Suggest we have fewer children for a few generations and consume less and it's absolutely impossible, against human nature. The One Child policy proves it.
We idolize people who change the world as entrepreneurs but say one person can't make a difference in changing culture.
Yet many cultures have chosen to lower birthrates through peaceful, voluntary means resulting in prosperity and stability. On a personal level, I've lowered my footprint by about 90 percent in a few years according to online calculators and every change improved my life. I figure most westerners could drop 75 - 80 percent with just the low-hanging fruit, improving their lives -- buying less junk, wasting less food, living closer to family, camping instead of visiting Timbuktu.
Changing course is a matter of belief and choice far more than technology. While it's not monolithic, this community seems to choose to believe living sustainably is harder than space travel. I'd argue space travel is more attainable when we aren't pushing the Earth's limits to sustain life and human society.