Re: What will happen to Voyager 1 when it encounters the Bow Shock? Thank

This answer was kindly provided by my colleague Stan Cowley (Professor of Solar-Planetary Physics at the University of Leicester, UK):

First a few words about the nature of the bow shock surrounding the heliosphere and why it occurs. Because the outer atmosphere of our Sun is very hot, much hotter than the Sun’s visible surface, it is not contained by the Sun’s gravity, but streams continuously outward into the solar system to form the solar wind outflow. The flow speed is such that a particular parcel of the gas usually takes about three days to reach the orbit of the Earth from near the surface of the Sun. As it moves away from the Sun, the flow speed of the gas stays roughly constant, but it becomes ever more rarefied as it expands outward into ‘space’. If the solar system was really surrounded by ‘space’ then the solar wind would flow outwards undisturbed forever. Ultimately, however, at a distance of roughly one hundred times the Earth’s orbital distance the solar wind runs into the ‘local interstellar medium’ (LISM), the highly rarefied gas that exists between the stars in our part of the Milky Way galaxy. When the solar wind meets the LISM, it is forced to slow down to much lower speeds, after which it is carried away by the LISM, which is moving past our solar system at a relative speed of a few tens of kilometres per second. However, because the solar wind outflow is supersonic, disturbances cannot travel back into the gas to tell it to slow down gradually to the speed of the LISM. Instead, the deceleration occurs abruptly at a shock wave across which the gas is slowed, compressed by factors of two or three, and heated. This shock is called the ‘termination shock’ of solar wind supersonic outflow, and it was crossed by Voyager 1 in December 2004. As the LISM approaches the expanding solar wind from the other side it is also forced to slow down, and since its flow relative to the solar system is again supersonic, the slow- down is also expected to occur via a shock wave. This anticipated shock is called the ‘bow shock’ because it will have the form of a bow wave of a boat moving through water, the ‘boat’ in this case being the solar system, and the ‘water’ the LISM.

It is not known exactly how much beyond the termination shock the bow shock will be, but there is some hope that Voyager 1 will reach it within the next 10-15 years before its power supply becomes too weak to allow continued communication with Earth. This would make the distance about half as far again as that of the termination shock. When Voyager 1 crosses the bow shock it will finally have moved into the undisturbed LISM that lies beyond our solar system. The gas will become faster-flowing than before, less dense, and cooler. Some naturally-occurring radio emissions may also be measured around the time of the shock crossing, caused by interactions of the gas particles with the shock. However, the physical integrity of the spacecraft, and its continued path away from the Sun, will not be affected by these changes at all. Although the solar wind and LISM flow very quickly relative to the spacecraft, and have been doing so ever since launch in 1977, these gases are so rarefied that the spacecraft effectively does not feel a thing. Their density is about one thousand billion billionth of the density of the Earth’s atmosphere at the Earth’s surface, such that it represents, in Earth terms, a really very good vacuum indeed.