APOD: Heliopause Electrostatic Rapid... (2016 Apr 16)

[APOD Robot]

Heliopause Electrostatic Rapid Transit System

Explanation: Want to take a fast trip to the edge of the Solar System? Consider a ride on a Heliopause Electrostatic Rapid Transit System (HERTS). The concept is currently being tested and it might take only 10 to 15 years to make the trip of over 100 Astronomical Units (15 billion kilometers). That's fast compared to the 35 years it took Voyager 1, presently humanity's most distant spacecraft, to approach the heliopause or outer boundary of the influence of the solar wind. HERTS would use an advanced electric solar sail that works by extending multiple, 20 kilometer or so long, 1 millimeter thin, positively charged wires from a rotating spacecraft. The electrostatic force generated repels fast moving solar wind protons to create thrust. 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 cruised toward the outer planets.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

[Guest]

Delicate structures hurtling through space sounds silly. Build a megawatt reactor and be done with it.

[bystander]

Click to play embedded YouTube video.

http://asterisk.apod.com/viewtopic.php?t=35830#p256393

[madtom1999]

I doubt any alien will be able to understand the message written on the wires.

[BMAONE23]

Any advanced civilization that encounters this craft would think ... man how backwards are these people, all their commercials indicated they were wireless

[Nitpicker]

Any advanced civilization that encounters this craft would think ... man how backwards are these people, all their commercials indicated they were wireless

The existence of our commercials should be enough to demonstrate our backwardness.

[neufer]

Click to play embedded YouTube video.

Heliopause Electrostatic Rapid Transit System

Explanation: Want to take a fast trip to the edge of the Solar System? Consider a ride on a Heliopause Electrostatic Rapid Transit System (HERTS).

The existence of our commercials should be enough to demonstrate our backwardness.

[Asterhole]

So this craft would possibly continue to accelerate along its journey (to where?). And supposing there was an exosolar destination - which would take hundreds, if not thousands of years to reach, how would it decelerate once it got there?

[neufer]

So this craft would possibly continue to accelerate along its journey (to where?). And supposing there was an exosolar destination - which would take hundreds, if not thousands of years to reach, how would it decelerate once it got there?

Unless one specifically wishes to ride down the heliotail (away from Scorpius towards Aldebaran) the craft would accelerate only out to about 11 billion miles. From there it could coast to another star and then decelerate by flying into the head winds of that star (Aldebaran an orange giant star located about 65 light years away).

<<The wind exerts a pressure at 1 AU typically in the range of 1–6 nPa (as compared with ~ 9,000 nPa for a light sail), although it can readily vary outside that range.

On 13 December 2010, Voyager 1 determined that the velocity of the solar wind, at its location 10.8 billion miles from Earth had slowed to zero. "We have gotten to the point where the wind from the Sun, which until now has always had an outward motion, is no longer moving outward; it is only moving sideways so that it can end up going down the tail of the heliosphere, which is a comet-shaped-like object," said Voyager project scientist Edward Stone.>>

---

<<The heliotail is the solar system's tail or can be understood as the tail of the heliosphere. Similarly it can be compared to a comet, which also has a tail (however a comet's tail does not stretch behind it as it moves, it is always pointing away from the Sun). A further explanation of the tail is a region where the Sun’s million mile per hour solar wind flows down and ultimately escapes the heliosphere, slowly evaporating because of charge exchange. The shape of this newly found tail by NASA's Interstellar Boundary Explorer (IBEX) is that of a four-leaf clover. Due to the particles in the tail, they do not shine, therefore it cannot be seen with conventional instruments. IBEX has made the first observations by using a technique called "energetic neutral atom energy" which is the process of measuring the neutral particles created by collisions at the solar system's boundaries.

The tail has been shown to contain fast and slow particles; the slow particles are on the side and the fast particles are encompassed in the center. The shape of the tail can be linked to the sun sending out fast solar winds near its poles and slow solar wind near its equator more recently. The clover-shaped tail moves further away from the sun, which makes the charged particles begin to morph into a new orientation.

[MarkBour]

In any case, this is a fascinating system for propulsion. I'm wondering what keeps the tethers extended? Is there a rotation? Wouldn't there be some drag on a rotation that would require continual energy addition to keep it going? Over time, would the tethers necessarily become damaged, or misshapen? Would the spacecraft be able to have some spares that could be deployed, and damaged tethers released?

Three systems were compared: ion-thrust, which we now know works well, a photonic reflective solar sail (L-Sail, perhaps?), and this E-Sail (that's a name they used for it that may stick). What are the comparisons of possible payloads? Why do they suggest that ion thrust would have to stop after about 5 AU? Can't we just build a bigger one and have it go longer? Would it be worth considering a combination: An E-Sail or solar sail that carried an ion-thrust ship to the heliopause, and then it used ion-thrust from there, for as long as it could. In such a case, turning on the ion-thrusters right away from Earth might also be fine, but I think that would most likely reduce the value of the sail, wasting its thrust.

What happens to the E-Sail past the heliopause? Has Voyager detected anything that would amount to a cross-wind once past that point? Currently, the Voyager I craft is sensing a thicker plasma. Surely the E-sail could be deactivated in that instance, it might even be worth it to try to retract the tethers until deceleration. Especially if interstellar space has enough particles and/or fields that might damage the system over a long journey. We would have no chance to communicate with it to tell it to extend again when the time is right, it would have to be robustly autonomous well beyond our current achievements. I mean, a system flying to the nearest star would have to take an action after a century in space (or longer), and too far from us to talk in any reasonable time frame.

But the APOD only referred to it as an interesting way to get to the heliopause. As far as I can tell, this system is not yet any kind of candidate for a true interstellar voyage. If it achieved say, 3 times the speed of Voyager, that's still (I compute) about 25,000 years to get to the nearest star at around 4.3 light years.

