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@zazoobred
Voyager 1 is moving away from the Sun at 17 km/s.
Earth moves around the Sun at ~29.78 km/s.
Currently, earth's location around the Sun is such that it is moving towards Voyager 1.
So, the distance between Earth and Voyager 1 is decreasing at slightly less than (29.78 - 17) = 12.78 km/s. Slightly less because Voyager 1 lies above the ecliptic and Earth is not moving at 28.78 km/s directly towards Voyager 1.
So, the RTT is decreasing by about 3-4 seconds per 12 hours.