Why is Mars considered so inhospitable

The idea of sending people to Mars in hopes of colonizing the planet has long been out of the realm of possibilities but big thinkers like Elon Musk are determined to keep trying (link).

From what you have learned from this class:

• Why is Mars considered so inhospitable?
• What would be needed to ensure humans could survive on Mars? In particular, where would we get our food, our oxygen, our water and our energy?
• If you were tasked with sending people to Mars, give a time-line of how they would get there, what would they bring, how long would they stay and what advice you would give anyone who wanted to go?