82 votes
Accepted

Why are so many space telescopes placed in LEO instead of at Lagrange Points? And why do we hear about Hubble more than any Langrange-orbit telescope?

The reason there are so few spacecraft placed at Lagrange points is that it's much harder to get there. Launching sizeable payloads to Earth escape velocities requires a very large vehicle and is ...
  • 9,884
55 votes
Accepted

What does the Sun-Earth-Moon system look like from the Sun-Earth L-2 point?

I treated this as a problem of geometry and came up with this: The sun is the large yellow disk. The earth is the largest black disk, obscuring most of the sun The left-hand dark-grey disk is the ...
45 votes
Accepted

Delta-v to hit the moon: is reaching Lunar L1 enough?

This is an excellent and fun question, bravo. First, the table is being read incorrectly, here's how to properly read it: For example, to go from LEO-Ken to EML1 is given as 3.77 km/s, not 0.77 km/s. ...
41 votes

Since L2 has no visible marker, how will James Webb's ground control determine its relative position and velocity for station keeping?

So how does JWST identify station keeping corrections? It doesn't. While the JWST does know where it is pointing, it does not know where it is in space. It doesn't need to. The JWST Flight Dynamics ...
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36 votes

Why doesn't JWST use ion thrusters?

Since the L2 point is unstable, JWST needs engines to maintain it's orbit. It uses mono-propellant engines which have given it a 10 year (maximum) lifespan. The JWST uses bi-propellant engines (...
  • 70.8k
30 votes
Accepted

This orbit looks wrong near a Lagrange point. Is it?

Is the orbit shown in the graphic wrong, or is my understanding of orbital mechanics lacking, having only been influenced by KSP? It's not an either-or question. The graphic is "wrong" from the ...
  • 70.8k
30 votes
Accepted

Does the distance to L2 vary?

Does the distance to L2 vary? Yes, it does. The Earth is in a more or less elliptical orbit about the Sun. I wrote "more or less" because the Moon is a factor, and also because Venus, ...
  • 70.8k
28 votes

Why won't JWST deploy in LEO where it is potentially serviceable?

JWST is being launched on an Ariane V with a cryogenic upper stage. That upper stage has to be used immediately to launch it on a trajectory to the Sun-Earth L2. The stage operates on batteries, and ...
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26 votes

This orbit looks wrong near a Lagrange point. Is it?

As noted by Mys_721tx in comments, this is the trajectory of the object J002E3 -- believed to be the S-IVB upper stage launched with the Apollo 12 mission -- and the animation is accurate. As David ...
26 votes

Is there something inherently more difficult about servicing satellites in the 2nd Sun-Earth Lagrangian point?

Distance certainly is a key factor. It took JWST a month to go from launch to L2 pseudo-orbit insertion. Another key factor is that to date, the only successful satellite servicing missions have been ...
  • 70.8k
25 votes

What does the Sun-Earth-Moon system look like from the Sun-Earth L-2 point?

The spacecraft that orbit at L2 usually don't just stay at the L2 point, but do what's called a Halo Orbit, or related Lissajous Orbit. Essentially they orbit the L2 point, instead of right at it. As ...
  • 120k
25 votes

What is the "mass" of a Lagrange point?

Lagrange points... can be orbited by asteroids, satellites, and any other useful or interesting object. Assuming two-body motion however... First a quick note; the stability of a Lagrange point ...
  • 147k
23 votes
Accepted

What are the sources of light at L2? How will the James Webb telescope be powered?

The planned orbit for the JWST is quite a large halo orbit around Sun-Earth L2. It's very roughly elliptical, with dimensions of about +/- 350,000 km "vertically" (perpendicular to the Earth's orbital ...
  • 147k
23 votes
Accepted

How Many Martian Lagrange points are there? ...And are they useful for satellites?

The Phobos and Deimos L1 and L2 points are virtually coexistant with the moons themselves. I have seen that the L1 point for Phobos is 10 km above the moon. At that altitude, just build a large tower. ...
  • 120k
23 votes

Why doesn't JWST use ion thrusters?

