Where's the spacecraft's origin? a planet's (Earth's?) surface? Then it first needs to get into orbit (I think it's 4.72 lbs of fuel/oxidizer per pound of cargo using current technology?). Once in orbit it needs to handle the planetary orbital debris. Then it'll leave the Van Allen belt, which protects against radiation, which makes the metal brittle. If it passes by Jupiter to pick up speed, it'll be exposed to the second highest source of x-rays in the solar system. Any spacecraft powerful enough to travel to another star system is not going to try using a Hohmann Transfer method...

It's not a given that a space craft will travel through the asteroid belt- it could easily pass at an angle to the solar plane, bypassing the asteroid belt. The next part is getting through the terminal shock and Heliopause. Apparently this is rather difficult for the voyager spacecraft. Not sure why, but apparently the voyager spacecraft has been bumping up against it for years. After that, we're effectively in "Interstellar Space", but we still need to get through the Oort cloud, which spans ~1/2 the distance to alpha centauri. Once in this region of space you're subjected to even more radiation than craft inside the solar system.

Depending on the type of transportation used, that matters as well. Current Warp theory states that if it were possible to travel at near-light speeds (or faster if you're into the whole Alcubierre drive and its improvements over the years), an accumulation of particles collects at the front of the ship and, bombarded with higher energy over the course of the distance travelled, become highly energetic. A sudden stop would impart momentum to the ionized particles at the front, effectively turning them into a bullet. So you get ALL the way to the planet, stop, and then potentially blow it up by shooting the shit out of it with a massive ion cannon, propelled by the energy picked up by your ship (turning around would have the same effect on earth).

.... I just realized you're talking only about neptune. Fuck it... I'm not deleting all that.

I will say, though, that Jupiter puts out a fuckton of xrays, and neptune is #3 xray producer in the solar system IIRC. So a spacewalk is probably out of the question around jupiter or neptune.

Because of the distance, solar power is likely not a good option for power, so some internal power source would be required.