First, some background.
For the sake of argument, since as far as I know from Canon, there are no limits on the distance a portkey can take you, if you knew from Muggle sources what space was like, could you devise a way to portkey to another solar body?
For example, let's look at Neptune's system. The only activity there was basically what we learned on the Voyager 2 flyby in 1989, and since, there hasn't been any more Muggle exploration, and neither are there plans to.
For a particularly enterprising Witch or Wizard, what's stopping them from taking a portkey out to Triton or Nereid? Presumably there are enough applicable charms to keep the explorers safe (the bubblehead and warming charms could conceivably allow them to breathe and stay comfortable in the hostile environments, and I assume there are spells or charms to allow physical constructs for protection and/or travel - transfiguration, for example).
In Canon, the requirements for international portkeys are the consent of the authorised person or people within the governing bodies of both the source and destination (example: France and England in the case of Arthur meeting Bill and Fleur). However, in Quiddich Through The Ages, there is a note about referee tampering in which at least one unlucky sap had his broom turned into a portkey which dumped him in the middle of the Sahara. However, going off-planet involves going somewhere without a (human) governing body to authorize incoming travellers, so I assume the same rules wouldn't apply, though I imagine there would still be some bureaucratic involvement from the local authorities prior to departure.
So from a strictly theoretical point or view, is it plausible? Could it be done? And if so, why hasn't it?
Having read the other "Wizards In Space" questions, I don't see the answers I'm looking for. The other questions don't address the plausibility of using spells and portkeys for extra-planetary exploration and instead only rule out apparition and brooms.
For the sake of argument, would it also be (theoretically) possible to add an extra-planetary location to the Floo Net (once you get there, that is, presumably by portkey)?