Electric Sails: Fast Probe to Uranus

by Paul Gilster on January 14, 2014

For years now Pekka Janhunen has been working on his concept of an electric sail with the same intensity that Claudio Maccone has brought to the gravitational focus mission called FOCAL. Both men are engaging advocates of their ideas, and having just had a good conversation with Dr. Maccone (by phone, unfortunately, as I’ve been down with the flu), I was pleased to see Dr. Janhunen’s electric sail pop up again in online discussions. It turns out that the physicist has been envisioning a sail mission to an unusual target.

Let’s talk a bit about the mission an electric sail enables. This is a solar wind-rider, taking advantage not of the momentum imparted by photons from the Sun but the stream of charged particles pushing from the Sun out to the heliopause (thereby blowing out the bubble’ in the interstellar medium we call the heliosphere). As Janhunen (Finnish Meteorological Institute) has designed it, the electric sail taps the Coulomb interaction in which particles are attracted or repulsed by an electric charge. The rotational motion of the spacecraft would allow the deployment of perhaps 100 tethers, thin wires that would be subsequently charged by an electron gun with the beam sent out along the spin axis.

Szames_sail

Image: The electric sail is a space propulsion concept that uses the momentum of the solar wind to produce thrust. Credit: Alexandre Szames.

The electron gun keeps the spacecraft and tethers charged, with the electric field of the tethers extending tens of meters into the surrounding solar wind plasma — as the solar wind ‘blows,’ it pushes up against thin tethers that act, because of their charge, as wide surfaces against which the wind can push. The sail uses the attraction or repulsion of particles caused by the electric charge to ride the wind, the positively charged solar wind protons repelled by the positive voltage they meet in the charged tethers.

One disadvantage that electric sails bring to the mix, as opposed to solar sails like IKAROS, is that the solar wind is much weaker — Janhunen’s figures have it 5000 times weaker — than solar photon pressure at Earth’s distance from the Sun. This has come up before in comments here and it’s worth quoting Janhunen on the matter, from a site he maintains on electric sails:

The solar wind dynamic pressure varies but is on average about 2 nPa at Earth distance from the Sun… Due to the very large effective area and very low weight per unit length of a thin metal wire, the electric sail is still efficient, however. A 20-km long electric sail wire weighs only a few hundred grams and fits in a small reel, but when opened in space and connected to the spacecraft’s electron gun, it can produce several square kilometre effective solar wind sail area which is capable of extracting about 10 millinewton force from the solar wind.

Computer simulations using tethers up to 20 kilometers in length have yielded speeds of 100 kilometers per second, a nice step up from the 17 kps of Voyager 1, and enough to get a payload into the nearby interstellar medium in fifteen years. Or, as Janhunen describes in the recent paper on a Uranus atmospheric probe, an electric sail could reach the 7th planet in six years. Janhunen sees such a probe as equally applicable for a Titan mission and, indeed, missions to Neptune and Saturn itself, but notice that none of these are conceived as orbiter missions. A significant amount of chemical propellant is needed for orbital insertion unless we were to try aerocapture, but the problem with the latter is that it is at a much lower technical readiness level.

A demonstrator electric sail mission, then, is designed to keep costs down and reach its destination as fast as possible, with the interesting spin that, because we’re in need of no gravitational assists, the Uranus probe will have no launch window constraints. As defined in the paper on this work, the probe would consist of three modules stacked together: The electric sail module, a carrier module and an entry module. The entry module would be composed of the atmospheric probe and a heat-shield.

At approximately Saturn’s distance from the Sun, the electric sail module would be jettisoned and the carrier module used to adjust the trajectory as needed with small chemical thrusters (50 kg of propellant budgeted for here). And then the fun begins:

About 13 million km (8 days) before Uranus, the carrier module detaches itself from the entry module and makes a ~ 0.15 km/s transverse burn so that it passes by the planet at ~ 105 km distance, safely outside the ring system. Also a slowing down burn of the carrier module may be needed to optimise the link geometry during flyby.

Now events happen quickly. The entry module, protected by its heat shield, enters the atmosphere. A parachute is deployed and the heat shield drops away, with the probe now drifting down through the atmosphere of Uranus (think Huygens descending through Titan’s clouds), making measurements and transmitting data to the high gain antenna on the carrier module.

