<p>On balance, no. Because in six months we're going to be there ourselves.</p>
<p>In astronomy not much changes over that time frame.</p>
<p>For example the light from GN-z11 left there about 13.4 billion years ago. Another half year more or less isn't likely to make much difference in our model of the universe.</p>

CC BY-SA 4.0
2024-04-21T19:10:04.857Z
Time-domain astronomy is a very broad and interesting field. Things getting in the way (Sun, Moon, Earth) are a problem for understanding what's happening. https://en.wikipedia.org/wiki/Time-domain_astronomy
John Doty
CC BY-SA 4.0
2024-04-21T19:17:01.697Z
"Because in six months we're going to be there ourselves." In that case, when we get there we could leave a space telescope there and continue on our way. .
KDP
CC BY-SA 4.0
2024-04-21T19:21:38.390Z
@KDP: And come back a year later to collect the film.
A. I. Breveleri
CC BY-SA 4.0
2024-04-21T19:23:27.167Z
"Another half year more or less isn't likely to make much difference in our model of the universe." - Sure, but when a neutrino burst is detected telescopes everywhere spring into action to try and locate where it came from. Same for when a gravitational wave is detected. Talking of which scopes at L4 and L5 potentially provide part of a very large base line GW detector, many orders of magnitude larger than LIGO. Also the larger baseline improves parallax measurements considerably and the comms links will be useful for future space exploration.
KDP
CC BY-SA 4.0
2024-04-22T17:25:28.090Z
*"In astronomy not much changes over that time frame."* ~~ Everybody gangsta until we miss a supernova because the Sun was in the way.
Mindwin Remember Monica
CC BY-SA 4.0
2024-04-22T17:25:56.707Z
@KDP Newton's First Law wants to have a chat with you.
Mindwin Remember Monica
CC BY-SA 4.0
2024-04-22T17:38:55.540Z
@MindwinRememberMonica LOL :P
KDP
CC BY-SA 4.0
2024-04-21T19:01:33.940Z
2024-04-21T19:01:33.940Z
A. I. Breveleri
<p>No, there are currently no plans to put a space telescope, or any other kind of satellite, at the Sun-Earth L3 <a href="https://en.wikipedia.org/wiki/Lagrange_point" rel="noreferrer">Lagrange point</a>.</p>
<p>The main problem is (as you mention) that it's impossible to communicate directly with a body that's always behind the Sun. So it's pointless putting anything at L3 if you don't have a relay station of some kind, probably at L4 or L5.</p>
<p>Such a relay needs to be fairly powerful. The side length of the L3, L4, L5 triangle is <span class="math-container">$\sqrt 3$</span> AU, and the distance from L4 or L5 to Earth is 1 AU, so the total distance from L3 to Earth via the relay is ~22.7 light-minutes. The line from L3 to L4/L5 passes within 0.5 AU of the Sun, where electromagnetic interference is more intense.</p>
<p>If you're going to L4/L5 you might as well put your 'scope there. It will be able to see most things behind the Sun that L3 can see. The L4 &amp; L5 points are gravitationally stable, unlike L1 / L2 / L3, which are saddle points. However, that's not necessarily an advantage, because it means that there's a fair quantity of dust and a few asteroids in the vicinity.</p>
<p>By the way, it takes a fairly long time to get to L3 / L4 / L5 if you don't want to use a huge amount of fuel. The easy way to get a spacecraft to a point on (or near) the Earth's orbit is to put the ship on a heliocentric orbit with a semi-major axis that's slightly larger or smaller than 1 AU. Gradually, the ship will move behind or ahead of Earth. That's the strategy which was used for the <a href="https://en.wikipedia.org/wiki/STEREO" rel="noreferrer">STEREO</a> mission. But this takes a while, due to the ship's synodic period relative to Earth.</p>
<p>If <span class="math-container">$T_1$</span> and <span class="math-container">$T_2$</span> are the orbital periods of two bodies (in concentric orbits), with <span class="math-container">$T_1&lt;T_2$</span>, their relative synodic period <span class="math-container">$S$</span> is given by
<span class="math-container">$$1/S = 1/T_1 - 1/T_2$$</span></p>
<p>The synodic period is the time for the angle between the bodies (as measured at the primary) to go through a full cycle, eg the time between succesive conjunctions or oppositions. To send a ship to L3 we need to wait for half a synodic period.</p>
<p>Eg, if the ship's orbital period is 1.1 years, then its synodic period relative to Earth is 11 years, so it takes 5.5 years to reach L3 with that orbit.</p>
<hr />
<p>I have an interactive 3D Lagrange point diagram on our sister site: <a href="https://space.stackexchange.com/a/57679/38535">https://space.stackexchange.com/a/57679/38535</a></p>

