Astronomy

What is Propeller Effect exactly?

What is Propeller Effect exactly?


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I find the original reference seems to be here. I still do not understand the mechanism. It is necessarily occur in neutron binaries? Anyone can introduce it in detail? This paper is old.


The magnetic field of the rapidly rotating neutron star interacts with the material coming from the other star in the binary. This results in a transfer of angular momentum, spinning the neutron star down but accelerating the material out of the system. The effect is somewhat like a garden sprinkler seen from above, with material flung out ("propelled") of the system.

I think there needs to be a quite delicate balance between the binary period (or separation) in the system and the strength of the magnetic field and rotation rate of the neutron star to form the propeller effect. It seems to happen most in the early stages of the X-ray binary when the neutron star is rotating rapidly. After the neutron star spins down by the torques exerted on it, the material from the secondary star in the binary forms either an accretion disk or is directly accreted onto the neutron star down the magnetic field lines.

The effect has also been seen, possibly only the once, in Cataclysmic Variable (CV) systems, which consist of a low-mass star (typically a K or M dwarf) and a magnetized white dwarf. The "poster child" for this type of propeller system in CVs is AE Aquarii A magnetic propeller in the cataclysmic variable AE Aquarii; Wynn et al 1997

The propeller theory seems to be alive and well as there over 800 citations to the original 1975 paper. A recent highly cited review discussing propellers and accreting neutron stars seems to be Alipar 2001


Boat Propellers, Custom Props, Boat Propeller Repairs, Prop Reconditioning

Customer Comment:
"I'm in the boat repair business and pride myself with the ability to set a boat up correctly. However, there comes a point where the boat needs more help to perform correctly than just the boat being set up and dialed in. Generally the only way to get that little extra edge is to have your propeller modified.
After re powering my Champion 203 with a Mercury 225 Pro XS, it was still not performing exactly as it should. I tried eight or ten different props and finally found which factory prop ran the best overall.
I then sent the prop to Steve at Steve's Custom Props. He immediately found that the one blade was 1/4 of a pitch off from the Factory! He then customized the prop and put his magic touch to it. All I can say is WOW. I got four to five more miles per hour, the hole shot is the best it has EVER been, and the midrange turning and handling is completely different! The boat had good bow lift before, but now it is incredible!
Having used many different Prop Shops over the years. There was one thing that made Steve's work stand apart. Most customers probably would not even notice this, but when the prop came back, it was polished and didn't have any grinding marks or other blemishes that many of the other shops don't take the time to correct. That impressed me as much as the performance difference!
If you want one of the best in the business to repair your prop, add more cup, or completely customize your prop, Steve is the guy to call!
Michael Bristow
North Texas Fiberglass

Steve,
I just wanted to thank you for your help getting my new prop 'dialed
in' ! I finally got the boat out after our last conversation and my
zx200 got up to 68.5mph with a full tank and 15mph winds chopping up
the lake. A low tank and slight ripples and I'll be screaming!
Thanks again,
Shawn


A Simple Explanation of React.useEffect()

I am impressed by the expressiveness of React hooks. You can do so much by writing so little.

But the brevity of hooks has a price — they’re relatively difficult to get started. Especially useEffect() — the hook that manages side-effects in functional React components.

In this post, you’ll learn how and when to use useEffect() hook.

1. useEffect() is for side-effects

A functional React component uses props and/or state to calculate the output. If the functional component makes calculations that don’t target the output value, then these calculations are named side-effects.

Examples of side-effects are fetch requests, manipulating DOM directly, using timer functions like setTimeout() , and more.

The component rendering and side-effect logic are independent. So it would be a mistake to perform side-effects directly in the body of the component.

How often the component renders isn’t something you can control — if React wants to render the component, you cannot stop it.

How to decouple rendering from the side-effect? Welcome useEffect() — the hook that runs side-effects independently of rendering.

useEffect() hook accepts 2 arguments:

  • callback is the callback function containing side-effect logic. useEffect() executes the callback function after React has committed the changes to the screen.
  • dependencies is an optional array of dependencies. useEffect() executes callback only if the dependencies have changed between renderings.