Thus we get atmospheric measurements of Uranus similar to what the Galileo probe was able to deliver at Jupiter, measuring the chemical and isotopic composition of the atmosphere. A successful mission builds the case for a series of such probes to Neptune, Saturn and Titan. Thus far Jupiter is the only giant planet whose atmosphere has been probed directly, and a second Jupiter probe using a similar instrument package would allow further useful comparisons. Our planet formation models, which predict chemical and isotope composition of the giant planet atmospheres, can thus be supplemented by in situ data.

Not to mention that we would learn much about flying and navigating an electric sail during the testing and implementation of the Uranus mission. The paper is Janhunen et al., “Fast E-sail Uranus entry probe mission,” submitted to the Meudon Uranus workshop (Sept 16-18, 2013) special issue of Planetary and Space Science (preprint).

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{ 34 comments }

andy January 14, 2014 at 15:01

This is an interesting development. Uranus seems to get a bad rep in terms of spacecraft destinations, probably because of the bad timing of the Voyager 2 encounter to visit the planet at the blandest season of its year (and probably also because of the stupid joke). Nevertheless Uranus mission would definitely be of interest as the outer giants seem to be the closest thing we have in this solar system to the low-density super-Earths that seem to be all over the galaxy, so observing one up close would be useful. The actual nature of Uranus and Neptune is rather poorly known, they are often called “ice giants” but it is not known what fraction of the mass is actually ice.

Furthermore the satellite system appears to be quite dynamic, with rings appearing and disappearing and apparently unstable satellite orbits. It would also be nice to be able to test theories about the equatorial ridge on Iapetus by observing whether Oberon also has one.

So hopefully this idea is viable and leads to an actual mission…

Michael January 14, 2014 at 15:46

The tethers could be made from nanotubes (today tech) with a metal coating to conduct charge which would also reflect light adding a bit more push through photon pressure. Normal electrostatic charge would be enough to open the tethers and keep them open as well.

Alex Tolley January 14, 2014 at 16:28

This is an interesting concept. At this point it seems rather theoretical. I’m bothered by the assumptions that the charged wires can be kept separated by dynamic tuning of their length. ( I see in later papers that the circumference is now a wire with tether controls and thrusters). Like wind sails, these electric sails will need to adjust for the variations in the solar wind. It might appear that sailing in front of a CME would be a great thrust advantage, but the forces would probably blow the sail inside out like an umbrella in a strong wind.

While the centrifugal approach to tensioning the sail looks attractive, I wonder whether using more conventional rigging like that of solar sails might be attractive. One paper suggests that a grid of wires isn’t feasible for deployment. Yet we have talked about large solar sails and I wonder whether this approach with booms, wires strung between them, and rigging might offer a more versatile sail. It would certainly ensure that an optimal spacing between the wires could be achieved, and each wire could be voltage controlled for thrust and maneuvering.

bret moore January 14, 2014 at 20:38

Small robots like little spiders rigging a web for the sails. 3D printed and fueled on the moon.

Lionel January 15, 2014 at 6:23

I have a question about how these highly charged wires interact: How can they be kept in a plane?

Alex Tolley above suggested CME might blow the sail inside-out like an umbrella.

But without considering forces from the CME, won’t the wires all be repelling each other and want to move out of a plane? Think about how four or more hydrogen atoms around the a central atom (e.g. carbon in methane) splay out to form a tetrahedron… won’t the charged wires repel each other, and tend to form a sea-urchin type shape?

It’s a very interesting concept and a neptune/uranus mission would be great

Alex Tolley January 15, 2014 at 11:56

A BOE calculation for a rigged square sail with tethers strung between 2 spars indicates that the same thrust for 100 tethers, each 20 km long could be achieved with a rigged sail of about 1/2 the dimensions. The cost is the extra mass of the spars.

However, one very interesting claim (Electric solar wind sail mass budget model) is that the thrust is a function of 1/r of the distance from the sun, rather than the 1/r^2 for a solar sail. I’m unfamiliar with the physics, but the idea is that the effective “reflective area” gets larger as the solar wind density decreases.

@Lionel – the solution to the repelling effects is that the charged tethers have minimal overlap of their fields. With a spacing of 200m apart to avoid field overlap, the reference sail would have field overlap for the inner 3 km. This would have the effect you are concerned about. The circumferential tether, centrifugal forces of the spinning sail and differential tether tensions is supposed to counteract that effect (theoretically).

Another issue is powering the electron guns that charge the tethers. The model sail uses solar panels, whose power declines as a 1/r^2 from the sun. Thus these panels must be adequate for the maximum powered range, perhaps no farther than Jupiter’s orbit of ~5AU. For missions in deep space where the claimed 1/r thrust would outperform a solar sail, nuclear power would be needed. At this point only the Russians may be planning spacecraft nuclear power systems large enough to meet the 375 KW needed for the reference design.

