Answer:
There are so many things that make human travel difficult.
Here are some ideas ;
1) Air and Water -Taking water and oxygen from the Earth is very expensive and very difficult. (Note that if there is lots of water, oxygen can be made from the water. Also, if there is carbon dioxide, oxygen can be made.)
2) Food - people will need to grow their own food on another planet - transportation to and from Earth is, again, too expensive. Plants need carbon dioxide and sunlight (real sunlight is best; artificial lighting will work, too.)
3) Energy - Human settlements will need to create their own energy. Energy can be created using the sun.
Note that these powers have their own advantages and disadvantages
4) Radiation - Radiation is harmful for people, plants and animals. On Earth, we are protected by a magnetic field and an atmosphere. Some planets and moons have neither; some have only one or the other. Very few have both. Some planets have very strong magnetic fields that create even more radiation. Crazy right .
5) Gravity - planets or moons with a lesser gravitational pull than the Earth will result in astronauts having smaller bone and muscle mass, which is a serious problem if they are to come back to Earth. The closer a planet's gravity is to Earth, the less of a problem this is.
6) Atmosphere - Some planets have no atmosphere while some have atmospheres that will crush people(Woah ) .It's easier to deal with low-pressure rather than high-pressure, but both are dangerous if there is an air leak.
7) Distance - the further a planet is from Earth, the harder it is to.
8) Temperature - The hotter or colder a planet is, the more energy it will take to change the temperature(and im not really sure how ).
Explanation:
