APOD: LightSail A (2015 Jun 19)

[APOD Robot]

LightSail A

Explanation: Hitching a ride to low Earth orbit, LightSail A accomplished a challenging test mission, unfurling its 32 square meter mylar solar sail on June 7. This dramatic image from one of the bread loaf sized spacecraft's fisheye cameras captures the deployed sail glinting in sunlight. Sail out and visible to Earthbound observers before its final orbit, LightSail A reentered the atmosphere last weekend. Its succesful demonstration paves the way for the LightSail B spacecraft, scheduled for launch in April 2016. Once considered the stuff of science fiction, sailing through space was suggested 400 years ago by astronomer Johannes Kepler who observed comet tails blown by the solar wind. But modern solar sail designs, like the one tested by LightSail A rely on the small but continuous pressure from sunlight itself for thrust.

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[Boomer12k]

Argggh...me hearties....host the main Solar Sail...the booty will soon be ours....arrggghh....

:---[===] *

[Nitpicker]

I was under the impression that solar radiation pressure, and the pressure exerted by the solar wind, were the same thing. But the explanation text suggests otherwise. Can someone please set me straight?

[neufer]

Once considered the stuff of science fiction, sailing through space was suggested 400 years ago by astronomer Johannes Kepler who observed comet tails blown by the solar wind. But modern solar sail designs, like the one tested by LightSail A rely on the small but continuous pressure from sunlight itself for thrust.

I was under the impression that solar radiation pressure, and the pressure exerted by the solar wind, were the same thing. But the explanation text suggests otherwise. Can someone please set me straight?

• Solar radiation pressure from light particles is quite different from (and much stronger than)
  • the pressure exerted from solar wind particles
    (or from the magnetic fields that they drag along with them).
  Johannes Kepler knew nothing about actual solar wind particles
  but guessed, rather, that comet tails were blown away by solar light particles.

<<Johannes Kepler put forward the concept of radiation pressure back in 1619
to explain the observation that a tail of a comet always points away from the Sun.>>

<<Solar radiation exerts a pressure on the sail due to reflection and a small fraction that is absorbed. The absorbed energy heats the sail, which re-radiates that energy from the front and rear surfaces.

The momentum of a photon or an entire flux is given by p = E/c, where E is the photon or flux energy, p is the momentum, and c is the speed of light. Solar radiation pressure is calculated on an irradiance (solar constant) value of 1361 W/m2 at 1 AU (Earth-Sun distance):

• perfect absorbance: F = 4.54 μN per square metre (4.54 μPa)
  perfect reflectance: F = 9.08 μN per square metre (9.08 μPa) (normal to surface)

Solar wind, the flux of charged particles blown out from the Sun, exerts a nominal dynamic pressure
of about 3 to 4 nPa, three orders of magnitude less than solar radiation pressure on a reflective sail.>>

[Nitpicker]

Thanks neufer.

[Nitpicker]

Question 2: If there was no solar wind (only solar radiation), would active comets in the solar system still display separated dust and ionised gas tails?

[Dad is watching]

My impression of a sail (solar or otherwise) is that it is a tensile structure. The force imparted on it is transferred to the supporting vessel thru tensile members. When I look at this photo, it seems that the sail is a rather limp and rumpled structure without tensile integrity. I am wondering how the energy caught by the wail isn't wasted on bending the fabric or just having the fabric flop around due to small differential pressures across the surface of the fabric? Was this photo taken prior to, or after, the main test were conducted and therefore give a false impression? Or was there a flaw in deployment of the sail?

[hoohaw]

MESSENGER, the mission to Mercury, made many tiny course corrections over years, simply by manipulating its orientation so light pressure from the Sun would vary.

[Nitpicker]

Based on a 9 micropascal radiation pressure and a 32 square metre sail, the propulsion force is equivalent to the weight of about 30 milligrams (0.001 ounces) on Earth. I've just read that LightSail A has a mass of about 5 kilograms, which means it would take almost 5 hours for this force to change its velocity by 1 metre per second. Perhaps not surprising the sail doesn't look taut.

(Of course after a year, it could be travelling at more than Mach 5.)

[k golem]

In order to be truly effective, spacecraft that "sail" on the solar wind in our solar system must also be able to utilize a rudder and keel. Or else steering or navigating back to the center of the solar system would be impossible.

[planetquinn]

I wonder about that too. A spacecraft could sail downwind but could it ever return? Are there be other forces out there to triangulate with? There would need to be 4 sources of wind--a tetrahedral grid--to navigate freely in space.

[Dad is watching]

In order to be truly effective, spacecraft that "sail" on the solar wind in our solar system must also be able to utilize a rudder and keel. Or else steering or navigating back to the center of the solar system would be impossible.

We thought about that too. As there is no real substrate in space against which a keel or rudder could act (like the sea or atmosphere), a rudder and keel would be useless. Perhaps return journeys of light sail ships could be planned to include free return trajectories from other solar system object, getting gravity assist to change direction and then use the light sail to steer or do mid-course corrections. Sail orientation would need to be adjustable to reduce drag and control light sail force moments. But for efficiency, the sail should be taught, right?

