On the contrary we already know how to geoengineer our planet but we are still working on interstellar travel.
In fact, we have already geoengineered our planet several times. The most obvious one is the increase in atmospheric CO2 and thus global warming caused by burning fossil fuels. This was an accident of course. Another example is the creation and then subsequent repair of the ozone hole by first releasing CFCs then stopping. Smaller examples include programs to seed clouds to prevent/encourage rain or even cloud formation. Beyond that I think we are pretty close (~50 years) to developing global weather pattern models accurate enough to predict the effect of various variables on rainfall, currents, jetstream, etc. It won't be long before we realize, for example, putting a desert in x location will cause y results. And things like creating deserts or forests or lakes? Yea, we can definitely do that already, we've just had very few good reasons to. If the political will existed we could probably start spraying sulfer into the upper atmosphere within the next 5 years which would result in a significant and quick (and cheap) temperature drop. If we really had to we probably are resourceful enough to be able to seed a watery planet with algae and bacteria that has been toughened in a space environment and will go on to start converting the planet's atmosphere to oxygen.
On the other hand, a star ship? That's a joke. Orion (the nuclear weapon propelled ship) is fraught with problems, not least of which is that 50% of the energy of each blast must be wasted by design. What material can withstand repeated nuclear explosions at close range without vaporizing, especially in vacuum? What kind of ultra magical structural material will be able to withstand and transmit the stresses of intermittent high-g acceleration? Will nukes even be enough to move such an enormous structure through space at close to light speed? It seems far more sensible to me to take all those uranium and plutonium cores, de-enrich them, and use them to create a huge number of fission reactors which would then fuel a fusion reactor which would drive the ship via heavy-ion plasma. That kind of technology is pretty far off, as is the kind of productivity that would make such a project even remotely affordable without bankrupting our planet. We spent $200 Billion in today's dollars getting a few people to the moon in a dinky can! I'm pretty sure it's gonna be really hard to send thousands of people at light speed across the dark expanse. I'm pretty sure geo engineering is a hell of a lot easier.