Forty years ago, the Voyager 1 and 2 missions began their journey from Earth to become the farthest-reaching missions in history. In the course of their missions, the two probes spent the next two decades sailing past the gas giants of Jupiter and Saturn.
And while Voyager 1 then ventured into the outer Solar System, Voyager 2 swung by Uranus and Neptune, becoming the first and only probe in history to explore these worlds.
This summer, the probes will be marking the 40th anniversary of their launch - on September 5th and August 20th, respectively.
Despite having travelled for so long and reaching such considerable distances from Earth, the probes are still in contact with NASA and sending back valuable data.
So in addition to being the most distant missions from Earth, they are the longest-running mission in history.
In addition to their distance and longevity, the Voyager spacecraft have also set numerous other records for robotic space missions. For example, in 2012, the Voyager 1 probe became the first and only spacecraft to have entered interstellar space.
Voyager 2, meanwhile, is the only probe that has explored all four of the Solar System's gas/ice giants - Jupiter, Saturn, Uranus and Neptune.
Their discoveries also include the first active volcanoes beyond Earth - on Jupiter's moon Io - the first evidence of a possible subsurface ocean on Europa, the dense atmosphere around Titan (the only body beyond Earth with a dense, nitrogen-rich atmosphere), the craggy surface of Uranus' "Frankenstein Moon" Miranda, and the ice plume geysers of Neptune's largest moon, Triton.
These accomplishments have had immeasurable benefits for planetary science, astronomy and space exploration. They've also paved the way for future missions, such as the Galileo and Juno probes, the Cassini-Huygens mission, and the New Horizons spacecraft.
As Thomas Zurbuchen, the associate administrator for NASA's Science Mission Directorate (SMD), said in a recent press statement:
"I believe that few missions can ever match the achievements of the Voyager spacecraft during their four decades of exploration. They have educated us to the unknown wonders of the universe and truly inspired humanity to continue to explore our solar system and beyond."
But what is perhaps most memorable about the Voyager missions is the special cargo they carry. Each spacecraft carries what is known as the Golden Record, a collection of sounds, pictures and messages that tell of Earth, human history and culture.
These records were intended to serve as a sort of time capsule and/or message to any civilisations that retrieved them, should they ever be recovered.
As noted, both ships are still in contact with NASA and sending back mission data. The Voyager 1 probe, as of the writing of this article, is about 20.9 billion km (13 billion mi; 140 AU) from Earth.
As it travels northward out of the plane of the planets and into interstellar space, the probe continues to send back information about cosmic rays - which are about four times as abundant in interstellar space than around Earth.
From this, researchers have learned that the heliosphere - the region that contains the Solar System's planets and solar wind - acts as a sort of radiation shield.
Much in the say that Earth's magnetic field protects us from solar wind (which would otherwise strip away our atmosphere), the heliopause protects the Solar planets from atomic nuclei that travel at close to the speed of light.
Voyager 2, meanwhile, is currently about 17.7 billion km (11 billion mi; 114.3 AU) from Earth. It is traveling south out of the plane of the planets, and is expected to enter interstellar space in a few years.
And much like Voyager 1, it is also studying how the heliosphere interacts with the surrounding interstellar medium, using a suite of instruments that measure charged particles, magnetic fields, radio waves and solar wind plasma.
Once Voyager 2 crosses into interstellar space, both probes will be able to sample the medium from two different locations simultaneously.
This is expected to tell us much about the magnetic environment that encapsulates our system, and will perhaps teach us more about the history and formation of the Solar System.
On top of that, it will let us know what kinds of hazards a possible interstellar mission will have to contend with.
The fact that the two probes are still active after all this time is nothing short of amazing. As Edward Stone - the David Morrisroe Professor of Physics at Caltech, the former VP and Director of NASA's Jet Propulsion Laboratory, and the Voyager project scientist - said:
"None of us knew, when we launched 40 years ago, that anything would still be working, and continuing on this pioneering journey. The most exciting thing they find in the next five years is likely to be something that we didn't know was out there to be discovered."
Keeping the probes going has also been a challenge since the amount of power they generate decreases at a rate of about four watts per year.
This has required that engineers learn how to operate the twin spacecraft with ever-decreasing amounts of power, which has forced them to consult documents that are decades old in order to understand the probes' software and command functions.
Luckily, it has also given former NASA engineers who worked on the Voyager probes the opportunity to offer their experience and expertise. At present, the team that is operating the spacecraft estimate that the probes will run out of power by 2030.
However, they will continue to drift along their trajectories long after they do so, traveling at 48,280 km per hour (30,000 mph), covering a single AU every 126 days.
At this rate, they will be within spitting distance of the nearest star in about 40,000 years, and will have completed an orbit of the Milky Way within 225 million years.
So it's entirely possible that someday, the Golden Records will find their way to a species capable of understanding what they represent.
Then again, they might find their way back to Earth someday, informing our distant, distant relatives about life in the 20th century.
And if the craft avoid any catastrophic collisions and can survive in the interstellar medium of space, it is likely that they will continue to be emissaries for humanity long after humanity is dead. It's good to leave something behind!
This article was originally published with Universe Today. Read the original article.