Copyright © 2009 by Gaylen Hinton. Published by The Mars Society with permission.

THE COLONIZATION OF MARS VIA A MARTIAN SPACE ELEVATOR

Gaylen Hinton Durham, North Carolina gaylenhinton@yahoo.com

ABSTRACT

An initial colony on Mars could be established through a Martian space elevator (MSE) that was sent from earth along with the colonists. The MSE and the colony could get into areosynchronous orbit (ASO) with the expenditure of less than 600 m/s of delta V. Once in ASO, the MSE would be made operational by simultaneously lowering a base station to Mars and extending the counterweight. The base station would be mobile in order to maneuver it to the right location. The colony would be based about 10° off of the equator in order to position the MSE where the Martian moons would not affect it.

It would probably not be practical to send an MSE to Mars unless there was a space elevator (SE) on earth first, due to the mass involved. In order for an earth-based SE to give a craft the delta V necessary to get to Mars, the craft would have to be released about 25,000 km past geosynchronous orbit (GSO). At that point the net force is only about 1/40g. Therefore huge loads, assembled at GSO, could be sent to Mars with minimal loading on the cable. Also, multiple loads could be sent and collected together in transit. Therefore, the initial colony could be composed of hundreds of people, the MSE, and all their supplies and equipment.

MARTIAN SPACE ELEVATOR BASICS

An areosynchronous orbit (ASO) is the altitude where the force of gravity and the centrifugal force of an object fixed with respect to the surface of Mars are equal:

This is less than half the elevation of a synchronous orbit on earth.

The load on the cables from the surface to ASO would be, for a cable of uniform thickness of mass λ/m:

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That is only about 1/5 of the specific strength required of a space elevator (SE) on earth. Because of this, it would not be necessary to have a tapered cable for an MSE. Cables of uniform thickness could be easily reeled out to construct the MSE. Also, because gravity on Mars is only .38g, the cables would only have to be 38% as thick to carry the same load as on earth. Actually, with the reduced loading of the cable on itself, the final cable would only have to be about 1/3 the size of an earth SE.

A serious problem for a Mars SE (MSE) is that there are two moons in the way. Phobos is less than half the ASO distance, and Deimos is just slightly outside ASO, but both would get in the way of the MSE and its counterweight.

The redeeming factor is that both moons only have an inclination of about one degree to the Martian equator. As they are only 15 – 25 km in diameter, these moons would only affect a narrow band around the equator.

Deimos is 23500 km from the center of Mars, and makes an angle of about one degree with the equator. So it actually travels

km on either side of the equator.

In order to make sure that that the MSE will always be clear of Deimos with a safety factor, and because the orbit of Deimos precesses somewhat with respect to the plane of the equator, we would want to have the closest point of the MSE at least 500km from the equatorial plane.

The moon Phobos is about 9500 km from the center of Mars. Its orbit makes an angle of 1.09 degrees with the equator. Therefore, it travels

km on either side of the equator – much less than Deimos.

Therefore, in order for the MSE to clear the moons of Mars, it needs to be positioned off the Martian equator. Figure 1 below shows the relationships between an MSE with a base station at latitude C above the Martian equator, a space station at ASO, the MSE counterweight, and the moon Deimos. Phobos is not shown because if the MSE can clear Deimos, it will easily clear Phobos.

We can see by this figure that the approximately one degree angle that Deimos makes with the equatorial plane requires a very much larger angle of latitude for the base station in order for the MSE to stay out of its path.

In the above figure, the length of the MSE tether from the surface to the ASO station is L, and it makes an angle B with the plane of the base station latitude. The radial distance of the ASO station from the center of Mars is r, and it makes an angle A with the equatorial plane. The

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radius of Mars is R, so the separation of the plane of the base station latitude from the plane of the equator is RsinC.

The outward force, CF, on the ASO station is provided by the centrifugal force on the station and counterweight. The gravitational force, mg, on the ASO station tends to counter that centrifugal force. The tension and gravitational forces are balanced when the ASO station assumes a position where:

We also have the relationship:

These two equations can be solved to get:

For a satellite in orbit, mg = CF. Therefore, if there were a massless cable going to Mars from an ASO satellite, then angle A would equal angle B. However, with any tension on the cable from the counterweight, the angles will not be equal.

