People often talk about visitors from space landing on earth, but NASA announced recently that they have managed to do the reverse: a human-manufactured object has, for the first time, left the local solar realm. Voyager 1 is now sailing between the stars...
http://voyager.jpl.nasa.gov/
The Voyager 1 spacecraft was launched by NASA on September 5, 1977 from Cape Canaveral, Florida aboard a Titan-Centaur rocket (just to confuse you, Voyager 2 launched on August 20 of the same year aboard a similar rocket) (and I met the Emin in that year).
The date of launch was important, because all the outer planets of the solar system (the “gas giants”) were perfectly lined up for the two Voyagers to visit each one as they travelled away from the solar system: (in order) Jupiter, Saturn, Uranus and Neptune + 48 of their moons. It took almost 2 years for them to reach Jupiter (“are we nearly there, yet?”). These are the dates of closest approach to each planet:
The stats are mind-boggling. Voyager 1 is travelling at a speed of a million miles a day (about 3.6 AU per year) (1 AU = 1 Astronomical Unit = the distance from the Earth to the Sun). Voyager 2 is a little slower, at about 3.3 AU per year. They were not the first spacecraft to head towards outer space fast enough to escape the pull of the Sun; Pioneer 10 and 11 were the first (and second) to do that, but on February 17, 1998, Voyager 1 passed Pioneer 10 to become the most distant human-made object in space. NASA reckons that, on September 2013, Voyager 1 was at a distance of 18.7 billion kilometers (125.3 AU) from the Sun.
Carl Sagan included a gift for extra-terrestrials:
http://voyager.jpl.nasa.gov/spacecraft/goldenrec.html
Voyager 1 is famous for containing a “Phonograph Record” (something that your dad & me played before they invented CDs). It is a 12-inch gold-plated copper disk “containing sounds and images selected to portray the diversity of life and culture on Earth ... selected for NASA by a committee chaired by Carl Sagan of Cornell University ... assembled 115 images and a variety of natural sounds. To this they added musical selections from different cultures and eras, and spoken greetings from Earth-people in fifty-five languages.” It also has symbols drawn on the surface:
It will be 40,000 years before Voyager 1 gets close to any other planetary system. Carl Sagan called it “launching a bottle into the cosmic ocean”, which I think is a tremendous image.
So, how do NASA know that Voyager is finally between the stars?
...with difficulty, is the answer. One instrument designed to do this (a solar plasma measurement device) failed back in 1980, which did not help. The ancient (in technology terms) devices on board do not help either. The transmitter on board is just 22 Watts (the computer speakers in front of me are stronger than that!). The signal takes more than 17 hours to travel to Earth, and by the time it gets here has been degraded to a billion-billionth of a watt (“Ollie, will you turn that game down, I’m trying to listen to Voyager”).
Anyway, where does the ‘Solar System’ end?
The Sun pours out ‘plasma’ (fantastically hot, electrically & magnetically charged gas) continually, and this radiates out into space, getting colder & slower as it goes (because it is expanding all the time). This is the equivalent for the Sun of a planet’s atmosphere, and the whole thing is called a ‘heliosphere’. There is also some plasma & stuff in space - not much, because the space between the stars is so much enormously bigger than the distance between our local planets - but there is some. Eventually, there is a point where the pressure (and at this distance it is more electrical & magnetic pressure rather than physical pressure) of the interstellar space equalises the pressure of the solar space. That point--which is many millions of miles deep--is called the ‘heliopause’ (the switch-over point where the Solar Realm stops & the Interstellar Realm begins) (NASA calls it the ‘Heliosheath’).
NASA reckoned that both Voyagers entered that x-over some time ago:
Voyager 1 crossed the termination shock in December 2004 at about 94 AU from the Sun while Voyager 2 crossed it in August 2007 at about 84 AU. Both spacecraft are now exploring the Heliosheath. (Then): While the exact location of the Heliopause is not known, it has been estimated that Voyager could reach this entry into interstellar space 10 years after crossing the Termination Shock.
Argument at NASA has been intense recently. Finally, they have made their mind up, and had a Press Conference last Thursday (12 September 2013). They reckon that Voyager 1 is finally in foreign lands: “data from the Voyager probe’s instruments shows that the spacecraft is now out of the influence of our sun and has been navigating the space outside our Solar System since August 25, 2012”. And here are some crucial statements from NASA physicist Gary Zank:
The material in which Voyager finds itself is created by our neighbouring stars and supernova remnants, and so Voyager in some very real sense is in material that is not from the medium in which it (was created) — and so we’ve truly crossed over.
Voyager is now exploring into truly unknown lands. We can do the same by exploring inside — that, for us, are true, unknown realms (and thus a bit scary).
--------- Alex Kemp