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   Stephen Hawking, one of the world's most famous scientists, believed that humans will one day have to move into space, "Once we step into space, our future should be safe," he said.

Today, many countries are all planning to send astronauts to Earth's closest neighbor: the moon. Some of these countries want to create space stations there within the next 10 years. These stations will prepare humans to visit and later live on Mars or other earthlike planets.

__①__ Robert Zubrin, a rocket scientist, thinks humans should colonize(殖民) space. He wants to start with Mars. Why? He thinks sending people to Mars will allow us to learn a lot of things—for example, the ability of humans to live in a very different environment. Besides, any progress we make in the fields of science, technology, medicine. and health will do good to people here on Earth.

__②__ Elon Musk, the CEO of Space X also believes we should colonize Mars. His long-term goal is to put one million people there if something bad happens to us here on Earth.

__③__ Many people say it's too expensive. Also, most space trips are not short. A one-way trip to Mars, for example, would take at least six months. People traveling this kind of distance could face many health problems. Besides, these first people would find life especially difficult in space. On the moon's surface, for example, the sun's rays are very dangerous. People would have to stay indoors most of the time.

__④__ No matter what different concerns there are, sending people into space seems certain. In the future, we might see cities on the moon or even new human cultures on other planets. First stop: the moon.