OFFICIAL29 According to the professor,what is the significance of having a satellite in orbit about 36,000 kilometers above Earth's surface?

[00:00.00]NARRATOR: Listen to part of a lecture in a structural engineering class.[00:05.90]MALE PROFESSOR: Today ,let's begin to look at structural engineering in the Space Age. [00:11.17]Uh, new problems...new possibilities mean we can think in new ways...find radically different approaches.[00:19.33]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?[00:31.29]FEMALE STUDENT: Well, the cost, right?[00:34.28]MALE PROFESSOR: Exactly. I mean, just taking the space shuttle up and back one time is hugely expensive. Uh, why?[00:42.46]FEMALE STUDENT: I-I guess a lot of it is for fuel, right? [00:46.00]To—to get the rocket going fast enough...[00:48.56]MALE PROFESSOR: OK...Fast enough to...?[00:51.63]FEMALE STUDENT: To, uh, escape Earth's gravity.[00:54.23]MALE PROFESSOR: Good. So we are burning up an enormous amount of fuel at every launch, just to get the rocket up to what's known as "escape velocity". [01:05.18]Now, escape velocity is around 11 kilometers a second, pretty fast. [01:12.59]But do we really have to go this fast?[01:16.13]FEMALE STUDENT: Well...yeah! I mean, how else can you, um, escape? [01:20.98]I mean, that's the whole point of escape velocity, right? [01:24.14]Otherwise gravity will pull you back down to the Earth....
[01:27.48]MALE PROFESSOR: Actually, that's a common misconception.[01:30.54] Escape velocity is simply the speed of an object that's, uh, let's say, shot out of a cannon, the minimum initial speed so that the object could later escape Earth's gravity on its own. [01:43.80]But that's just if there's no additional force being applied. [01:48.59]If you keep on supplying force to the object, keep on pushing it upward. [01:53.76]It could pull away from Earth's gravity at any speed.[01:58.53]FEMALE STUDENT: Even really slow? [02:00.46]So you're saying, like, if you had a ladder tall enough, you could just climb into space?[02:06.28]MALE PROFESSOR: Yeah—uh, well, theoretically. [02:08.68]I mean, I can see some practical problems with the ladder example. [02:12.72]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! [02:22.29]MALE PROFESSOR: No. I was thinking more along the lines of an elevator...[02:27.80]FEMALE STUDENT: Wait, you're serious?[02:29.01]MALE PROFESSOR: Sure. An elevator. [02:31.40]That's a new idea to most of us, but in fact it's been around for over a century. [02:36.92]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.[02:47.45]FEMALE STUDENT: But wait, elevators don't just rise up. [02:49.76]They have to hang on some kind of wire or track or something.[02:53.09]MALE PROFESSOR: Uh, true—and for decades that's exactly what's prevented the idea from being feasible, or even just taken seriously: [03:01.19]Um, where do we find a material strong enough, yet light weight enough, to act as a cable or track. [03:07.51]I mean, we're talking 36,000 kilometers here—[03:11.59]and the strain on the cable would be more than most materials could bear.
[03:15.22]But a new material developed recently has a tensile strength higher than diamond, yet it's much more flexible. [03:23.58]I'm talking about carbon nanotubes.
[03:27.22]FEMALE STUDENT: OK. I've read something about carbon nanotubes.[03:31.99] They are strong, alright, but aren't they just very short little cylinders in shape?[03:36.42]MALE PROFESSOR: Ah, yes. But these cylinders cling together at a molecular level. [03:41.73]You pull out one nanotube or row of nanotubes, and its neighbor's come with it, and their neighbors, and so on. [03:50.49]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.[04:02.38]FEMALE STUDENT: OK. Fine. But what's going to hold this ribbon up and keep it reach enough to support an elevator car?[04:09.10]MALE PROFESSOR: Well, we definitely have to anchor it at both ends. [04:12.50]So what we need is a really tall tower here on the ground right at the equator and a satellite in geostationary orbit around the Earth. [04:22.30]There's a reason I mentioned that figure of 36,000 kilometers.[04:27.73]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 planet Earth. [04:39.43]So once you are 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. [04:51.18]And there you have it![04:53.07]FEMALE STUDENT: So you really think this is a possibility?[04:56.65] Like, how soon could it happen?[04:58.55]MALE PROFESSOR: Well, the science fiction writer Arthur C. Clarke talked about building a space elevator back in the 1970s. [05:05.27]And when someone asked him when he thought this idea might become a reality, his reply was, "Probably about fifty years after everybody quits laughing."