NASA begins testing electronic sail technology for deep space probes

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Artist's concept of the E-Sail in operation(Credit: NASA/MSFC)

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It took Voyager 1 over 30 years to reach interstellar space, but scientists at NASA's Marshall Space Flight Center in Huntsville, Alabama, are testing a new technology that could cut that time by two thirds. The Heliopause Electrostatic Rapid Transit System (HERTS) or E-sail concept is a novel form of propellant-less propulsion that relies on a series of wires to catch the solar winds. The technology promises to reduce the travel time from Earth to the heliopause – about 123 AU (18 billion km, 11 billion mi) from the Sun – to under 10 years.

Deep space travel is very slow, with the outer regions of the Solar System taking decades to reach using chemical rockets and complicated slingshot orbits. One solution is to do away with rockets altogether and replace them with solar sails – giant mylar sails spread on gossamer threads to catch the solar winds, which are the constant stream of protons and other charged particles that stream from the Sun at 400 to 750 km/s (900,000 to 1.7 million mph).

The idea is already being tested in Earth orbit, but it's not a perfect solution. The huge sheets of plastic aren't easy to deploy even in zero gravity, and operating them requires a complex assembly of miniature reels and winches. In addition, the force of the solar winds is proportional to the distance from the Sun, and once a solar sail is more than 5 AU (464 million mi, 747 million km) from the Sun, the acceleration falls off markedly.

Based on the work of Dr. Pekka Janhunen of the Finnish Meteorological Institute, the E-Sail concept consists of a small unmanned payload containing instruments and a power source. From this radiate 10 to 20 electrically-charged, bare aluminum wires that are about one millimeter thick, 12.5 mi (20 km) long and weighing only a few grams.

NASA engineer Bruce Wiegmann, principal investigator for the HERTS E-Sail, demonstrates the long, thin wires that will construct the E-Sail(Credit: NASA/MSFC/Emmett Given)

These wires are extended from the spinning spacecraft after launch. Centrifugal force causes the wires to extend and stiffen into a large circular web spinning at one revolution per hour, which is electrostatically charged to repel the fast-moving, positively-charged protons. The repulsion pushes the spacecraft along like a sail boat running before the wind.

The analogy is actually quite accurate because the principle is the same as that of a sailing vessel. The E-Sail can even "trim" its electrostatic sails by modulating each wire's voltage as the sail rotates. This way, the spacecraft can be steered in the same way that a boat can by trimming its sails.

The wires may seem a very small surface, but they generate an electric field that extends for tens of meters like invisible sail cloth. The effective sail area generated by the wires increases as the spacecraft moves away from the Sun. At one AU (93 million mi, 150 million km) the effective surface area is 232 mi2 (601 km2), but at 5 AU this area becomes more than 463 mi2 (1,200 km2). According to NASA engineer Bruce Wiegmann, principal investigator for the HERTS E-Sail, the system can continue to accelerate for distances up to 16 to 20 AU (1.5 billion mi, 2.4 billion km to 1.9 billion mi, 3 billion km) from the Sun, which is three times the distance of a mylar solar sail's effective acceleration range.

Within a controlled plasma chamber tests will examine the rate of proton and electron collisions with a positively charged tether(Credit: NASA/MSFC/Emmett Given)

The E-Sail is currently being tested in NASA's High Intensity Solar Environment Test system, which is a controlled plasma chamber that simulates the vacuum and plasma of interplanetary space. The purpose of the tests are to improve modelling data that will be needed to scale up the technology for a proper spacecraft.

To improve the models, the engineers are working to determine the rate of proton and electron collisions on a positively charged wire, as well as the degree of proton deflection and learning how many electrons are absorbed by the wires.

The latter is very important because the E-Sail probe is a closed system flying through a hard vacuum, which means that if it builds up a static electric charge, it will hold onto it like a Leyden jar in a school physics lab. If it builds up too much of a charge, then the sails lose their effectiveness.

To keep the electrostatic charge properly balanced, the E-Sail has an electron gun similar to the ones used in old-fashioned cathode ray tubes to create an image. However, instead of running a television screen, this gun squirts electrons into space to neutralize any charge build up.

For the test, the wires used are stainless steel rather than aluminum. According to the engineers, steel is denser than aluminum, but its non-corrosive properties make it a good substitute because it won't degrade during the tests.

NASA says that the E-Sail technology is still in its infancy and that it will be over a decade before it becomes practical. In the meantime, more tests and improved models are needed as well as work on developing deployment mechanisms.

"As the team studied this concept, it became clear that the design is flexible and adaptable," says Wiegmann. "Mission and vehicle designers can trade off wire length, number of wires and voltage levels to fit their needs – inner planetary, outer planetary or heliopause. The E-Sail is very scalable."

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