EU project to build Electric Solar Wind Sail

December 9, 2010

The European union has selected the Finnish Meteorological Institute to lead an international space effort whose goal is to build the largest and fastest man-made device.

The electric sail is a Finnish invention which uses the solar wind as its thrust source and therefore needs no fuel or propellant. The solar wind is a continuous plasma stream emanating from the Sun.

The working principle of the so-called electric solar wind sail was invented in 2006 by Finnish Meteorological Institute researcher Pekka Janhunen.

In December 8-9, 2010, the kickoff meeting of the electric sail EU project was held at the Finnish Meteorological Institute. The meeting gathered space scientists and engineers from Finland, Estonia, Sweden, Germany and Italy. The ESAIL project will last for three years, its EU funding contribution is 1.7 million euros and its goal is to build the laboratory prototypes of the key components of the electric sail. In the EU evaluation, the ESAIL project got the highest marks in its category.

The electric solar wind sail may enable faster and cheaper access to the solar system. In the longer run it may enable an economic utilisation of asteroid resources. A related but simpler device (the so-called plasma brake) can be used for deorbiting satellites to address the issue. The working principles of the electric sail and the plasma brake will be tested in the coming years by the Estonian ESTCube-1 and the Finnish Aalto-1 nanosatellites.

According to estimates, a full scale electric sail will produce one newton continuous thrust and weigh only 100 kg. In certain missions the performance level of the electric sail is 100-1000 times larger than that of present chemical rockets and ion engines. The electric sail consists of long and thin metallic tethers which are kept in a high positive potential by an onboard solar-powered electron gun. The charged tethers repel solar wind protons so that the solar wind flow exerts a force on them and pushes the spacecraft in the desired direction.

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1 / 5 (1) Dec 09, 2010
I'm positive the original concept for this is long before 2006. I know I heard of people theorizing solar sail propulsion in the mid 1990's, and it was almost certainly mentioned much earlier by someone, probably Dyson or Sagan or somebody.

Anyway, 1 newton for 100kg is pretty good when you think about it. That comes to accelerating the space craft by 36m/s per hour. Doesn't look like a big deal, but it costs "nothing" and do that over days, weeks, or months and you could achieve some really, really high velocities.

After 1 month it would be moving ~25.92km/s (93,312km/hr).

After 2 months it would be moving about twice as fast, but then it would be much farther from the sun by then and eventually the inverse square law would reduce the amount of acceleration you would have available.

It could even pass voyager or pioneer within a decade or so, because it's top speed would be tens of times greater than theirs.
1 / 5 (1) Dec 09, 2010
The acceleration would still be a rate of 2cm/s per hour even by the time it passed Pluto.

Ok, by the time we get that far out we're talking "relatively negligible" acceleration, but in reality it would still be around 175.2 m/s per year, which is still about one half of a mach per year.

One should be able to calculate the exact velocity of the craft (neglecting solar fluctuations,) given only the initial velocity and the acceleration at it's initial position, since the future force of acceleration will always be proportional to the inverse squared law.

I haven't done this yet, but the maximum potential velocity by the time it reaches Pluto SHOULD make the New Horizons space craft look like a turtle by comparison.

Note that it comes to ~0.22621 A.U.* in the first month alone, and in the second month it would move another 0.56552 A.U.** because it would be moving so much faster.

So it could reach Mars in about 2.5 months or so, and would reach Pluto in under 7 years.
5 / 5 (1) Dec 09, 2010
Uh, this isn't a 'traditional' solar sail, with acres and acres of reflective film, it is more like a 'sea anchor'...

Presumably, it will use solar energy to charge the tethers ? And 'tip' discharge points to control the configuration and keep the tethers taut ??
1 / 5 (1) Dec 09, 2010
I think I must have mishandled a decimal or something earlier when I was doing those numbers in my head. I think it's a lot slower rate fo acceleration with respect to the A.U. than I was initially using. I am working on a more accurate approximation currently.

apparently, I forgot to convert meters to kilometers, or vice versa, and also screwed up something else. So much of that above is wrong.

