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{{Short description|Proposed spacecraft propulsion device}}
{{distinguish|magnetic sail|photon sail}}
[[File:Heliopause Electrostatic Rapid Transit System - HERTS.jpg|thumb|300px|The Heliopause Electrostatic Rapid Transit System (HERTS) is a spacecraft concept using an electric sail]]

An '''electric sail''' (also known as an '''electric solar wind sail''' or an '''E-sail''') is a proposed form of [[spacecraft propulsion]] using the [[dynamic pressure]] of the [[solar wind]] as a source of thrust. It creates a "virtual" sail by using small wires to form an [[electric field]] that deflects solar wind protons and extracts their momentum. The idea was first conceptualised by [[Pekka Janhunen]] in 2006 at the [[Finnish Meteorological Institute]].<ref>[https://archive.today/20121212003450/http://www.wipo.int/pctdb/en/wo.jsp?WO=2007099201 Electric Sail For Producing Spacecraft Propulsion.] Patent filed on 2 February 2007; ''PatentScope''.</ref>

== Principles of operation and design ==
[[File:ESTCube solar wind infograph.jpg|thumb|upright=1.4|Principles of operation of an electrical sail]]
The electric sail consists of a number of thin, long and conducting [[electrodynamic tether|tethers]] which are kept in a high positive potential by an onboard [[electron gun]].<ref name="HERTS 2015"/> The positively charged tethers deflect solar wind protons, thus extracting momentum from them. Simultaneously they attract electrons from the solar wind plasma, producing an electron current. The electron gun compensates for the arriving electric current.

One way to deploy the tethers is to rotate the spacecraft, using [[centrifugal force]] to keep them stretched. By fine-tuning the potentials of individual tethers and thus the solar wind force individually, the spacecraft's [[aircraft attitude|attitude]] can be controlled.

E-sail missions can be launched at almost any time with only minor variations in travel time. By contrast, conventional [[gravity assist|slingshot]] missions must wait for the planets to reach a particular alignment.<ref name=tr1401/>

[[File:ESTCube orbiidil 2.jpg|thumb|Artist's [[Rendering (computer graphics)|rendering]] of [[ESTCube-1]], launched in May 2013, which was intended to be the first satellite to test an electric sail.]]
The electric solar wind sail has little in common with the traditional [[solar sail]]. The E-sail gets its momentum from the [[solar wind]] ions, whilst a photonic sail is propelled by [[photon]]s. Thus, the available pressure is only about 1% of photon pressure; however, this may be compensated by the simplicity of scale-up. In the E-sail, the part of the sail is played by straightened conducting tethers (made of wires) which are placed [[radius|radially]] around the host ship. The wires are electrically charged and thus an [[electric field]] is created around the wires. The electric field of the wires extends a few dozen metres into the surrounding solar wind plasma. The penetration distance depends on the solar wind plasma density and it scales as the plasma [[Debye length]]. Because the solar wind electrons affect the electric field (similarly to the photons on a traditional solar sail), the effective electric radius of the tethers is based on the electric field that is generated around the tether rather than the actual tether itself. This fact also makes it possible to manoeuvre by regulating the tethers' electric charge.

A full-sized sail would have 50–100 straightened tethers with a length of about {{cvt|20|km|mi}} each.
<ref>{{Cite journal | last1 = Janhunen | first1 = P. | title = Electric Sail for Spacecraft Propulsion | doi = 10.2514/1.8580 | journal = Journal of Propulsion and Power | volume = 20 | issue = 4 | pages = 763–764 | year = 2004 | s2cid = 122272677 }}</ref>

<ref>{{Cite journal | last1 = Janhunen | first1 = P. | last2 = Sandroos | first2 = A. | doi = 10.5194/angeo-25-755-2007 | title = Simulation study of solar wind push on a charged wire: Basis of solar wind electric sail propulsion | journal = Annales Geophysicae | volume = 25 | issue = 3 | pages = 755 | year = 2007 |bibcode = 2007AnGeo..25..755J | url = http://hal.archives-ouvertes.fr/docs/00/31/82/98/PDF/angeo-25-755-2007.pdf | doi-access = free }}</ref><ref>{{cite web |url=http://www.electric-sailing.com |title=The electric solar wind sail by Pekka Janhunen |access-date=2008-04-18 }}</ref> Compared to a reflective solar light sail, another propellantless deep space propulsion system, the electric solar wind sail could continue to accelerate at greater distances from the Sun, still developing thrust as it cruises toward the outer planets. By the time it reaches the [[ice giant]]s, it may have accumulated as much as {{cvt|20|km/s|mph km/h}} velocity, which is on par with the ''[[New Horizons]]'' probe, but without [[gravity assist]]s.