[k golem]

So we need an element that will interact with dark matter or the massive Higgs Boson so that we can push against these things and turn the solar sail into a solar wing.

[neufer]

Question 2: If there was no solar wind (only solar radiation),
would active comets in the solar system still display separated dust and ionised gas tails?

Good question! I'm not sure.

My guess is that the tails would reduce to one continuous broad tail with the colorful ion portion on the outside.

http://burro.astr.cwru.edu/Academics/As ... ttail.html
http://www2.ess.ucla.edu/~jewitt/tail.html

<<CO+ absorbs sunlight and fluoresces, emitting energy at a wavelength of 4200 Angstroms, which is blue light.>>

Now, you like to do math.

How many 4200 Angstrom photons would one CO+ ion have to scatter
in order to achieve an escape velocity of say... 30 km/s

[Keyman]

First, as a charter member of the Planetary Society, thanks for acknowledging our privately funded project. We are very proud of what we accomplished. This was a test flight, and there were a lot of things that we learned that will need to be fixed before next April when LightSail B heads up (on a SpaceX Falcon Heavy (~crossing fingers~)). There are a number of highlighted links in the text, that will provide you more information, than I can here, and I'd encourage you to check them out.

Thanks to neufer for your clarifications on radiation pressure vs. solar wind. (As an aside, I always tell my wife that the TV *is* louder when you turn off the light at night because the light pressure has been holding back the sound waves.)

Solar sailing can work much the same way wind sailing does. NASA has been doing some studies which:

found that transparent refractive objects may settle into a position where they feel a force that is perpendicular to the incoming light direction. This transverse force is akin to the lift experience by an airplane wing and other airfoils. Thus they call it “optical lift”. Objects in their experiments simultaneously thrust forward. The combination of lift and thrust allows for the optical steering of optical wings.http://www.nasa.gov/offices/oct/early_s ... sails.html

JAXA has actually done it:

Liquid crystal panels on the edges of the sail can change their surface reflection of sunlight by using low amounts of electricity to turn on or off. The "on" setting creates a mirror-like reflection that pushes the spacecraft forward, while the "off" setting has a more diffuse reflection that redirects the pressure of sunlight in all directions, lessening the force against the sail.http://www.space.com/8896-solar-sail-sp ... -time.html

Third...come join us http://sail.planetary.org/index.html

[Coil_Smoke]

I had recently heard a story of the failure of a solar sailing craft. I think it was a comunications issue. Light Sail A seems like a success. Was there a recent failure of Light Sail craft ?

[Chris Peterson]

My impression of a sail (solar or otherwise) is that it is a tensile structure. The force imparted on it is transferred to the supporting vessel thru tensile members. When I look at this photo, it seems that the sail is a rather limp and rumpled structure without tensile integrity. I am wondering how the energy caught by the wail isn't wasted on bending the fabric or just having the fabric flop around due to small differential pressures across the surface of the fabric? Was this photo taken prior to, or after, the main test were conducted and therefore give a false impression? Or was there a flaw in deployment of the sail?

A normal sail, used in an atmosphere (as on a boat), is tensile, but not flat. Sails are designed to be curved (camber) with carefully designed leading and trailing edges. They act like wings, generating lift, and that is an important aspect of their efficiency and ability to provide not just propulsion, but also direction. But a light sail is operating in a medium where you can't have lift in a significant way, so I'd expect the most efficient structure to be a flat one. A loose solar sail will still "fill with the wind" after a while and become taut, but without the entire surface being normal to the incident radiation it won't be producing maximum thrust. Of course, since these sails in practice would utilize tacking, they wouldn't produce maximum thrust at all times in any case. A bit of camber probably wouldn't matter all that much.

[Chris Peterson]

In order to be truly effective, spacecraft that "sail" on the solar wind in our solar system must also be able to utilize a rudder and keel. Or else steering or navigating back to the center of the solar system would be impossible.

Unless the craft achieves solar escape velocity, it will remain in a closed orbit around the Sun. Design the trajectory correctly, and there's no reason it can't return to any desired perihelion. Remember, it's not starting from zero velocity.

[Chris Peterson]

Question 2: If there was no solar wind (only solar radiation), would active comets in the solar system still display separated dust and ionised gas tails?

I don't see how. They'd have a dust trail (formed by solar radiation pressure), but no ion tail at all. Gas would expand spherically around the nucleus and largely disperse. Ultimately you might have a slightly higher density in a toroidal volume defined by the comet's orbit, and it could be partly ionized, although detecting it would be challenging, and it wouldn't be structured like a tail.

[neufer]

Question 2: If there was no solar wind (only solar radiation),
would active comets in the solar system still display separated dust and ionised gas tails?

I don't see how. They'd have a dust trail (formed by solar radiation pressure), but no ion tail at all. Gas would expand spherically around the nucleus and largely disperse. Ultimately you might have a slightly higher density in a toroidal volume defined by the comet's orbit, and it could be partly ionized, although detecting it would be challenging, and it wouldn't be structured like a tail.

