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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.

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  • $\begingroup$ Related question on our sister site: space.stackexchange.com/q/8513/38535 & links therein $\endgroup$
    – PM 2Ring
    Apr 21 at 6:16
  • $\begingroup$ NASA is so uninterested in L3 that it's not even in Horizons ssd.jpl.nasa.gov/api/… $\endgroup$
    – PM 2Ring
    Apr 21 at 6:27
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    $\begingroup$ 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? $\endgroup$
    – KDP
    Apr 21 at 7:12
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    $\begingroup$ You might as well put your comments in a formal answer. They seem resonable and informative. $\endgroup$
    – KDP
    Apr 21 at 7:18
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    $\begingroup$ " 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. $\endgroup$
    – Flater
    Apr 22 at 4:56

2 Answers 2

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No, there are currently no plans to put a space telescope, or any other kind of satellite, at the Sun-Earth L3 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 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 $T_1<T_2$, 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

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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.

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  • $\begingroup$ 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. en.wikipedia.org/wiki/Time-domain_astronomy $\endgroup$
    – John Doty
    Apr 21 at 19:10
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    $\begingroup$ "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. . $\endgroup$
    – KDP
    Apr 21 at 19:17
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    $\begingroup$ @KDP: And come back a year later to collect the film. $\endgroup$ Apr 21 at 19:21
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    $\begingroup$ "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. $\endgroup$
    – KDP
    Apr 21 at 19:23
  • $\begingroup$ "In astronomy not much changes over that time frame." ~~ Everybody gangsta until we miss a supernova because the Sun was in the way. $\endgroup$ Apr 22 at 17:25

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