Stuff in orbit is always falling down toward the planet, but always missing on account of its sideways motion. The closer the orbiter is to the planet, the harder it is to miss it as it is falling. But it is still possible, provided there is enough sideways motion.
I propose that a VLO {very low orbit) device be constructed and set into orbit around the moon. I choose the moon for its lack of atmosphere, which otherwise would slow the VLO and make it crash. The VLO would be set into orbit about 2 feet (or 1 meter) off the ground. A path would need to be carefully cleared and marked, as a rock in the way would have tragic consequences. The device would need to be very shiny, to reflect rays which might push it down toward the surface.
A railgun might be used to start the VLO. The railgun would need to be quickly moved out of the way as the VLO completed its first orbit. The VLO would be good for all the usual sort of things very low lunar satellites are good for, except it would be better, because it is lower.-- bungston, Sep 11 2003 Earth Space Web http://www.halfbake...a/Earth-Space_20WebInspiration! [bungston, Oct 05 2004, last modified Oct 17 2004] Light mill http://math.ucr.edu...ill/light-mill.htmlThat's how they chop light rays into photons. [kbecker, Oct 05 2004, last modified Oct 17 2004] Wonderfully weird and directionless. Have a pastry.-- ato_de, Sep 11 2003 The thing would be going at about 6000 km/h and complete a circle every hour and 50 minutes.
It would take a big rail gun. You could also start it in a high orbit and gradually decrease the altitude.-- AO, Sep 11 2003 Make one end shiny and other end black. Launch it in a plane that is parallel to the plane of the earth orbit, but still around the moon. Now you have a light mill (link) circling the moon.To launch float it on magnetic rails and let the sun do the acceleration until orbital speed is achieved. Then pull off the carriage real fast to make it fly.-- kbecker, Sep 11 2003 "The device would need to be very shiny, to reflect rays which might push it down toward the surface." Hello?
And after you answer that one: Why?-- phoenix, Sep 11 2003 [phoenix] is technically correct, absorbing a photon in a black surface just picks up a single push. Reflecting it gives a double push because of the recoil when it goes back. To eliminate the push make the device from a cylinder of glass with just coated ends.Will look nice from earth when the device just comes around the moon: "Ahhh, look, Bungston's rod is rising"
To answer [phoenix]' last question: Because!-- kbecker, Sep 11 2003 I for one don't need to see [bungston]'s rod rising.-- Cedar Park, Sep 12 2003 I've seen the videos. It's not much, really...-- DeathNinja, Sep 12 2003 A very low orbit baby.. I love what you've got Let's get constructing, we can... Weld hot No more atmosphere, baby.. Tide is today Girl, I can clear and mark...Okay This thing is very shiny, baby..To reflect the sun You pull the trigger of my gun Rail gun, (rail gun), rail gun Rail gun, (rail gun), rail gun...-- thumbwax, Sep 12 2003 "all the usual sort of things very low lunar satellites are good for" - name one! (Especially given that we don't currently have anyone on the Moon.)-- DrCurry, Sep 12 2003 I really like the glass rod concept, and the physics assistance. Much better. I highly recommend the light mill link. [Deathninja], I am cancelling your membership to my website. Don't care if you're paid up for this month.
Ahh, [Curry, Curry, Curry]. The myriad uses of a very low lunar satellite. Where to begin?-- bungston, Sep 12 2003 The sky is falling....the sky is falling...-- tatju, Sep 12 2003 On reading the light mill paper, I am not sure that this would work on the moon since it seems that a small amount of residual gas is necessary for the vanes to turn. Or perhaps the lunar atmosphere is not actually a vacuum, but just very thin, so that this small amount of gas exists there?-- bungston, Sep 12 2003 The undisturbed lunar atmosphere exists, but is much more rarified than anything we've managed to create on earth. There are details in the "light mill" article that more deeply explain the physics of the radiometer (that it doesn't have much to do with thermal pressure, but rather thermal creep at the edges).
Please, enlighten us on the multiple uses of an extremely low orbit lunar satellite moving at 6000 km/hr?-- Freefall, Sep 12 2003 You could play that game where you stand on the moon as the satellite approaches and jump just before it hits you.-- AO, Sep 12 2003 1: Anyone able to view the VLO could use it as a clock. 2: The VLO could be used to relay messages to the far side of the moon and vice versa, without stringing up cumbersome phone lines. 3: An LED strip on the VLO could change such that as it sped by, the illusion of a message would be left on the viewer's eye (like those strips which appear to be an eye of seen while turning the head). This could be used for advertising purposes. 4. Slight orbital variations of the VLO could be used as a rapid meteor impact detector. 5. The VLO would be a mascot for the new lunar program, and would be depicted on T-shirts, caps and stationery. Elementary school sports teams would be named the V-Los, or maybe the VLO Riders. 6. The VLO would invigorate the "space race", stimulating nations to invest money into building and launching lower, faster VLO devices. To save on clearing the paths, these would orbit in parallel. 7. Once VLO technology had matured, different companies could offer objects which could be launched along the VLO trajectory. Sports cars, golf clubs, candy bars and other consumer goods would be set into very low orbit as publicity stunts, as well as to test durability at very high speeds under lunar conditions.
