A number of science fiction stories, including novels by Arthur C, Clarke, Robert Forward, and Kim Stanley Robinson, have involved space elevators on Earth and Mars. These have all been set many decades in the future; the elevator cables were meters and even kilometers in diameter and massed in the billions of tons. In some of them, the horrific destruction caused by the cable breaking and falling was an important story element.

The proposed HighLift Space Elevator bears very little resemblence to the space elevators of science fiction. Rather than a cable as big around as a sequoia, it will be about the size of a sheet of paper. Rather than huge trains running up and down on magnetic tracks, climbers the size of a dump truck will grip the ribbon between two rollers and pull themselves up.

• Nothing is strong enough to make a Space Elevator.
  Carbon nanotubes (CNT)
• What about conservation of angular momentum? When an elevator ascends the ribbon, it must be accelerated eastward because the Earth's rotation represents a larger eastward velocity the higher you go. The required eastward force on the ascending elevator would have to be provided by a corresponding westward force on the ribbon, possibly requiring rockets at intervals along the cable.
  If you go through the math quantitatively, the angular momentum for the climbers requires a pound or so of force over the one-week travel time, and we do that easily with our many tons of material in the anchor and the counterweight.

  The quantities really are tiny, but just to be complete, a climber going up pushes the entire elevator slightly to the east, causing it to lean. However, the ribbon recovers for the same reason that it stays up in the first place. Centripetal acceleration is acting on the upper two-thirds pulling it outward, and the lost angular momentum is replaced very quickly (essentially as fast as it is lost). The ribbon will never loose enough angular momentum to even deflect a single degree let alone fall. The extra angular momentum is stolen from the Earth's rotation; we will have to worry about this effect slowing down the Earth and making the day longer if we ever decide to ship Australia into space.

• What if it breaks?
  • How easy would it be to break?
    Very difficult. The planned position of the elevator avoids hurricanes, lightning and other extreme weather. The ribbon is engineered to be twice as strong as it needs to be to support itself and any planned cargo attached to it.
  • What if it falls?
    The majority, the long end out in space, gains enough speed that it burns up in the atmosphere, with the lower portion falling into the sea. It will not fall on top of anyone.
  • For the portion that doesn't burn up in a fall- what effect will it have on the environment?
    Honestly, it will make a little bit of a mess. But New York City tickertape parades have made bigger messes. Comparatively it will put much less dust, dirt, debris and chemicals into the environment than wildfires of the American west, any one of the large expendable rockets, or a month of natural meteors hitting Earth. The ribbon is light (7.5 kg/km) so, any pieces that fall to earth will slow down, in the air, to about the same terminal velocity as that of an open newspaper page falling. It will not have enough momentum to cause mechanical damage when it comes down. We have considered other health risks such as inhalation of very small fragments and believe this will not be a problem but we are conducting studies to make sure this isn't a problem. Since we are aware of the possible problems now we can design the elevator to avoid these problems.
  • How large a wave/disturbance would it generate?
    The wave/disturbance would be nonexistent. As above, there just isnít enough mass, even in later, larger, ribbons, to generate such energy dispersion. There might be a small amount of light as a line in the sky as the ribbon burns up but after that it will be a few pieces of black film fluttering to Earth. Because of the size, distribution and winds, it is conceivable that only a few people would even see the event in any way and just as few would find actual pieces of the ribbon.
  • How much warning would there be from the time of a break and the time it would take for the lower portion to splash down?
    Depending on exactly what happened it could be a few hours to weeks.
  • What would happen to the surviving portion?
    The ribbon that fell to Earth could be recovered for study but because of the amount and distribution it would be difficult to find many pieces. The pieces that do land would eventually degrade but not for a very long time. Keep in mind that this is mostly a stable form of carbon; it doesnít do anything. The debris would resemble long hair and would probably be broken up in interactions with animals, plants, wind, fish and waves. In fiber form it would be much too large to inhale and would probably work its way through a digestive system unaffected. The only debris we have any concern about is if it were reduced to nanotube size. This we don't understand yet so we will study this to see if there is a problem and then probably also design the ribbon to remain in larger pieces if it re-enters.
  • What would happen to anything climbing the ribbon at the time it broke?
    The short answer is that some payloads will fall (below the break and below 24,000 km altitude), some will enter low orbit (below the break and between 24,000km and GEO) and some will be tossed to high Earth orbit (above GEO) depending on where the payloads are and where the break is. However, what happens also depends on the reaction of the payload. In other words, what happens depends on lots of factors. Escape pods or re-entry vehicles may be required depending on designs.
• Will high winds pose a problem?
  Analysis shows that the proposed ribbon (of 1cm width below 10km altitude) would break at 71.5 m/s (159 mph) or a Category 5 hurricane. The proposed location of the ribbon is not in a hurricane or high wind area.
• What if lightning strikes the ribbon?
  The best way to deal with lightning is to avoid it hence the proposed location of the ribbon is in an area that receives little to no lightning.
• Won't the ribbon "short out" the atmosphere? There is a voltage difference of several hundred kilovolts between the Earth's surface and the ionosphere. The (presumably conducting) ribbon would at least provide a path to reduce the global potential and might even short it out, forcing the ribbon to carry a large current. Would this effect the construction of the highway? How might a shorting out of the global circuit effect the Earth's meteorology?
  It is correct that there are substantial electrical potential in Earth's atmosphere. However, doing a quantitative analysis of the situation, we found the currents that will flow through the ribbon will be minimal due to the resistance of the ribbon, cross-sectional area of the ribbon, the distance the potentials are separated by, and the poor coupling between the ionosphere and the ribbon. The total effect of the ribbon on the potentials can be seen in conventional tethered balloons or kites.
• Will the ribbon produce an electrical current? The last space shuttle-tether experiment, which unspooled about 12 miles of cable, generated thousands of volts of electrical potential and kilowatts of power, burned through the insulation of the cable, and generated a tremendous explosive arc of electricity, that snapped the tether. Now imagine a 60,000-mile-long cable and its electrical-generating capacity and you begin to see the disastrous potential.
  