Since the separation of the MSE cable from the equatorial plane is , and we want that separation to be at least 500 km, we would have,

Without any initial tension from the cable, mg = CF, so

At ASO, the mg force is only 1/36 of what it is at the surface. Therefore, if we assumed sufficient tension in the MSE to support a 15,000 kg load at the surface, that tension would balance 540,000 kg at ASO. So with a 540,000 kg ASO station and the ability to lift 15,000 kg, we would have . Substituting this in we get:

If we assumed , then the ASO station could have a mass of 270,000 kg or double cables (for a moving MSE) with a mass of 540,000kg then,

 

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  As  a  load  is  being  carried  up  the  MSE,  its  weight  will  also  affect  the  position  of  the  cable.  Figure  2  below  shows  a  load  on  the  cable  of  the  MSE.  

  For  a  load   being  carried  up  the  MSE,  with  a  tension  T  on  the  cable,  figure  2  gives  the  following  relations,  with    being  the  weight  of  the  load  at  the  point  shown:  

 

  These  can  be  solved  to  give,  

 

  A  load  at  the  distance  L’  from  the  base  station  pulls  the  cable  from  the  plane  of  the  base  station  latitude  towards  the  equatorial  plane  by  the  amount .  With  no  load  at  all,  the  cable  would  be  a  distance  of    from  the  plane  of  the  base  station.    (  can  be  determined  by  the  initial  conditions  using  equation  2  –  typically  about  .007.)  So  the  difference,     ,  with  the  load  m’  on  the  cable  is:    

 

  But  to  the  first  approximation,     ,  so  we  would  have:  

 

  With  the  maximum  load    at  the  surface,  but  it  diminishes  as    as  the  load  rises.  Therefore,  at  any  height,  

 

Substituting  into  equation  3,  we  get,  

 

The  above  equation  can  be  iterated  to  show  that  the  maximum  movement  due  to  the  load  on  the  cable  is  when  the  load  is  about  1600  km  up.    This  movement  from  the  maximum  load  would  be  70  to  90  km,  depending  on  the  initial  location  of  the  base  station.  

  Also,  this  same  basic  equation  can  give  an  estimate  of  the  effect  of  the  weight  of  the  cable  itself  on  the  position  of  the  MSE  station.  Another  15  km  or  so  would  be  added  to  any  movement  caused  by  the  load.    

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  Therefore,  during  worst  case  conditions,  about  100  km  needs  to  be  added  to  the  displacement  of  Deimos  in  order  to  clear  the  MSE.  With  the  410  km  maximum  movement  of  the  moon  itself,  500  km  of  clearance  would  not  leave  enough  of  a  safety  factor.  Likely  the  use  of  the  MSE  would  need  to  be  restricted  during  a  Deimos  alignment  of  maximum  excursion  in  order  to  maintain  a  sufficient  safety  factor.  In  addition,  the  use  of  the  MSE  would  have  to  be  timed  to  dampen  any  oscillations,  rather  than  increase  them.  

  In  any  event,  depending  upon  the  initial  mass  of  the  ASO  station,  the  MSE  would  need  to  be  positioned  about  10  or  12  degrees  north  or  south  of  the  equator.  

  An  alternate  plan  to  an  off-­‐equator  MSE  would  be  to  put  the  base  on  a  movable  platform  with  wheels.  In  order  to  avoid  the  moons  (up  to  25  km  across)  the  platform  would  have  to  be  able  to  move  north  and  south  at  least  30  km.  That  would  restrict  the  location  of  a  Mars  colony  to  a  location  with  a  smooth  surface  for  30km.  Also,  this  plan  would  put  very  strict  requirements  on  maintenance,  reliability  and  backup  systems  for  the  moving  platform.  If  the  mobile  base  ever  failed  to  move  properly  just  one  time,  the  MSE  would  be  destroyed.    

  The  moons  of  Mars  make  their  maximum  excursions  from  the  equatorial  plane  once  each  orbit.  However,  that  maximum  is  not  at  the  same  place  each  time,  relative  to  the  surface,  due  to  the  different  orbital  and  rotational  periods.  Relative  to  the  surface  of  Mars,  those  maximum  excursions  appear  to  rotate  around  the  planet.    

