Re: Will the probes Voyage, pioneer etc. become comet like

Comets are thought to orbit in a very large, diffuse, spherical halo around the solar system. They are weakly held by the sun's gravity, but still held. Most of them don't ever visit the inner solar system.

One idea which I like is that passing stars disturb the orbits of the things, and that's what knocks them into the inner solar system. Think of it this way: if you're out on the street or highway, walking too close to the traffic lane, and a big lorry goes by you tend to get knocked into the ditch! The cometary halo is well out in the traffic lane, and every few millenia a lorry goes by. Of course, some of the comets get run over or knocked further out into traffic, but we never see those.

Once the comets are knocked into the inner solar system, they can

• Crash into the Sun, or into a planet as happened with Jupiter a few years ago;
• Get slung around the Sun and back out into the halo, or even out of the Sun's neighborhood altogether;
• Get captured, by interaction with one or more of the larger planets, into a "close" orbit around the sun. A close orbit can bring the comet back as often as each 75 years (Comet Halley) or every two millenia (Comet Hale-Bopp) or even longer periods.

As to whether the Voyager and Pioneer probes will ever come back, the short and simple answer is NO. The Voyager probes were accelerated to solar escape velocity (that is, they were speeded up to the point where the sun could no longer hold them) by their encounters with Jupiter and Saturn. Voyager is still sending back good data from the outer solar system, and it's hoped that it will have reached true interstellar space before its power runs out in about 2020; if so, it will be the first on-site information we have ever had from outside the solar system.

It's possible that one of the probes will be picked up by another civilization (or even by us; they're not moving all that quickly), but not at all likely. Consider how small the probes are compared to the space between the stars, and you will see that they're unlikely to come within a light-year of another star before the end of the universe.

"O Lord, your sea is so large and my boat is so small."

               Dan Berger
               Bluffton College
               http://cs.bluffton.edu/~berger/