The Voyager 1 and 2 spacecraft were launched in the 1970s. Three things made these missions possible. The first was the close conjunction of the gas giant planets which made possible a “Grand Tour" of several of them by a single spacecraft. The second was the mathematical discovery of the gravity assist manoeuvre by Michael Minovitch whilst he was a researcher at the Jet Propulsion Laboratory. The third was the maturation of spacecraft technology, in particular the Radioisotope Thermonuclear Generators (RTGs) which had been proven by the earlier SNAP (Systems for Nuclear Auxiliary Power) program and Pioneer spacecraft, allowing a Plutonium-238 power source to provide power in deep space for several decades.
The Voyager spacecraft are now heading out into interstellar space, although what defines interstellar space is a matter of debate. But as of 5th January 2015 Voyager 1 was at a distance of 19,570,500 km or 130.82 Astronomical Units (AU) from the Sun, where 1 AU is the mean distance between the Sun and the Earth. The roundtrip light time from the Sun was around 36 minutes and 16 seconds. Similarly, Voyager 2 was at a distance of 16,106,670 km or 107.67 Astronomical Units from the Sun. The roundtrip light time from the Sun was around 29 minutes 50 seconds. Recall that the nearest stars are just over 4 light years away.
Voyager Spacecraft (image credit: Adrian Mann)
Both of the Voyager spacecraft are in their ‘Voyager Interstellar Mission (VIM)’ phase to extend the exploration of the solar system beyond the neighbourhood of the outer planets to the outer limits of the Sun's sphere of influence and beyond. They are continuing to help characterise the outer solar system environment and to search for the heliopause boundary, the outer limits of the Sun’s magnetic field and outward flow of the solar wind.
The Voyager probes are moving at around 17 km per second relative to the Sun. on the current paths, Voyager 1 is heading towards the constellation Camelopardalis, the Giraffe, and will arrive in the nearest star system in that direction in around 40,000 years, flying within 1.6 light years of the nearest one. Voyager 2 is headed towards the constellation Andromeda, towards the star Ross 248, which it may get near also in around 40,000 years.
Will the spacecraft ever arrive at those destinations? The escape velocity from the Sun is around 618 kilometres per second very close to its surface (around 0.00465 AU). or from the Earth is around 42.1 kilometres per second. But then the Earth already has a velocity of around 30 km/s so in reality, any spacecraft trying to escape need only achieve a speed of around 12 kilometres per second. The escape velocity is given by the inverse square root of the distance, so the further away you are from the source of the gravitational pull, the smaller the escape velocity.
Let’s look at what happens if you calculate the escape velocity at different radii from the Sun. At 1 AU it is 42.2 kilometres per second as already mentioned. But what about at 108 AU and 130 AU; the approximate distances of the Voyager 2 and Voyager 1 spacecraft? We find that the escape velocities drop to around 4.05 kilometres per second and 3.69 kilometres per second respectively. If we go out even further to 200 AU or 500 AU the escape velocity drops to 2.98 kilometres per second and 1.89 kilometres per second respectively. At 1,000 AU, the escape velocity would be as low as 1.33 kilometres per second. Escape velocity falls to1 kilometre per second only once a spacecraft reaches as far as 1,774 AU, still short of the Oort cloud where comets are believed to exist in large numbers.
The Voyager probes performed gravity assists as they passed the gas giants. The gravitational potential energy associated with those planets and other objects in the solar system will act to pull the spacecraft back and slow the velocity, but this is not sufficient to offset the large kinetic energy the spacecraft already have as they move away from the Sun and head towards a mathematical infinity. This is defined by their hyperbolic excess velocity. So, with their initial velocity and the help of the gas giants, the Voyager probes will indeed keep on going towards those distant stars.
One day, we may build faster spacecraft and overtake them. I think we then have several options. We could retrieve them; put them into a museum on Earth for visitors to enjoy. Alternatively, we could just leave them be, and let them continue on their long journeys, however long it takes. We may decide to upgrade their power supplies, however, as they will be long dead. Perhaps we could upload some Americium-241 which has a half-life of 500 years compared to the meagre 87 years of Plutonium-238. Then after 432 years, they will need something else, but let us leave that to our imaginations.
Kelvin F.Long, Executive Director i4iS