  The  ratio  between  Mars’  rotation  and  Deimos’  orbital  period  is   .  Therefore,  if  a  point  on  the  surface  and  Deimos  were  initially  aligned,  after  one  Martian  day  (sol)  Deimos  will  be    orbits  behind.  In  order  to  line  up  again  it  would  take,  

 

  Therefore,  every  5.3384  sols  Deimos  would  be  in  a  position  to  potentially  interfere  with  an  MSE.  However,  if  the  initial  alignment  was  with  the  location  of  maximum  excursion  of  Deimos  from  the  equatorial  plane  (410  km),  then  at  the  second  alignment,  Mars  would  have  rotated    radians.  (That  is  121.8°  past  the  point  of  maximum  excursion.)  So,  at  the  nth  alignment,  Mars  would  have  rotated    radians,  and  the  distance  of  Deimos  from  the  equatorial  plane  would  be:  

         km       (3)  

  Knowing  the  distance  of  the  mobile  MSE  from  the  equator,  and  the  diameter  of  the  moon,  we  can  compare  to  the  distances  of  equation  (3)  to  see  when  there  would  be  an  interference  with  Deimos.  By  going  through  all  the  values  of  n  from  one  alignment  to  hundreds,  we  can  determine  how  many  sols  there  are  between  times  of  interference.    

  The  worst  case  scenario  (for  a  mobile  MSE  near  the  point  of  maximum  departure)  would  be  a  move  every  sixteen  sols  (every  third  alignment)  for  five  or  six  times,  followed  by  a  wait  of  267  sols.  For  a  mobile  MSE  on  the  equator,  there  would  be  thirty  one  

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alignments  (165  sols)  until  a  move  was  required,  followed  occasionally  by  another  move  sixteen  sols  later.  

  The  ratio  of  Phobos’  orbit  to  Mars’  rotation  is  3.2171.    Doing  the  same  exercise  for  Phobos  as  Deimos  gives,  

 

and  the  distance  of  departure  from  the  equatorial  plane  at  the  nth  alignment:  

 

  By  going  through  the  values  of  n,  we  can  determine  how  many  sols  between  times  of  interference  with  Phobos.  Typically,  there  would  be  seven  to  eleven  alignments  between  required  moves,  with  an  occasionally  longer  wait.  That  would  mean  that  a  mobile  MSE  would  have  to  move  twice  each  three  to  five  day  period.  (Once  to  move  away  from  Phobos,  and  then  again  to  move  back.)  

  Perhaps  a  compromise  between  an  off-­‐equator  MSE  and  a  mobile  MSE  could  be  made.  A  lot  of  movement  is  required  for  a  mobile  MSE  to  avoid  Phobos,  but  only  half  the  latitude  is  required  to  avoid  it  completely  compared  to  Deimos.  An  MSE  could  be  placed  far  enough  from  the  equator  to  avoid  dealing  with  Phobos,  but  still  have  a  mobile  base  to  deal  with  the  occasional  moves  required  by  Deimos.    

  Even  if  the  initial  MSE  was  set  with  a  fixed  base,  because  the  orbit  of  Deimos  precesses  with  respect  to  the  equatorial  plane,  eventually  the  base  platform  may  have  to  be  made  mobile  to  avoid  it.    

  With  any  SE,  the  minimum  counterweight  mass  is  when  there  is  no  lump  weight  at  all,  but  simply  a  cable  extended  outward  from  the  synchronous  orbit.  The  total  tension  for  the  MSE  would  be  the  maximum  allowable  load  on  the  cable.  

  From  equation  (1)  we  know  that  it  takes    of  force  to  balance  a  uniform  cable  at  ASO.  However,  we  can  use  that  same  equation  to  determine  how  far  a  cable  of  uniform  thickness  would  need  to  extend  out  from  ASO  to  provide  the  initial  tension.  The  initial  tension  at  the  surface,  and  therefore  the  maximum  load  carrying  capacity  of  the  MSE  would  be,  

 

  If  we  assumed  that  the  maximum  allowable  load  on  the  cable  was     ,  (or  the  allowable  specific  strength  was     ),  we  can  set  F    in  equation  1  and  change  the  limits  of  integration  out  to  a  distance  r:  

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  If  the  maximum  allowable  load  on  the  cable  was   ,  then   .  

