鈥淰OYAGER 1 has entered the final lap of the race to interstellar space,鈥� exults Edward Stone. And this time he is sure that the spacecraft he helped launch nearly 30 years ago has finally reached the very fringes of our solar system.
Voyager 1 was supposed to have reached this milestone nearly three years ago but the evidence was inconclusive. Now it has officially crossed the 鈥渢ermination shock鈥�, the region where the speed of the solar wind drops abruptly from supersonic to subsonic, and has entered the shell of dense solar wind called the heliosheath that separates our solar system from interstellar space. 鈥淭his is big news,鈥� says Eric Christian, the programme leader for Voyager 1 at NASA鈥檚 headquarters in Washington DC.
The Voyager twins, 1 and 2, were launched in 1977 to explore the outer planets. In 1998, Voyager 1 became the most distant spacecraft, overtaking the Jupiter probe Pioneer 10. Then, in November 2003, Stamatios Krimigis of Johns Hopkins University in Laurel, Maryland, reported that Voyager 1 had gone past the termination shock sometime in August 2002 (快猫短视频, 8 November 2003, p 12). At the time, the spacecraft was 85 astronomical units from the sun.
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He still stands by his claim, which was based on estimates of the solar wind speed. But many scientists disagreed because Voyager 1 had not seen a concomitant and significant increase in the sun鈥檚 magnetic field, a sign that solar particles are slowing down and bunching together at the termination shock. 鈥淚 do not agree with Krimigis,鈥� says Donald Gurnett of the University of Iowa in Iowa City, a member of the Voyager 1 team. 鈥淲e should have seen the magnetic field increase and we did not.鈥�
Now, Voyager鈥檚 magnetometer has spotted a telltale two-and-a-half-fold increase in the magnetic field. At the same time as the magnetic field soared, Voyager 1鈥檚 cosmic ray detector (CRD) saw an increase in galactic cosmic rays 鈥� highly energetic charged particles from distant supernovae. This ruled out the possibility that solar flares were responsible for the increase in the magnetic field detected by Voyager 1. Had the craft merely encountered a shock front generated by a solar flare, the galactic cosmic rays would have decreased, says Alan Cummings of the California Institute of Technology in Pasadena, who analyses data from the CRD.
Further confirmation of Voyager 1鈥檚 position came from the orientation of the lower energy cosmic rays hitting its detectors. Outside the termination shock, the turbulent and strong solar magnetic field should toss these cosmic rays around, making it seem like they are coming from all over rather than from specific directions. This is exactly what Voyager 1 saw, says Cummings.
All this happened in the middle of December 2004, when Voyager 1 was about 94 AU from the sun. 鈥淭his time the entire Voyager 1 team agrees we have crossed the termination shock,鈥� says Stone of Caltech. He presented the data at an American Geophysical Union meeting in New Orleans this week.
But there is one piece of data that still puzzles the Voyager 1 team. It concerns the so-called anomalous cosmic rays (ACR), that enter the solar system as neutral particles from interstellar space but get ionised by photons from the sun or by the solar wind and are accelerated at the termination shock. According to earlier predictions, Voyager 1 should have seen dramatically higher numbers of high-energy ACRs as it crossed the termination shock, but did it not.
Stone and Cummings suspect that these high-energy ACRs are only created in the more turbulent regions of the shock front, rather than all along it as had been previously thought. 鈥淲e think ACRs are still coming from the termination shock, but from a different part than Voyager came through,鈥� says Cummings. Where exactly the ACRs are created remains a mystery.
鈥淎t the same time as the magnetic field soared, Voyager鈥檚 detectors saw an increase in galactic cosmic rays鈥�
Nonetheless, researchers are excited that Voyager 1 could reach interstellar space before it runs out of fuel.