• Like the comet's neutral Hydrogen cloud

• Welcome to Astronomy at ASU!

Chapter 15-7: A comet is a chunk of ice and dust that partially vaporizes as it passes near the Sun

• Surrounding the coma is a very large hydrogen envelope, perhaps 10 million kilometers in size
  This material comes from breakdown of molecules by high-energy photons from the Sun
  The great extent of this envelope can be seen in photographs of its UV emission lines

[Coil_Smoke]

I had recently heard a story of the failure of a solar sailing craft. I think it was a comunications issue. Light Sail A seems like a success. Was there a recent failure of Light Sail craft ?

Well, I found an answer to my own question...

LightSail Solar Sail Spacecraft Goes Silent Again
by Mike Wall, Space.com Senior Writer | June 05, 2015 01:53pm ET

[Pin It] Artist's concept of The Planetary Society's LightSail spacecraft with its sail deployed. The cubesat has encountered several difficulties on its debut flight.
Credit: The Planetary Society
View full size image

A tiny cubesat has fallen silent in orbit for a second time, just two days before it was supposed to deploy its solar sail.

The nonprofit Planetary Society's LightSail spacecraft has not communicated with Earth since Wednesday afternoon (June 3), shortly after an apparently successful solar-panel deployment, mission team members said.

"Mission managers believe the cubesat's batteries are in a safe-mode-like condition designed to protect the electronics until power levels are safe for operations," The Planetary Society's Jason Davis wrote in a mission update Wednesday evening.

[Keyman]

I had recently heard a story of the failure of a solar sailing craft. I think it was a communications issue. Light Sail A seems like a success. Was there a recent failure of Light Sail craft ?

There were a number of "communication issues" with LightSail A. And in reality, it was never in an orbit high enough to allow actual "sailing". This wasn't "by design" but by necessity. Finding a ride to orbit in the time frame (and expense allowance) required wasn't possible. But it was 'only a test flight' anyway, more to check out the actual operation of the craft. Can it communicate? (Yes, sometimes, mostly.) Can we deploy the sails? (Yes, mostly - which would/will have gone better if/when the communication glitches are resolved.) We had only two ground stations from which to communicate, so a more constant/regular set of solutions could not be sent. There were extension processes to make the sail more taut...if we'd been able to get the signals through.

In that it was a test flight, it was a 'success'. We learned a lot of things form the stuff we could test. It didn't "sail" because it wasn't meant to...this time.

But it was up there, and... https://www.youtube.com/watch?v=d8NUCOMmCpY
We did that.

[Keyman]

I had recently heard a story of the failure of a solar sailing craft. I think it was a comunications issue. Light Sail A seems like a success. Was there a recent failure of Light Sail craft ?

Well, I found an answer to my own question...

LightSail Solar Sail Spacecraft Goes Silent Again
by Mike Wall, Space.com Senior Writer | June 05, 2015 01:53pm ET

[Pin It] Artist's concept of The Planetary Society's LightSail spacecraft with its sail deployed. The cubesat has encountered several difficulties on its debut flight.
Credit: The Planetary Society
View full size image

A tiny cubesat has fallen silent in orbit for a second time, just two days before it was supposed to deploy its solar sail.

The nonprofit Planetary Society's LightSail spacecraft has not communicated with Earth since Wednesday afternoon (June 3), shortly after an apparently successful solar-panel deployment, mission team members said.

"Mission managers believe the cubesat's batteries are in a safe-mode-like condition designed to protect the electronics until power levels are safe for operations," The Planetary Society's Jason Davis wrote in a mission update Wednesday evening.

Yep. That happened. Jason Davis has a series of daily blogs on the the whole mission, and tweets in between. #LightSail But you can access them all here; http://sail.planetary.org/missioncontrol

[Grizzly]

My impression of a sail (solar or otherwise) is that it is a tensile structure. The force imparted on it is transferred to the supporting vessel thru tensile members. When I look at this photo, it seems that the sail is a rather limp and rumpled structure without tensile integrity. I am wondering how the energy caught by the wail isn't wasted on bending the fabric or just having the fabric flop around due to small differential pressures across the surface of the fabric? Was this photo taken prior to, or after, the main test were conducted and therefore give a false impression? Or was there a flaw in deployment of the sail?

My understanding was that the sail was not fully deployed, but that this was deliberate. The test was to see whether it could get as far as it did in the unfolding / deployment process. There was some discussion of attempting full deployment prior to it returning to earth but I am not sure whether this was attempted.

[Dad is watching]

Of course, since these sails in practice would utilize tacking, they wouldn't produce maximum thrust at all times in any case.

We had to think about this a lot and discuss it over supper. We couldn't figure out how to tack a light-sail ship without inducing a turning moment that could not be overcome without a substrate to work against. After a while tho, we figured that the spacecraft would be equipped with a gyro system that could counteract the turning moment, thereby replacing the need for the substrate and keel.

Thanks Chris