Put your side-effect logic into the callback function, then use the dependencies argument to control when you want the side-effect to run. That’s the sole purpose of useEffect() .

2. The dependencies of useEffect()

dependencies argument of useEffect(callback, dependencies) lets you control when the side-effect runs. When dependencies are:

A) Not provided: the side-effect runs after every rendering.

B) An empty array [] : the side-effect runs once after the initial rendering.

C) Has props or state values [prop1, prop2, . state1, state2] : the side-effect runs only when any depenendecy value changes.

Let’s detail into the cases B) and C) since they’re used often.

3. The side-effect on component did mount

To invoke a side-effect once after the component mounting, use an empty dependencies array:

useEffect(. []) was supplied with an empty array as a dependencies argument. When configured in such a way, the useEffect() is going to execute the callback just once, after initial mounting.

Even if the component re-renders with different name property, the side-effect runs only once after the first render:

4. The side-effect on component did update

Each time the side-effect uses props or state values, you must indicate these values as dependencies:

The useEffect(callback, [prop, state]) invokes the callback after the changes are being committed to DOM and if and only if any value in the dependencies array [prop, state] has changed.

Using the dependencies argument of useEffect() you control when to invoke the side-effect, independently from the rendering cycles of the component. Again, that’s the essence of useEffect() hook.

Let’s improve the Greet component by using name prop in the document title:

name prop is mentioned in the dependencies argument of useEffect(. [name]) . useEffect() hook runs the side-effect after initial rendering, and on later renderings only if the name value changes.

useEffect() can perform data fetching side-effect.

The following component FetchEmployeesByQuery fetches the employees list over the network. The query prop filters the fetched employees:

useEffect() starts a fetch request by calling fetchEmployees() async function after the initial mounting.

When the request completes, setEmployees(fetchedEmployees) updates the employees state with the just fetched employees list.

On later renderings, if the query prop changes, useEffect() hook starts a new fetch request for a new query value.

Note that the callback argument of useEffect(callback) cannot be an async function. But you can always define and then invoke an async function inside the callback itself:

To run the fetch request once when the component mounts, simply indicate an empty dependencies list: useEffect(fetchSideEffect, []) .

Some side-effects need cleanup: close a socket, clear timers.

If the callback of useEffect(callback) returns a function, then useEffect() considers this as an effect cleanup:

Cleanup works the following way:

A) After initial rendering, useEffect() invokes the callback having the side-effect. cleanup function is not invoked.

B) On later renderings, before invoking the next side-effect callback, useEffect() invokes the cleanup function from the previous side-effect execution (to clean up everything after the previous side-effect), then runs the current side-effect.

C) Finally, after unmounting the component, useEffect() invokes the cleanup function from the latest side-effect.

Let’s see an example when the side-effect cleanup is useful.

The following component <RepeatMessage message="My Message" /> accepts a prop message . Then, every 2 seconds the message prop is logged to console:

Open the demo and type some messages. The console logs every 2 seconds any message that’s been ever typed into the input. However, you need to log only the latest message.

That’s the case to clean up the side-effect: cancel the previous timer when starting a new one. Let’s return a cleanup function that stops the previous timer:

Open the demo and type some messages: only the latest message logs to console.

useEffect(callback, dependencies) is the hook that manages the side-effects in functional components. callback argument is a function to put the side-effect logic. dependencies is a list of dependencies of your side-effect: being props or state values.

useEffect(callback, dependencies) invokes the callback after initial mounting, and on later renderings, if any value inside dependencies has changed.

Because useEffect() hook heavily relies on closures, you might need to get them well too. Also be aware of stale closures issue.

The next step to mastering useEffect() is to understand and avoid the infinite loop pitfall.

Still have questions about useEffect() hook? Ask in the comments below!


Why a propeller has angled blades

Propeller blades are fixed to their hub at an angle, just as the thread on a screw makes an angle to the shaft. This is called the pitch (or pitch angle) of a propeller and it determines how quickly it moves you forward when you turn it, and how much force you have to use in the process. Sometimes (and this can be confusing) the distance a propeller moves you forward as it turns through one complete revolution is also called its pitch, but it's easy to see that the angle of the blades and how far they move you forward in a single rotation are related.