Therefore,  if  we  wanted  the  maximum  cargo  capacity  on  the  MSE  to  be  15,000  kg,  we  would  have,  with  a  cable  of  strength   ,      

 

  So  the  cables  of  the  MSE  would  have  a  total  mass  of    

 

  If  the  MSE  were  made  with  movable  cables,  there  would  be  one  cable  going  up  and  another  coming  down,  so  the  total  mass  would  be  double  that.  Also,  a  tapered  cable  would  require  more  mass.  

  As  an  elevator  car  left  the  surface  of  Mars  and  got  closer  and  closer  to  ASO,  the  gravitational  force  on  that  car  would  diminish  as   .  Because  of  that,  with  a  cargo  capacity  of  15,000  kg  at  the  Martian  surface,  there  could  be  a  total  load  of  six  elevator  cars  of  10,000  kg  each,  evenly  spaced  between  Mars  and  the  ASO.  

THE  COLONIZATION  OF  MARS  VIA  A  MARTIAN  SPACE  ELEVATOR  

  If  a  complete  MSE  was  sent  from  earth  with  the  first  Mars  colony,  then  all  the  equipment,  supplies,  habitats,  and  colonists  could  be  lowered  directly  down  to  the  base  station  from  ASO.  A  tremendous  amount  of  effort  would  be  saved  because  everything  could  come  down  to  the  surface  with  little  or  no  concern  for  aerodynamics,  reentry,  landing,  or  protection.    

  The  fact  that  the  cables  of  an  MSE  might  have  a  mass  of  200,000  kg  or  more  seems  daunting.  All  of  the  mass  of  the  equipment,  supplies,  habitats  and  colonists  would  be  added  to  those  200,000  kg.  Such  a  large  amount  of  mass  would  be  prohibitive  if  it  had  to  be  sent  to  Mars  via  chemical  rockets.  However,  if  there  was  an  SE  on  earth  first,  then  all  of  that  mass  could  be  easily  sent  to  Mars  to  provide  the  first  colony  with  everything  they  need  to  succeed.  

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  With  an  SE  on  earth,  there  is  no  energy  cost  to  send  a  spacecraft  to  Mars  beyond  getting  it  to  GSO.  In  fact,  energy  is  gained  by  sending  a  load  from  GSO  to  the  release  point  where  it  would  be  slung  toward  Mars.  Typically,  a  load  bound  for  Mars  would  need  to  be  released  about  25,000  km  beyond  GSO.  That  would  give  it  the  delta  V  necessary  to  escape  earth  and  travel  to  Mars.  At  25,000  km  past  GSO  the  net  force  on  any  object  is  only  about  1/40  g.  A  200,000  kg  load  would  only  apply  5,000  kg-­‐force  on  the  cable  at  the  release  point.  

  Therefore  huge  loads,  assembled  at  GSO,  could  be  sent  to  Mars  with  minimal  loading  on  the  SE  cable.  Also,  multiple  loads  could  be  sent  and  collected  together  in  transit.  Therefore,  the  initial  colony  could  be  composed  of  hundreds  of  people,  the  MSE,  and  all  their  supplies  and  equipment.  A  drawing  of  the  complete  Mars  colony  en  route  to  Mars  is  shown  below  in  Fig.  3.  

  Dozens  or  even  hundreds  of  loads  of  equipment,  supplies,  and  people  could  be  sent  up  the  SE  to  be  assembled  together  at  a  space  station  at  GSO.  Once  assembled  and  prepared,  the  much  larger  colonial  loads  could  be  sent  to  Mars  via  the  SE.  

  The  same  technology  used  to  create  a  SE  (carbon  nanotubes?)  could  also  create  large,  inflatable,  rotating  space  craft  that  would  bring  the  colonists  to  Mars  with  artificial  gravity,  and  all  the  comforts  of  home.  Also,  this  same  technology  could  be  used  to  make  lightweight  habitats  for  the  Martian  colonists  on  the  surface.  

  The  cables  and  equipment  necessary  to  make  the  MSE  would  be  sent  along  with  the  rotating  space  habitat,  or  be  sent  in  separate  loads  and  collected  together  in  transit.  In  any  event,  by  the  time  the  colonists  reached  Mars,  everything  would  be  assembled  together  in  one  large  load.  As  shown  in  Fig.  3,  there  would  be  a  reel  of  cable  to  lower  a  mobile  base  station  to  Mars  and  another  reel  to  extend  the  counterweight  further  out.    

