TPO29听力第6篇，自己做的听力稿，供T友参考

TPO29-6
Listening to a lecture of a structural engineering class

:Today, let’s begin to locate our structural engineering in space age（太空时代）. New problems, new possibilities mean we can be thinking new ways to find radically different approaches. So, let’s consider……, well, what would you say is the biggest obstacle today of putting structures, equipments, people, anything really, into space?
: Well, the cost, right?
: Exactly. I mean just taking space shuttle up and back one time is hugely expensive. En, why?
: I guess a lot of its fuel right? ...To get the rocket going fast enough?
: Ok, fast enough to…？
: To escape earth gravity.
: Good. So we are burning up an enormous amount of fuel at every launch just to get the rockets up to what’s we’ve known “escape velocity”. Now, the escape velocity is around 11 kilometers a second, pretty fast! But, do we have to go this fast?
: Yeah, I mean how else can you escape? I mean that’s the whole point of escape velocity right? Otherwise, gravity would pull you back down to the earth.
: Actually, that’s the common misconception. Escape velocity is simply the speed of an object that’s, let’s say, a shot out of a cannon, the minimum initial speed so that an object could later escape earth gravity on its own. But that’s just there is no additional force being applied. If you keep on supplying force to the object, keep on pushing it upwards, it could pull away from earth gravity at any speed.
: Even really slow? So you are saying like if you have a ladder tall enough, you can just climb into space?
: Yes. Well, theoretically. I mean I can see some practical problems with the ladder example, like you might be a little bit tired out at the first a few kilometers or so, especially involved the oxygen tank you should be holding up with you.
: No, I am thinking more along the lines of an elevator.
: Wait, you serious?
: Sure, an elevator. That’s a new idea for most of us, but it’s been around for a century. If we could power such an elevator with solar energy, we could simply rise it up into space for a fraction of cost of the trip by rockets or shuttle.
:But wait, elevator don’t just rise up. They have to hang up some kind of wire or tracker or something.
: True. And for decades, that’s exactly what’s prevented the idea from being feasible or even just taken seriously. Where we do find materials strong enough, yet light weighed enough to act as a cable or track. I mean we are talking about 36,000 kilometers here, and the strain on the cable would be more than most materials could bear, but a new material developed recently has a ten-thousand strain higher than diamond yet it’s more flexible. I’m talking about Carbon Nanotubes.
: Ok. I’ve read something about Nanotubes. They are strong alright, but aren’t they just very short little cylinders in shape?
:Yes, but these cylinders cling together at the molecular level. You pull out one nanotube or a roll of nanotubes, and its neighbors come with it, and their neighbors and so on. You can actually draw out a 36,000 kilometers strand or ribbon of nanotubes stronger than steel yet maybe a thousand of thickness of human hair.
: Ok fine, but what’s gonna hold this ribbon up and keep it rigid enough to support an elevator car?
:Well, we definitely have to anchor it at both ends. So what we need is a really tall tower here on the ground, right up at the equator and a satellite in a geostationary orbit around the earth. There is a reason I mentioned the figure 36,000 kilometers, that’s about how high an object would have to be, orbiting straight up from the equator, to constantly remain directly above the exact same spot on the rotating planetary earth. So once you are in this geostationary orbit, right over the tower, just lower your carbon nanotube cable down from the satellite, tap the tower here on the earth and then there you have it!
: So you really think this is a possibility. Like how soon could it happen?
:well the science fiction writer #### talked about building a space elevator back in 1970s, and when someone asked him when he thought this idea might become to reality and his reply was probably about 50 years, after everybody quits laughing.