  There are several aspects to this question. An electrical might be produced in several ways: 1) due to the electrical properties of Earth's atmosphere, 2) collection of the space plasma and 3) due to the movement of the ribbon relative to the natural magnetic fields near Earth.
  
  1): Earth's atmosphere consists of voltage differentials and charge build-up in association with storm activity, in the ionosphere and even on a clear blue-sky day. The last is easiest, the voltage potential set up there can be many volts but is over a large distance and based on the resistance of the ribbon would produce little current or difficulty. Storms produce high voltage differences, enough to produce lightning which can damage the ribbon. We have selected an anchor location in an area that has essentially no lightning and very little storm activity. With our mobile anchor we will be able to avoid the rare storm. In the upper atmosphere there can also be charge and voltage potential build up. In the upper atmosphere (ionosphere) the density of plasma is low and there will be poor coupling between the plamsa and our ribbon. We will have some charge build up and small current flow but it will be comparable to a small household battery. One area we need to investigate further is sprites or breakdown events in the extreme upper atmosphere.
  
  2) In space there are plasmas that can deposit charge on satellites and our ribbon. As above, the density of this plasma is very low and a small amount of charge will be collected on our ribbon. We also expect both positive and negative charge to be deposited on the ribbon which would reduce teh net charge build-up. If one charge preferentially builds up this will raise the potential of the ribbon, pushing away more of this charge and attracting the opposite charge which will limit the net voltage build-up. The total charge and voltage we expect and the length and resistance of the ribbon again make the currents much smaller than what you could get from common small batteries.
  
  3) The third place where currents can be produced is when a wire is dragged through a magnetic field. This is how commercial electric generators work. To produce electricity you need a strong magnetic field and a high velocity. This has been demonstrated on orbital tethers traveling at 11,000mph through Earth's magnetic field, high voltages have been produced (1000's of volts). (3a) In our case our ribbon is essentially stationary relative to Earth's magnetic field so again very little current will be produced. (3b) The upper end of the ribbon will be in space and be moving relative to the interplanetary magnetic field but since this magnetic field is very weak little current is produced here as well.
  
