Thursday, 21 March 2013

Now, Voyager?

There's an interesting article on Bad Astronomy about whether or not the space probe Voyager 1 has left the solar system and become the first human-made artifact to enter interstellar space. As you'll see from the correction to the post, the answer's not entirely clear, mostly due to which definition of "interstellar space" you choose. "Interstellar space" could start:
a) at the point where the solar wind slows to a stop as its constituent particles collide with the particles that exist between the stars 
b) at the point where the sun's influence becomes so weak that the prevailing magnetic field changes direction.
Voyager seems to have reached a) but not b). NASA and JPL have apparently chosen reaching b) as their working definition of "leaving the solar system."

Other definitions are available. For example, as of now (ish), Voyager 1 is 123.6 astronomical units (AU) away from the sun (click here for a more precise estimate).

The Oort cloud (the hypothetical spherical cloud of icy bodies surrounding the sun, thought to be the place where long-period comets originate), could be up to 50,000 AU from the sun, or over four hundred times further away from the sun than Voyager 1 is now.

Nobody's ever observed the Oort cloud directly, but assuming that the theories are correct and it does exist, then surely a vast spherical shell of icy material, centred on the sun, is part of the solar system.

Or maybe the boundary lies even further out, at the point where the gravitational influence of the sun is no greater than that of the nearest star.

Nature seems so irreducibly nuanced and subtle that one neat cut-off point may be too much to ask of her. Don't let's ask for the moon. We have the stars (although maybe not just yet).


Meridian said...