CC BY-SA 4.0
2024-04-21T11:01:52.723Z
[+1] There's a cool GIF of how the STEREO pair economically got into orbits both slightly larger and smaller than 1 AU in [Does the arrow in this STEREO trajectory animation point heliocentric prograde, or towards the Sun?](https://space.stackexchange.com/q/35557/12102).
uhoh
CC BY-SA 4.0
2024-04-21T11:04:14.313Z
@uhoh I also have a STEREO answer: https://space.stackexchange.com/a/56140/38535 ;)
PM 2Ring
CC BY-SA 4.0
2024-04-21T11:12:58.150Z
Looking at the diagram, the L4 and L5 points don't look particularly stable but fortunately someone in the comment mentions that the Coriolis force helps to stabilise them. Nice work though ;_)
KDP
CC BY-SA 4.0
2024-04-21T13:11:16.100Z
@KDP Yes, it's a bit unintuitive why maxima in the effective potential can be stable. This answer by Qmechanic has details on the role of the Coriolis force: https://physics.stackexchange.com/a/363801/123208
PM 2Ring
CC BY-SA 4.0
2024-04-21T09:58:02.253Z
2024-04-21T10:04:12.070Z
2024-04-21T10:04:12.070Z
PM 2Ring
<p>Are there any plans to place a telescope satellites on the far side of the Sun at the L3 Lagrange point?</p>
<p>I think it would be useful for a number of reasons. It would cover our blind spot for incoming meteors and open our chances of seeing some rare astronomical events like supernovae that may be obscured from our Earth viewpoint. It would also provide a wide baseline for more accurate parallax measurements of distances to astronomical objects as well as for experiments studying GR effects like frame dragging.</p>
<p>I am pretty sure that communicating with said satellite may be a problem due to having the Sun in the line of sight, but maybe this issue can be overcome by having additional telescope satellites at the L4 and L5 Lagrange points that could provide a communication link to and from the L3 telescope, while providing additional coverage. With the additional telescopes there is also the potential to effectively have a large version of the LIGO experiment for measuring gravitational waves. The communication links between the telescopes could also provide a useful communication hub for future space missions and probes that may end up on the far side of the Sun. The telescopes would not have to be as fancy or as expensive as the Hubble or James Webb space telescopes to provide significant benefits.</p>

CC BY-SA 4.0
2024-04-21T06:16:36.593Z
Related question on our sister site: https://space.stackexchange.com/q/8513/38535 & links therein
PM 2Ring
CC BY-SA 4.0
2024-04-21T06:27:37.327Z
NASA is so uninterested in L3 that it's not even in Horizons https://ssd.jpl.nasa.gov/api/horizons.api?format=text&MAKE_EPHEM=NO&COMMAND=lagrange
PM 2Ring
CC BY-SA 4.0
2024-04-21T06:59:46.030Z
Well, they haven't read my post yet, lol :-) Maybe the Chinese or the Russian's might be interested in having information that no else has access to as well as claiming bragging rights, like China attempting to be the first to land a probe on the dark side of the moon ;-)
KDP
CC BY-SA 4.0
2024-04-21T07:12:40.620Z
Maybe L4 and L5 might be more practical as I think they are more stable and possibly provide nearly as much coverage of the far side?
KDP
CC BY-SA 4.0
2024-04-21T07:18:31.810Z
You might as well put your comments in a formal answer. They seem resonable and informative.
KDP
CC BY-SA 4.0
2024-04-21T20:30:59.897Z
So we'd know if the other side of the Sun shines?;-)
Simon Crase
CC BY-SA 4.0
2024-04-22T04:56:41.080Z
_" It would cover our blind spot for incoming meteors"_ I don't quality to even be an amateur astronomer but my gut is telling me that if something remains out of sight, hiding behind the sun for a significant portion of its travel time, that it's very unlikely for it to be on a collision course with the Earth.
Flater
CC BY-SA 4.0
2024-04-22T05:31:04.800Z
If its something kicked out the asteroid belt and is on a collision course with the sun and it takes six months to arrive, we will be in the line of fire when by the time it arrives. Unlikely, but possible. On the other hand, 'it' might be something that doesn't want us to see it coming ;-)
KDP
CC BY-SA 4.0
2024-04-22T13:01:19.223Z
Note that if my math is correct, the Sun masks about 1/230 000 of the celestial sphere (about 0.0004%), so you would need to be pretty unlucky for an interesting event to happen right behind the Sun **and** last less than half a day (as about 12 hours later what was behind the Sun becomes visible). Of course the influence of the Sun on what you may see "close to behind it" may exceed beyond just the size of the Sun itself, but it's still a small and time-limited "hole" in what we can see. Still, curious as we are, the L4/L5 options are much easier than L3.
jcaron
CC BY-SA 4.0
2024-04-22T16:53:13.450Z
The *real* question is "Is it worth the big chunk of the limited amount of money that we spend on space exploration?"
RonJohn
<space-telescope><lagrange-point>
Worthwhile to put a telescope on the far side of the Sun?
KDP
# Worthwhile to put a telescope on the far side of the Sun?