In order to minimise damage to the thin tethers from [[micrometeoroid]]s, the tethers would be formed from multiple strands, 25–50 micrometers in diameter, welded together at regular intervals. Thus, even if one wire were severed, a conducting path along the full length of the braided wire would remain in place. The feasibility of using [[ultrasonic welding]] was demonstrated at the University of Helsinki in January 2013.<ref>[http://www.scienceworldreport.com/articles/4394/20130110/superthin-wire-electric-sail-space-propulsion-esail.htm Superthin wire for electric sail space propulsion engineered], Mark Hoffman, ''Science World Report'', 10 Jan 2013.</ref>

== Development history ==
[[File:E-sail tale.jpg|thumb|upright=1.4|Graphical overview electric sail development]]

[[Academy of Finland]] has been funding electric sail development since 2007.<ref>{{cite web| title=Suomen Akatemia Rahoituspäätökset (Academy of Finland Funding decisions)| url=http://webfocus.aka.fi/ibi_apps/WFServlet?ekaLataus=0&IBIF_ex=x_RahPaatYht_report2&IBIAPP_app=aka_ext&UILANG=fi&SANAHAKU=&ETUNIMI=Pekka&SUKUNIMI=Janhunen&SUKUPUOLI=FOC_NONE&HAKU=FOC_NONE&ORGANIS=FOC_NONE&TUTKDI=FOC_NONE&TMK=FOC_NONE&PAATVUOSI_A=2001&PAATVUOSI_L=2017&LAJITTELU=PAATOS&TULOSTE=HTML| access-date=Jan 2, 2022| archive-date=August 24, 2018| archive-url=https://web.archive.org/web/20180824135010/http://webfocus.aka.fi/ibi_apps/WFServlet?ekaLataus=0&IBIF_ex=x_RahPaatYht_report2&IBIAPP_app=aka_ext&UILANG=fi&SANAHAKU=&ETUNIMI=Pekka&SUKUNIMI=Janhunen&SUKUPUOLI=FOC_NONE&HAKU=FOC_NONE&ORGANIS=FOC_NONE&TUTKDI=FOC_NONE&TMK=FOC_NONE&PAATVUOSI_A=2001&PAATVUOSI_L=2017&LAJITTELU=PAATOS&TULOSTE=HTML| url-status=dead}}</ref>

To test the technology, a new [[European Union]]-backed electric sail study project was announced by the FMI in December 2010.<ref>{{cite news|last1=Dillow|first1=Clay|title=EU-Backed 'Electric Sail' Could Be the Fastest Man-Made Device Ever Built|url=http://www.popsci.com/technology/article/2010-12/eu-backed-electric-sail-could-be-fastest-man-made-device-ever-built|work=Popular Science|date=December 9, 2010|language=en}}</ref>  The EU funding contribution was 1.7 million euros. Its goal was to build laboratory prototypes of the key components, it involved five European countries and ended in November 2013.<ref>{{Cite web|url=http://www.electric-sailing.fi/fp7/|title=Electric Solar Wind Sail EU FP7 project|website=www.electric-sailing.fi |access-date=Jan 2, 2022}}</ref> In the EU evaluation, the project got the highest marks in its category.<ref>{{Cite web|url=http://www.electric-sailing.fi/|title=E-sail |access-date= |website=www.electric-sailing.fi}}</ref><ref name="physorg2010">{{cite web|url=http://www.physorg.com/news/2010-12-eu-electric-solar.html |title=EU project to build Electric Solar Wind Sail |publisher=Physorg.com |access-date=2014-01-12}}</ref> An attempt was made to test the working principles of the electric sail in low Earth orbit in the [[Estonia]]n nanosatellite [[ESTCube-1]] (2013-2015), but there was a technical failure and the attempt was unsuccessful. The piezoelectric motor used to unfurl the sail failed to turn the reel. In subsequent ground-based testing, a likely reason for the failure was found in a slipring contact which was likely physically damaged by launch vibration.

An international research team that includes Janhunen received funding through a 2015 NIAC Phase II solicitation for further development at NASA's Marshall Space Flight Center.<ref name="HERTS 2015">{{cite news |last=Wall |first=Mike |url=http://www.space.com/31063-electric-sail-solar-wind-space-exploration.html?adbid=10153148079961466&adbpl=fb&adbpr=17610706465 |title='Electric Sails' Could Propel Superfast Spacecraft by 2025 |work=Space.com |date=9 November 2015 |access-date=2015-11-10 }}</ref><ref>{{cite news |url=https://spaceref.com/science-and-exploration/electric-solar-sail-concept-inftoduction/ |title=Electric Solar Sail Concept Introduction |work=NASA |publisher=SpaceRef |date=17 August 2015 |access-date=2025-02-24 }}</ref> Their research project is called 'Heliopause Electrostatic Rapid Transit System' (HERTS).<ref name="HERTS 2015"/><ref>[http://www.nasa.gov/feature/heliopause-electrostatic-rapid-transit-system-herts HERTS] program at NASA (2015)</ref> The Heliopause Electrostatic Rapid Transit System (HERTS) concept is currently being tested. For HERTS, it might take only 10 to 15 years to make the trip of over 100 [[astronomical unit]]s (15 billion kilometers). In the HERTS concept, multiple, 20 kilometer or so long, 1 millimeter thin, positively charged wires would be extended from a rotating spacecraft.

