NARRATOR：

Now listen to part of a lecture on the topic you just read about.

MALE PROFESSOR：

Asteroid colonization is not a very practical idea.

Each of the points in the reading has a serious downside.

First, while low gravity on an asteroid would make landing and taking off relatively easy, low-gravity environments also present certain risks.

In a low-gravity environment, people start losing muscle mass and their bone density becomes lower.

Even astronauts who spend just a few months in spaceships, which are low-gravity environments, suffer from health problems like muscle and bone density loss.

Imagine the health problems that long-term colonists would experience on asteroids.

Second, the availability of valuable metals might make an asteroid colony seem like a profitable idea, but that’s not the whole picture.

You have to consider additional factors.

One thing is the costs: the costs of supporting a colony and of transporting the metals are likely to be high and will reduce the profits.And furthermore, there’s no guarantee that the price for which you can sell the metals will remain the same.

If precious metals are mined in large quantities, it would increase the supply of the metals, which could end up lowering their market price.

So mining on asteroids may not be very profitable.

Third, even if some asteroids are easy to reach, they may not be easy to return from.

Asteroid orbits—the paths on which asteroids travel through space—can be unusual.

Some orbital paths come close to Earth, but then move away from Earth—often a great distance away from Earth.

So even if an asteroid gets close to Earth at one point, making it easy for colonists to get to the asteroid, it does not stay close to Earth.

It can actually travel much farther away from Earth than a planet like Mars.

Getting back from an asteroid that travels that far would be a challenge.