Audacious
& Outrageous: Space Elevators
from science@NASA
Inspired partly by science
fiction, NASA scientists are seriously considering
space elevators as a mass-transit system for the next
century.
Listen
to this story (requires RealPlayer)
Sept.
7, 2000 -- "Yes, ladies and gentlemen, welcome
aboard NASA's Millennium-Two Space Elevator. Your
first stop will be the Lunar-level platform before we
continue on to the New Frontier Space Colony
development. The entire ride will take about 5 hours,
so sit back and enjoy the trip. As we rise, be sure to
watch outside the window as the curvature of the Earth
becomes visible and the sky changes from deep blue to
black, truly one of the most breathtaking views you
will ever see!"
Does this sound like the Sci-Fi
Channel or a chapter out of Arthur C. Clarke's,
Fountains of Paradise? Well, it's not. It is a real
possibility -- a "space elevator" -- that
researchers are considering today as a far-out space
transportation system for the next century.
Above: Artist Pat
Rawling's concept of a space elevator viewed from the
geostationary transfer station looking down along the
length of the elevator toward Earth. [more
information]
David Smitherman of NASA/Marshall's
Advanced Projects Office has compiled plans for such
an elevator that could turn science fiction into
reality. His publication, "Space Elevators: An
Advanced Earth-Space Infrastructure for the New
Millennium," is based on findings from a space
infrastructure conference held at the Marshall Space
Flight Center last year. The workshop included
scientists and engineers from government and industry
representing various fields such as structures, space
tethers, materials, and Earth/space environments.
"This is no longer science
fiction," said Smitherman. "We came out of
the workshop saying, 'We may very well be able to do
this.'"
A space elevator is essentially a long
cable extending from our planet's surface into space
with its center of mass at geostationary Earth orbit
(GEO), 35,786 km in altitude. Electromagnetic vehicles
traveling along the cable could serve as a mass
transportation system for moving people, payloads, and
power between Earth and space.
Current plans call for a base tower
approximately 50 km tall -- the cable would be
tethered to the top. To keep the cable structure from
tumbling to Earth, it would be attached to a large
counterbalance mass beyond geostationary orbit,
perhaps an asteroid moved into place for that purpose.
"The system requires the center
of mass be in geostationary orbit," said
Smitherman. "The cable is basically in orbit
around the Earth."
Four to six "elevator
tracks" would extend up the sides of the tower
and cable structure going to platforms at different
levels. These tracks would allow electromagnetic
vehicles to travel at speeds reaching thousands of
kilometers-per-hour.
Conceptual designs place the tower
construction at an equatorial site. The extreme height
of the lower tower section makes it vulnerable to high
winds. An equatorial location is ideal for a tower of
such enormous height because the area is practically
devoid of hurricanes and tornadoes and it aligns
properly with geostationary orbits (which are directly
overhead).
Above: Equatorial base
sites are essential for space elevators because they
align properly with geostationary orbits. In Arthur C.
Clarke's novel, Fountains of Paradise, engineers built
a space elevator on the mythical island of Taprobane,
which was closely based on Sri Lanka, a real island
near the southern tip of India. Clarke made one
important change to the geography of Sri Lanka/Taprobane:
he moved the island 800 km south so that it straddles
the equator. At the moment, Sri Lanka lies between 6
and 10 degrees north.
According to Smitherman,
construction is not feasible today but it could be
toward the end of the 21st century. "First we'll
develop the technology," said Smitherman.
"In 50 years or so, we'll be there. Then, if the
need is there, we'll be able to do this. That's the
gist of the report."
Smitherman's paper credits Arthur C.
Clarke with introducing the concept to a broader
audience. In his 1978 novel, Fountains of Paradise,
engineers construct a space elevator on top of a
mountain peak in the mythical island of Taprobane
(closely based on Sri Lanka, the country where Clarke
now resides). The builders use advanced materials such
as the carbon nanofibers now in laboratory study.
"His book brought the idea to
the general public through the science fiction
community," said Smitherman. But Clarke wasn't
the first.
As
early as 1895, a Russian scientist named Konstantin
Tsiolkovsky suggested a fanciful "Celestial
Castle" in geosynchronous Earth orbit attached to
a tower on the ground, not unlike Paris's Eiffel
tower. Another Russian, a Leningrad engineer by the
name of Yuri Artsutanov, wrote some of the first
modern ideas about space elevators in 1960. Published
as a non-technical story in Pravda, his story never
caught the attention of the West. Science magazine ran
a short article in 1966 by John Isaacs, an American
oceanographer, about a pair of whisker-thin wires
extending to a geostationary satellite. The article
ran basically unnoticed. The concept finally came to
the attention of the space flight engineering
community through a technical paper written in 1975 by
Jerome Pearson of the Air Force Research Laboratory.
This paper was the inspiration for Clarke's novel.
