, though a negligible change in the magnetic field presented scientists with a curveball and suggested the heliopause was far more complex than we thought.
Then, in December of 2018, Voyager 2 joined its sibling in interstellar space, around 11 billion miles (18 billion km) from Earth. Though second to the punch, Voyager 2 may prove more fruitful to scientists because its Plasma Science Experiment is still functional, while Voyager’s failed way back in 1980. This instrument gathers data on the electric current of solar winds to deduce their speed, density, temperature and pressure, with its readings indicating a sharp decline as it exited the heliopause.
Here, beyond the Sun’s influence, the Voyager probes are unable to draw on solar power, and instead use radiothermal generators powered by plugs of plutonium fuel. These play a crucial role in keeping their electronics from freezing up in the bleak harshness of interstellar space, and keeping their instruments in working order.
Earlier this year, NASA began the process of switching off the heating implements for various instruments aboard the Voyager probes to ration out what’s left of the plutonium fuel stocks. The radiothermal generators are depleting at a rate of four watts each year and are currently operating at around 60 percent of their original output.
At this rate, they are expected to cease working entirely around 2025, so NASA is intent on squeezing every last bit of interstellar science out of them that it can. This will come via the cosmic ray, plasma, magnetometer and low-energy charged particle instruments that continue to collect invaluable data.
But the trajectories of the Voyager probes will draw them further and further away, long after its fuel stocks are depleted. In around 300 years, Voyager 1 is expected to reach the mysterious Oort cloud, a massive bubble of icy debris generally acknowledged as the edge of the solar system. Voyager 2, meanwhile, will reach the brightest star in our sky, Sirius, in about 296,000 years.
So it might be 42 years in, but in a way, the Voyager journey is just beginning.
In next week's edition of "Into the great unknown" we look at Rosetta, the first mission ever to orbit and land on a comet. For more on pioneering space probes, check out previous instalments from the series.