Propellers look like screws, so how are the two connected? A screw converts the turning motion of your hand into forward motion that drives the screw's body (and anything it's attached to) firmly into the wall. The angle of the thread on a screw determines how much force you have to use to turn it. A screw with a steep thread (and fewer turns along its length) will be harder to turn but will go into the wall faster, while one with a shallow thread (and more turns along its length) is easier to rotate but you have to turn it more times to drive it in. If you find screws confusing, think of a screw standing upright on its flat end (like the photo above) and imagine you're an ant walking up the thread from the bottom the top, so the thread is like a zig-zag path winding up a hillside. The more gently the path winds (the shallower the thread), the easier it is to climb (the less force your body needs to exert), but the further you'll walk and the longer it will take. Like gears, pulleys, and levers, screws are examples of simple machines&mdashdevices that multiply (or otherwise transform) forces.

Propellers are similar to screws but not exactly the same, because they're doing a totally different job. The purpose of a screw is to hold something like a shelf to a wall and minimize the amount of force you need to drive it into a solid material such as wood or plasterboard with a screw, the driving force is pretty much constant. But the purpose of an airplane propeller is to make more or less thrust (driving force) at different points of a flight (during takeoff, for example, or steady cruising). The angle of a propeller's blades and its overall size and shape affect the thrust, and so too does the speed of the engine. Another difference is that while a screw is moving into a simple, solid material and meeting a more or less constant force of opposition, a propeller is moving in a fluid airstream and there all kinds of extra factors to take into consideration. For example, although a propeller makes thrust to move you forward, it also produces drag that tends to hold you back and slow you down, and the amount of drag it makes depends on the angle of the blades. These sorts of things make propellers far more complex than simple wood screws!

Photo: This electric desk fan (we're looking down from above) has blades set at an angle to the central motor shaft, just like a propeller. The blades have a large area, much like marine propellers, because they're designed to move a large volume of air at a relatively low motor speed. You don't want a fan to spin too quickly and skid across your desk. Unlike with a plane propeller, drag isn't an issue, so it doesn't really matter how big the blades are.


Marine Engines

II.A Torque or Power versus Speed

The propulsion engine is a device for producing the torque required by the propulsor that converts the torque into thrust. Almost universally, the propulsor is a screw propeller. The torque characteristic of the engine must therefore coordinate with that of a propeller. Since the torque is transmitted by a rotating shaft, this last statement implies that the torque–revolutions per minute (rpm) characteristic of the propeller determines the acceptability of the torque–rpm characteristic of the engine. In other words, the power–rpm characteristic must be acceptable, and this conveys equivalent information, since power is the product of torque and rpm. Power–rpm is used in the subsequent discussion.

Figures 1 and 2 show typical power–rpm curves for a propeller and a diesel engine ( Fig. 1 ) or a turbine engine ( Fig. 2 ). (Both propeller and engine curves are somewhat idealized, but serve well for discussion.) In both instances, the engine power equals that of the propeller at what is presumably rated power and rpm for both the engine and its load. Since the engine power clearly exceeds the required propeller power at rpm below rated, the engine in both instances is able to accelerate the propeller from rest to the rated condition.

FIGURE 1 . Matched characteristics of diesel engine and marine propeller.

FIGURE 2 . Matched characteristics of turbine engine and marine propeller.

Both Figs. 1 and 2 show a family of propeller curves to indicate a range within which the characteristic of a particular propeller may fall. The change from one curve to another of the family occurs because of a change in propeller pitch, or if the pitch is fixed, from any factor that changes the resistance characteristic of the ship. The point illustrated is that the engine characteristic must be suitable for the family, and not just for a single propeller characteristic. The curves demonstrate that a shift of propeller curve to the left (increase in pitch or increase in resistance) without a compensating shift upward of the engine curve—which may not be possible without overloading the engine—must cause the engine power and rpm both to decrease. The turbine suffers a much smaller decrease than the diesel. The advantage this conveys to turbine propulsion is a minor one, however, and both types of engines match the marine propulsive load characteristic quite well.