Enamored as I am of the Benford’s beamed sails for high acceleration, I’m wondering if it might be possible to focus the solar wind to increase its local density at the sail. I’m thinking of some sort of -ve charged device in solar orbit to bend protons in the solar wind into a more parallel path towards the eSail, increasing the proton density and therefore thrust. Probably infeasible, but just thinking speculatively.

torque_xtr January 15, 2014 at 14:15

Another consideration, would the cables be charged over all their length, or only the ends of them would be charged because of the repulsion? This could reduce the effective area, unless the probe is a charged dandelion flower. And would the inflated magsail with superconducting coil produce higher accelerations? It would be a better design still if it could also provide a decceleration against gas giant’s magnetic field…

On the other hand, a _really_ huge loop of cable in the same field, and a joule heat dissipator, may provide the same for electrosail, or even serve as a power source for some kind of high thrust plasma rocket, but this goes into speculative and low-TRL tech. (PS just estimated the numbers and found them unrealistic, ~10^3 km wide superconducting cables needed)

Alex Tolley January 15, 2014 at 15:57

@torque_xtr – the whole cable is charged. At 1 AU, the repulsion radius around the cable is about 100m (however that is calculated). As I mentioned, what I find interesting is that this radius increases as the solar wind decreases (charge leakage/neutralized by the electrons) making the sail area increase as the solar wind decreases.

Peter Popov January 15, 2014 at 18:14

Let me focus attention to a different issue. How do you design a heat shield to withstand entry at 100 km/s? Say typical shields designed to date operate in the range 10-20 km/s. The kinetic energy at 100 km/s is 25 to 100 times more, respectively. Besides, Titan’s atmosphere is denser, which should impacts the shied performance adversely?

I liked reading all the comments regarding keeping the wires oriented and thus functioning, changing direction, etc.

Eniac January 15, 2014 at 20:08

The design parameter that determines acceleration is the charge/mass density on the wires. It benefits from higher voltage and lower cross-section (both linearly. It is limited by specific tensile strength, which is why carbon nanotubes are by far the most desirable material, as someone else has pointed out.

The charge maintenance current goes up with increasing voltage and down with decreasing cross section. Thus, higher voltage comes with a trade-off, but thinner wires win on both counts.

I have not dug into the literature, but I sure hope that Janhunen has considered the tensile forces on the wires. Any wire can be charged only so much until it breaks under the force of it’s self-repulsion. This is likely to be the critical limitation for such systems.

What is exciting is that the same sort of device also enables Lorentz-turning, i.e. you could imagine going into a forced orbit around a strongly magnetic body such as Jupiter. This may conceivably provide the means of stopping at the destination that we were worried about.

Michael January 16, 2014 at 15:39

@Alex Tolley January 15, 2014 at 11:56

‘However, one very interesting claim (Electric solar wind sail mass budget model) is that the thrust is a function of 1/r of the distance from the sun, rather than the 1/r^2 for a solar sail. I’m unfamiliar with the physics, but the idea is that the effective “reflective area” gets larger as the solar wind density decreases.’

The way I see it is that although the wind is still been accelerated out to mars (dropping off) it is still getting weaker in terms of momentum per area. I am wondering if it is a typo.

‘Enamored as I am of the Benford’s beamed sails for high acceleration, I’m wondering if it might be possible to focus the solar wind to increase its local density at the sail.’

Turning the design on its head and making the craft negative and then the more massive positively charged particles will be attracted to from further away increasing the catchment area.

@Peter Popov January 15, 2014 at 18:14

‘Let me focus attention to a different issue. How do you design a heat shield to withstand entry at 100 km/s?’

The probe that entered Jupiter’s atmosphere was going about 47 km/s, so say at 100 km/s that is only about 4 times as much energy to be got rid of, no mean feat I may add.

@Eniac January 15, 2014 at 20:08

‘It is limited by specific tensile strength, which is why carbon nanotubes are by far the most desirable material, as someone else has pointed out.’

A very, very large voltage would be required to remove sufficient bonding electrons to cause the ‘wiring’ to rupture but having said that if they occurred around a nanotube within a short distance of each other it could at least weaken the material.

Looking a little closer at the design it looks as if it needs more ring stays, which is ok because they too can be charged and then less tether mass is required. Also when two outer tethers join the inner ring stay only one tether need be joined to the next ring stay and so on reducing mass.