  If you add up all f these currents and voltages you end up with nothing that will be problematic with the exception of the lightning. So as long as we avoid lightning and the storms associated with it we will get very little current flow or trouble from this area. You can think of most of this in terms of a conventional TV tower. Related to most of the topics discussed above (1 and 3a) we will experience the same voltages and millions times less (due to the relative sizes) current as a TV tower. Several colleagues had hoped that we might produce electric power from the ribbon but from what we have found power generation from the ribbon will take a lot of work and design modifications to produce much of anything.
  -Dr. Brad Edwards
  
• Will an oscillation bring the ribbon down?
  There will be a 7 hour natural frequency which can be actively damped with the anchor station.
• Will radiation degrade the components?
  The carbon fiber composites are good for 1,000 years in orbit.
• Will the ribbon effect bird/other species migration?
  Unlikely due to the small size and passive nature of the system.
• What about things hitting the ribbon?
  • Will the large amount of orbital debris be a problem?
    Low Earth objects can and will pose a serious problem. However active avoidance can be used to avoid objects that would cause damage. On average, an object would have to be avoided every 14 hours.
  • Don't all existing satellites cross the Equator, where they could hit the ribbon? Each and every satellite in Earth orbit (except for those in an exact geostationary orbit) crosses the equator twice during each and every orbit. Unless that elevator is really good at dodging, one of these satellites is going to run into it.
    We are good at dodging, and we will avoid the satellites. We are tracking them and will have days to weeks warning. We will move the anchor about a kilometer each day to avoid the debris.
  • Is the ribbon in the path of any existing launch programs?
    Tentatively no.
  • Is it in any existing flight paths?
    One of the nice things about our anchor site is that it is in the middle of nowhere, approximately 400 miles from shipping or plane routes.
• The Leonids and the Space Elevator
  The Leonids are a trail of dust and debris left by the Tempel-Tuttle comet as it traverses our solar system each 33 years. The last passage was in 1998 and the next is expected in 2031. The dust and debris left by the comet passage disperses and leaves the neighborhood of Earth on a timescale of years though some debris always remains. The flux density of the debris can fluctuate by 10,000 from year to year. The Leonid debris also has a distribution of debris that includes dust particles and objects up to 10 cm in diameter.
  
  
  An article published by McNeil, Lai and Murad, Charge Production due to Leonid Meteor Shower Impact on Spacecraft Surfaces, discusses the impact probability on spacecraft and the details of the Leonid debris. The flux density from McNeil is shown in figure 1. This flux density is for a Leonid shower with a peak visual flux of 1000 meteors per hour. The standard peak is 10 to 20 and the largest in 1966 was 160,000. We can calculate the probability of impact on the elevator ribbon based on the 1000 peak number and then scale from there.
  
  Objects larger than about 10 cm have a finite possibility of destroying the ribbon. Objects as large as 5 cm in diameter has a small chance of destroying the ribbon. If we consider a weighted probability function we might approximate the likelihood of destruction with the likelihood of impact by a 10 cm or larger object. In our baseline, with densities of 3 gm/cm3 (estimate for the Leonid debris), this relates roughly to a mass of about 1500 gm (4/3*3.141*5^3*3) or a flux density of 10-17/m2 s. A typical Leonid shower lasts roughly 2 hours or 7200 s and the total area of the proposed elevator ribbon is 108m2. This gives us a probability of damage leading to destruction for each annual passage through the Leonid debris of roughly 1/100,000 for the showers with peak visual rates of 1000/hour. For a more standard shower the probability would be 50 to 100 times less. For the largest likely event (possibly in 2031) the probability would go up by 160 to a 1/625 possibility of severe damage. These are rough estimates and more accurate calculations are required. However, these estimates indicate that until 2031 the danger is probably minimal. By 2031, modifications and mitigation techniques could be implemented to improve survivability.
  
  -Dr. Brad Edwards
• Would the elevator be susceptible to a terrorist attack?
  Yes- so is practically everything. An attack on the ribbon is unlikely because of the anchor station's isolation and the relatively small amount of casualties that would result. Its main protection is being so hard to get to, there is no way to sneak up on it. However if everyone has equal access to the benefits of the space elevator, there is substantially less risk.

  The anchor is located in the equatorial Pacific 400 miles from any air or shipping lanes. The ribbon would also have restricted airspace around it. The ribbon and anchor would be protected like any other valuable piece of property, in this case probably by the U.S. military.

Updated: RGM 2/6/2003 2:37:14 PM