The discussion of power versus speed, including the labeling of Figs. 1 and 2 , has implied that rotational speeds of propeller and engine are the same. However, this implication is true only for low-speed diesel engines, engines whose piston stroke is long enough that rpm is in a range suitable for an efficient propeller. Propeller speed must be accommodated because limits of thrust loading (maximum feasible thrust per unit of propeller disk area), coupled with limits of acceptable angle of attack of blades with respect to water, dictate a limited range of speeds over which the propeller will be acceptably efficient. While reciprocating engines can be designed with matching speed, acceptably efficient turbine designs lie between one and two orders of magnitude in speed above efficient propeller speed. Fortunately, mechanical gearing is readily adapted to match diverse engine and propeller speeds. If the gearing can be thought of as part of the engine, then it is indeed correct to speak of the two speeds as if they were one.


Should You Sharpen Boat Propellers?

Some boaters are actively sharpening their props. This is not necessary and can ruin your prop. If you alter the edge, it will affect how it propels your boat. If done correctly it can increase speed. This would only be relevant for racing. Normal boaters should never need to do this.

If you have damage to your propeller, you can look into filing it. Perhaps if you hit rocks or a log and caused nicks and scratches to appear. These could be filed smooth. But the blades themselves should not require any sharpening. This will only increase potential harm in the event of an accident. Also it further wears down the prop. Eventually it will need to be replaced because it’s unable to do the job properly.

If your propeller has been very badly dulled by an accident, have it repaired. A professional propeller shop can handle this type of work. Repair or replace the damage.


RC Plane Props Basics

You'd be forgiven for just thinking of your rc plane's prop as the thing that pulls the plane along, but understanding a bit about exactly how propellers actually work is no bad thing.

Simply put, props are nothing more than vertically mounted rotating wings. The prop's job is to convert the motor power in to thrust, to pull/push the plane through the air. Thrust is generated in exactly the same way as lift is generated by the wing, and that's why props have a profile airfoil section.

The 'twist' in the propeller is there to create the essential Angle of Attack of each blade, just like a wing has an AoA. The twist is greater towards the hub of the prop because of varying airspeeds along the length of the blades, and hence varying thrust generation. The picture to the right approximately illustrates how the Angle of Attack varies along the blade length.

This difference in thrust occurs because the tips of the prop blades move faster than the inner portions of the blades, so the AoA has to change accordingly along the length of the blades more thrust is generated at faster speeds, just like more lift is generated over a faster moving wing. At slower speed (i.e. nearer to the hub of the propeller), the AoA has to be greater to generate a similar amount of thrust being generated at the faster moving tips.

RC Propeller Size Labelling

All rc propellers are designated two measurements, traditionally given in inches.

The first number is the arc diameter created by the spinning prop i.e. propeller length from tip to tip. The second number is the pitch and this is the harder of the two to understand - but we'll give it a go.

Essentially, diameter determines the 'grunt' and pitch determines the speed of the plane.
Pitch indicates how far, in inches, that propeller will move through the air per single revolution of the engine (i.e. every single complete turn of the prop). However, the pitch measurement must only be taken as a guideline because real-life factors influence the actual distance eg the prop material, its condition, efficiency, air density on the day etc.
So pitch measurement is really only a theoretical value, but it is good enough to help you choose the right size propeller for your airplane's performance requirements.

One way to understand propeller pitch is to imagine the gauge of two different screw threads, coarse and fine, both being screwed into a piece of wood at the same rotational speed. The screw with the coarse thread will cut into the wood faster than the fine threaded screw will.
It's the same for propellers 'cutting' through the air (hence the reason why propellers are sometimes called airscrews).

In the illustration below, the two arrow lines represent the path of each propeller tip. You can see that the higher pitch prop (eg 10x8) takes only one and a half turns to cover the same distance that the lower pitch prop (eg 10x4) takes 3 turns to. So, with both engines and props spinning at identical RPM, and everything else being equal, the higher pitch prop will travel further in the same amount of time - hence a faster flying plane.

