A. To help students understand what is required to launch a satellite
B. To describe new materials now being used to explore space
C. To describe a potential technology for space exploration
D. To show how ideas from science fiction often develop into actual technologies
NARRATOR:Listen to part of a lecture in a structural engineering class.
MALE PROFESSOR:Today, let's begin to look at structural engineering in the Space Age.Uh, new problems... new possibilities, um, mean we can think in new ways... find radically different approaches.So let's consider, uh- well... What would you say is the biggest obstacle today to putting structures, equipment, people, uh-anything, really-into space?
FEMALE STUDENT:Well, the cost, right?
MALE PROFESSOR:Exactly. I mean, just taking the space shuttle up and back one time is hugely expensive. Uh, why?
FEMALE STUDENT:I-I guess a lot of it's for fuel, right? To-to get the rocket going fast enough...
MALE PROFESSOR:[encouragingly] OK... fast enough to...?
FEMALE STUDENT:To, uh, escape Earth's gravity.
MALE PROFESSOR:Good! So we're burning up an enormous amount of fuel at every launch, just to get the rocket up to what's known as "escape velocity."
Now escape velocity is around 11 kilometers a second-pretty fast. But-do we really have to go this fast?
FEMALE STUDENT:[confused] Well... yeah! I mean, how else can you... um, escape?I mean, that's the whole point of escape velocity, right?Otherwise, gravity will pull you back down to the Earth...
MALE PROFESSOR:Actually, that's a common misconception.Escape velocity is simply the speed of an object that's, uh, let's say, uh, shot out of a cannon... the minimum initial speed so that the object could later escape Earth's gravity on its own.But that's just if there's no additional force being applied.If you keep on supplying force to the object, keep on pushing it upward, it could pull away from Earth's gravity at any speed.
MALE STUDENT:Even really slow?[light dawning] So you're saying, like, if you had a ladder tall enough, you could just climb into space?
MALE PROFESSOR:Yeah-uh, well, theoretically. I mean, I can see some practical problems with the ladder example.Uh, like, you might get just a little bit tired out after the first few thousand kilometers or so- uh, especially with all the oxygen tanks you'd have to be hauling up with you!
No, I was thinking more along the lines of an elevator...
MALE STUDENT:[continues chuckling for a second, then does double take] Wait, you're serious?
MALE PROFESSOR:Ah, sure. An elevator- uh, that's a new idea to most of us, but in fact it's been around for over a century.If we could power such an elevator with solar energy, we could simply rise up into space- for a fraction of the cost of a trip by rocket or shuttle!
MALE STUDENT:But wait-elevators don't just "rise up"- they have to hang on some kind of wire, or track, or something...
MALE PROFESSOR:Uh, true-and for decades that's exactly what's prevented the idea from being feasible, or even just taken seriously: Um, where do we find a material strong enough, yet light weight enough, to act as a cable or track.I mean, we're talking 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 tensile strength higher than diamond, yet it's much more flexible.I'm talking about carbon nanotubes.
MALE STUDENT:OK, I've read something about carbon nanotubes-they're strong, alright, but aren't they just very short little cylinders in shape?
MALE PROFESSOR:Ah, yes, but- these cylinders cling together at a molecular level.You pull out one nanotube, or row of nanotubes, and its neighbors come with it. And their neighbors, and so on.So you could actually draw out a 36,000-kilometer strand or ribbon of nanotubes, stronger than steel, but maybe a thousandth the thickness of a human hair.
FEMALE STUDENT:OK, fine, but what's gonna hold this ribbon up? And keep it rigid enough to support an elevator car?
MALE PROFESSOR:Well, we'd definitely have to anchor it at both ends.So what we'd need is a really tall tower here on the ground, right at the equator, and a satellite in geostationary orbit around the Earth.
There's a reason I mentioned that figure of 36,000 kilometers- that's about how high an object would have to be orbiting, uh, straight up from the equator, to constantly remain directly above the exact same spot on the rotating planet Earth.So once you're in this geostationary orbit, right over the tower, just lower your carbon nanotube cable down from the satellite, tether it to the tower here on Earth, and there you have it!
FEMALE STUDENT:So you really think this is a possibility? Like, how soon could it happen?
MALE PROFESSOR:Well, the science fiction writer Arthur C. Clarke talked about building a space elevator back in the 1970s.And when someone asked him when he thought this idea might become a reality, his reply was, "Probably about 50 years after everybody quits laughing."
Professor: Today let’s begin to look at structural engineering in the Space Age. Uh, new problems…new possibilities mean we can think in new ways, find radically different approaches. So let’s consider…uh, well, what would you say is the biggest obstacle today to putting structures, equipment, people …uh, anything really, into space?
文章开门见山引出主旨 structural engineering in Space. 然后着重描述New possible 以及different approaches。
C选项即为文章核心new approaches to space的同义替换，是对全文主旨最准确的概括