Tuesday, May 7, 2013

One step closer to solar wind-powered spacecraft

Unit set up for sun simulation tests. This side, which normally faces sun, contains a protective heat shield with solar cells for power supply. Photograph: Roland Rosta
Unit set up for sun simulation tests. This side, which normally faces sun, contains a protective heat shield with solar cells for power supply.     Photograph: Roland Rosta
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One step closer to solar wind-powered spacecraft

A little over a year ago, a research team started to develop a vital part of a Finnish invention – an electric solar wind sail for interplanetary journeys. Now, a prototype has been successfully manufactured and tested.
Published 2013-03-25Anneli Waara


“ We are happy and proud to now have demonstrated that the unit not only can be designed, although it was certainly challenging, but also that it can be manufactured and that it satisfies its demanding specifications so that revolutionary electric sail spacecraft can be realised ”, says Greger Thornell, who has worked on the project with his colleagues Johan Sundqvist and Sven Wagner at Ångström Space Technology Centre, ÅSTC, Uppsala University.

The electric solar wind sail is a groundbreaking new concept for efficient propellantless space journeys in interplanetary space, based on tapping momentum from the solar wind by up to 20 km long, thin, electrically charged radial wires stretched by centrifugal force. Full-scale electric sails could produce up to one Newton of continuous thrust and weigh 100–200 kg, and thus enable fast and economical access to the solar system.

The technology is being developed by the EU FP7 project ESAIL, the main goal of which is to develop and demonstrate laboratory prototypes of the key components of the sail.

With the successful manufacturing and rigorous space environment testing of a prototype unit to be used at the end of each of the sail’s spokes to hoist the sail and monitor the rig, an important step has now been taken. Basically, the unit is an autonomous device with many of the features of a large-scale satellite, including communication capabilities, power supply, propulsion system, onboard computer, and thermal control, but it weighs only about half a kilogram and it must work in a wide solar distance range from Earth to the outer asteroid belt.

Other teams involved in the subproject are: Nanospace AB, Uppsala; Alta S.p.A, Pisa, Italy; DLR-Bremen, Germany, and Tartu University, Estonia. The ESAIL project is led by the Finnish Meteorological Institute where the electric sail was invented.
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Additional Informations

The electric solar wind sail, or electric sail for short, is a propulsion invention made in 2006 at the Kumpula Space Centre. This website is for the EU FP7 project, which is developing the E-sail towards prototype phase. See also the electric sailing science and technology pages. The next step after ESAIL project will be SWEST: Solar Wind Electric Sail Test. The SWEST proposal was filed to EU in Nov 23 2011. (click on the image to visit esail's project webpage)
The electric solar wind sail, or electric sail for short, is a propulsion invention made in 2006 at the Kumpula Space Centre. This website is for the EU FP7 project, which is developing the E-sail towards prototype phase. See also the electric sailing science and technology pages.
 
The next step after ESAIL project will be SWEST: Solar Wind Electric Sail Test. The SWEST proposal was filed to EU in Nov 23 2011.
 
(click on the image to visit esail’s project webpage)
Chemical Propulsion @ 50 N ethane H2O2 bipropellant engine. ALTA - Space Propulsion Systems and Services (click on the image to visit ALTA's webpage)
Chemical Propulsion @ 50 N ethane H2O2 bipropellant engine.
ALTA – Space Propulsion Systems and Services
(click on the image to visit ALTA’s webpage)
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Solar Wind Energy Source Discovered

Solar wind flows away from the sun at speeds up to and exceeding 500 km/s (a million mph). More
Solar wind flows away from the sun at speeds up to and exceeding 500 km/s (a million mph).                             Click on the image for more
An artist's concept of the Wind spacecraft sampling the solar wind. Justin Kasper's science result is inset.
An artist’s concept of the Wind spacecraft sampling the solar wind. Justin Kasper’s science result is inset.

March 8, 2013: Using data from an aging NASA spacecraft, researchers have found signs of an energy source in the solar wind that has caught the attention of fusion researchers. NASA will be able to test the theory later this decade when it sends a new probe into the sun for a closer look.
The discovery was made by a group of astronomers trying to solve a decades-old mystery: What heats and accelerates the solar wind?

The solar wind is a hot and fast flow of magnetized gas that streams away from the sun’s upper atmosphere.  It is made of hydrogen and helium ions with a sprinkling of heavier elements.  Researchers liken it to the steam from a pot of water boiling on a stove; the sun is literally boiling itself away.

But,” says Adam Szabo of the NASA Goddard Space Flight Center, “solar wind does something that steam in your kitchen never does.  As steam rises from a pot, it slows and cools.  As solar wind leaves the sun, it accelerates, tripling in speed as it passes through the corona. Furthermore, something inside the solar wind continues to add heat even as it blows into the cold of space.

Finding that “something” has been a goal of researchers for decades.  In the 1970s and 80s, observations by two German/US Helios spacecraft set the stage for early theories, which usually included some mixture of plasma instabilities, magnetohydrodynamic waves, and turbulent heating.  Narrowing down the possibilities was a challenge. The answer, it turns out, has been hiding in a dataset from one of NASA’s oldest active spacecraft, a solar probe named Wind.

