Tailor-Made Plants Using
Next-Generation Molecular Scissors
Lifecycles of Lithium-Ion Batteries:
Understanding Impacts from
Material Extraction to End of Life
Gabrielle G. Gaustad
Building Smarter Water Systems
The Electric Solar Wind Sail (E-sail):
Propulsion Innovation for
Solar System Travel
Sini Merikallio and Pekka Janhunen
Functional Natural Materials for
Michael H. Ramage
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The E-sail will enable space travel and exploration with
higher speed, better mass economy, and at less cost.
The electric solar wind sail (E-sail) is a novel propulsion concept that
enables fast and economic space travel in the solar system. For propulsion
it utilizes a continuous particle stream from the Sun (i.e., solar wind) by
deploying long, electrically conductive charged tethers, which through elec-
tric force interaction are pushed by the charged solar wind particles, mainly
protons (Janhunen et al. 2010). The E-sail thus provides constant thrust
without fuel consumption, enabling more ambitious space missions than cur-
In this paper we explain how the E-sail works and review some advantages
and challenges of the technology. We then describe some specific possibili-
ties that it opens for solar system travel and exploration: asteroid mining of
water and metal ores, support for a manned Mars presence, and the reduction
of space debris.
The Electric Solar Wind Sail: Overview
The physical principle of the E-sail was discovered in 2004 (Janhunen 2004)
and the technical concept in 2006 (Janhunen 2010a). The E-sail is currently
Sini Merikallio and
The Electric Solar Wind Sail (E-sail):
Propulsion Innovation for Solar System Travel
Sini Merikallio is a student at the University of Helsinki Faculty of Veterinary
Medicine; she was previously a scientist at the Finnish Meteorological Institute, Earth
Observations. Pekka Janhunen is research manager, Space Research and Observation
Technologies, Finnish Meteorological Institute.
under development by
the Finnish Meteoro-
logical Institute (https://
NASA (the Heliopause
Electrostatic Rapid Tran-
sit System, HERTS),
and the European Space
Agency (ESA; unpub-
The possible applica-
tions of the E-sail are
numerous and promis-
ing. It may be used to
support manned Mars
flight (Janhunen et al.
2015), tow an Earth-
threatening 3 million
ton asteroid to a more
benign track ( Merikallio
and Janhunen 2010), or
deliver a probe to Mer-
cury within a year with-
out any gravity assists
(Quarta et al. 2010).
It will be ideal for a cometary rendezvous ( Quarta et
al. 2016), fast planetary entry probe ( Janhunen et al.
2014), or asteroid investigations and sample returns
(Quarta and Mengali 2010a; Quarta et al. 2014).
Travel ling toward the edges of the Solar system, the
E-sail will make it possible to reach the heliosheath in
15 years (Quarta and Mengali 2010b), a feat that took
the Voyager spacecraft 27 and 30 years (Decker et al.
2008; Stone et al. 2005).
How It Works
Thrust for the E-sail is produced by the interaction of
charged tethers with solar wind particles: deflected by
the electric potential surrounding the tethers, the par-
ticles transfer some of their momentum to the E-sail.
Solar wind consists mainly of hydrogen and helium
nuclei, and a comparable number of electrons. All of
these contribute to the thrust of the E-sail, although
most of the wind’s momentum is a function of the more
massive positively charged particles.
Figure 1 shows an artist’s impression of an E-sail
design; the size of the solar wind particles and spacecraft
is hugely exaggerated, and the numbers of tethers, pro-
tons, and electrons are not representative. Wire tethers
are deployed from the spacecraft and their extension
maintained by centrifugal force due to rotation of the
The produced thrust of an E-sail is inversely propor-
tional to its distance from the Sun, F α (1/r) (Janhunen
et al. 2010), in contrast to the traditional photonic solar
sail, for which F α (1/r2). The reason behind this is that,
with greater distance from the Sun and a corresponding
attenuation of the solar wind, the effective area around
the charged E-sail wires increases. In other words, the
impact of the wire potential extends farther from the
sail as the plasma density dwindles, resulting in better
performance than with photonic sails, for which the
area of the sail stays constant.
