Moonlight drive

    The data from the ageing Voyager probes are illuminating the edge of the Solar System.

    Someone in the NASA media-relations office knows their music. A press release from the agency last month stated that the twin Voyager spacecraft were poised to Break on Through to the Other Side — referring to the probes' approach to the edge of the Solar System, but also to a 1967 hit from the US band The Doors. NASA pointed out to journalists that the missions were launched 35 years ago and was no doubt hoping for some (more) positive coverage to mark the anniversary. What's more, on 13 August, Voyager 2 became the longest-operating spacecraft, beating the record of Pioneer 6, which was launched in December 1965 and returned its final signal some 12,758 days later. (Voyager 2, counterintuitively, was launched two weeks before Voyager 1, but the latter is now the farthest from the Sun.)

    The spin doctors can be excused this time. Voyager is a truly great mission, and one that reporters still find hard to resist — some of them have been happily writing about its discoveries ever since the two craft launched in 1977. It is the science story that keeps on giving: the deep, hazy atmosphere of Saturn's moon Titan; the volcanoes of Jupiter's moon Io; the large, unusual magnetic field of Uranus; and the geysers of Triton, the frozen world that orbits Neptune — all discovered and lapped up by an eager public as the probes skimmed past the outer planets.

    Still, their work is not done. Even though the probes are now more than 15 billion kilometres away from the Sun, their handlers on the ground remain in near-daily contact, as the spacecraft continue to send back useful information — now about the farthest reaches of the Solar System. Last year, NASA even coaxed the ageing and radiation-blasted parts of Voyager 1 into performing a series of rolls to have a proper look around. It was curious because some of the data being sent back from the spacecraft seemed to suggest that the edge of the Solar System was nearby. Levels of high-energy cosmic rays, which originate far beyond our corner of space, had spiked. And the number of lower-energy particles that come from closer to home seemed to dip.

    The results of the latest tests, which are published on page 124, have surprised many. If Voyager 1 truly is near the point where the heliosphere — the bubble of charged particles from the Sun — fades to interstellar grey, then it should have found solar particles that have been buffeted by the winds of deep space, generated by supernovae that exploded long ago elsewhere in the Galaxy. In fact, the particles it found had effectively been becalmed.

    The implications of the discovery for our understanding of the structure of the Solar System, and how it changes as it whizzes through space, are profound. As a News story on page 20 explains, the find could mean that astronomers will have to rethink their models of the heliopause, the boundary at which the outward pressure of the heliosphere is balanced by the inward push of outer space. Or it could mean that Voyager 1 is still some distance from the heliopause.

    That would no doubt disappoint the NASA press office, which is eager to announce that at least one probe has entered a new realm of discovery — and before the batteries of the spacecraft run out, in a decade or so. But it should not lose heart. Like the Voyager probes, The Doors are still going, albeit not as strongly and with their best work probably behind them. If the heliopause is farther away than we thought, and the reach of the solar wind longer than we realized, then the Voyager twins still have many years remaining as Riders on the Storm, and some way to go before they reach The End.

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    Moonlight drive. Nature 489, 6 (2012). https://doi.org/10.1038/489006a

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