lagrange Points

[ruko]

The Webb telescope will be stationed at the L2 Lagrange point. Some of the texts and diagrams tell/show the object is in orbit around this L2 point and the sun. My question is: Is it really in orbit around the L2 point? How can something orbit a mass less point?

Thanks

[spartan45]

Here is a list of satellites orbiting Lagrange point 2 (L2). There are also others at L1.

The Herschel Space Observatory was a space observatory built and operated by the European Space Agency (ESA) and the largest infrared telescope ever launched until the James Webb space telescope. The observatory was carried into orbit by a Ariane 5 in May 2009, reaching the second Lagrange point (L2) of the Earth–Sun system, 1,500,000 kilometres (930,000 mi) from Earth, about August 2009 and active till 2013.The observatory sifted through star-forming clouds—the "slow cookers" of star ingredients—to trace the path by which potentially life-forming molecules, such as water, form.

https://en.wikipedia.org/wiki/Herschel_Space_Observatory

 

Gaia, is a European Space Agency (ESA) space observatory orbiting sun-Earth L2 Lagrange point arrived 19 Dec 2013 measuring the positions, distances and motions of stars with unprecedented precision.

https://en.wikipedia.org/wiki/Gaia_(spacecraft)

 

At L₂ Gaia assumed a Lissajous orbit instead of a Halo orbit. Lissajous orbits include components in this plane and perpendicular to it, and follow a Lissajous curv. Halo orbits also include components perpendicular to the plane, but they are periodic, while Lissajous orbits are usually not.

https://en.wikipedia.org/wiki/Lissajous_orbit

 

The Russian Space Observatory called Spectrum-X-Gamma SRG. SXG (Spektr-RG) completed a 100 day cruise to L2 on 21 October 2019 where it entered an orbit around the Sun; circling the Sun-Earth L2 Lagrange point about 1 million miles from Earth. One of its tasks is to enable study of galaxy clusters.

https://en.wikipedia.org/wiki/Spektr-RG

[spartan45]

The diagram above, (drawn using information from the ‘How Does The James Webb Space Telescope Work’ video after 22mins in and Wikipedia referenced below), showing the five Lagrange points. L1 & L2 are relatively stable points in space where gravitational and centrifugal forces are well balanced enabling satellites to be placed in what is known as ‘Halo’ orbits around them.  L3, opposite, L4 60⁰ forward and L5 60⁰ behind Earth in its orbit are relatively unstable Lagrange point’s not suitable placing satellites in Halo orbits. The Lagrange points illustrated can found in all such planetary orbits.

How Does the James Webb Space Telescope Work? - Smarter Every Day (1 Oct 2021)

https://www.youtube.com/watch?v=4P8fKd0IVOs

 

James Webb Halo orbit

https://en.wikipedia.org/wiki/Halo_orbit

 

A very recent text book (I can’t remember its title) suggested the Moon may have been formed by a planet which had been at a Lagrange point in the Sun-Earth orbit. The planet’s orbit became unstable and it hit the Earth with a slow glancing blow, debris left by the collision became the Moon.       

[spartan45]

The symmetry of Lagrange points around the orbits of solar system planets seems good evidence that once a gravity field has propagated at the speed of light the static gravity field then in place results in instant speed for gravity having to be used in all calculations.   

Ref: Ask google, bing, etc ‘What speed of gravity is used in space probe trajectory calculations’ and look for this forum among the top results.

[OceanBreeze]

 

That’s a nice question, and although I see Spartan provided a comprehensive answer, I would like to add a little to that. It is natural to think that for something to orbit a particular point, there must be a mass at that point. But that is not the case with a satellite orbiting a Lagrange point. So how does it do it?

The answer is that the Lagrange point itself is not stable; it moves around in a celestial dance caused by the relative motion between the Earth, the moon and the Sun. It is a classic three-body problem. As a result, it is not possible to place a satellite at the calculated Lagrange point, because it is only stable along one axis.

Any satellite placed at the Lagrannge point will  join in the celestial dance and will sometimes get a little ahead of the moon as it orbits the Earth, and sometimes it will fall behind, and the influence of the sun will add instability in the vertical direction due to the Earth’s tilt. This results in what we call a Halo orbit, or even a Lissajous orbit for the satellite around the Lagrange point.

Actually, this works to our advantage, as these Halo orbits can be hundreds or even thousands of kilometers wide. Consequently, we can place hundreds of satellites at a single Lagrange point instead of only one, if the point was absolutely stable. It is also necessary for ground control stations to monitor the position of the satellite to make sure it does not drift too far away from the Halo orbit, and make occasional adjustments using the booster rockets on the satellite. Again, I like your question because it shows you are a thinker!