Launched in 1994, Wind is so old that it uses magnetic tapes similar to old-fashioned 8-track tapes to record and play back its data.  Equipped with heavy shielding and double-redundant systems to safeguard against failure, the spacecraft was built to last; at least one researcher at NASA calls it the “Battlestar Gallactica” of the heliophysics fleet. Wind has survived almost two complete solar cycles and innumerable solar flares.

After all these years, Wind is still sending us excellent data,” says Szabo, the mission’s project scientist, “and it still has 60 years’ worth of fuel left in its tanks.

Using Wind to unravel the mystery was, to Justin Kasper of the Harvard-Smithsonian Center for Astrophysics, a “no brainer.” He and his team processed the spacecraft’s entire 19-year record of solar wind temperatures, magnetic field and energy readings and …

I think we found it,” he says.  “The source of the heating in the solar wind is ion cyclotron waves.
Ion cyclotron waves are made of protons that circle in wavelike-rhythms around the sun’s magnetic field.  According to a theory developed by Phil Isenberg (University of New Hampshire) and expanded by Vitaly Galinsky and Valentin Shevchenko (UC San Diego), ion cyclotron waves emanate from the sun; coursing through the solar wind, they heat the gas to millions of degrees and accelerate its flow to millions of miles per hour. Kasper’s findings confirm that ion cyclotron waves are indeed active, at least in the vicinity of Earth where the Wind probe operates.

Ion cyclotron waves can do much more than heat and accelerate the solar wind, notes Kasper.  “They also account for some of the wind’s very strange properties.

The solar wind is not like wind on Earth.  Here on Earth, atmospheric winds carry nitrogen, oxygen, water vapor along together; all species move with the same speed and they have the same temperature.  The solar wind, however, is much stranger.  Chemical elements of the solar wind such as hydrogen, helium, and heavier ions, blow at different speeds; they have different temperatures; and, strangest of all, the temperatures change with direction.

We have long wondered why heavier elements in the solar wind move faster and have higher temperatures than the lighter elements,” says Kasper.  “This is completely counterintuitive.

The ion cyclotron theory explains it: Heavy ions resonate well with ion cyclotron waves. Compared to their lighter counterparts, they gain more energy and heat as they surf.
The behavior of heavy ions in the solar wind is what intrigues fusion researchers. Kasper explains:

When you look at fusion reactors on Earth, one of the big challenges is contamination.  Heavy ions that sputter off the metal walls of the fusion chamber get into the plasma where the fusion takes place.  Heavy ions radiate heat. This can cool the plasma so much that it shuts down the fusion reaction.

Ion cyclotron waves of the type Kasper has found in the solar wind might provide a way to reverse this process. Theoretically, they could be used to heat and/or remove the heavy ions, restoring thermal balance to the fusing plasma.

I have been invited to several fusion conferences to talk about our work with the solar wind,” he says.

The next step, agree Kasper and Szabo, is to find out if ion cyclotron waves work the same way deep inside the sun’s atmosphere where the solar wind begins its journey.  To find out, NASA is planning to send a spacecraft into the sun itself.

Solar Probe Plus, scheduled for launch in 2018, will plunge so far into the sun’s atmosphere that the sun will appear as much as 23 times wider than it does in the skies of Earth. At closest approach, about 7 million km from the sun’s surface, Solar Probe Plus must withstand temperatures greater than 1400 deg. C and survive blasts of radiation at levels not experienced by any previous spacecraft.  The mission’s goal is to sample the sun’s plasma and magnetic field at the very source of the solar wind.

With Solar Probe Plus we’ll be able to conduct specific tests of the ion cyclotron theory using sensors far more advanced than the ones on the Wind spacecraft,” says Kasper.  “This should give us a much deeper understanding of the solar wind’s energy source.

The research described in this story was published in the Physical Review Letters on February 28, 2013: “Sensitive Test for Ion-Cyclotron Resonant Heating in the Solar Wind” by Justin Kasper et al. 

Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit: Science@NASA
  • Read straight from NASA
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Sensitive Test for Ion-Cyclotron Resonant Heating in the Solar Wind

Plasma carrying a spectrum of counterpropagating field-aligned ion-cyclotron waves can strongly and preferentially heat ions through a stochastic Fermi mechanism. Such a process has been proposed to explain the extreme temperatures, temperature anisotropies, and speeds of ions in the solar corona and solar wind. We quantify how differential flow between ion species results in a Doppler shift in the wave spectrum that can prevent this strong heating. Two critical values of differential flow are derived for strong heating of the core and tail of a given ion distribution function. Our comparison of these predictions to observations from the Wind spacecraft reveals excellent agreement. Solar wind helium that meets the condition for strong core heating is nearly 7 times hotter than hydrogen on average. Ion-cyclotron resonance contributes to heating in the solar wind, and there is a close link between heating, differential flow, and temperature anisotropy.

© 2013 American Physical Society

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