@DavidHammen's answer goes a long way towards answering, especially in that the space telescope's bus was finalized quite a long time ago when ion propulsion was much less a proven long-term reliable ...
  • 147k
23 votes

Does the distance to L2 vary?

JPL Horizons has data for 8 Lagrange points: ID# Name Designation IAU/aliases/other 31 SEMB-L1 Lagrange 32 SEMB-L2 Lagrange 34 SEMB-L4 Lagrange 35 SEMB-L5 Lagrange 3011 EM-L1 Lagrange 3012 ...
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22 votes
Accepted

Is this what station keeping maneuvers look like, or just glitches in data? (SOHO via Horizons)

You are most likely seeing an artifact of how JPL represents its ephemerides for fast numerical computation. JPL integrates the equations of motion over time. This inevitably results in mismatches ...
  • 70.8k
20 votes

Ordering of the Lagrange points

According to Wikipedia, L1-L2-L3 were discovered first, by Euler, prior to Lagrange's work, and L1 is "the most intuitively understood" of them.
20 votes

Is there something inherently more difficult about servicing satellites in the 2nd Sun-Earth Lagrangian point?

Getting to L2 in a timely fashion is more challenging than going to the Moon. The Apollo missions took about 3 days to get to the Moon. 4.5 days is how long the LRO took to orbit the Moon when ...
  • 120k
19 votes
Accepted

Reverse Lunar Space Elevator

Here's a pic of the tether proposed by Liftport. Go 160,000 km beyond EML1 and drop a payload from that point. It will follow an elliptical path whose perigee grazes low earth orbit. A 3 km/s burn at ...
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19 votes

How many satellites can stay in a Lagrange point?

Lagrange points as I understand it are points in space between 2 objects where the gravitational pull between them is effectively equal. A quick check of Wikipedia's Lagrangian point or any article ...
  • 147k
19 votes

What is the "mass" of a Lagrange point?

When we consider the mass something is orbiting about, we assume a gravitational potential corresponding to an inverse-square force, which corresponds to either a point mass or, by Newton's shell ...
18 votes

Why won't JWST deploy in LEO where it is potentially serviceable?

The James Webb Space Telescope will not be deployed in Low Earth Orbit because there is too great a risk of the optics being damaged by debris. [T]he environment around the ISS is not suitable for ...
  • 23.8k
17 votes

What does the Sun-Earth-Moon system look like from the Sun-Earth L-2 point?

A reasonably accurate1 rendering can be made with Celestia. Here we are at 1,500,000 km from Earth's center, on August 26, 2018 (the next full moon as I write this), at approximately the Earth-Sun L2 ...
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17 votes

Why are so many space telescopes placed in LEO instead of at Lagrange Points? And why do we hear about Hubble more than any Langrange-orbit telescope?

To add to the existing good answer about the practicalities of launching to Lagrange points, it's also worth considering why the missions which have gone that far are using the unstable Lagrange ...
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14 votes

Are there interstellar Lagrange points?

Lagrange points do exist between stars. In case of single stars, they are too far away from the stars to have any practical effect. However, in case of the binary stars, the Roche Lobe has its apex ...
  • 1,560
14 votes
Accepted

Would it be possible to Boost ISS to L1 or Lunar Orbit?

Before getting to the technical feasibility of moving the ISS, I feel obligated to point out that operating it at L1 or lunar orbit is impractical for a few reasons: The ISS is designed for the ...
14 votes

Is L2 at a distance where the Earth totally eclipses the Sun?

No. The Earth's umbra extends 1.4 million km. The L2 point is at 1.5 million km. Even if perfectly aligned, the Earth will only show as an annular eclipse, shading about 70% of the sun's disk. However,...
14 votes
Accepted

Gravitational field of the Moon and Earth

Because the gravitational field is a field, there's a couple ways this question could be interpreted: The magnitude of the gravitational accelerations due to the Earth and Moon are equal The ...
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13 votes
Accepted

Any satellites in Earth-Sun L3 point?

TL;DR No, there are no sats there today, and no declared plans from any of space agencies to do that. Here's why: An Earth-Sun $L_3$ point is an unfortunate place for a satellite to be in. First ...
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