Forty years ago, the Voyager probes 1 and 2 were launched. I remember it happening, along with the feelings of pride and excitement that mankind had been able to construct and launch such things. It was less than a decade from the first moon landing, and it felt as though space was progressively, and quite rapidly, opening up to us all. Those were optimistic days.
The launch time was chosen very carefully, so as to take advantage of a rare planetary line-up to gain acceleration as they passed each of several planets over the years. This manoeuvre has come to be known as slingshot, and is used extensively in films and books as well as for real. Anyway, this series of relatively close passes also meant that we were treated, at increasing intervals, to images of planets with details which at the time had never been seen. These remote places, mere points of light to the naked eye, suddenly became real places, and we saw how familiar things like weather patterns, volcanoes, and water appeared throughout our solar system.
The two probes are still travelling outwards, still gathering new information, and still sending signals back to Earth. These signals now take 16 hours for Voyager 2, and nearly 20 hours for Voyager 1, and are fantastically weak compared to the strength at take-off. One of the many scientific spinoffs has been the development of ever more accurate equipment to listen to these distant voices. But the lifetime of the battery power is finite. From 2020 onwards, the scientific instruments will be turned off one by one to prolong the on-board power, and after 2025 none will be operational. From then on, the spacecraft will simply continue on as complicated pieces of metal. Their current velocity will be broadly the same, as there is hardly any gravitational drag.
Since 2013, Voyager 1 has officially been classed as travelling through interstellar space, as opposed to the volume of space directly linked to our sun. You could liken this to the atmosphere which surrounds a planet, attenuating in stages to interplanetary space – and indeed the region is now called the heliosphere. The very fact that such a boundary region exists was not recognised before Voyager 1’s instrument data was analysed. Our present understanding is that in this zone, the constant stream of particles pouring outwards from our sun – the solar wind – ceases to have a clear direction of flow and becomes turbulent. You could liken it to air flow around the speed of sound, but the density of particles is so thin that there is no hazard to navigation! In this region, Voyager 1 is encountering as many particles from other stars as it does from ours – the boundary zone acts as a buffer shielding our entire solar system from too much stuff passing casually in.
Voyager 2, though launched first, has taken a slightly different trajectory, and is now a few years behind Voyager 1. Currently it is still in the heliopause – the boundary zone – and will emerge in a few years. Both craft will – in around 300 years or so – begin to traverse a region called the Oort Cloud. This is a vague and fuzzy shell largely inhabited by comets and similar celestial debris, which occasionally get disturbed enough to drop down to the inner system and make themselves known. It is possible that one of the Voyagers will get close enough to interact with one of these objects, but hugely unlikely given the sheer volume of space concerned.
Other things being equal, they will come out the other side of the Oort cloud in about 30,000 years… and still the nearest star will be our sun. It will take about 40,000 years, give or take, before they will be nearer another star than our own sun. This last figure highlights just how far it is from one star to the next, compared with the distances from a star to the associated planets. Right now, the closest star to us is Alpha Centauri, but by that time another star will be our nearest neighbour, Gliese 445. But even that won’t be very close – the point of nearest approach is about 1.6 light years.
Both Voyager craft carry “The Golden Record”, looking not unlike an old LP vinyl record, containing a diverse collection of information about us. I remember there being considerable controversy about this as launch time approached back in 1977. There were earnest debates about the content – should Johnny B. Goode be part of our interstellar welcome pack? Was it improper to have pictures of a naked man and woman? But there were also more basic questions. Did we wanted to make our existence known to other possible life forms? Should we include what are effectively navigation instructions telling whoever finds them how to find us? Those who are curious can look up the exact list of what we sent on these golden disks here, and even listen to the audio content here.
For me, the Voyager craft have been a background feature of life from my late teens. For some people, they have been the focus of their entire working lives. And so far, they are the only two objects that we have built which have escaped the gravity well of our sun, and are now at large in the galaxy.