Some features for the mission:
- total mass < 3.1t (or < 2.1t as per one slide below, could be a typo?), 270kg of scientific payload (including 150kg atmospheric entry probe)
- two 300W RTG for power supply, 380W at mission end
- 3.75m HGA and X/Ka band downlink, data rate ≥ 32kbps
- launched directly into Earth-Neptune transfer orbit, with departure C3 ≥ 158 km^2/s^2 and 15 years of transfer
- annual launch window, primary in 2033
- Triton gravity assist to reduce capture ΔV, finally enter Triton 2:1 resonant orbit
The slide about the transfer trajectory shows a propulsion stage of nearly 50,000kg. This suggests that the launch vehicle is CZ-9, which has the exact TLI capability.
Source to the attached pics