My question is relatively simple in the essence that it takes few words to ask, but scientifically may be a nightmare. In 2000, the Goldstone Deep Space Communications Complex sent a message to Pioneer 6, a solar observation probe somewhere out there in space, and successfully communicated with it for a few hours. The question is this: Is there any feasible way of communicating with 6 if one acquired the right antenna and sent the right commands to it? If so, how would one do it, or even find out how to do it?
The main obstacles:
- you need access to a large antenna. There are a few large dishes (25 m) operated by amateurs. The ISEE-3 effort was given time on the (even larger) Arecibo dish.
- you need to know the communications protocol. For ISEE, NASA assisted by giving information and hardware (a custom transmitter, IIRC). As of 2000, NASA still knew how to contact Pioneer 6, so chances are they'll be able to tell you everything you need to know.
- you need to know where the spacecraft is. JPL Horizons only has ephemerides until 1999, it seems. I don't know if NASA has better information somewhere else.
- the probe still has to work, obviously. The ISEE example shows it's possible, and these old designs should be pretty robust (no microelectronics to go haywire from a single cosmic ray strike, for example)
Pioneer 6 is in a heliocentric orbit with a period of 311 days. Periapsis 0.81 AU, Apoapsis 0.98 AU. It had a close approach to Earth in 1988. Every ~year it'll pass us at a distance of less than 0.2 AU, your best timeframe for contact is just before and after closest approach (not during, as it'll be on a straight line between us and the Sun at that time).
Pioneer 6 was started 1965. The spacecraft was constructed for an operation of 6 month only. The last communication was 35 years after the start. The degrading of the solar cells reduces the available power for communication. The chance is very small that everything of the electronics needed for a communication is still functional.