There is adventure afoot, with controversy to boot. The two Voyager spacecraft were launched by NASA in 1977, and, having explored the outer planets, are now on a path heading out of the Solar System. Voyager 1 is more than 13 billion kilometres away, more than 85 times farther from the Sun than the Earth is. At that distance, argue Krimigis et al.1 on page 45 of this issue, the spacecraft has encountered the so-called termination shock of the solar wind, where the solar wind begins to merge with the interstellar medium. But on page 48, McDonald et al.2 argue equally convincingly that the termination shock still lies ahead. Either way, Voyager 1 has entered a region of our Solar System that has never yet been explored.

The outer atmosphere of the Sun expands continuously into space as a supersonic, flowing plasma of charged particles, known as the solar wind. The wind's velocity is typically between 400 and 750 kilometres per second: in comparison, the speed of sound near the Earth is only 30–50 kilometres per second. As two plasmas do not readily penetrate each other, the supersonic plasma of the solar wind carves out its own volume inside the plasma in the interstellar medium, a volume that is known as the heliosphere.

At some point in the outer heliosphere, however, the solar-wind plasma must begin to merge with the plasma of the interstellar medium. As with all supersonic flows, this merger begins with a shock transition at which the speed drops abruptly, from supersonic to subsonic — just like the shock that precedes a supersonic plane and causes a sonic boom. This is the termination shock of the solar wind (Fig. 1). It surrounds the entire heliosphere, and is by far the largest shock transition in the Solar System. Estimates3 of the distance to the termination shock from the Sun range from 85 to 120 au (1 au, or astronomical unit, is the mean distance between the Earth and the Sun, equivalent to around 150 million kilometres).

Figure 1: Fantastic voyage.
figure 1

NASA/VOYAGER

Voyagers 1 and 2 have flown on different trajectories past the outer planets of the Solar System, and Voyager 1 may1 — or may not2 — have reached the termination shock, at 85 au from the Sun. The termination shock occurs where the supersonic plasma of the solar wind begins to slow down as it encounters the interstellar medium at the bounds of the Solar System. The solar wind and the interstellar medium do not merge easily, so further out, beyond the termination shock, there is the true boundary between the solar wind and the interstellar medium — the heliopause. Further out still, if the Solar System is itself moving supersonically relative to the interstellar medium, there may be a large bow shock.

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The termination shock is expected to be a fascinating astrophysical object and a prodigious accelerator of energetic particles. Interstellar neutral gas penetrating into the inner heliosphere becomes ionized, is picked up by the solar wind and carried into the outer heliosphere. These particles are then accelerated, with their energies increased by a factor of more than 10,000, to form an energetic particle population known as the anomalous cosmic-ray component. It is likely that this acceleration occurs at the termination shock. The accelerated particles can, in turn, exert pressure that alters the structure of the termination shock. In this sense, the termination shock should be similar to shocks in the interstellar medium that result from supernovae, which are also expected to be strong accelerators of energetic particles.

If Voyager 1 has encountered the termination shock, there should be several distinct signatures in the data collected by the spacecraft. (Voyager 2 currently trails Voyager 1 by about 20 au, and has not yet reached the possible location of the termination shock.) The most obvious signature would be the drop in the solar wind speed, with a resulting increase in density and magnetic-field strength. Also, many accelerated particles, particularly the anomalous cosmic rays, should be present.

These two different signatures are the basis for the controversy between Krimigis et al.1 and McDonald et al.2. The plasma detector on-board Voyager 1, which would have provided a direct measure of the solar wind speed, has not operated for some years. To compensate, Krimigis et al. have performed a clever analysis of low-energy particles, and conclude that the solar wind speed has dropped, as required. They also see an increase in the number of these low-energy particles, and of the right types of particle, as would be expected near the termination shock. In contrast, measurements of anomalous cosmic rays at higher energies by McDonald et al. imply that these particles must be accelerated at a distance beyond Voyager 1's current position. McDonald et al. do see a substantial increase in the intensity of ions and electrons at around 85 au, but argue that it is only a precursor of the termination shock that still lies ahead.

In fact, the termination shock is not expected to be stationary. Its location will vary with changing heliospheric conditions, which in turn respond to changing solar conditions. Indeed, Krimigis et al. argue that Voyager 1 only penetrated beyond the termination shock for around 200 days, after which the shock moved outwards again, leaving the spacecraft back in the supersonic solar wind. The termination shock may well be moving outwards for the next few years, in which case the chase is on again to encounter and study the largest shock transition in the Solar System.

The question is, of course, who's right? Has Voyager 1 already encountered the termination shock? Personally, I tend to agree with Krimigis et al.1 that their data can most readily be explained if the termination shock had been crossed. The data of McDonald et al.2 might then suggest that the termination shock has a more complex shape than expected, or that there is some other means of accelerating energetic particles, beyond the location of Voyager 1. Neither explanation is certain, and we must hope that Voyager 1, on its march ever outwards, will encounter this interesting region again soon.

Once the termination shock has definitely been passed, the adventure enters a new phase. The region of subsonic solar wind beyond the termination shock will be fascinating, characterized by turbulence and particle acceleration, and by many unusual plasma phenomena. The Voyager spacecraft move at 3–4 au per year, and will eventually encounter the heliopause (estimated to be around 150 au from the Sun), which is the boundary that separates the solar-wind plasma from interstellar plasma (Fig. 1). Then we will have truly penetrated the interstellar medium. The Voyagers might eventually reach a 'bow shock', caused by the heliosphere itself moving supersonically through the interstellar medium. But both craft will have exhausted their power supply long before that, in around 2020. After a 40-year mission, they will drift quietly into the pristine interstellar space beyond.