After having a more in depth look at the design I do have several issues with it, firstly the electron gun beam will have an effect of trying to pull the tethers together by attraction, but that could be countered by ring stays or rotation as he has stated. Secondly due to the high positive charge on the craft electrons will be attracted to the craft at very high velocity which could cause localised heating of the ‘wiring’ as electrons are very small. Thirdly the electron beam used will have a force pushing against the sail reducing forward motion, maybe charges been shot off to the side would be better.

And a neat space weather forecaster, don’t get caught short!

http://www.ips.gov.au/Solar/1/4

Eniac January 16, 2014 at 23:44

@Michael: Tensile strength is needed to keep the wire intact against the electrostatic repulsion it exerts on itself, on the large scale. It has nothing to do with bonding electrons being removed.

The electron gun does not cause attraction. To the contrary, the removal of electrons leads the cables to repulse each other. The gun itself also produces thrust, which you would direct towards the back where it will help. Not sure if the help will be significant or not. Electrons are very light, but then the solar wind is also very weak, so the propulsion due to the electron beam may well be of similar magnitude to that of the sail itself.

@Alex Tolley: I am a little surprised that the “repulsion radius” is given as 100 m. The biggest it could possibly be is the Debye length of the surrounding plasma, which last I checked was around 7 meters or so (Here it is given as 10 m for solar wind: http://en.wikipedia.org/wiki/Debye_length#Debye_length_in_a_plasma), significantly less than 100. In fact it should be much smaller even than that, because a lot of the reflection will be grazing.

Eniac January 16, 2014 at 23:57

There is also no mention in articles I saw about the Debye length that it increases further from the sun, as has been claimed for the “repulsion radius”. I am now wondering if the authors even consider Debye screeing in their calculations. Put simply, the Debye length determines how far away the effect of a local charge can reach before it is neutralized by plasma polarization. There is no way a wire can have a larger propulsion cross section than the Debye length. If that is the claim, I smell something fishy here.

Alex Tolley January 17, 2014 at 10:40

@Eniac

“Electric solar wind sail mass budget model”, p85

The previous assertion about the overlapping negligibility
between electric potential structures of different tethers
can be justified as follows. At 1 AU the potential structure
radius is approx 100 m under average solar wind conditions (Janhunen,
2009). In all E-sail models considered in this paper
the distance between the tether tips is 2xpi×20 km/100 =
1257 m. Thus, under usual conditions, overlapping affects
about 200 m/1257 m = 16% of the tether length. The electric
potential structures scale as proportional to the solar distance
r because they are proportional to the plasma Debye
length which goes as approx 1/sqrt(n), where n is the plasma density
and napprox 1/r^2. Therefore, at 4 AU the overlapping can affect
approx 64% of the tether length. Near the main spacecraft, where
the tethers are close one to the other, they form an effectively
impenetrable obstacle to solar wind ions such that ions are
reflected back. A model of how the thrust behaves inside the
overlap region is not yet available. We roughly estimate that
on average, within the overlapping region, the thrust is 60%
of the free tether value. This implies that at 1 AU the thrust
would be reduced by approx 6% by the overlapping effect and at
4 AU the reduction would be approx 25 %.

Did I misunderstand the meaning of potential structure
radius is approx 100 m
?

stephen January 17, 2014 at 17:35

How well can it handle dust and debris?

Michael January 17, 2014 at 22:06

@stephen January 17, 2014 at 17:35

‘How well can it handle dust and debris?’

It is a very open structure so a dust collision is very unlikely, but if it were to hit a wire at 100km/s it will break for sure. A wire repair device could be made to walk the wires and reattach them, say using twin cables very close together as the base for reattachment. ultimately it will be a trade off between a probability of an impact and the added mass of the repairs devices.

Eniac January 18, 2014 at 0:33

@Alex: Thanks for the quote. I cannot tell from this paragraph what exactly the potential straucture radius is supposed to be. It is obvious, though, that the auther is aware of the importance of the Debye length, and he states clearly that this increases with 1/sqrt(n) and is thus roughly proportional to the distance from the sun. I get this now.

What remains is the fact that the Debye length is ~10 m at 1 AU, and this does not jibe with the 100 m potential structure radius. Given the definition of the Debye length, I do not see how any potential can extend beyond it, and accounting for grazing reflection at the periphery should reduce the effective sail area even further.