So you can see that selecting a different propeller pitch size is going to significantly change your airplane's performance, with speed being the primary factor.

The diameter of the propeller (10" in the example above) will also effect how the airplane flies, but also how the engine runs and, again, following your engine manufacturer's recommendations is the place to start.

Be Noise Aware!

Prop diameter directly influences the amount of thrust generated, but an ever-increasing and non-performance related issue these days, linked to rc airplanes, is that of noise.

A faster turning propeller (and props can easily turn in excess of 10,000 RPM) generates a lot of noise as the tips cut through the air. In fact, when you hear an rc airplane flying it's more than likely the propeller that you're hearing more than the engine.

A larger diameter prop reduces the engine's RPM at any given power setting, because there is more for the engine to turn over and hence more work to do. Slower turning props generate less noise, so larger diameter props run quieter than smaller diameter props, all else being equal.

In this environmentally-sensitive world that we live, this is a serious consideration when selecting a propeller, especially if your flying site is 'noise sensitive' (eg close to houses etc.).

IC Propeller Size Recommendations

As already mentioned, following the prop size recommendations made by your engine manufacturer should always be your first point of reference. But there are generally recognised prop size ranges for each engine size and these are the sizes to choose if you're unsure about propeller selection.

The following propeller size chart (© Top Flight, reproduced with permission) is easy to use select your IC engine displacement along the bottom scale, then follow the vertical line up to the shaded area to give the prop size range for that engine.

Although this chart is related to Top Flight's Power Point range of props, the size ranges suit all brands.

EP Propeller Sizes

Matching a prop to an IC engine is easy if you follow the general recommendations outlined in the above chart, which have long been accepted in the hobby. Fitting an incorrect prop would mean the engine would still run, but your plane would perform poorly.

But with the advent of electric power (EP), propeller selection became a whole new minefield!

EP prop selection is much more critical because different combinations of motors, ESCs and battery packs can generate huge differences in operating speeds and loads.

As with IC, electric motor manufacturers give a specific propeller size range for their motors but it's more critical that the range is adhered to. Over-propping can do irreparable damage to electric motors and particularly ESCs, because an oversized propeller will force the motor to work harder than it was designed to.

If you put an oversize prop on an IC engine, the engine will likely stop running. No harm done. But put an oversize prop on an electric motor and the motor will just keep on trying to turn the prop.
The motor will draw more and more current as it tries to keep up with its Kv rating (the number of RPM it has been designed to turn, per each volt fed into it). With too big a propeller, the motor will just keep working harder and harder to spin the extra load, until something (likely the ESC) overheats and catches fire.

Too small a propeller on an EP motor won't do any damage, but you won't get the required performance from your plane. The motor will draw less current and the plane will likely be seriously under-powered.

Use a Watt Meter

The only accurate way to know whether or not your EP propeller is resulting in the correct current draw through the ESC is to use a Watt meter connected between battery pack and ESC, as the video below shows.

Watt meters don't cost much money and they are simple to operate a test takes only a few minutes and will give you solid peace of mind. Personally, if you're an EP flyer, I would say that a Watt meter is as essential as your battery charger!

Number of Propeller Blades

Most propellers used in the rc flying hobby have two blades, but props with three or even four blades are available.

Two-bladed propellers are commonly used because they are relatively efficient and easy and cheap to produce but sometimes an rc airplane will call for more blades, particularly where a scale look is required.

Adding more blades decreases the overall efficiency of the prop because each blade has to cut through more turbulent air from the preceding blade. In fact, a single blade propeller is the most efficient but these are rarely (almost never!) seen in our hobby although they have been experimented with. A single blade prop must be balanced with a counterweight on the other side of the hub to the blade, otherwise the plane would shake itself to pieces as soon as the prop was turning!

If choosing a three or four bladed propeller over a two bladed one, a very general rule of thumb is to decrease the prop diameter by an inch and increase the pitch by an inch. That said, fuselage and ground clearance issues might dictate which propeller size you can and can't have on your plane. As with everything, trial and error is going to play a part in your propeller selection.

Beware the Biting Prop!

Never ever underestimate the potential for an rc plane propeller to do serious damage.

