It’s the great cosmic game of “are we there yet”?
The Voyager 1 spacecraft has almost left the Solar System — but not quite, according to a set of papers published online in Science on 27 June.
Voyager 1, launched in 1977, is now 18.6 billion kilometres from the Sun and getting farther away every day. In 2004, it crossed from the part of space dominated by the Sun’s influence into a transition region where the solar wind mixes turbulently with interstellar gas. Space physicists are now eagerly waiting for Voyager 1 to exit this transition region and enter true interstellar space.
But the change hasn’t been as clear-cut as some might hope. The Science papers describe a peculiar couple of weeks during summer 2012, when the probe experienced some kind of shift — but not the final exit from the Solar System.
The first hint came on 28 July 2012. The number of charged particles flooding from the Sun dropped suddenly and dramatically — only to recover a few days later. For nearly a month, Voyager 1 saw particle counts rise and fall and finally stabilize for good on 25 August.
As the ion counts were dropping, the magnetic field strength was rising. In late July it suddenly nearly tripled, reaching the highest level seen since 2004. The magnetic field, too, fluctuated for weeks before finally settling down in late August. The probe must have crossed an ill-defined boundary into a slightly different region of space, Voyager scientists write. They dub it the ‘heliospheric depletion region’.
The Voyager scientists announced the basics of this change at a conference last December, but the Science papers provide a far more detailed look — as well as hint at other changes seen by the probe. In late March, for instance, Voyager 1 saw a rise in proton intensity that probably traces back to solar flares that erupted earlier that month. It is the farthest that anyone has ever measured the influence of a solar eruption.
The abruptness of the heliospheric depletion region came as something of a shock. Earlier modelling work had suggested that the transition into interstellar space might be accompanied by gradual changes in cosmic rays or the magnetic field.
Voyager 1 and its twin, Voyager 2, explored the outer Solar System in the 1970s and 1980s. Voyager 2, which detoured past Uranus and Neptune, is not as distant from the Sun as Voyager 1 is.
Although the particle measurements suggest that Voyager 1 is seeing much more influence from interstellar space than from the Sun, one big change remains to be seen. Scientists expect the direction of the magnetic field to shift when the probe reaches true interstellar space. Project scientist Ed Stone, of the California Institute of Technology in Pasadena, who Nature profiled last month, thinks that could happen any day now. He has persuaded the Deep Space Network to devote up to 10 hours a day to tracking the faint signal from Voyager 1 so as not to miss the final exit.
You can follow its current cosmic-ray measurements, updated every six hours, at the project’s home page.