Space Elevators involve lowering a tether from synchronous orbit down to the surface of a planet, then electromechanically hauling payload up the tether to space. While theoretically possible, the concept has been shown to be infeasible on Earth until the development of mass-produced ultra-lightweight materials with specific tensile strengths of ~40 MPa/kg/m^3 (~20 times stronger than Kevlar). Such strength is within the theoretical limits of Carbon Nanotubes (CNTs), but it is not known when practical commercially available CNTs will reach this required strength. On Mars, however, the lower surface gravity and lower synchronous orbit altitude allow a space elevator to be built from materials with specific strengths of only ~5 MPa/kg/m^3, which is within the range of existing CNTs, provided such materials could be mass produced. The required tether mass and length is also significantly reduced from 9,000 tonnes and 155,000 km at Earth to only 1,500 tonnes and 70,000 km at Mars. The driving engineering limits for construction of a space elevator will be compared between Earth and Mars, and an industrial/economic analysis will be presented to quantify the project scale, timeline, cost, and expected economic activity Mars will likely have to support before a Martian space elevator would become a profitable investment. Presented at the 2018 Mars Society Conference in Pasadena California.

1. Why the Solar System’s First
Space Elevator Will Likely be
Martian
Max Fagin
Made In Space, Inc.
NASA Ames Research Center
Image credit: Erik Wernquist, “Wanderers”

2. Space Elevator Basics
Concept
Space Elevators can be as efficient as a motor + solar array:
30-60% efficient
~$5 of electricity per kg
By contrast…
Rockets can only be as efficient as an explosion:
<0.1% efficient
~$15,000 of rockets per kg
Synchronous Orbit
Low Orbit
To other planets, asteroid etc.
Need ~55 MJ per kg to Escape Earth

3. Space Elevator Basics
History
1960: Concept formalized by Yuri Artsutanov as “Electric
Railroad to the Stars” but no known materials had sufficient
strength.
…but no one really knows.
1970-80’s: Carbon Nano Tubes (CNTs) first characterized as
theoretically capable of reaching the required strength.
Today: Millimeter scale CNTs of the required strength
are regularly grown in labs, but we have no idea how to
scale up production to industrial levels (1k-10k tonnes,
~10,000 km).
2035-2065: Predicted range when CNT experts say we will
have solved the problem…
1865: Idea proposed by Konstantin Tsiolkovsky
Image Credit: Roger Gilbertson

4. Space Elevator Basics
Elements
Apex Station
- Scrapyard
- Interplanetary Transport
Anchor Station
Synchronous Station
- Sat deployment and recovery
Climbers
- Lifting Capacity
- Tether Maintenance
- Tether Dynamics Control
Tether

5. Space Elevator Basics
To be self supporting, the tether must be able to survive gravity (~r -2) and centrifugal force (~r)
Tether must bulge in the middle and tapper at the ends.
Thether Thickness = A(r) ∝ exp
𝜌
σy
GM
1
R
+
4π2
GMT2 R −
r2
2
−
1
r
Tether Material Properties Planet Properties
𝜌 = Density M = Mass
σy = Yield Stress R = Radius
T = Length of Day
r
AsyncAsurf
Cross Sectional Area at Synchronous Altitude
Cross Sectional Area at Surface
“Taper Ratio” = τ =
Async
Asurf
=
Tether

6. Space Elevator Basics
Taper Ratio is determined by material:
Strength per density = Pa/(kg/m3) = “Yuri” = (m/s)2
Steel: 0.1 MYuri, Taper ~10200
Kevlar: 2 MYuri, Taper ~108
CNT: 30 MYuri, Taper ~5
Tether

7. Space Elevator Basics
Tether
Taper Ratio =
Async
Asurf
= exp
𝜌
σy
GM
R
1 − R
GMT2
4π2
−
1
3
2
+ 1 +
R
2
GMT2
4π2
−
1
3
Tether Material Properties Planet Properties
𝜌 = Density M = Mass
σy = Yield Stress R = Radius
T = Length of Day
r
AsyncAsurf
Taper Ratio ∝
exp Planet Radius
exp Planet Mass1/3
exp Day Length−4/3
It is currentally 250 times easier to build a space elevator on Mars than on Earth

8. Mars vs. Earth
1,000
10,000
100,000
1,000,000
10,000,000
1
10
100
1,000
10,000
1 2 4 8 16 32 64 128
TetherMass(Tonnes)
TaperRatio(Asynchronous/Asurface)
Tether Material Specific Strength (MPa/kg/m3 , MYuri)
Space Elevator Construction Requirements
CNT(Realistic)
CNT(TheoreticalLimit)
CNT(Current)
Kevlar
Tether Mass

