If they continue on their current trajectories, what is the probability Voyager 1 or 2 will crash into a planet or star in the (distant) future?
Mark Thompson
Tewkesbury, Gloucestershire, UK
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Both Voyager 1 and Voyager 2 have reached interstellar space. Currently, Voyager 1 is located 23 billion kilometres from Earth, while Voyager 2 has made it to a distance of 19 billion kilometres.
Since both have already passed by the planets in our solar system, they aren’t expected to crash into a planet or star for some time. However, the trajectories of the probes are influenced by the gravitational forces of the stars they pass, which could cause them to change direction.
For now, Voyager 1 is moving in the general direction of the constellation Ophiuchus, while Voyager 2 is heading towards Sagittarius. Still, the vast distances between stars mean the probes are unlikely to approach any specific star system for tens of thousands of years.
Both Voyager 1 and Voyager 2 are powered by radioisotope thermoelectric generators, which convert the heat generated by the decay of plutonium-238 into electricity. These are expected to provide enough power to keep the Voyagers’ instruments running until around 2025. After that, the probes will no longer be able to communicate with Earth, but they will continue to travel through space.
Luce Gilmore
Cambridge, UK
The chances of collision between a space probe and a macroscopic obstacle in interstellar space are vanishingly small.
That includes entire solar systems. Our solar system is about 60 astronomical units (AU) in diameter (up to Neptune) and the nearest star system, Alpha Centauri, is about 270,000 AU away. This means that, at the distance of Alpha Centauri, our solar system would occupy only three-billionths of the surface area of a sphere of that radius – quite a small target. If a probe did pass through a star system, it would almost certainly trace a hyperbolic orbit and exit unscathed, as did the extrasolar asteroid Oumuamua.
However, macroscopic obstacles aren’t the only sort in interstellar space: there are cosmic ray particles too. In any volume of space, the energy density due to starlight is only twice that of cosmic rays, so they are relatively abundant.
Cosmic ray particles are energetic and when they hit a solid object, they will knock an atom sufficiently hard that it generates a cascade of secondary impacts. In a structure as small as a typical space probe, this will result in a shower of atoms ejected into space at high speed. This amounts to stochastic “spallation” that will, given sufficient time, abrade any probe to nothing. Such is the fate of the five probes that are bound for the stars so far.
Damir Blazina
Chester, UK
Both Voyagers will eventually collide with something, most likely the remnants of a star, but this will take an extraordinarily long time.
The Voyagers’ highest chances of collision were in the solar system, around the planets and in the asteroid and Kuiper belts, which they have already safely traversed. Their trajectories were carefully planned to maximise the chances of safe passage.
The next highest chance of collision will occur in some 300 years, when both spacecraft are expected to enter the Oort cloud, a hypothesised cloud of icy planetesimals surrounding the sun at distances of around 2000 to 200,000 times that between the sun and Earth. The Oort cloud constitutes trillions of objects and the traverse will take some 30,000 years, but models suggest that the objects there are a few hundred metres to a few kilometres in size and are separated by millions of kilometres. The probes are most likely to sail though untouched.
Afterwards, the probes will be in empty space, where there is nothing to collide with. For a collision to occur, they would need to come close enough to a star and approach at the right velocity to be drawn towards it by gravity, after which they could collide with the star itself or with one of its orbiting planets or other bodies. Considering that star systems are usually separated by several light years, this is extremely unlikely.
Calculations in a from Coryn Bailer-Jones and Davide Farnocchia indicate that in the next 11 million years, Voyager 1’s closest approach to another star will be at a distance of around 0.3 parsecs and a relative speed of 46 kilometres per second, while Voyager 2 will come no closer than 0.5 parsecs at a relative speed of over 70 km/s. (A parsec is some 31 trillion kilometres). This is too far and too fast to be significantly affected by the stars’ gravitational fields, so the probes won’t collide.
However, the spacecraft lack the velocity to escape the galaxy, meaning they will eventually pass close enough to some star to be drawn towards it. The same paper estimates that the timescale for one of them to collide with a star is in the order of 1020 years. This is six orders of magnitude longer than the time by which all stars in the universe will have exhausted their fuel, so the collision will be with a stellar remnant such as a white dwarf or a black hole.
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