After using a calculator and paper I figure that during the range from 1a.u. to 1.1 a.u it would take 5,723,842 seconds (66.248 days,) accelerating to 52.27km/s (188,172 km/hour) by the time it was 1.1 a.u. from the Sun.

Since most of the acceleration takes place in the first tenth of an A.U. it will actually only take it a few days to travel the next tenth of an A.U.

Additionally, because it is moving faster as it passes through each successive segment of space, it spends less time in that region, so it doesn't accelerate as much nor as long from 1.1a.u. to 1.2a.u., etc.
not rated yet Dec 09, 2010
Ah crap.

What the heck is wrong with me today?

Those numbers aren't right either. LOL!


When I was intending to use approximations with 1/10th of an A.U. I accidentally plugged in a full A.U....yeah...


for linear acceleration with initial velocity zero, we have:

distance = (1/2)at^2

where "a" is acceleration and "t" is time.

In an attempt to get a better approximation, I wanted to break the distances into segments that were more representaive of the fact acceleration is changing, so I used 0.1 a.u. segments.

This should give distance = 14,959,787,070meters = 0.1a.u.

The approximation for the linear average of acceleration along this segment came to 0.0091322m/s^2.

Therefore the time to reach 1.1 a.u. from the sun, starting at 1 a.u. SHOULD actually be:

1,810,037.69s = 20.9495days

Which gives final velocity of: 16.52981 km/s.

and average v of 8.26490 km/s during that time.

Maybe my brain is finally awake today. goodness.
not rated yet Dec 09, 2010
Ok, I've worked out a table to display the approximations I've been working on with graphics calculator. Hopefully this will keep some formatting.

Distance(a.u.)_Elapsed Time(days)_Velocity(km/s).

Earth 1a.u._ 0 time_ 0 velocity.

1.1_ 20.9495 days_ 16.53km/s.

1.2_ 29.85 days_ 22.375km/s.

1.3_ 36.961days_ 26.324km/s.

1.4_ 43.188days_ 29.288km/s.*

1.5_ 48.8715days_ 31.632km/s.

1.6_ 54.185days_ 33.548km/s.

1.7_ 59.226days_ 35.152km/s.**

1.8_ 64.058days_ 36.518km/s.

1.9_ 68.724days_ 37.698km/s.

2.0_ 73.255days_ 38.730km/s.

* Mars at Perihelion.
** Mars at Aphelion is slightly less than this.

As you can see, the rate of acceleration drops off quickly, but even at a distance of 2 a.u. from the sun it would still gain about 1km per second for every tenth of an a.u. it crosses.

Even if acceleration stopped at 2 a.u., I find the craft could go from Earth to 40a.u. (pluto) in just 4 years 312 days 2 hours.
not rated yet Dec 09, 2010
3.0_ 114.53days_ 45.168km/s.

4.0_ 151.716days_ 47.957km/s.

5.0_ 187.238days_ 49.530km/s.

6.0_ 221.842days_ 50.543km/s.

7.0_ 255.861days_ 51.251km/s.

8.0_ 289.473days_ 51.774km/s.

Keep in mind, this is no gravity assist.

After 8 A.U. it could still continue accelerating, but it would do so very slowly. It would take the next 5 a.u. worth of acceleration just to gain one more km/s worth of velocity, and then after that it probably would not gain another km/s worth of velocity until some time after passing Pluto.
not rated yet Dec 10, 2010
YOU guys! Are you compensating for relativistic effects?
5 / 5 (1) Dec 10, 2010
"I'm positive the original concept for this is long before 2006."
-People have been thinking long and hard about how to use the solar wind for space propulsion since the early 1970s, both the photon component and the particle component. This particular concept as described above dates from 2006.

"YOU guys! Are you compensating for relativistic effects?"
-Not necessary at these velocities.

For ideas about what to do when you have cheap interplanetary propulsion, see the books by John S. Lewis. I suggest you start with "Rain of Iron and Ice".

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