9. [CELLRANGE]
[CELLRANGE]
[CELLRANGE]
[CELLRANGE]
[CELLRANGE]
[CELLRANGE]
[CELLRANGE]
[CELLRANGE]
[CELLRANGE]
[CELLRANGE]
[CELLRANGE]
Asteroids
Mars vs. Earth
Accessible Destinations
If Mars is serving as an exporter to the outer solar system/asteroids, a space elevator is a
means of reducing ΔV requirements to every destination.
Destination Detach Altitude (km) Required ΔV (km/s)
Asteroids 39,172 – 73,404 1.198 - 3.296
Earth 51,094 3.236
Venus 78,343 6.073
Jupiter 93,246 4.087
Saturn 113,854 4.461
Uranus 131,736 4.093
Neptune 136,897 3.733
(Diagram to scale)

10. Mars vs. Earth
Construction Requirements
Assuming tether is to be manufactured:
- In synchronous orbit
- Using a carbon rich (C-type) asteroid as feedstock
- Density 2700 kg/m3
- 3% Carbon Content
- 80% Process Yield
- Present day cutting edge CNT bundle strength (~7 MYuri) and manufacturing rates
- 5000 CNT reactors working at 1.0 kg/month/reactor (2019 Industry expert prediction by 2019)
Mars Earth
Tether Mass 4,000 tonnes 13,000,000 tonnes
Tether Length 69,000 km 151,000 km
Required Asteroid Mass 170,000 tonnes 540,000,000 tonnes
Required Asteroid Diameter 50 m 725 m
Manufacturing Time 35 years 210,000 years
*Industry expert prediction of achievable by end of 2018

11. Mars vs. Earth
Terrestrial Concerns
On Earth, base station, climbers and tether must survive:
- Hurricanes
- Wind shear
- Lightning strikes
- Atomic oxygen in the ionosphere
- Induced ionospheric currents (CNTs are conductive)
- NIMBYs
On Mars, none of these problems apply.
Tethers and climbers can be optimized
for one environment (space).
Equator
Image Credit: ISEC

12. Potential Issues
Power Availability
Climbers will probably use solar power to ascend the tether.
1370 W/m2 of solar power available on Earth, only 590 W/m2 at Mars (57% reduction)
57% irradiance reduction at Mars, but also 62% reduction in gravity.
A climber designed for Earth would be +14% faster on Mars.
Image Credit: ISEC

13. Potential IssuesWhat about Phobos?
You are Here
Phobos
(to scale)
LAX

14. Potential Issues
Phobos Properties
Size 27 x 22 x 18 km
Mass 1013 tonnes
Altitude 5820-6100 km
Inclination ~1.0˚
Period 7.6 hrs
Relative Velocity 1,470 m/s
~Mach 4!
What about Phobos?
Solution:
“The Twang”
a.k.a. “The Clarke Oscillation”
Fly-by every 0.31 sols
Close pass every ~4 sols
Collision inevitable every ~14 sols.

15. Potential IssuesWhat about Phobos?
Phobos
(To Scale)
100 km Tether
Keep Out Cylinder
Phobos / Space Elevator Impact Risks
Jan 01 - Feb 28, 2030
Equator
Phobos orbital tracks
(60 sols)
Close pass every ~4 sols. Collision is inevitable every ~14 sols.

16. Potential IssuesWhat about Phobos?
5820
5840
5860
5880
5900
5920
5940
5960
5980
6000
6020
6040
6060
6080
6100
6120
6140
6160
6180
6200
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
TetherAltitude(km)
Phobos Centroid Distance from Tether (km)
Phobos / Space Elevator Impact Risks
Jan 01 - Feb 28, 2030
Collision
Flyby
Closepass
Phobos
(To Scale)
Close Approaches
Collisions can be avoided provided one climber every ~14 sols is devoted to collision avoidance

17. Potential IssuesWhat about Phobos?

18. What if it falls down?
It almost certainly won’t. But IF it did…

19. Summary
Image credit: Erik Wernquist, “Wanderers”
- Space Elevator construction is 250-to-30 times easier on Mars than on Earth
- Construction only requires space infrastructure (minimal planet support)
- Not clear at present when (if ever) CNTs will reach the required strength for
building a space elevator for Earth (Maybe start in 2060?)
- CNT industry is already at the point where it could construct enough CNTs
for a Martian elevator in ~35 years even with zero further improvements.
- So which will happen first? Mars export economy or Earth rated CNTs?
Questions?

20. Mars vs. Earth
1,000
10,000
100,000
1,000,000
10,000,000
1
10
100
1,000
10,000
1 2 4 8 16 32 64 128
TetherMass(Tonnes)
TaperRatio(Asynchronous/Asurface)
Tether Material Specific Strength (MPa/kg/m3 , MYuri)
Space Elevator Construction Requirements
CNT(Realistic)
CNT(TheoreticalLimit)
CNT(Current)
Kevlar
Tether Mass

21. Backup Slides: CNT’s
CNT’s have been made that satisfy specific strength requirements.
So what’s the hold up?
Requires 2000 tonnes of defect free CNTs.
Would take ~20,000 years to grow
Strong?
Large Amounts? Fast?
Pick One

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