  That  large  colonial  assembly  would  be  aero-­‐captured  into  a  large  elliptical  orbit  around  Mars.  Then,  using  a  bi-­‐elliptic  transfer,  the  assembly  would  be  moved  into  an  ASO  around  the  equator  using  less  than  600  m/s  of  delta  V.  Using  common  fuels,  that  orbital  maneuver  could  be  accomplished  by  having  less  than  20%  of  the  initial  mass  as  fuel.    

  Once  in  ASO,  a  base  station  would  be  lowered  towards  Mars  at  the  same  time  the  counterweight  was  sent  further  out.  The  two  processes  would  be  coordinated  in  order  to  keep  the  assembly  at  ASO  until  the  base  station  reached  earth.    

  Unfortunately  the  base  station  would  reach  Mars  at  the  equator,  but  it  needs  to  be  located  about  600  km  north  or  south  of  the  equator.  Therefore,  after  a  mobile  base  station  reached  the  surface,  it  could  be  driven  to  the  proper  location,  although  that  would  require  a  600  km  free  path.  Another  possibility  would  be  to  have  thrusters  on  the  base  station,  and  those  thrusters  would  move  the  station  into  its  position  before  it  even  touched  the  ground.  Even  large  propellers  could  serve  as  the  thrusters  to  locate  the  base  station  at  its  correct  longitude.      

  Once  in  the  correct  location,  the  base  station  would  be  firmly  anchored  in  place.  Then  the  correct  tension  could  be  adjusted  on  the  MSE  cables  with  the  counterweight,  and  

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the  colony  would  be  ready  to  start  unloading.  At  that  point  the  MSE  would  be  fully  operational.    

  While  the  equipment  in  the  assembly  at  ASO  was  being  unloaded  down  to  the  surface,  the  majority  of  the  colonists  would  remain  in  the  rotating  space  craft.  That  rotating  craft  would  remain  as  a  permanent  part  of  the  ASO  space  station.  It  could  have  a  permanent  manned  presence,  or  simply  be  an  empty  station  waiting  for  occupants  to  arrive.  However,  the  colonists  would  likely  leave  it  as  an  empty  shell,  taking  everything  they  can  down  to  the  surface.    

  One  of  the  first  loads  to  come  down  the  MSE  would  have  to  be  a  crane.  Unlike  loading  and  unloading  in  zero-­‐g,  people  in  space  suits  could  not  man-­‐handle  large  objects  on  Mars.  A  10,000  kg  piece  of  equipment  on  Mars  would  weigh  the  same  as  a  3800  kg  load  on  earth.  Therefore,  the  crane  would  have  to  move  and  position  each  load  that  came  down  the  MSE.  

  Once  the  habitats  came  down  the  MSE  and  were  assembled  by  the  initial  crew,  then  the  rest  of  colonists  could  begin  to  come  down  and  also  work  on  the  surface.  Each  person  could  then  start  setting  up  his  own  work  and  living  areas.    

  Having  a  large  initial  colony  greatly  enhances  the  possibility  of  success.  First  of  all,  with  more  people  there  could  be  more  professions  and  disciplines,  with  their  respective  equipment,  represented.  Therefore  there  would  be  more  likelihood  that  all  needed  skills  and  materials  would  be  present  on  Mars.  Also,  with  larger  numbers,  the  possibilities  of  personality  conflicts  are  reduced.    

  The  only  practical  way  to  set  up  an  MSE  is  to  send  it  in  its  completed  form  from  earth.  Even  a  colony  of  one  million  people  on  Mars  would  still  be  so  resource-­‐limited  that  it  is  unlikely  that  they  could  ever  build  an  MSE  on  their  own.    

  The  MSE  would  facilitate  much  more  economical  transportation  to  and  from  earth  because  no  rockets  would  be  needed.  A  load  would  simply  be  sent  out  on  the  MSE  cable  about  20,000  km  farther  than  the  ASO  station  and  released.  That  would  give  the  load  sufficient  delta  V  to  get  back  to  earth.    

  The  initial  cost  of  the  MSE  would  be  part  of  the  setup  cost  of  the  colony,  and  would  probably  be  paid  for  by  the  elimination  of  the  reentry/landing  vehicles  that  would  otherwise  be  required.    