That's all I have right now. I think it adequately makes the case. Like the personal computer and internet, other uses for the VLO will be devised once it exists.-- bungston, Sep 12 2003 2 feet off the ground. Ha! I'd expect at least under an inch for a _real_ low orbit device.-- Worldgineer, Sep 12 2003 This might entail extensive excavations (beyond moving mountains) as the moon is probably not perfectly round +/- 2 feet.-- FarmerJohn, Sep 12 2003 If the transparent cylinder model is used, excavations might be limited to a simple tunnel the diameter of the VLO. Mountains would be left in place. In addition, rocks weigh considerably less on the moon, making excavations less difficult. I grant that some extensive survey work will be needed before setting the VLO in motion.-- bungston, Sep 12 2003 Or you could have just said you meant 2 feet off the highest point.-- Worldgineer, Sep 12 2003 where is the fun in that?-- ato_de, Sep 12 2003 Explosive VLOs would help with that plan. Although at 6000 km/h explosives might be superfluous.-- bungston, Sep 12 2003 If you use a laser to very precisely determine the orbital altitude at various points along the path, it seems you could produce an extremely sensitive gravity wave detector. If you're serious about making such a detector, the lunar VLO may actually be more cost-effective than an earth based solution for a similar sensitivity.-- lurch, Sep 12 2003 Ooooooh. There you go. Application.-- Madcat, Sep 12 2003 Application? Buhhhh!
Go to NASA if you want to throw around applications. This site is reserved for avant garde engineers and scientists.-- kbecker, Sep 12 2003 I don't know the exact maths (never my strong point), but considering Kepler's third law, I think 6000 klicks per hour is being very conservative.-- waugsqueke, Sep 13 2003 The formula you want is v = sqrt(G * M / R) where v is orbital velocity, G is the universal gravitational constant, M is the mass of the moon, and R is the radius of the orbit.
M = 7.349 * 10^22 kg R = 1738.1 km G = 6.67300 * 10^-11 m^3 kg^-1 s^-2
It goes slower than a low earth orbit because the mass is not very big in this case.-- AO, Sep 14 2003 [AO] - It took me 9 months to revisit this idea, but thank you for the equation. I did the math and found a speed of 53117 meters/second, or 191222 km/ hour. Which means a full circuit would take 32 seconds.
I was thinking about the VLO orbiter because I was pondering how to illuminate the inside of Pellucidar, or some other celestial-sized hollow body, and still have day and night. An object inside a very large sphere should still orbit the center of mass of that sphere. Thus a lunimous device like the VLO orbiter, orbiting about 1 km inwards from the inner surface of the sphere should provide a change of night and day for the inhabitants. Very short nights and days.-- bungston, Jul 19 2004 Umm... how exactly will this lumnous body produce a day and night? I mean I suppose the inverse square law and all proves that it will be somwhat darker on the far side than on the near side...
Or will this luminous body be illuminated on only one side? I'd imagine you'd have as much luck setting it up so it's got a polar orbit, and shines in through the 800 mile wide holes above the north and south poles.
In general, orbiting inside a hollow body is dangerous, as the masscons would be killer.-- ye_river_xiv, Jan 04 2007 A one sided luminosity would be necessary. Given that each day would be only a few minutes long, I agree that a different mechanism for illumination would be necessary.-- bungston, Jan 05 2007 //An object inside a very large sphere should still orbit the center of mass of that sphere.// Actually, an object inside of a hollow sphere experiences no gravity, and thereby has nothing to orbit. I'll refrain from the math demo unless it is requested.
//I did the math and found a speed of 53117 meters/second, or 191222 km/ hour. Which means a full circuit would take 32 seconds.// You neglected to convert the radius of the moon from kilometers to meters. Had you done so, you would have come up with about 6026 km/hr, as [AO] said. (A professor used to tell me: "Nothing screws up a good math problem like a bit of arithmetic.")-- lurch, Jan 05 2007 Here again I have neglected the edifcation provided by lurch, and stumbled about ingorant for the past 4 years. Meters, kilometers - the passage of years lets me concede they are not the same.
But the object inside a hollow sphere experiencing no gravity - that does not seem right. An object in the exact center of Pellucidar would experience no gravity as the pull from all sides would cancel out, just as the object falling thru a tunnel piercing the earth will oscillate back and forth until coming to rest at the center.