(Asked by: username_0 on 2024-04-21)

Are there any plans to place a telescope satellites on the far side of the Sun at the L3 Lagrange point?

I think it would be useful for a number of reasons. It would cover our blind spot for incoming meteors and open our chances of seeing some rare astronomical events like supernovae that may be obscured from our Earth viewpoint. It would also provide a wide baseline for more accurate parallax measurements of distances to astronomical objects as well as for experiments studying GR effects like frame dragging.

I am pretty sure that communicating with said satellite may be a problem due to having the Sun in the line of sight, but maybe this issue can be overcome by having additional telescope satellites at the L4 and L5 Lagrange points that could provide a communication link to and from the L3 telescope, while providing additional coverage. With the additional telescopes there is also the potential to effectively have a large version of the LIGO experiment for measuring gravitational waves. The communication links between the telescopes could also provide a useful communication hub for future space missions and probes that may end up on the far side of the Sun. The telescopes would not have to be as fancy or as expensive as the Hubble or James Webb space telescopes to provide significant benefits.

**Tags:** space-telescope, lagrange-point
> **Comments:**
> **username_1**: Related question on our sister site: https://space.stackexchange.com/q/8513/38535 & links therein  
> **username_1**: NASA is so uninterested in L3 that it's not even in Horizons https://ssd.jpl.nasa.gov/api/horizons.api?format=text&MAKE_EPHEM=NO&COMMAND=lagrange  
> **username_0**: Well, they haven't read my post yet, lol :-) Maybe the Chinese or the Russian's might be interested in having information that no else has access to as well as claiming bragging rights, like China attempting to be the first to land a probe on the dark side of the moon ;-)  
> **username_0**: Maybe L4 and L5 might be more practical as I think they are more stable and possibly provide nearly as much coverage of the far side?  
> **username_0**: You might as well put your comments in a formal answer. They seem resonable and informative.  
> **username_2**: So we'd know if the other side of the Sun shines?;-)  
> **username_3**: _" It would cover our blind spot for incoming meteors"_ I don't quality to even be an amateur astronomer but my gut is telling me that if something remains out of sight, hiding behind the sun for a significant portion of its travel time, that it's very unlikely for it to be on a collision course with the Earth.  
> **username_0**: If its something kicked out the asteroid belt and is on a collision course with the sun and it takes six months to arrive, we will be in the line of fire when by the time it arrives. Unlikely, but possible. On the other hand, 'it' might be something that doesn't want us to see it coming ;-)  
> **username_4**: Note that if my math is correct, the Sun masks about 1/230 000 of the celestial sphere (about 0.0004%), so you would need to be pretty unlucky for an interesting event to happen right behind the Sun **and** last less than half a day (as about 12 hours later what was behind the Sun becomes visible). Of course the influence of the Sun on what you may see "close to behind it" may exceed beyond just the size of the Sun itself, but it's still a small and time-limited "hole" in what we can see. Still, curious as we are, the L4/L5 options are much easier than L3.  
> **username_5**: The *real* question is "Is it worth the big chunk of the limited amount of money that we spend on space exploration?"  

---

## 2 answers

### Answer by username_1 on 2024-04-21. Score: 21 | Accepted answer

No, there are currently no plans to put a space telescope, or any other kind of satellite, at the Sun-Earth L3 [Lagrange point](https://en.wikipedia.org/wiki/Lagrange_point).