Left: In 1895 Konstantin
Tsiolkovsky looked at the Eiffel Tower in Paris and
imagined it attached to a "celestial castle"
at the end of a spindle shaped cable, with the
"castle" orbiting the earth in a
geosynchronous orbit. The modern vision of a 50 km
space tower -- the necessary anchor for any space
elevator -- is far taller than the Eiffel Tower. [more
information]
Pearson, who participated in the
1999 workshop, envisions the space elevator as a
cost-cutting device for NASA. "One of the
fundamental problems we face right now is that it's so
unbelievably expensive to get things into orbit,"
said Pearson. "The space elevator may be the
answer."
The workshop's findings determined
the energy required to move a payload by space
elevator from the ground to geostationary orbit could
remain relatively low. Using today's energy costs,
researchers figured a 12,000-kg Space Shuttle payload
would cost no more than $17,700 for an elevator trip
to GEO. A passenger with baggage at 150 kg might cost
only $222! "Compare that to today's cost of
around $10,000 per pound ($22,000 per kg)," said
Smitherman. "Potentially, we're talking about
just a few dollars per kg with the elevator."
During the workshop, issues
pertinent to transforming the concept from science
fiction to reality were discussed in detail.
"What the workshop found was there are real
materials in laboratories today that may be strong
enough to construct this type of system," said
Smitherman.
Smitherman listed five primary
technology thrusts as critical to the development of
the elevator. First was the development of
high-strength materials for both the cables (tethers)
and the tower.
In
a 1998 report, NASA
applications of molecular nanotechnology,
researchers noted that "maximum stress [on a
space elevator cable] is at geosynchronous altitude so
the cable must be thickest there and taper
exponentially as it approaches Earth. Any potential
material may be characterized by the taper factor --
the ratio between the cable's radius at geosynchronous
altitude and at the Earth's surface. For steel the
taper factor is tens of thousands -- clearly
impossible. For diamond, the taper factor is 21.9
including a safety factor. Diamond is, however,
brittle. Carbon nanotubes have a strength in tension
similar to diamond, but bundles of these
nanometer-scale radius tubes shouldn't propagate
cracks nearly as well as the diamond tetrahedral
lattice."
Above: Carbon nanotube (CNT)
is a new form of carbon, equivalent to a flat graphene
sheet rolled into a tube. CNT exhibits extraordinary
mechanical properties: the Young's
modulus is over 1 Tera-Pascal and the estimated tensile
strength is 200 Giga-Pascals. [more
information]
Fiber materials such as graphite,
alumina, and quartz have exhibited tensile strengths
greater than 20 GPa (Giga-Pascals, a unit of
measurement for tensile strength) during laboratory
testing for cable tethers. The desired strength for
the space elevator is about 62 GPa. Carbon nanotubes
have exceeded all other materials and appear to have a
theoretical strength far above the desired range for
space elevator structures. "The development of
carbon nanotubes shows real promise," said
Smitherman. "They're lightweight materials that
are 100 times stronger than steel."
The second technology thrust
was the continuation of tether technology development
to gain experience in the deployment and control of
such long structures in space.
Third was the introduction of
lightweight, composite structural materials to the
general construction industry for the development of
taller towers and buildings. "Buildings and
towers can be constructed many kilometers high today
using conventional construction materials and
methods," said Smitherman. "There simply has
not been a demonstrated need to do this that justifies
the expense." Better materials may reduce the
costs and make larger structures economical.
Fourth
was the development of high-speed, electromagnetic
propulsion for mass-transportation systems, launch
systems, launch assist systems and high-velocity
launch rails. These are, basically, higher speed
versions of the trams now used at airports to carry
passengers between terminals. They would float above
the track, propelled by magnets, using no moving
parts. This feature would allow the space elevator to
attain high vehicle speeds without the wear and tear
that wheeled vehicles would put on the structure.
Left: A computer model of
a maglev -- or magnetically levitated -- launch
vehicle. Maglev technologies are essential for future
space elevators. [more
information]
Fifth was the development of
transportation, utility and facility infrastructures
to support space construction and industrial
development from Earth out to GEO. The high cost of
constructing a space elevator can only be justified by
high usage, by both passengers and payload, tourists
and space dwellers.
During a speech he once gave,
someone in the audience asked Arthur C. Clarke when
the space elevator would become a reality.
"Clarke answered, 'Probably
about 50 years after everybody quits laughing,'"
related Pearson. "He's got a point. Once you stop
dismissing something as unattainable, then you start
working on its development. This is exciting!"
Related Links:
Science
@ NASA Headlines!
Highway2Space.com
- NASA/Marshall web site about space transportation
research
Space
Towers -- from NASA/Marshall's
"Liftoff" web site
Space
Elevator Concept -- from NASA/Marshall's
Flight Projects Directorate
NASA
applications of molecular nanotechnology-
learn more about carbon nanfibers and how they may
be used with space elevators.
Nanotechnology
Gallery - more information about carbon
nanfibers.
