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Voyager I Finally Did It!

Voyager I Finally Did It!


Launched in 1977, the two Voyager probes have seen some strange and unexpected sights as they cruised through our solar system. After passing the planets, the probes have continued outward towards the farthest reaches of the solar system. In the summer of 2012, Voyager I was now 18.2 billion km away from us, more than three times the distance between the sun and Pluto. The solar system however by definition consists not only of the planets, but of the volume in space to which the sun’s particles extend, or in other words the volume in space which is impacted by the sun. The question everybody was asking, was how long would it take Voyager I to leave the solar system and enter interstellar space? And what would Voyager find when it got to interstellar space?

In July and August 2012, Voyager recorded a change in the solar wind (outward flow of charged particles). The speed of motion of the particles fell to zero and at the same time the energy content in the particles increased. Such changes might suggest that Voyager I was now in interstellar space, some scientists declared. However these observations were not what physicists expected to find at the solar system/interstellar space boundary. The drop in speed of the charged particles was “totally and completely unanticipated” (Nature May 23/13 p. 427) Scientists also expected that charged particles would come from many directions in space once interstellar space was entered rather than just one direction (as from the sun). However this has not been observed. The magnetic field showed no change in direction. A technical article on these observations (Nature September 6/12) concluded that the expectations as to what interstellar space is like, may have to be reassessed: “perhaps necessitating a new theoretical formulation of the interaction of the solar wind with the local interstellar medium.” (p. 126)

As late as March 2013, the Voyager project director Edward Stone, contradicted a statement by the American Geophysical Union that Voyager I had left the solar system. Not long after, on September 12/13, a formal statement was published by mission scientists that Voyager I had indeed left the solar system on August 25/12 as previously suspected. Apparently the probe’s instruments revealed that Voyager was now surrounded by charged particles much different from those coming from the sun. On this basis scientists decreed that Voyager I had indeed left the solar system. Scientists’ expectation that the direction of the particles should change however still has not been fulfilled and scientists do not know why. Possibly their ideas about interstellar space need major revision. Meanwhile engineers estimate that Voyager I has about six more years of fuel left, plenty of time, we hope, for more exciting observations.

Margaret Helder
February 2015

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