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. [1]
The electric sail consists of a number of thin, long and conducting tethers which are kept in a high positive potential by an onboard electron gun. [2] 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 attitude can be controlled.
E-sail missions can be launched at almost any time with only minor variations in travel time. By contrast, conventional slingshot missions must wait for the planets to reach a particular alignment. [3]
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 photons. 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 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 20 km (12 mi) each. [4]
[5] [6] 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 giants, it may have accumulated as much as 20 km/s (45,000 mph; 72,000 km/h) velocity, which is on par with the New Horizons probe, but without gravity assists.
In order to minimise damage to the thin tethers from micrometeoroids, 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. [7]
Academy of Finland has been funding electric sail development since 2007. [8]
To test the technology, a new European Union-backed electric sail study project was announced by the FMI in December 2010. [9] 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. [10] In the EU evaluation, the project got the highest marks in its category. [11] [12] An attempt was made to test the working principles of the electric sail in low Earth orbit in the Estonian 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. [2] [13] Their research project is called 'Heliopause Electrostatic Rapid Transit System' (HERTS). [2] [14] 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 units (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, [15] [16] the Finnish Aalto-1 nanosatellite, currently in orbit, will test the electric sail for deorbiting in 2019. [17] [18] [19] [20] [21]
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. [22] [23]
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 flows and magnetic fields. [24] Instead, inside a planetary magnetosphere, the electric sail may function as a brake, allowing deorbiting of satellites. [25]
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? ] 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 could be used for changing the trajectory. Interstellar ships approaching a sun might use solar wind flow for braking. [25]
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 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 watts, producing about 0.5 newtons accelerating the craft by about 1 mm/s2. The craft would reach a velocity of about 20 km/s (45,000 mph; 72,000 km/h) by the time it reaches Uranus, 6 years after launch. [3] [27] The downside is that the electric sail cannot be used as a brake, so the craft arrives at a speed of 20 km/s (45,000 mph; 72,000 km/h), limiting the missions to 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. [3]
The magnetopause is the abrupt boundary between a magnetosphere and the surrounding plasma. For planetary science, the magnetopause is the boundary between the planet's magnetic field and the solar wind. The location of the magnetopause is determined by the balance between the pressure of the dynamic planetary magnetic field and the dynamic pressure of the solar wind. As the solar wind pressure increases and decreases, the magnetopause moves inward and outward in response. Waves along the magnetopause move in the direction of the solar wind flow in response to small-scale variations in the solar wind pressure and to Kelvin–Helmholtz instability.
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric entry.
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV. The composition of the solar wind plasma also includes a mixture of materials found in the solar plasma: trace amounts of heavy ions and atomic nuclei of elements such as carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron. There are also rarer traces of some other nuclei and isotopes such as phosphorus, titanium, chromium, and nickel's isotopes 58Ni, 60Ni, and 62Ni. Superimposed with the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field. The boundary separating the corona from the solar wind is called the Alfvén surface.
Solar sails are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large surfaces. A number of spaceflight missions to test solar propulsion and navigation have been proposed since the 1980s. The first spacecraft to make use of the technology was IKAROS, launched in 2010.
A magnetic sail is a proposed method of spacecraft propulsion where an onboard magnetic field source interacts with a plasma wind to form an artificial magnetosphere that acts as a sail, transferring force from the wind to the spacecraft requiring little to no propellant as detailed for each proposed magnetic sail design in this article.
Nuclear pulse propulsion or external pulsed plasma propulsion is a hypothetical method of spacecraft propulsion that uses nuclear explosions for thrust. It originated as Project Orion with support from DARPA, after a suggestion by Stanislaw Ulam in 1947. Newer designs using inertial confinement fusion have been the baseline for most later designs, including Project Daedalus and Project Longshot.
Laser propulsion is a form of beam-powered propulsion where the energy source is a remote laser system and separate from the reaction mass. This form of propulsion differs from a conventional chemical rocket where both energy and reaction mass come from the solid or liquid propellants carried on board the vehicle.
BepiColombo is a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) to the planet Mercury. The mission comprises two satellites launched together: the Mercury Planetary Orbiter (MPO) and Mio. The mission will perform a comprehensive study of Mercury, including characterization of its magnetic field, magnetosphere, and both interior and surface structure. It was launched on an Ariane 5 rocket on 20 October 2018 at 01:45 UTC, with an arrival at Mercury planned for on 5 December 2025, after a flyby of Earth, two flybys of Venus, and six flybys of Mercury. The mission was approved in November 2009, after years in proposal and planning as part of the European Space Agency's Horizon 2000+ programme; it is the last mission of the programme to be launched.
