Answer: (Do not Copy Word for Word, Text taken from Websites in Answer)
The primary argument calling for space colonization is the long-term survival of human civilization and terrestrial life. By developing alternative locations off Earth, the planet's species, including humans, could live on in the event of natural or human-made disasters on our own planet. As stated by the text, “It is conceivable that some great unexpected mass of matter should presently rush upon us out of space, whirl sun and planets aside like dead leaves before the breeze, and collide with, and utterly destroy every spark of life upon this earth...some new disease that will destroy not 10 or 15 or 20 percent of the earth's inhabitants as pestilences have done in the past, but 100 percent, and so, end our race. And finally there is the reasonable certainty that this sun of ours must someday radiate itself toward extinction.” It is very likely that all life on Earth would cease to exist due to mass extinction from a disease or radiation. In order to prevent this, space colonization would be necessary if we were to want to continue human life and other organisms.
