Madsen Pirie

There is some debate among scientists as to whether the space probe Voyager 1 has now left the solar system and is in interstellar space.  It comes down to definitions and where lines are drawn.  There is no point like Checkpoint Charlie in divided Berlin where it used to tell us “You are now leaving the American Sector.”  Just as Earth’s gravity never ceases to exert influence, so the sun’s influence persists beyond the solar system.  We speak of the heliopause where the number of cosmic rays increases while the intensity of energetic particles from the sun declines.

“A big change occurred on 25 August last year, which the GRL paper’s authors say was like a ‘heliocliff.’  Within just a few days, the heliospheric intensity of trapped radiation decreased, and the cosmic ray intensity went up as you would expect if it exited the heliosphere.”

The probe, launched with its twin to study the outer planets, gave us wonderful new insights into Jupiter, Saturn, Uranus and Neptune.  Now it is 18bn km from Earth, and to all intents and purposes in interstellar space.  Its batteries, powered by plutonium, will last perhaps 10-15 years before its instruments shut down.

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