4 of the 5 spacecraft listed in the answer to this question have a Right Ascension of 18.5 hours, plus or minus 1.4 hours. That seems a bit of a coincidence. Is there a reason why spacecraft leave the Solar System with such a Right Ascension?
It's a coincidence caused by the position of the planets they visited and the moment they visited them. Pioneer 10 only visited Jupiter so it could be launched any time they wanted. The other 3 had to have a reasonably close conjunction of at least Jupiter and Saturn.
In the Voyager timeframe, there was a rare 5-planet lineup of Jupiter, Saturn, Uranus, Neptune and Pluto. Everything launched to take advantage of that, had to go in the same general direction.
- Voyager 1 passed Saturn on November 12, 1980
- Pioneer 11 passed Saturn on September 1, 1979
- Voyager 2 passed Saturn on August 25, 1981
That's less than 2 years, or 1/15 of Saturn's orbital period. So they were still quite close at that point.
Pluto, being the slowest of them all, is still in the same general area 30 years later. Jupiter, being the fastest of the group, was on the other side of the solar system just a few years earlier.
This graph shows their trajectories:
The image doesn't tell the whole story: what you don't see is that all 4 probes have different inclinations.
- Pioneer 10 is on the ecliptic plane
- Pioneer 11 is slightly above it
- Voyager 1 is far above
- Voyager 2 is far below, due to its Triton flyby at Neptune