The limits of the solar system lie between about 85 and 120 astronomical units away, where one astronomical unit (AU) is about 150 million kilometres or the distance between the Earth and the Sun. At these distances the solar wind - a supersonic plasma of charged particles from the Sun - collides with the interstellar plasma of outer space to create a “termination shock”. Theory predicts that the speed of the solar wind will decrease abruptly in this region, and that there will also be an increase in the strength of the magnetic field. The shock should also accelerate particles from the interstellar medium to high energies.

Tom Krimigis of the Johns Hopkins University in Maryland and colleagues analyzed energetic particles – such as protons and electrons - in the 85 AU region with the LECP instrument on Voyager 1 (S Krimigis et al. 2003 Nature 425 45). In mid 2002, they observed that the intensity of these particles increased by a factor of about 100 over a period of about 7 months. Moreover, they calculated that the solar wind decreased from supersonic (above 300 kilometres per second) to subsonic (less than 50 kilometres per second) speeds during this time.

However, Frank McDonald of the University of Maryland and colleagues argue that cosmic-ray measurements from another Voyager instrument show that Voyager has only reached a precursor region and not the actual termination shock itself (F McDonald et al. 2003 Nature 425 48). Their argument is based on the energy distribution of the cosmic rays and measurements of the magnetic field. “The absence of any significant increase in magnetic field strength means that we have not entered the shock area,” McDonald told PhysicsWeb. “We have seen the foothills but not the big mountain yet.”

Which explanation is correct? “I tend to agree with Krimigis and colleagues that their data can most readily be explained if the termination shock had been crossed,” writes Len Fisk of the University of Michigan in the issue of Nature in which the two papers appear. Another possibility is that the termination shock is moving and that Voyager 1 passed through it, only for the shock to overtake the spacecraft again. Both teams are now analyzing further data.