Kurzgesagt – In a Nutshell

Sources – Skyhooks

We would like to thank the following experts and researchers for their scientific support:

  • Prof. Matthew Caplan

Assistant Professor of Physics at Illinois State University

  • Malik K. (Matterbeam)



‘Tether’ and ‘skyhook’ are occasionally used interchangeably in the literature (and sources referenced here), and authors each have their own definitions for these terms which vary with the proposed design. Some authors use ‘tether’ and ‘skyhook’ to refer exclusively to components of the launch system, i.e. the long cable and end-points for spacecraft attachment respectively. Others use ‘tether’ to refer to nonrotating designs and ‘skyhook’ to refer to rotating designs and their variants. ‘Skyhook’ is also used to describe nonrotating variants, and in certain cases, variants with carefully chosen rotations which could be stationary in an appropriately chosen reference frames. Ultimately, this technology is an exploratory phase and lacks rigorous definition of terms, and care should be taken to notice how exactly these terms are being used by any given author.

– Rockets need to reach a velocity of about 40,000 km/h to escape from Earth.

#Earth: By the numbers, retrieved, 2019


– There is a promising technology that has been tested successfully in orbit.

#YES2 team claims a space tether world record, 2007


– A cable and a weight. So called tethers.

#How an Earth Orbiting Tether Makes Possible an Affordable Earth-Moon Space Transportation System, 1994


#History of the Tether Concept and Tether Missions: A Review, 2013


– This concept is known as the Skyhook.

#Satellite Elongation into a True "Sky-Hook", 1966


#How an Earth Orbiting Tether Makes Possible an Affordable Earth-Moon Space Transportation System, 1994


– It works even better if we make it spin!

#A non-synchronous orbital skyhook, 1977


#History of the Tether Concept and Tether Missions: A Review, 2013


– A rotating tether of just the right length, and in just the right orbit slows down its tip relative to the ground at the bottom, and speeds it up at the top like a catapult.

#Design Analysis of a Capture Mechanism for Rendezvous Between a Space tether and Payload, 2006



The design we use in the video is called a “momentum exchange-tether”. It’s super effective, but this type of tether is a bit tricky. To receive a maximum performance transporting the payload, the tether needs to be placed very accurately with the right speed in Earth’s orbit and it would need elaborate mechanics to catch the payloads. So exact calculations are key here.

As we describe multiple designs and occasional variations of them throughout this video, the presentation of detailed design specs for each are the beyond the scope of our video.

– Specialized fibers already exist that can survive the extraordinary stresses a skyhook would be faced with.

#Aramid fibers (kevlar and technora), 2011


#The Second Young Engineers’ Satellite: Innovative Technology through Education, 2010


#Conceptual Design and Analysis of Space Tether Transportation System With Electrodynamic Propulsion, 2015


#The Hoytether™: A Space-Survivable Tether Structure, 2018


– The tether’s tip is dashing through the atmosphere at around 12,000 km/h.

Reference for calculation:

#Hypersonic Airplane Space Tether Orbital Launch System, 2000, S. A3-4



We took the numbers from the source above as reference and calculated that a direct Earth to Mars tether would have its tip at the lowest point of its rotation travel at a velocity of 3310 m/s over the ground, which corresponds to exactly 11,916 km/h.

It’s also interesting that certain designs, such as the one proposed by Moravec (1977), consider a tether whose rotation is chosen such that its instantaneous velocity when touching the atmosphere is minimized, and effectively zero at the tip during the moment of maximum extension into the atmosphere.

– So it will dip to a height of 80 to 150 kilometers and no lower.

#Hypersonic airplane space tether, 2000, A3 14/15


– It is possible to balance the payloads coming in and being sent off.

#Tethers in space handbook, second edition, 1989, S.117


– We can recover energy by slowly pushing against Earth’s magnetic field or with small electric or chemical engines that regularly correct the tether’s position.


By electric engines, we mean ion thrusters:

#The Incredible Ions of Space Propulsion, 2000


– Tethers could make trips between the planets fast, straightforward and low-cost.

#Mars-earth rapid interplanetary tether transport (MERITT) system: I. Initial Feasibility Analysis, 1999


– The mars tether could pick up a vehicle traveling at only about 1,000 km/h.


We want to lower the spacecraft caught by the Mars tether as close to the surface as possible, so it can land there with minimal extra requirements in terms of wings or parachutes. We deem a release altitude of 10 km the safest minimum to avoid our tether tip crashing into hills and mountains.

The tether length needs to be as long as possible to decrease the g-forces crews and payload experiences.

If we take a tether with a radius of 600km, orbiting at 610km above Mars, it would rotate with 3059m/s. That means our crew and cargo would have to deal with only 1.59g of acceleration, which is even less than in a roller coaster on Earth.

And since we put our tether in a 610 km high orbit, our tether can drop the spaceships and cargo 10 km above the martian surface. This distance guarantees that the tether tip won’t crash into higher surface features. Crew and cargo will arrive in the martian atmosphere at a velocity of 252m/s or 907.2km/h, which is almost the speed of airplanes here on Earth. So, it should be manageable to get spacecraft to the ground. And of course speed them up to this velocity, so they can be picked up by the tether.

– Tethers could shorten trips between both planets from 9 months to 5 or 3, if the constellations are right.

#Interplanetary Mission Design Handbook: Earth-to-Mars Mission Opportunities and Mars-to-Earth Return Opportunities 2009-2024, 1998, Appendix E


#Mars-earth rapid interplanetary tether transport (MERITT) system: I. Initial Feasibility Analysis, 1999, S. 14/15



The 3 month figure is for a spacecraft boosted by a 4km/s rotation velocity tether in Low Earth Orbit (as in the MERITT study) that further accelerates by another 1,000m/s using onboard propulsion. It is calculated with the help of this trajectory planning tool:

#Easy Porkchop, retrieved 2019


– ...and reduce the scale of the rockets required by 84 to 96%.


Savings with tether vs rocket using kerosene-oxygen engines: 96.3%

Savings with tether vs rocket using hydrogen-oxygen engines: 84%

How did we calculate this?:

A tether-boosted rocket only needs to catch a tether from Earth and land on Mars.

DeltaV to Tether = 4600m/s. DeltaV to Mars landing (using parachutes): free.

If this rocket uses kerosene-oxygen engines, it needs 3.5 kg of fuel per kg of empty mass.

In comparison, a kerosene rocket would need 96,6 kg:

1-3,5 kg / 96,6 kg= 96,3% savings


Merlin-1D build by SpaceX

#Inside SpaceX’s Revolutionary Merlin Engines That Could Take Us to Mars, 2017


#SpaceX Unveils Plans To Be World’s Top Rocket Maker, 2011


In comparison a hydrogen rocket would need 23 kg:

1-3,5 kg / 23 kg= 84% savings



#RS-25 Engine, 2019


– Phobos is the perfect point for super tethers just under 6,000 km long.

#Space Colonization Using Space-Elevators from Phobos,


#Phobos: Facts About the Doomed Martian Moon, 2017



There are two numbers appearing in the context of the Mars-Phobos proximity constantly. While the distance between the core of Mars and Phobos is 9378 km, 6000 km is the distance between their surfaces. So the latter number is more relevant to us and our tether-project.

Further reading:

– History of tested tethers:

#History of the Tether Concept and Tether Missions: A Review, 2013