Pioneer 6 to 9 are a series of scientific probes launched 1965-1968 that orbit the Sun at about 0.75 to 1.2 AU. Would it still be possible to contact Pioneer 7 or 8?
This question asks a similar question about Pioneer 6. This answer muses about the possibility of contacting 7 or 8. The Wikipedia page for the spacecraft series states that attempts to contact Pioneer 9 in 1987 failed, so presumably there is no point in trying with that spacecraft again. But the last contacts with 6-8 (in the 1980s) were successful.
Not to be confused with Pioneer 10 and 11, which are inactive and on a trajectory out of the solar system.