Michael January 18, 2014 at 4:24

@Eniac January 16, 2014 at 23:44

‘Tensile strength is needed to keep the wire intact against the electrostatic repulsion it exerts on itself, on the large scale. It has nothing to do with bonding electrons being removed.’

I misunderstood what you had written, you meant the structural integrity of the wiring.

‘The electron gun does not cause attraction. To the contrary, the removal of electrons leads the cables to repulse each other.’

I agree on the latter part of your statement but not to the former, the electrons also have an attractive force towards the positively charge wiring, the wiring will want to follow it and collapse the structure.

‘The gun itself also produces thrust, which you would direct towards the back where it will help.’

Dr Janhunen has the electrons going forwards in the design and the way you suggest it may cause the solar wind to become neutral before coming near the craft, better they go off to the sides.

Michael January 18, 2014 at 4:27

Hi Paul

Is it possible after these discussions to get Dr Janhunen to clear up some misconceptions and resolve some of the queries.

Paul Gilster January 18, 2014 at 10:07

Ideally, it would be best to have Dr. Janhunen write a follow-up essay that answers the various issues raised here. I’ll see what I can do.

Alex Tolley January 18, 2014 at 12:03

@Stephen – the cables are a modification of the Hoytether design that they call a Heytether. You may recall that the Hoytether design is a lattice type design that was suggested by Brad Edwards for the space elevator. The design specifically addresses hits by objects smaller than the cable width.

Since the structure is spinning, a complete tether break could be fairly catastrophic as the section outside the break would try to extend itself beyond the circumference and potentially destabilize the sail dynamics. The probability is extremely low given the tether design.

Pekka Janhunen January 18, 2014 at 12:31

Paul asked me to comment, so I try to answer some of the
questions. Overall I found all comments interesting without major
misconceptions.

@Michael Nanotubes might be ideal, but how to bond together nanotube
cables to make a multiline tether.

@Alex Tolley Power consumption scales by solar wind density which
scales as 1/r^2 i.e. in the same way as solar panel power.

@Peter Popov The 100 km/s speed came from somewhere else, the
Uranus mission would move slower. The Uranus mission paper is meant to
describe a relatively near-term concept, not push for the limits of
the tech.

@Michael,Eniac When an electron beam is shot into solar wind plasma,
as soon as beam density drop below background, it only creates a small
negative potential well of the order of background electron
temperature (10 eV) which is enough to displace a requirement amount
of background electrons. Thrust provided by the electron beam is
insignificant because of the low mass of electrons so where the gun is
directed is not so important. The tethers warm up only slightly
because of the accelerated electron bombardment they receive from the
solar wind.

@Eniac The region of influence can indeed extend beyond Debye
length. The relevant quantity is so-called effective Debye length
where the electron temperature is replaced by the tether voltage
e*V0. So the effective Debye length is by factor sqrt(e*V0/Te) larger
than the normal one. In our case this factor can be as large as
45. This is seen in plasma simulations, and for an independent account
see e.g. Hanspeter Schaub
research page
and search for ‘effective Debye length’. According
to simulations, the plasma sheath is roughly factor 2 smaller than the
effective Debye length.

Eniac January 18, 2014 at 13:42

Pekka Jahunen: Thanks for the clarification. I can’t say I understand the concept of the much larger “effective” Debye length, and the link you gave led me only to a brief mention, not an explanation or reference. If this is an established concept, perhaps you have a literature reference?

Aside from that, everything now makes sense.

Eniac January 18, 2014 at 14:31

@Michael: The whole point of the electron beam is to get rid of the electrons, to shoot them away to infinity. They will be gone so fast that any attractive effect they might have while still in the vicinity is negligible. Whether there is a significant propulsion effect depends on the product of three ratios: 1) the ratio of the electron beam stopping voltage to that of the solar wind protons, 2) The number of electrons absorbed for each proton reflected, and 3) the electron/proton mass ratio, which is about 1/2000.

The electron beam voltage will have to be higher than the wire voltage, otherwise the electrons would come back. So ratio 1) should be quite a lot larger than 1. However, a good electrostatic sail will have a small ratio 2), and ratio 3) is naturally small, so I am inclined to believe Dr. Jahunen’s assertion that electron beam thrust is insignificant.

Eniac January 18, 2014 at 14:46

Thinking about electron gun propulsion some more gave me these thoughts: Electron gun propulsion is so attractive because it can produce thrust without stored reaction mass. The reaction mass is taken from the surrounding plasma. The problem is low energy efficiency due to the small electron mass.

