Monday, September 10, 2012

Has Voyager I Left the Solar System?

A burning question on some astronomy forums concerns the Voyager I spacecraft (see photo) and whether it has left the solar system. Only recently the JPL team behind this marvelous craft celebrated its 35th anniversary, all the more remarkable given its longevity and what it’s done: delivered some of the most amazing images of the planets before the arrival of the Hubble Telescope. (Voyager I reached Jupiter in 1979, and Saturn in 1980)

It is also notable for being the first space craft that played a lead role in a movie: this was in the film “Star Trek I” (“Star Trek: The Motion Picture") from 1979 which launched the famous 1960s science fiction series film run. In the film, the Voyager I appeared as “V-GER” an apparently alien entity that poses a challenge to the crew of Enterprise, and which they only surmount when Spock figures out it’s actually Voyager I – but with an alien ‘remake’ that confers a fairly nasty "disposition".

Though now 11.3 billion miles from Earth, and far beyond Pluto (which many take to be the ‘boundary’ of the solar system, the fact is that Voyager is still at least two years from actually being considered to have left the solar system. The reason has to do with its trajectory through the heliosphere, or the region associated with the Sun’s magnetic influence including the extent of its charged particle stream called “the solar wind”.

A particular boundary called the “heliopause” is the one of most interest in determining the departure of a craft under powered flight (and under gravitational influence). The heliopause is the interface where the solar wind is stopped by the interstellar medium. One can therefore think of it as a three dimensional region or surrounding “envelope” at which the solar wind's strength is no longer sufficient to overcome the stellar winds of the external stars. In technical terms, this absence of counter-pressure signals the end of the solar system.

From the computations of celestial mechanics, we expect Voyager 1 to cross the heliopause by 2014. The exact time of crossing should be indicated by a sharp drop in the temperature of charged particles, a change in the direction of the magnetic field, and an increase in cosmic rays (Voyager detect 9% enhancement in one month), following a more gradual increase of 25% from Jan. 2009 to Jan. 2012) All this suggested it was approaching the heliopause.

According to JPL Director Edward Stone, once this threshold is crossed Voyager I will become the first craft to “leave the bubble”.

This is remarkable! It means that despite the fact manned space missions have likely been abandoned forever (no money, and when money is available it'll likley be used for more weapons, wars) we have actually dispatched a craft that will have - after 37 years - departed from the solar "bubble" into the distant stellar firmament. Will actual aliens pick up on it, traks it and then track it back to us?

Who knows? But it's kind of an exciting prospect to consider....assuming those ETs are friendlies!

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