A new satellite launched in June 2017,<ref name="Aalto Univ 2016">{{cite web|url=https://wiki.aalto.fi/display/SuomiSAT/Summary |title=Aalto-1 is the first Finnish nanosatellite project |work=[[Aalto University]] |access-date=2016-04-25 |url-status=dead |archive-url=https://web.archive.org/web/20141223055310/https://wiki.aalto.fi/display/SuomiSAT/Summary |archive-date=2014-12-23 }}</ref><ref>{{Cite web|url=http://www.aalto1.fi/tarina.html|title=Tämä domain on varattu &#124; aalto1.fi|website=www.aalto1.fi}}</ref> the [[Finland|Finnish]] [[Aalto-1]] [[nanosatellite]], currently in orbit, will test the electric sail for deorbiting in 2019.<ref>{{Cite web|url=https://www.iltalehti.fi/kotimaa/a/cb58c54f-8c93-437b-ac78-340878478946|title=Ensi yönä kello 00:51 taivaalla kiitää tähdenlento - Kyseessä on epäonnisen suomalaissatelliitin viimeinen matka|website=www.iltalehti.fi}}</ref><ref>{{Cite web|url=https://en.ilmatieteenlaitos.fi/press-release/121643|title=EU project to build Electric Solar Wind Sail - Finnish Meteorological Institute|website=en.ilmatieteenlaitos.fi}}</ref><ref>{{Cite web|url=https://www.err.ee/|archive-url=https://web.archive.org/web/20130131165800/http://uudised.err.ee/index.php?06270176|url-status=dead|title=uudised|archive-date=January 31, 2013|website=ERR}}</ref><ref name="FirstFinn">{{cite web|url=http://www.aalto.fi/en/current/news/2016-03-02/|title=Aalto-1 satellite is ready for space|date=2 March 2016|publisher=Aalto.fi|access-date=25 April 2015}}</ref><ref>{{Cite web|url=http://www.baltic-course.com/eng/Technology/?doc=102574|title=ESTCube-1 sends its last words: "Long live Estonia!"|last=курс|first=The Baltic Course - Балтийский|website=The Baltic Course {{!}} Baltic States news & analytics|access-date=2016-04-24}}</ref>

In 2017, [[Academy of Finland]] granted Centre of Excellence funding for 2018–2025 to a team that includes Janhunen and members from universities, to establish a ''Finnish Centre of Excellence in Research of Sustainable Space''.<ref>{{Cite web |url=http://www.aka.fi/globalassets/40akatemia/coes-2018-2025.pdf |title=List of units selected to the Centre of Excellence programme 2018–2025 |access-date=2017-11-23 |archive-date=2017-12-01 |archive-url=https://web.archive.org/web/20171201033941/http://www.aka.fi/globalassets/40akatemia/coes-2018-2025.pdf |url-status=dead }}</ref><ref>{{Cite web|url=https://www.aalto.fi/en/news|title=News &#124; Aalto University|website=www.aalto.fi|date=15 December 2023 }}</ref>

== Intrinsic limitations ==

Almost all Earth-orbiting satellites are inside Earth's magnetosphere.  However, the electric sail cannot be used inside planetary [[magnetospheres]] because the solar wind does not penetrate them, allowing only slower [[Plasma (physics)|plasma]] flows and [[magnetic field]]s.<ref>{{cite web|title=Electric Sails" Could Allow Us To Reach the Farthest Recesses of Space|date=October 30, 2017|publisher=Futurism|access-date=May 23, 2023|url=https://futurism.com/electric-sails-could-allow-us-to-reach-the-farthest-recesses-of-space}}</ref> Instead, inside a planetary magnetosphere, the electric sail may function as a brake, allowing deorbiting of satellites.<ref name=sciam/>

Like for other solar sail technologies, while modest variation of the thrust direction can be achieved by inclining the sail, the thrust vector always points more or less radially outward from the [[Sun]]. It has been estimated{{by whom|date=January 2016}} that maximum operational inclination would be 60°, resulting in a thrusting angle of 30° from the outward radial direction. However, like with the sails of a ship, [[Tacking (sailing)|tacking]] could be used for changing the trajectory. Interstellar ships approaching a sun might use solar wind flow for braking.<ref name="sciam">{{Cite news|url=https://www.scientificamerican.com/article/spacecraft-solar-sail/|title=Sail E-way: Spacecraft Riding the Solar Wind on Electric-Field Sails Could Cruise at 180,000 km/h|last=Ashley|first=Steven|work=Scientific American|access-date=2018-07-21|language=en}}</ref>