There are countless stories of model pilots losing fingers, or suffering horrendous lacerations to skin on their hands and arms. Even a small size plastic propeller can hurt and cut skin, so imagine what the bigger ones can do.

Always take great care around a spinning prop and treat it with the utmost respect. Keep hands and fingers well clear and never become complacent.
If you want some gory evidence of what props can do, just Google "rc propeller injuries" and you'll soon see. Keep safe!

Well hopefully this article has given you an understanding of propellers used on rc airplanes, and an idea of how to select the right size propeller for your model.
Remember to follow your engine/motor manufacturer recommendations whenever you can, and use a Watt meter if you are going to experiment with different propeller sizes for EP rc planes.

Related Pages

RC propeller balance.

Model airplane engines.

Breaking in a glow plug engine.


San Andreas Fault Looks Like a Big Propeller

Everyone that lives along the San Andreas Fault has “the big one” in the back of their mind always, whether they care to admit it or not. For Californians, the question is not if but when another devastating earthquake will occur on the famously shaky ground where they make their homes. Earthquake drills are a regular event at schools, and all businesses have a plan for such an emergency. As a matter of fact, the community as a whole just recently took part in a huge initiative to boost awareness and make sure everyone knows what to do when the time comes. This event was called the Great California ShakeOut. However, even as Californians live in fear and prepare for when that day arrives, scientists continue to try to gain a better understanding of just what happens to make that shaky ground so shaky to begin with.

For years, the San Andreas fault was looked at like most other faults. It is the edge where two huge tectonic plates meet, leaving it fated to always be susceptible to major earthquakes. But new findings have caused seismologists to start to change their visualization of one of the world’s most troublesome fault lines. Now, they believe that the fault is actually vertical. The fault actually continues down into the Earth’s mantle in a shape that resembles a huge propeller. This helps them explain some of the disparities of who feels what when a major earthquake happens. The vertical nature of the fault can cause some areas very close in proximity to one another to suffer completely different experiences. Depending on where a city falls on the propeller determines how severe the shaking will be at that point. This effect has long been documented, although an explanation for the behavior was never arrived at. In 1989, this was observed when the city of Watsonville, south of the fault, experienced almost twice as bad shaking as the city of San Jose. San Jose is located north of the fault line, but both cities are almost exactly the same distance from epicenter of the earthquake. On a normal fault line, both cities would have experienced similar shaking because the seismic waves move outward in a circle from the center. The propeller shape of the San Andreas fault makes it unique and causes this strange phenomenon where the epicenter no longer determines where the damage will be most catastrophic.

The good news about the discovery is it helps scientists understand what is going on beneath the Earth’s surface better, and these findings will no doubt cause them to take a closer look at other fault lines all over the world. Hopefully, this better understanding will lead to better chances of predicting earthquakes and knowing what areas are in the most danger when they do happen.

For the record, the propeller shape is only for helping people visualize what the San Andreas fault line looks like. Some of the more outlandish conspiracy sites (we’re not naming any names, but PakAlert Press), are probably feverishly working on their article exposing that a giant alien spacecraft with huge propellers is lurking beneath the Earth’s surface, leftover from some previous civilization. The earthquakes are them cranking up the machine of course.

It sounds crazy, but just watch. Someone will go there. Better stick with Common Sense Conspiracy to get all of your information.


How To Do Such Stars Effect

I've been quite struggling to find a good tuto to get stars as in the below picture. Would really appreciate if someone could give me some tips / advise how to achieve such effect in photoshop.

Thank you so much in advance.

Attached Thumbnails

Edited by kzar, 28 March 2021 - 07:22 AM.

#2 qswat72

Edited by qswat72, 28 March 2021 - 07:30 AM.

#3 LuscombeFlyer

I find this discussion a bit amusing!

Reflector owners seek ways to reduce or eliminate the diffraction effects of the secondary mirror holder, while refractor owners are looking for a method to simulate the same!

#4 WoodlandsAstronomer

I find this discussion a bit amusing!

Reflector owners seek ways to reduce or eliminate the diffraction effects of the secondary mirror holder, while refractor owners are looking for a method to simulate the same!