Advantages and Challenges
The E-sail requires no propellant, and discharging of
the wires by the solar wind thermal electrons can be
counter acted by an electron gun powered by solar panels
of a modest size. To enable maneuvering and trajectory
control, the E-sail thrust can be steered by controlling
the voltage of individual tethers and thus changing the
plane of the E-sail’s rotation. At 1 astronomical unit
(au) of distance from the Sun, approximately 2,000 km
FIGURE 1 Artist’s impression of an electric solar wind sail showing the spacecraft from which dozens
of tethers (green) are deployed. The whole structure rotates in a cartwheel fashion around the space-
craft to keep the tethers centrifugally stretched. Also shown are solar wind particles (protons [+] and
electrons [e−]) and their tracks affected by the electric charge of the tethers. The widths of the tethers
and the size of the spacecraft are greatly exaggerated. Image by Alexandre Szames/Antigravite.
of E-sail tether are required to produce 1 newton (N)
of thrust. This can be achieved with, for example, 100
tethers, each 20 km long, spun out centrifugally from
the spacecraft. There are no technological showstoppers
in sight for producing an E-sail like this.
Space is dense with tiny dust particles that threat-
en the integrity of the E-sail. The risk of this micro-
meteorite impact is mitigated by weaving the E-sail
tether into a 2–3 cm wide mesh-like structure of several
wires so that isolated damages in constituent wires do
not jeopardize the whole (Seppänen et al. 2011).
E-sail tethers need to be lightweight, conductive,
resistant to micrometeoroid impacts, and able to with-
stand the tension and pull created by the centrifugal
acceleration. The number and lengths of the tethers can
vary. Their diameter is restricted by the need to limit
surface area so as not to generate excessive thermal elec-
tron current. Such current would need to be cast off
by the electron gun, the use of which decreases perfor-
mance by increasing power system energy consumption.
Given mechanical (tensile strength, surface area, and
weight) and availability (workability and industrial sup-
ply) requirements, the material currently under consid-
eration for the tethers is 25–50 μm diameter aluminum
alloy wire. Each kilometer of the tether weighs 10 g
(Seppänen et al. 2013), resulting in a total tether mass
of just 20 kg for a 2,000 km E-sail. The whole propulsion
unit—including supporting structures, electron guns,
power systems, and design margins—weighs 50–200 kg
(Janhunen et al. 2013), far less than the weight of cur-
rently used propellant technologies. These features give
the E-sail a significant advantage, especially in sample
return missions and campaigns with many targets.
In the future, carbon nanotube technology might
further enhance the E-sail by allowing the manufacture
of longer, more lightweight yet durable and conduc-
tive tethers (Lee and Ramakrishna 2017; Monthioux
et al. 2017).
Asteroid Mining: Rocket Fuel from Water
The E-sail will permit very low cost freight carriage in
the solar system and thus enable affordable asteroid
mining operations. It can be used for the transporta-
tion of mining equipment to asteroids and return of the
mined products. One E-sail can make several trips to
and from asteroids during its estimated 10 years of life.
The technology can be easily multiplied and operations
could proceed on several asteroids simultaneously.
In addition to relatively rich heavy metal ores in aster-
oids, our interest was raised by another reserve: an abun-
dant number of water-bearing asteroids on near-Earth
orbits (Elvis 2014) that can be readily accessed by the
E-sail (Quarta 2014). The water can be separated from
the asteroid material by using a two-part container (fig-
ure 2) in which the water is evaporated from the asteroid
regolith in the first chamber and then pressure driven
into the other chamber to condense into ice ( Janhunen
et al. 2015). The temperatures of the containers can be
controlled by their surface albedos and infrared emis-
sivities (i.e., coating by colored metal or white paint)
or by using additional
shades, heat pumps,
or solar- powered heat
elements. Once filled,
the second con tainer
can be separated
and hauled to the
orbit of the Earth or
The resulting water
can be split into
hydrogen and oxygen,
which form a potent
spacecraft fuel when
liquefied. This process
which in space is read-
ily available via solar
panels. Currently all
FIGURE 2 Illustration of a two-chamber unit that can be used in situ to extract water from asteroid
regolith. Asteroid material is heated in the first chamber (left) so that water in the material vaporizes.
