The passage lists three advantages of establishing colonies on asteroids in our solar system.However, the professor doesnt agree with these opinions and he gives us three reasons in response.
First, the passages idea is that due to the low gravity on the asteroids,spaceships can land safer.Also, this special environment will allow a spacecraft to carry more equipment and take off with less fuel.But, the professor points out that low gravity will bring health risks to astronauts.They will suffer because of mass loss and low bone density.Their muscle wont function as well as on Earth.So,with serious health problems, it is unrealistic to establish colonies and live there in the long term.
Second,when it comes to mining valuable metals on asteroids to cover the expense of traveling in the universe.However, it doesnt take the cost of transportation and supports for the colones into consideration, for which this traveling is unworthy.Also,there is no guarantee that the price will stay stable,because increasing supply of valuable metal will lower the market price.Thus,it may seem to be less profitable than we may think.
Third,there is a serious question about whether it is the same easy to return Earth as to reach asteroids.The professor says that these asteroids will move away and they are only close to Earth at a certain point.It is possibly that these astronauts have to travel far more away to return Earth ,compared with getting back from Mars.