  Martian  landing  vehicles  are  very  costly,  complicated,  and  risky  machines  comprised  of  heat  shields,  parachutes,  thrusters,  aerodynamic  surfaces,  and  maneuvering  devices.  Eliminating  landing  vehicles  would  eliminate  a  tremendous  complication  and  cost  for  a  Mars  colony.  The  landing  vehicles  would  be  one  the  most  difficult  parts  of  a  non-­‐MSE  colonization  effort.  Also  any  provision  for  vehicles  to  return  to  space  from  the  surface  would  add  a  great  deal  more  to  the  complication.  

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  In  addition,  using  an  MSE  to  lower  the  colony  to  the  surface  would  eliminate  any  possibility  that  a  load  missed  its  landing  site  and  ended  up  in  an  unretrevable  location.  Even  if  every  landing  hit  its  designated  site,  there  would  still  be  transportation  issues  in  bringing  each  load  to  the  colony  site.  On  the  other  hand,  the  MSE  would  deliver  every  load  to  the  exact  same  spot.    

  With  a  200,000  kg  MSE,  and  a  location  of  12°  north  or  south  latitude,  the  initial  mass  of  the  whole  colony  assembly  could  be  540,000  kg.  With  an  even  larger  initial  mass,  the  MSE  could  be  located  farther  away  from  the  equator,  or  the  equipment  and  habitats  could  be  in  separate  loads  from  the  colonists.      

  After  the  majority  of  the  colonists  and  the  equipment  on  the  first  load  were  lowered  to  the  surface,  those  additional  loads  could  dock  with  the  ASO  station.  Those  loads  would  initially  be  in  a  non-­‐equatorial  synchronous  orbit.  Near  the  point  of  maximum  departure  from  the  equatorial  plane,  they  would  be  captured  as  needed  by  the  ASO  station  and  their  cargo  lowered  down  to  the  Martian  surface.    

  Even  with  a  mass  of  200,000  kg,  the  MSE  could  still  end  up  being  only  be  a  small  portion  of  the  total  mass  of  the  initial  colony.  Therefore,  the  MSE  could  end  up  having  less  total  mass  than  the  mass  of  the  required  reentry  vehicles  for  landing  the  same  colony.    

  The  bottom  line  is  that  an  MSE  could  end  up  being  simpler,  cheaper,  less  risky,  and  require  less  mass  than  using  reentry  vehicles  for  colonizing  Mars.  

  Because  the  MSE  would  have  already  paid  for  itself  in  the  initial  colonization  effort,  any  subsequent  use  would  just  involve  the  incremental  cost  of  operating  it.  Therefore,  goods  and  materials  could  be  shipped  from  Mars  to  earth  rather  cheaply.  The  primary  cost  would  be  a  container  capable  of  surviving  earth  reentry.  

  Future  transports  from  earth  could  dock  with  the  ASO  station  by  entering  a  non-­‐equatorial  synchronous  orbit,  just  like  any  multiple  loads  from  the  original  colony.  After  unloading  its  cargo,  the  earth  transport  could  then  be  reloaded  with  cargo  and  passengers  from  Mars,  and  then  be  sent  back  to  earth.    

  However,  even  with  an  MSE  and  an  SE  on  earth,  the  practical  launch  windows  would  still  be  about  two  years  apart.  Therefore,  there  would  be  a  flurry  of  activity  around  those  launch  windows,  but  little  use  between  them.    

CONCLUSION  

  Space  elevators  are  coming.  The  necessary  material  will  be  available  sooner  or  later.  It  may  be  carbon  nonotubes,  or  it  could  be  some  other  allotrope  of  carbon  like  graphene,  colossal  carbon  tubes,  linear  carbyne,  or  polycumulene.  Even  lowly  polyethylene,  the  material  of  garbage  bags,  could  be  made  strong  enough  to  make  space  elevators  if  the  molecular  chains  were  long  enough  and  the  chains  aligned.  Several  polymers  of  boron  could  also  be  strong  enough  to  make  a  space  elevator.  One  way  or  other,  it  will  happen.  

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  An  MSE  would  be  the  most  practical  means  of  getting  a  large,  well  equipped  colony  established  on  Mars,  and  would  provide  the  means  for  economical  transportation  back  and  forth  from  earth.  It  is  the  opinion  of  the  author  that  Mars  will  be  colonized  by  an  MSE  long  before  it  could  be  colonized  by  any  other  means.  

 

Figure 1

 

            Figure  2  

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Figure  3