The main problem is (as you mention) that it's impossible to communicate directly with a body that's always behind the Sun. So it's pointless putting anything at L3 if you don't have a relay station of some kind, probably at L4 or L5.

Such a relay needs to be fairly powerful. The side length of the L3, L4, L5 triangle is $\sqrt 3$ AU, and the distance from L4 or L5 to Earth is 1 AU, so the total distance from L3 to Earth via the relay is ~22.7 light-minutes. The line from L3 to L4/L5 passes within 0.5 AU of the Sun, where electromagnetic interference is more intense.

If you're going to L4/L5 you might as well put your 'scope there. It will be able to see most things behind the Sun that L3 can see. The L4 & L5 points are gravitationally stable, unlike L1 / L2 / L3, which are saddle points. However, that's not necessarily an advantage, because it means that there's a fair quantity of dust and a few asteroids in the vicinity.

By the way, it takes a fairly long time to get to L3 / L4 / L5 if you don't want to use a huge amount of fuel. The easy way to get a spacecraft to a point on (or near) the Earth's orbit is to put the ship on a heliocentric orbit with a semi-major axis that's slightly larger or smaller than 1 AU. Gradually, the ship will move behind or ahead of Earth. That's the strategy which was used for the [STEREO](https://en.wikipedia.org/wiki/STEREO) mission. But this takes a while, due to the ship's synodic period relative to Earth.

If $T\_1$ and $T\_2$ are the orbital periods of two bodies (in concentric orbits), with $T\_1, their relative synodic period $S$ is given by
$$1/S = 1/T\_1 - 1/T\_2$$

The synodic period is the time for the angle between the bodies (as measured at the primary) to go through a full cycle, eg the time between succesive conjunctions or oppositions. To send a ship to L3 we need to wait for half a synodic period.

Eg, if the ship's orbital period is 1.1 years, then its synodic period relative to Earth is 11 years, so it takes 5.5 years to reach L3 with that orbit.

---

I have an interactive 3D Lagrange point diagram on our sister site: <https://space.stackexchange.com/a/57679/38535>

> **Comments:**
> **username_9**: [+1] There's a cool GIF of how the STEREO pair economically got into orbits both slightly larger and smaller than 1 AU in [Does the arrow in this STEREO trajectory animation point heliocentric prograde, or towards the Sun?](https://space.stackexchange.com/q/35557/12102).  
> **username_1**: @uhoh I also have a STEREO answer: https://space.stackexchange.com/a/56140/38535 ;)  
> **username_0**: Looking at the diagram, the L4 and L5 points don't look particularly stable but fortunately someone in the comment mentions that the Coriolis force helps to stabilise them. Nice work though ;_)  
> **username_1**: @KDP Yes, it's a bit unintuitive why maxima in the effective potential can be stable. This answer by Qmechanic has details on the role of the Coriolis force: https://physics.stackexchange.com/a/363801/123208  

---

### Answer by username_6 on 2024-04-21. Score: 7

On balance, no. Because in six months we're going to be there ourselves.

In astronomy not much changes over that time frame.

For example the light from GN-z11 left there about 13.4 billion years ago. Another half year more or less isn't likely to make much difference in our model of the universe.

> **Comments:**
> **username_7**: Time-domain astronomy is a very broad and interesting field. Things getting in the way (Sun, Moon, Earth) are a problem for understanding what's happening. https://en.wikipedia.org/wiki/Time-domain_astronomy  
> **username_0**: "Because in six months we're going to be there ourselves." In that case, when we get there we could leave a space telescope there and continue on our way. .  
> **username_6**: @KDP: And come back a year later to collect the film.  
> **username_0**: "Another half year more or less isn't likely to make much difference in our model of the universe." - Sure, but when a neutrino burst is detected telescopes everywhere spring into action to try and locate where it came from. Same for when a gravitational wave is detected. Talking of which scopes at L4 and L5 potentially provide part of a very large base line GW detector, many orders of magnitude larger than LIGO. Also the larger baseline improves parallax measurements considerably and the comms links will be useful for future space exploration.  
> **username_8**: *"In astronomy not much changes over that time frame."* ~~ Everybody gangsta until we miss a supernova because the Sun was in the way.  
> **username_8**: @KDP Newton's First Law wants to have a chat with you.  
> **username_0**: @MindwinRememberMonica LOL :P  

---

