|
|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
Orbital mechanics question DSCOVER L1 orbit
In sci.space.policy message -
september.org, Sun, 15 Feb 2015 10:15:33, Jeff Findley posted: It looks like an orbit around the sun, but it's not. It's correct to state that L1 is the location in space where the gravitational forces on a satellite from the sun and the earth cancel out. Approximating orbits by circles : for an object on the Sun-Earth line to remain on that line at a constant distance from the barycentre, it must have the same angular velocity as the Earth. It is well-known - IIRC, it was even taught in schools - that the centripetal force needed for circular motion is M w^2 r in obvious notation. As 1 AU is about 10^8 miles and L1 is about 10^6 miles inwards from Earth, the forces most certainly do not cancel out. Moreover, L1 & L2 were discovered by Euler, who should have also discovered L3 except that it was semantically off-topic. -- (c) John Stockton, nr London, UK. Mail via homepage. Turnpike v6.05 MIME. Web http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms and links; Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc. |
#12
|
|||
|
|||
Orbital mechanics question DSCOVER L1 orbit
On 15-02-18 00:36 , JF Mezei wrote:
DSCOVER stll orbiting earth after it is at L1. I understand the principle of why one would say the satellite is still orbiting earth, since over the course of a year, it technically goes around it. And also because the Earth still has a significant gravitational pull on the satellite, because L1 is near us, as astronomical distances go. If the Earth were to be removed, the satellite's path would change significantly -- it would become an ellipse around then Sun, with the old L1 position being the aphelium, but the perihelium would not be very much closer to the Sun. If the Sun were to be removed, the satellite's path would change drastically; it would become an ellipse around the Earth, with its old L1 position being the apogee. I'm not sure, but I suspect the perigee would be so low that the satellite would reenter. Can't the same be said of the other planets in solar system ? Yes, if one stretches the point. But the greater distances between the Earth and the other planets mean that the influence of the Earth on their motion is much less than its influence on the satellite at L1. Also, for the massive planets like Jupiter, it is more a case of them influencing the Earth's path. At one point, does stating the satellite still orbits earth basically bring back the old ideas of everything revolving around the earth ? No, that question was about which one is moving, and which one is not, because that was thought to have theological importance. Considering the gravitational coupling between the Earth and the Sun, it is far more correct to say that the Sun is still, and the Earth goes around it, than to claim the opposite. For an LEO object, slowing down would cause it to drop towards earth. Accelerating would caus it to climb to higher orbit from earth. For DSCOVER, if , once at L1, it were to slow down, Relative to what? It is not enough to just say "slow down", that does not specify how the velocity changes. would it fall down towards sun, or towards earth ? Wouldn't that determine which body it orbits ? If you were to reduce DSCOVER's angular velocity around the Sun, it would "fall toward the Sun" in the sense that its orbit around the Sun would become more elliptical, with the L1 position as the aphelium, and it would no longer keep pace with the Earth. -- Niklas Holsti Tidorum Ltd niklas holsti tidorum fi . @ . |
#13
|
|||
|
|||
Orbital mechanics question DSCOVER L1 orbit
|
#14
|
|||
|
|||
Orbital mechanics question DSCOVER L1 orbit
On 15-02-18 13:41 , Jeff Findley wrote:
For the last time, a classic "orbit" is a two body simplification to orbital mechanics. That's *not* what we have here! Sun-earth-satellite at L1 is a *three* body problem. We have not denied it. I really wish you would all stop talking about a satellite at L1 like it was in a classic circular orbit. It seems to me that the discussion is indeed about how the motion is different from a two-body orbit, and how it is similar. What do you want us to do, just say "it's a three-body problem" and then shut up? It's not even close to that! The radius of the halo orbit around L1 is on the order of 1 million km, which is less than 1% of the radius of the orbit of the L1 locus around the Sun. The latter orbit is the main part of the satellite's motion relative to the Solar System. I would say that the orbit of a satellite at L1 is quite close to a circular orbit around the Sun. In fact, to stay at L1 it has to use a halo orbit, which is not at all like a classic circular or elliptical orbit. Google "L1 halo orbit", I already gave a link in an earlier post. -- Niklas Holsti Tidorum Ltd niklas holsti tidorum fi . @ . |
#15
|
|||
|
|||
Orbital mechanics question DSCOVER L1 orbit
On 15-02-18 09:22 , JF Mezei wrote:
