Not at all! You're still thinking "in the box". You can divert a comet's path with a relatively miniscule amount of thrust, as long as you do it far enough away. A change of a fraction of a degree out in the Oort cloud would completely alter the path of a comet, and be relatively simple to do in terms of energy expenditure (you could do it with a conventional chemical engine, or even using the comet's own volatiles as reaction mass, if you wanted to get fancy). Calculating the exact change needed is the tricky part, and it's not THAT tricky.
In terms of melting the Martian polar ice caps, that's even simpler. Seed them with a genetically engineered plant that can thrive in the cold sparse atmosphere, but which (here's the key) is dark. You don't need to make that many, because they'll spread on their own (think ivy or kudzu). You lower the albedo of the poles, they retain more of the energy they receive from the sun, and they begin to melt. Soon, the amount of CO2 in the atmosphere is large enough to get a greenhouse effect going, and the process accellerates.
I'm not saying it would be a cake-walk, or quick, but it's certainly not a prospect that can legitimately be dismissed out of hand. Check out The Mars Society for a group of folks who have thought this stuff out a lot more than I have.
Transhumanist, thank you for pointing out those FACTS.
I also will point out that energy is an engineering problem and not an insurmountable one, especially when dealing with the environments we are talking about. The solar system has an almost limitless supply of energy in several forms.
As you pointed out comets contain volatiles for fuel production and also the gas giants. Solar power is available in three forms, the light itself, the solar wind, and heat. H3 is available on our moon for fusion power and who knows how many other planets or moons.
Carlr if the problem is of a religious predisposition I should inform you that I am both a Christian and my father is a Minister. I have found no wording in the Revelation that would preclude human life from exiting this rock. I in fact think it is in keeping with God's original and first message to us "Be fruitful and multiply".
If in fact your argument is not along the religious impute, than I sincerely apologize and ask you to explain yourself more fully.
Thank you.
The other point you make about a plant that can live in a cold,dark environment requires a plant that does not exist.If it is a water/carbon based life it will freeze and die.You would have to "create"new matter in the form of new genes that would allow a plant with characteristics opposite any that have ever populated the Earth.
No matter how you slice it,life on Earth can only be logically explained as the result of an intelligent design.
Just contemplate if you will all the factors that have to be in place for life to exist as we know it.
Not just exist but to do it in a practical,efficient manner.