I wonder if, analogously, we could use a proton gun and deploy a long, negatively charged wire made of a proton conductor to pick up the reaction mass. It would retain the advantage, but add a factor of 2000 to thrust/power. Similar to an ion drive, but without the propellant tank. We’d also still get thrust from the solar wind, picking up the momentum of the absorbed protons.

Michael January 18, 2014 at 15:25

Thanks Paul and yourself Pekka for taking the time to answer some of our questions.

@Pekka Janhunen January 18, 2014 at 12:31

‘Michael Nanotubes might be ideal, but how to bond together nanotube
cables to make a multiline tether.’

Individual nanotubes can be joined together, end-to-end, by capillary action to make a much larger nanotube, also if metals are used they can used to fill the interior of the nanotube allowing a good electrically conductive path.

http://www.materialsviews.com/the-world%C2%B4s-smallest-pipettes-capillary-action-in-carbon-nanotubes/

Now in order to protect the cable from a hit multi-lines of nanotubes can held together every now by rings of nanotubes or materials that hold them together.

http://www.nanoscience.gatech.edu/paper/2006/06_AM_2.pdf

Michael January 19, 2014 at 2:53

Just another thought, if the tethers were offset like a spiral then as the positively charged particles move along between the tethers electric fields and are reflected they would impart a centrifugal spin to the wiring helping maintain its open structure.

Michael January 19, 2014 at 5:45

An article on Debye length in plasmas

http://hanspeterschaub.info/Papers/Parker2006.pdf

Pekka Janhunen January 19, 2014 at 6:29

@Eniac

For a long time I saw from simulations that the sheath becomes much larger than the thermal Debye length. In my view the physical reason is that electrons are accelerated by the potential well so inside the sheath their effective ‘temperature’ and thus the effective Debye lenght becomes higher. A couple of years ago I randomly met Hanspeter Schaub in a conference and he was using the term. I don’t think it’s in widespread use, however.

Siguier et al. 2013 made laboratory measurements of a positively biaised wire in streaming plasma. They scanned the sheath with a moving Langmuir probe. Their sheath size is in agreement with my simulation. Their goal was not to study the E-sail but rather the physics of the electrodynamic tether, but anyway the experiment is relevant.

One could also use a negatively charged tether (Janhunen, 2009). The main issue is that field emission of electrons tends to occur from the wire surface which limits the usable voltage. The number of collected ions is too small to fill the tank of an ion engine. The solar wind thrust dominatese because the ions have high speed (400-800 km/s, much larger than a viable ion engine) and because the number of deflected protons is much larger than the number of protons which actually hit the thin tether.

Pekka Janhunen January 19, 2014 at 8:05

I should add that in the ionosphere a negative tether is what we plan to use, however, because it uses less power. That’s the ionospheric “plasma brake” application for satellite deorbiting. In the ionosphere, relatively low voltage (~1 kV) is not a problem since the voltage is anyway order of magnitude higher than the bulk ion kinetic energy of 4 eV. Above I referred to the solar wind case.

ljk January 20, 2014 at 10:41
Michael January 20, 2014 at 15:47

@Pekka

Do you have the mass of the tether structure to hand, I was just wondering what type of materials you were planning to use and whether more modern materials and techniques could be applied to it. From what I can see it is a sound concept.

Eniac January 23, 2014 at 0:13

@Pekka Janhunen: Thanks for the illuminating response. This is exciting research about the large sheath size and extended effective Debye length. Hopefully it will bear out.

You are right, of course: Electron field emission severely limits the potential of a proton collection wire. I had forgotten about that.

ljk January 27, 2014 at 21:44

The Majestic Realm of a Forgotten World: The Alluring Mysteries of Uranus

By Leonidas Papadopoulos

Having been the object of neglect from space agencies on one hand, and hilarity from the general public on the other, Uranus still remains one of the most mysterious places in the Solar System.

There are currently 22 planetary spacecraft scattered throughout the Solar System, actively exploring almost every part of the Sun’s planetary family. Yet, one glaring omission from this long list of space exploration targets has been the planet Uranus, ever since NASA’s Voyager 2 spacecraft paid a brief visit there, 28 years ago this month, in January 1986.

Although it shares many similarities with neighboring Neptune, Uranus is an interesting peculiarity on its own. And even though Voyager 2′s fly-by has provided us with the bulk of our current knowledge of the planet, a greater series of even more intriguing questions about this enigmatic cyan-tinted ringed world remain unanswered to this day.

Full article here:

http://www.americaspace.com/?p=50762

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