On 15-02-18 01:32, Niklas Holsti wrote: If you were to reduce DSCOVER's angular velocity around the Sun, it would "fall toward the Sun" in the sense that its orbit around the Sun would become more elliptical, with the L1 position as the aphelium, and it would no longer keep pace with the Earth. If there is no variation in distance between sun and earth, would it be correct to state that its only velocity vector would be tangential to the sun ? (forgetting the halo orbit around L1). Yes, if we are talking about the satellite's velocity vector with respect to the Sun, and if we indeed we forget about the halo orbit, and about the slight eccentricity of the Earth's orbit. If slowing down means the satellite reduces altitude to sun, doesn't that imply that it's orbit is solely around the sun ? And if it increases it speed (or fires thrusters to push itself towards earth) it would rise above L1 and its orbit would then switch from being around the sun to being around the earth which would bring it into highly elliptical orbit around earth ? Here I would agree with Jeff: the satellite's orbit is influenced by both the Sun and the Earth (and other bodies like the Moon), so it cannot be categorized into either "an orbit around the Sun" or "an orbit around the Earth". It is a mixture. The way I picture it, at L1, the satellite is orbiting only the sun with the earth acting as a static magnet that keeps the satellite at L1 altitude to compensate for its speed being too low to stay at L1. Yes... except that "magnet" is of course wrong, as the force is gravity, not magnetism. If at L1, it is not freefalling around the earth, can it really be considered to be in orbit around the earth ? See above about Jeff's comment. -- Niklas Holsti Tidorum Ltd niklas holsti tidorum fi . @ . |
#16
|
|||
|
|||
Orbital mechanics question DSCOVER L1 orbit
On 15-02-18 21:26 , JF Mezei wrote:
On 15-02-18 14:09, Niklas Holsti wrote: The radius of the halo orbit around L1 is on the order of 1 million km, is the movement around L1 technically an orbit since L1 has no mass ? Depends on how you technically define an "orbit". In the definition at http://en.wiktionary.org/wiki/orbit, gravitation and mass are not necessarily required. -- Niklas Holsti Tidorum Ltd niklas holsti tidorum fi . @ . |
#17
|
|||
|
|||
Orbital mechanics question DSCOVER L1 orbit
If it is this complex, would one expect that keeping a craft in this point
is goind to use more fuel? It seems to me that we are dealing with some very complex forces here and although the tendency is for things to stay there, any other forces woould not need to be that poweful to make it come out of the point and start behaving differently. Brian -- From the Sofa of Brian Gaff Reply address is active "Niklas Holsti" wrote in message ... On 15-02-18 09:22 , JF Mezei wrote: On 15-02-18 01:32, Niklas Holsti wrote: If you were to reduce DSCOVER's angular velocity around the Sun, it would "fall toward the Sun" in the sense that its orbit around the Sun would become more elliptical, with the L1 position as the aphelium, and it would no longer keep pace with the Earth. If there is no variation in distance between sun and earth, would it be correct to state that its only velocity vector would be tangential to the sun ? (forgetting the halo orbit around L1). Yes, if we are talking about the satellite's velocity vector with respect to the Sun, and if we indeed we forget about the halo orbit, and about the slight eccentricity of the Earth's orbit. If slowing down means the satellite reduces altitude to sun, doesn't that imply that it's orbit is solely around the sun ? And if it increases it speed (or fires thrusters to push itself towards earth) it would rise above L1 and its orbit would then switch from being around the sun to being around the earth which would bring it into highly elliptical orbit around earth ? Here I would agree with Jeff: the satellite's orbit is influenced by both the Sun and the Earth (and other bodies like the Moon), so it cannot be categorized into either "an orbit around the Sun" or "an orbit around the Earth". It is a mixture. The way I picture it, at L1, the satellite is orbiting only the sun with the earth acting as a static magnet that keeps the satellite at L1 altitude to compensate for its speed being too low to stay at L1. Yes... except that "magnet" is of course wrong, as the force is gravity, not magnetism. If at L1, it is not freefalling around the earth, can it really be considered to be in orbit around the earth ? See above about Jeff's comment. -- Niklas Holsti Tidorum Ltd niklas holsti tidorum fi . @ . |
#18
|
|||
|
|||
Orbital mechanics question DSCOVER L1 orbit
|
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Orbital mechanics question (around sun) | John Doe | Space Shuttle | 4 | July 8th 10 10:49 PM |
Orbital Mechanics Question | [email protected][_2_] | Astronomy Misc | 3 | October 17th 07 12:56 AM |
Orbital mechanics question | [email protected] | Astronomy Misc | 2 | October 16th 07 06:31 PM |
Orbital mechanics question (moon) | John Doe | Space Shuttle | 8 | October 18th 04 06:49 PM |
orbital mechanics question | Bill Clark | History | 0 | August 2nd 03 05:38 PM |