== Applications ==
* Fast missions (>{{sp}}{{cvt|50|km/s|mph km/h|disp=sqbr}} or {{convert|10|AU|ly pc|abbr=in|lk=in|disp=sqbr}}) out of the [[Solar System]] and [[heliosphere]] with small or modest [[Payload (air and space craft)|payload]]
* As a brake for a small [[interstellar probe]] which has been accelerated to high speed by some other means such as [[Beam-powered propulsion|laser lightsail]]<ref>{{Cite journal|arxiv = 1603.03015|doi = 10.1016/j.actaastro.2016.07.005|title = Combining magnetic and electric sails for interstellar deceleration|year = 2016|last1 = Perakis|first1 = Nikolaos|last2 = Hein|first2 = Andreas M.|journal = Acta Astronautica|volume = 128|pages = 13–20|bibcode = 2016AcAau.128...13P|s2cid = 17732634}}</ref>
* Inward-spiralling missions to study the Sun at a closer distance
* Two-way missions to inner Solar System objects such as [[asteroids]]
* Off-[[Lagrange point]] solar wind monitoring spacecraft for predicting [[space weather]] with a longer warning time than 1 hour

=== Fast missions to planet Uranus ===
Janhunen et al. have proposed a mission to [[Uranus]] powered by an electric sail. The mission could reach its destination in about the same time that the earlier [[Galileo (spacecraft)|''Galileo'']] space probe required to arrive at [[Jupiter]], just over one fourth as far away. ''Galileo'' took 6 years to reach Jupiter at a cost of $1.6 billion, while ''[[Cassini-Huygens]]'' took 7 years to get to Saturn and cost almost as much. The sail is expected to consume 540 [[watt]]s, producing about 0.5 [[newton (unit)|newton]]s accelerating the craft by about 1&nbsp;mm/s<sup>2</sup>. The craft would reach a [[velocity]] of about {{cvt|20|km/s|mph km/h}} by the time it reaches Uranus, 6 years after launch.<ref name=tr1401>{{cite web|author=Emerging Technology From the arXiv January 9, 2014 |url=http://www.technologyreview.com/view/523426/new-form-of-spacecraft-propulsion-proposed-for-uranus-mission |title=New Form of Spacecraft Propulsion Proposed For Uranus Mission &#124; MIT Technology Review |publisher=Technologyreview.com |access-date=2014-01-12}}</ref>
<ref>{{Cite journal|arxiv = 1312.6554|doi = 10.1016/j.pss.2014.08.004|title = Fast E-sail Uranus entry probe mission|year = 2014|last1 = Janhunen|first1 = Pekka|last2 = Lebreton|first2 = Jean-Pierre|last3 = Merikallio|first3 = Sini|last4 = Paton|first4 = Mark|last5 = Mengali|first5 = Giovanni|last6 = Quarta|first6 = Alessandro A.|journal = Planetary and Space Science|volume = 104|pages = 141–146|bibcode = 2014P&SS..104..141J|s2cid = 118329908}}</ref> The downside is that the electric sail cannot be used as a brake, so the craft arrives at a speed of {{cvt|20|km/s|mph km/h}}, limiting the missions to [[Planetary flyby|flybys]] or [[atmospheric entry]] missions. Braking would require a conventional chemical rocket.

The proposed craft has three parts: the E-sail module with solar panels and reels to hold the wires; the main body, including chemical thrusters for adjusting trajectory en route and at destination and communications equipment; and a research module to enter Uranus's atmosphere and make measurements for relay to [[Earth]] via the main body.<ref name=tr1401/>

== See also ==
*[[Electrodynamic tether]]
*[[Magnetic sail]]

== References ==
{{Reflist|2}}

== External links ==
*[https://www.youtube.com/watch?v=xuqYvEcgJsA Heliopause Electrostatic Rapid Transit System]
*[http://www.electric-sailing.fi/ FMI's official E-sail page]
*[http://www.electric-sailing.fi/publications.html List of original scientific publications]
*[http://www.space.fmi.fi/ Finnish Meteorological Institute/Space Research]
* {{cite news |  title=Electric Solar Wind Sail Could Power Future Space Travel In Solar System | date=2008-04-17 | url =https://www.sciencedaily.com/releases/2008/04/080415162612.htm | work =[[ScienceDaily]] |  access-date = 2008-10-15  }}

{{spacecraft propulsion}}

[[Category:CubeSats]]
[[Category:Finnish inventions]]
[[Category:Spacecraft propulsion]]
[[Category:Solar sailing]]
[[Category:Interstellar travel]]