An asymmetric solar wind termination shock

On 30 August 2007 Voyager 2 began to cross the termination shock, a boundary produced by the inter-action of the Sun with the rest of the Galaxy, where the supersonic solar wind abruptly slows as it presses outward against the surrounding interstellar matter. Five Letters in this issue present the data that the probe sent back. The Voyager 2 crossings occurred about 1.5 billion kilometres closer to the Sun than those of Voyager 1, illustrating the asymmetry of the heliosphere. The results from the plasma experiment, low-energy particle, cosmic ray, magnetic field and plasma-wave detectors reveal a complex and dynamic shock, reforming itself in hours rather than days. The cover graphic of Voayer 2 on the brink of entering interstellar space is by Henry Kline of JPL. It may be decades before another probe crosses the termination shock but remote observations can now bridge the gap — as shown by Wang et al. who report measurements of energetic neutral atoms in the heliosheath from the STEREO A and B spacecraft that complement the Voyager in situ observations made at the same time. In News & Views, J R Jokipii puts the Voyager findings into context. For more on the on Voyager odyssey, see page 24, and the Author page, and go to the movie on http://www.nature.com/nature/videoarchive/voyager.