The heliosphere is the magnetosphere, astrosphere, and outermost atmospheric layer of the Sun. It takes the shape of a vast, tailed bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstellar medium. The "bubble" of the heliosphere is continuously "inflated" by plasma originating from the Sun, known as the solar wind. Outside the heliosphere, this solar plasma gives way to the interstellar plasma permeating the Milky Way. As part of the interplanetary magnetic field, the heliosphere shields the Solar System from significant amounts of cosmic ionizing radiation; uncharged gamma rays are, however, not affected. Its name was likely coined by Alexander J. Dessler, who is credited with the first use of the word in the scientific literature in 1967. The scientific study of the heliosphere is heliophysics, which includes space weather and space climate.
An interstellar probe is a space probe that has left—or is expected to leave—the Solar System and enter interstellar space, which is typically defined as the region beyond the heliopause. It also refers to probes capable of reaching other star systems.
Electrodynamic tethers (EDTs) are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy. Electric potential is generated across a conductive tether by its motion through a planet's magnetic field.
Spacecraft electric propulsion is a type of spacecraft propulsion technique that uses electrostatic or electromagnetic fields to accelerate mass to high speed and thus generating thrust to modify the velocity of a spacecraft in orbit. The propulsion system is controlled by power electronics.
Pekka Janhunen is a Finnish space physicist, astrobiologist, and inventor. He is a research manager at the Finnish Meteorological Institute's Space and Earth Observation Centre and a visiting professor at the University of Tartu, Estonia. He is also senior technical advisor at Aurora Propulsion Technologies, a startup company operating in the space sector. Janhunen studied theoretical physics at the University of Helsinki and earned his Ph.D. in space plasma physics simulations in 1994. He has also published a theory on the origin of multicellular life. Janhunen is best known for his electric solar wind sail invention.
Energetic Neutral Atom (ENA) imaging is a technology used to create global images of otherwise invisible phenomena in the magnetospheres of planets and throughout the heliosphere.
Fast, Affordable, Science and Technology Satellite-Huntsville 01 or FASTSAT-Huntsville 01 of the NASA. FASTSAT-HSV 01 was flying on the STP-S26 mission - a joint activity between NASA and the U.S. Department of Defense Space Test Program, or DoD STP. FASTSAT and all of its six experiments flying on the STP-S26 multi-spacecraft/payload mission have been approved by the Department of Defense Space and Experiments Review Board (USA-220).
Aalto-1 is a Finnish research nanosatellite, created by students of Aalto University. Based on the CubeSat architecture, it was originally scheduled to be launched in 2013, it was launched on 23 June 2017. It is Finland's first student satellite project and indigenously produced satellite. As of 2021, the satellite is operational.
ESTCube-1 is the first Estonian satellite and first satellite in the world to attempt to use an electric solar wind sail (E-sail). It was launched on 7 May 2013 aboard Vega VV02 carrier rocket and successfully deployed into orbit. The CubeSat standard for nanosatellites was followed during the engineering of ESTCube-1, resulting in a 10×10×11.35 cm cube, with a volume of 1 liter and a mass of 1.048 kg.
The ISEE-1 was an Explorer-class mother spacecraft, International Sun-Earth Explorer-1, was part of the mother/daughter/heliocentric mission. ISEE-1 was a 340.2 kg (750 lb) space probe used to study magnetic fields near the Earth. ISEE-1 was a spin-stabilized spacecraft and based on the design of the prior IMP series of spacecraft. ISEE-1 and ISEE-2 were launched on 22 October 1977, and they re-entered on 26 September 1987.
The ISEE-2 was an Explorer-class daughter spacecraft, International Sun-Earth Explorer-2, was part of the mother/daughter/heliocentric mission. ISEE-2 was a 165.78 kg (365.5 lb) space probe used to study magnetic fields near the Earth. ISEE-2 was a spin-stabilized spacecraft and based on the design of the prior IMP series of spacecraft. ISEE-1 and ISEE-2 were launched on 22 October 1977, and they re-entered on 26 September 1987.
A plasma magnet is a proposed spacecraft propulsion device that uses a dipole magnetic field to capture energy from the solar wind. The field acts as a sail, using the captured energy to propel the spacecraft analogously to how the wind propels a sailing vessel. It could accelerate a vessel moving away from the sun and decelerate it when approaching a distant star at the end of an interstellar journey. Thrust vectoring and steering could be achieved by manipulating the dipole tilt for any type of magnetic sail.
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