#5 Tapio

I'm satrisfied that I have only scopes (refractor, SCT) that don't make star spikes.

But if you absolutely want them you can do it in software too (I find gswat's methdod more 'natural).

In Photoshop you can use Astronomy Tools actions and among more useful tools there is also Star Diffraction Spikes tool.


OrbitalHub

“August 19, 2008. A propeller-shaped structure created by an unseen moon appears dark in this image obtained by NASA’s Cassini spacecraft of the unilluminated side of Saturn’s rings. The propeller is marked with a red arrow in the top left of the annotated version of the image. The Encke Gap of Saturn’s A ring is in the lower right of the image. The A ring is the outermost of Saturn’s main rings. The moon, likely about a kilometer (half a mile) across, can’t be seen at this resolution. However, it is larger than other “propeller” moons and has cleared ring material from the bright (because they are less opaque) wing-like regions to its left and right in this image. Disturbed ring material close to the moon blocks more sunlight and appears like a dark airplane propeller.

Several density waves are visible in the ring, particularly in the upper left. A spiral density wave is a spiral-shaped accumulation of particles that tightly winds many times around the planet. It is the result of gravitational tugs by individual moons whose orbits are in resonance with the particles’ orbits at a specific distance from Saturn. A propeller’s appearance changes with viewing geometry, and this image shows the way a propeller looks when viewed from the unilluminated side of the rings. The dark structure at the center of this propeller corresponds to the bright structure seen in Sunlit Propeller, which was imaged from the sunlit side of the rings.

This image is part of a growing catalogue of “propeller” moons that, despite being too small to be seen, enhance their visibility by creating larger disturbances in the surrounding fabric of Saturn’s rings. Cassini scientists now have tracked several of these individual propeller moons embedded in Saturn’s disk over several years.

These images are important because they represent the first time scientists have been able to track the orbits of objects in space that are embedded in a disk of material. Continued monitoring of these objects may lead to direct observations of the interaction between a disk of material and embedded moons. Such interactions help scientists understand fundamental principles of how solar systems formed from disks of matter. Indeed, Cassini scientists have seen changes in the orbits of these moons, although they don’t yet know exactly what causes these changes.

Imaging scientists nicknamed the propeller shown here “Santos-Dumont” after the early Brazilian-French aviator Alberto Santos-Dumont. The propeller structure is 5 kilometers (3 miles) in the radial dimension (the dimension moving outward from Saturn which is far out of frame to the lower right of this image). It is 65 kilometers (40 miles) in the azimuthal (longitudinal) dimension. Scale in the original image was about 2 kilometers (1 mile) per pixel. The image has been rotated and contrast-enhanced to aid visibility. The cropped inset of the propeller included here was magnified by a factor of four.

This view looks toward the northern, unilluminated side of the rings from about 45 degrees above the ring plane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera. The view was acquired at a distance of approximately 310,000 kilometers (193,000 miles) from Saturn and at a sun-Saturn-spacecraft, or phase, angle of 121 degrees.”

“After almost 20 years in space, NASA’s Cassini spacecraft begins the final chapter of its remarkable story of exploration: its Grand Finale. Between April and September 2017, Cassini will undertake a daring set of orbits that is, in many ways, like a whole new mission. Following a final close flyby of Saturn’s moon Titan, Cassini will leap over the planet’s icy rings and begin a series of 22 weekly dives between the planet and the rings.

No other mission has ever explored this unique region. What we learn from these final orbits will help to improve our understanding of how giant planets – and planetary systems everywhere – form and evolve.

On the final orbit, Cassini will plunge into Saturn’s atmosphere, sending back new and unique science to the very end. After losing contact with Earth, the spacecraft will burn up like a meteor, becoming part of the planet itself.

Cassini’s Grand Finale is about so much more than the spacecraft’s final dive into Saturn. That dramatic event is the capstone of six months of daring exploration and scientific discovery. And those six months are the thrilling final chapter in a historic 20-year journey.”


Watch the video: Rolling Shutter Explained Why Do Cameras Do This? - Smarter Every Day 172 (January 2023).