Pressure gradient drives the water vapor into the second chamber (right), where it cools and condenses.
baking unit condensation
centrifugal force and pressure
by heat pump
heating of oven by solar panels or direct absorption
the fuel used by a space-
craft has to be lifted from
the surface of the Earth
and carried throughout
the mission, requiring
enormous fuel mass frac-
tions. As an example,
NASA’s Juno mission
to Jupiter, launched in
2011, had a liftoff mass of
3,625 kg, of which propel-
lant accounted for more
than 2,000 kg. We have
come up with an approach
to address this challenge,
as described in the next
EMMI: Manned Mars
Flights Facilitated by
In 2015 we proposed the
Mars Initiative (EMMI;
Janhunen et al. 2015). The
idea behind EMMI is to
mine water from asteroids
and bring it to space-based
“gas stations” in the orbits of Earth and Mars where it
can be turned into rocket fuel. Such stations—with two
on the way to/from Mars (figure 3)—can significantly
facilitate manned Mars exploration in the near future.
Orbital fuel tank refills will allow for smaller tanks
and thus considerably lighter spacecraft. Moreover, the
spacecraft that lifts passengers and cargo from the sur-
face of the Earth into orbit can be different from that
which taxis between Earth and Mars. This will reduce
the design requirements of both vehicles, as the one
carrying passengers from Earth will not need to have
capabilities for long-term life support, and the traverse
shuttle will not need to survive atmospheric entry and
launch vibrations and thermal loads. In addition, the
availability of virtually free fuel on the Martian orbit
will increase mission safety and enable speedy returns
The asteroid-extracted water can also be used in life
support as a source of potable water and even oxygen
for breathing. Thick water layers around manned space-
craft and surface habitation modules can function as a
radiation protection shield during the long traverses
between Earth and Mars.
These spacecraft can be operated at a fraction of the
current estimated Mars colonization costs: once in place,
the EMMI is estimated to run on a budget comparable to
the maintenance costs of the International Space Station
(ISS). Moreover, launchers used for setting up EMMI can
be of the same scale as those used for building the ISS.
A spin-off from the E-sail technology, a plasma brake,
can be used to bring small satellites down from their
orbits at the end of their viable life (Janhunen 2010b,
2014; Orsini et al. 2018). It can be attached to existing
satellites and space debris with, for example, harpoons.
Advantages of the plasma brake are low weight, poten-
tially low cost, and high safety, as it can be operated
without any volatiles, explosives, or inflammables.
A plasma brake payload is currently flying on a
low Earth orbit (LEO) CubeSat mission, the Finnish
Aalto-1, and waiting to be tested using a short (100 m)
FIGURE 3 Schematic presentation of E-sail–facilitated Manned Mars Initiative (EMMI). At the
heart of EMMI are asteroid mining operations: water from an asteroid (bottom) is transported to the
planetary orbit and refined into liquid oxygen/liquid hydrogen LOX/LH2 fuel, which can be used
for transportation to and from Mars. Pictures of the planets and asteroid surface are by NASA and
not presented at scale.
Asteroid mining for water
baking unit condensation
centrifugal force and pressure
by heat pump
heating of oven by solar panels or direct absorption.
E-sail tether (Kestilä et al. 2013). It is important to note
that the relative speed of the spacecraft and ionosphere
(~7 km/s) is not comparable to the solar wind speed
(~400 km/s). However, as the tether voltage is varied in
sync with the rotation of the satellite, the E-sail effect
will be observable in changes in the CubeSat’s rota-
With Aalto-1, researchers are looking forward to
verifying, and measuring, the E-sail force in real space
Summary and Discussion
The design, production, and testing of electric solar
wind sail prototypes are making good progress. E-sail
technology could be available for solar system research
within 10 years and, if successful, may revolutionize the
way space travel and exploration missions are conceived
and executed. The E-sail will enable affordable continu-
ous manned Mars presence, considerably decrease travel
times in the solar system, make it possible to tackle space
debris, and help facilitate asteroid mining operations.
The E-sail thus holds great promise for accessing both
scientific and economical treasures of the solar system.
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