An influx of water on Venus What if a moon like triton or a frozen extraterrestrial body containing lots of frozen ice were set on a collision course with Venus.  Would an inlfux of huge amounts of H2O precipitate a possible terraforming of venus in such a case?
This is a tricky question because a lot of pros and cons come into play. Terraforming isn't a &quot;yes or no&quot; answer as much as it's a pros vs cons answer and as you'll see.  This answer might be too broad for this site, but that's for the powers that be to decide.
The short answer is mostly no.   There's several reasons why a large impact wouldn't be ideal and probably, adding water to Venus hurts more than it helps because water is a strong greenhouse gas.
The longer answer:
Any collision of that size is problematic.   It doesn't take that large an object to kill nearly all life on Earth or set the oceans boiling.   Just 20 or 30 miles across might be big enough for that.   A moon over a thousand miles across would vaporize the surface of Venus and create enormous blowback, losing a lot of the material into space that you wanted to add.   An impact that large might create a moon around Venus - and that in and of itself might not be so bad, unless the moon's orbit was unstable and then you could have a inner solar-system rogue moon that might destroy Earth one day.
An impact of the size of Triton on Venus could also create a lot of debris that might threaten Earth with multiple dangerous near Earth objects that would need to be tracked and perhaps deflected.   So, it's not a good idea to hit Venus with something extreemly massive.
Moving Moons is also hugely difficult.   Moons with the amount of water you're considering are enormously massive and energy intensive to move and worse, Moons are in their planet's gravity wells, so you have two gravity wells to work against.   You know the old joke, I used to walk 10 miles to school, uphill both ways.  Gravity wells actually are like that - uphill in both the towards the sun and away from the sun.   Sending something into Venus requires a lot of acceleration.
A quirk about gravity wells is that, far away is much easier.   It's much easier to push a distant Kuiper belt object with a highly eccentric orbit into Venus than it is to push a much closer asteroid belt object or gas giant planet's Moon into Venus.  Probably orders of magnitude easier.
It's still an enormous amount of energy to move anything that massive, but a distant, eccentric orbit Kuiper belt object would be the place to start.
Also, many small impacts is almost certainly better than 1 large impact in terms of retaining the materials.  Unless your goal is to blow away some of Venus' atmosphere, which might not be a bad idea, but lets save that for another question.
Part II.  Just add Water.
Venus needs water, but it probably needs other things first.   It's too hot and water on Venus would evaporate and become a greenhouse gas.   Adding that much water to Venus would only increase it's runaway greenhouse effect and that's the opposite of what you want.
An additional problem is that the icy bodies you mention also have CO2 and adding more CO2 to Venus would only increase it's problem of too much CO2.
Venus needs to be colder.  That's step one.   It's vast amount of atmospheric CO2 needs to be reduced.  That's also step one.   While water is necessary for photosynthesis, there's no reliable way to make photosynthesis happen on Venus unless it gets colder first, or the photosynthesis is done in air-ship but that's probably too little, too slow and too expensive.
Venus is also quite dark on it's surface due to constant cloud cover so artificial light would probably be required too.  If you could get photosynthesis going on Venus on a large scale, then adding lots of water would help, but that's a pretty big if.  Water in tandem with photosynthesis would be a nice first step.  Water by itself would do very little and probably make things worse.
Another problem, though it might be slow enough not to matter when you're talking oceans of water (and Triton has perhaps 5-10 times as much water as all the oceans on Earth), so this might not be an issue at all in your scenario, but Venus, like Mars, loses water into space.  It's gravity is too weak for it's temperature and lighter molecules like Water, Methane and Ammonia escape from Venus into space and that would be true even if you gave Venus a strong magnetic field.  It's a product of temperature and escape velocity, called Jeans escape.
There are other theoretical methods.
Deliver a hundred million billion tons of calcium to Venus' surface or mine it from Venus' crust might work and let the calcium absorb the CO2.  Delivering that much calcium would be next to impossible and mining on Venus would be enormously difficult due to it's very high temperature, so neither of those are easy.
Most rocky planets have plenty of Oxygen stored in the rocks that make up their crust and Venus already has plenty of Nitrogen. What Venus really lacks is hydrogen, so any hydrogen rich elements might be useful in the long run, but the first step is some kind of process to reduce the CO2 and get the planet to begin to cool down.  Maybe a sun-shield but one that size would be a very expensive project and very far outside of our ability right now.
Removing what's basically an ocean's mass of CO2 is very difficult.  CO2 is a very stable molecule and not easily removed from an atmosphere.