But would not an offset object (here the interior orbiting device as proposed) experience more pull from the nearer wall than the farther (inverse square and all)?-- bungston, Feb 29 2012 The object experiences more pull from a given mass on the near side, but more of the mass of the sphere is on the far side.
Think of four equal masses, spaced 90 degrees around a circle. Put, on a line between two of them, an object, closer to one than the other. That nearer body excerts more pull than the farther. However, the two to the sides cancel out each others lateral pull, but also add resultant force away from the near object. Once you extend this to a sphere (and do a bit of calculus) all the resultant forces cancel out.-- MechE, Feb 29 2012 For the math, check out the Wikipedia article on "Shell Theorem". Read that (kinda poorly written, IMHO), then jump to "Gauss's law for gravity" (much better written, but that closed-surface integration can snag the unwary).
I did this demo once before here, but it seems to be gone. Your hunch about inverse square distance is correct, but it has another effect you've neglected.
OK, you are about to be placed inside the shell of a hollow planet. You will not be in the center, but far off to one side. You are given a laser pointer - not some puny dollar-store thing, but a megawatt-class laser with beams coming out *both* ends. Be careful with it!
Now, taking your Darth Maul laser-pointer, you point it in any direction you please, and move it around to draw a shape. You will notice that if you point one end in a direction where the shell is close, then the other end is directed at a spot far away. Consequently, when you draw your shape, the close end draws a small shape; the far end draws a big shape - but they subtend identical angles.
If you now calculate the gravitation exerted on you by the two indicated areas, the close (small) one exerts more force on you per unit mass than the far (big) one - because of the inverse square law as you had presumed. However, the "per unit mass" caveat, when applied to the smallness/bigness of the two opposed figures, cancels precisely.
It works for any opposed pair of shapes you can draw, from any position inside the shell, as long as the shell is uniform.
(This is one of those things where you might wonder how you'd ever make the jump from theory to practice - but you can actually do the same math with respect to an extended disk in space. And it works - sometimes. This math has been used to determine the mass of an accretion disk around a black hole. That worked, as far as we can tell. However, when tried as a way of measuring the mass distribution of a spiral galaxy, the results were very strange. Further investigation led to the hypothecation of "dark matter" to correct reality into proper obeisance.)-- lurch, Feb 29 2012 That is great, you two. Thank you for taking the time.-- bungston, Feb 29 2012 The satellite should be golf-ball shaped, for Al.-- MaxwellBuchanan, Mar 01 2012 Perhaps you could launch many of these, aiming in different directions, as a sort of security system keeping others from landing on our moon. Just make sure to time them carefully so that they don't collide.-- Worldgineer, Mar 01 2012 //Explosive VLOs would help with that plan. Although at 6000 km/h explosives might be superfluous.//
6000 km/h gives 1.4 MJ/kg of kinetic energy. TNT delivers 4.184 MJ/kg. The kinetic energy would be transferred more effectively than that of an explosive, especially if the orbiter were dense and hard, but it would seem that explosives would still add significant destructive energy. Either way, it'd be pretty damned devastating.
Unfortunately, the annotation describing the plan in question seems to have gone.-- spidermother, Mar 02 2012 Well, the most obvious use for communications would be to stick Post-it notes on it, and someone on the other side of the moon could read it.
Or a more high tech approach would be a layer of phosphorus and a scanning cathode ray would write the message on the layer..it would remain visible long enough to circumnavigate the moon, at a wild guess..-- not_morrison_rm, Mar 12 2012 Worldgineer, presumably you could launch the different directions at slightly different heights. what would be fun is a series of orbits which are slightly oval in nature, presumably not possible with these weight ratios etc. then you could set up complex collisions.-- PainOCommonSense, Mar 16 2012 It would be super cool to experience the orbit in person.-- Ling, Mar 16 2012 /experience it/ I think the great speed without the rushing wind would be sort of surreal - like you were watching it on a screen.-- bungston, Mar 16 2012 I was thinking of this idea today because of lurch's explanation about weightlessness inside a shell planet, and the Darth Maul laser pointer.
Consider such a planet with a colony on the exterior surface. There is a door in the floor of one building. On opening it we see people floating around down there; they are on the interior side.
I wonder what the experience would be of coming up through that door from the weightless side.-- bungston, Mar 16 2012 Sounds heavy, man.-- Alterother, Mar 16 2012 //coming up through that door from the weightless side.// huh... cool. I'd never thought of it quite that way - it would feel like an anti-gravity repulsion field was trying to keep you away from the door. (Actually gravity toward everywhere except the non-existent mass in the shaft to the doorway, so the imbalance would *seem* like a repelling field.)
"Shut the door so I can get out!"-- lurch, Mar 17 2012 random, halfbakery