[neufer]

Three systems were compared: ion-thrust, which we now know works well, a photonic reflective solar sail (L-Sail, perhaps?), and this E-Sail (that's a name they used for it that may stick).

The Dawn spacecraft is propelled by three xenon ion thrusters.
Each has a specific impulse of 3,100 s and produce a thrust of 90 mN.

A photonic reflective solar sail would have to have at least 10,000 m²
of sail to produce the same thrust as a Dawn ion thruster.

The E-Sail would have to have a radius of at least 2 km
to produce the same thrust as a Dawn ion thruster.

[daddyo]

Neat idea. Someone should figure out the probability of a cut wire, if significant what you'd do to counter the loss of weight.

[Chris Peterson]

Neat idea. Someone should figure out the probability of a cut wire, if significant what you'd do to counter the loss of weight.

The probability seems vanishingly low, to the point that such a failure would represent a complete loss of mission, not something for which a recovery plan would be considered.

[daddyo]

First search found this interesting micrometeorite probability http://www.propagation.gatech.edu/ECE63 ... orite.html

If you look at the second image (plot) and assume any micrometeorite on the chart can cut a wire, given it's huge average speed, it might be pretty easy to figure out the chance of a hit if you know the wire diameter and total length. Fun little calculation. So I did it:

Area of fanned out wires, if using a guessed 22 gauge wire (0.65 mm diameter) and counted 20 wires in video = 0.00065 m * 20 km wires * 20 wires = 260 m^2

Micrometeorite flux for smallest particle in plot, although higher if you integrate all particle sizes, and happen to be about same size as guessed wire size = 100 meteorites/m^2/year

Hits over 15 year mission = 100 meteorites/m^2/year * 260 m^2 fan area * 15 year mission = 390k hits

That unfortunately sounds really bad for a successful mission, maybe survive about 1/2 hour. Maybe it needs to be a metallic sheet instead.

[JohnD]

Even if the HERTS was spinning - it was, but very slowly in the video - the wires would bow in the direction of travel, away from the Sun, in reaction to the thrust. The area exposed to the solar wind would be reduced and the thrust also. Has than been included in the calculations, when the video showed then straight out from the hub?
JOhn

[neufer]

Even if the HERTS was spinning - it was, but very slowly in the video - the wires would bow in the direction of travel, away from the Sun, in reaction to the thrust. The area exposed to the solar wind would be reduced and the thrust also. Has than been included in the calculations, when the video showed then straight out from the hub?

We are talking about a very lightweight spacecraft and thrusts of just ~90 mill-iNewtons.

At the Earth the gravitational tug of the Sun is just ~8mN per kilogram so a 90 mN thrust can totally negate the Solar pull for an 11 kg spacecraft.

Once that Solar pull is negated a 11 kg spacecraft can fly off in a straight line at 2π AU per year.

A spacecraft less than 11 kg will fly off much faster.

The Dawn spacecraft is propelled by three xenon ion thrusters.
Each has a specific impulse of 3,100 s and produce a thrust of 90 mN.

A photonic reflective solar sail would have to have at least 10,000 m2
of sail to produce the same thrust as a Dawn ion thruster.

The E-Sail would have to have a radius of at least 2 km
to produce the same thrust as a Dawn ion thruster.

[daddyo]

Maybe that's the trick, looking at that meteorite curve drop off at very small sizes, extremely small vehicles with hair width wires might work. Wires also double as a highly focused high gain antenna to earth.

Wonder how much power could be generated onboard given that current flow.

[JohnD]

For once, neufer, I think you misunderstand. I don't dispute the ability of this device to fly, just the shape it will assume in flight and the effect of that shape on the thrust it will have.

Assume an equal thrust per inch/meter of wire length.
The wire is free at the outer end, attached to the centre of mass at the other.
The moment of the thrust will depend on the distance from the CoM, and be progressively greater the further out that point is.
The result will be that the wire will not project straight out from the CoM, but have a curved shape (Possibly a catenary curve) which will bow the rosette into a shuttlecock shape. That will have a smaller face area to the solar wind, so the thrust will be less than expected from a flat, wider rosette.

If the CoM were displaced to "hang below" the rosette, like the woman on a parachute, and a few extra tethers were included to take the intermediate strain on the wires, then the flat shape could be maintained, at the cost of extra weight. The tethers would be non-conducting, of course.

John

[neufer]

For once, neufer, I think you misunderstand. I don't dispute the ability of this device to fly, just the shape it will assume in flight and the effect of that shape on the thrust it will have.

The "wind force" only amounts to ~ 90 milliNewtons by my estimation.

It would take very little spinning centrifugal force to make that seem irrelevant to a ~4km wide structure.

[daddyo]

It would be neat to model the wire shape, although I think catenaries are suspended by more than one point. Probably similar to a windmill.

There's probably a solution for this whole concept if you work on it long enough, just work smarter!

[JohnD]

For once, neufer, I think you misunderstand. I don't dispute the ability of this device to fly, just the shape it will assume in flight and the effect of that shape on the thrust it will have.

The "wind force" only amounts to ~ 90 milliNewtons by my estimation.

It would take very little spinning centrifugal force to make that seem irrelevant to a ~4km wide structure.

90mN, neufer? Overall? Per metre??
And the central mass less than a kgrm?
John

[neufer]

The "wind force" only amounts to ~ 90 milliNewtons by my estimation.

It would take very little spinning centrifugal force to make that seem irrelevant to a ~4km wide structure.

90mN, neufer? Overall?
And the central mass less than a kgrm?

Yes. The solar wind is really only effective on tiny charged particles.