A spacecraft at the Sun Earth Lagrange point one (1% of an AU, or one Solar diameter, from Earth towards the Sun) would have the Sun as its background. Is the Sun emitting strongly enough at wavelengths otherwise ideal for communication, that it requires important design changes compared to spacecrafts in other orbits? What wavelength would be ideal from SEL-1? What are the challenges, if any, and solutions to discern data from Solar noise at SEL-1?

  • $\begingroup$ There are/have been several spacecraft in "halo and/or Lissajous orbits" about Sun-Earth L1 (e.g. DSCOVR, SOHO...) but you are saying at L1 rather than around it. I'm hoping that @DavidHammen will elaborate further on this answer and subsequent comments there, about the cost of being in one of those orbits versus being "at L1" itself. In the mean time, just to double check, you mean to ask about a location where the spacecraft stays very close to the Sun-Earth line rather than far enough away that conventional DSN comms will still work? $\endgroup$ – uhoh Apr 12 '18 at 10:17

A spacecraft at or within a few degrees of the Sun-Earth L1 point would be unable to communicate with the Earth due to interference from solar radiation. Moreover, antenna operators very much do not like having their antenna pointed toward the Sun. Similarly, a spacecraft at or within a quarter of a degree of the Earth-Moon L2 point would be unable to communicate with the Earth due to blockage by the Moon.

Even without those communications concerns, spacecraft do not operate at the unstable Lagrange points. They instead fly in some kind of pseudo-orbit about the desired Lagrange point. One reason is stationkeeping costs. A spacecraft in such an orbit needs to perform a small number of smallish stationkeeping operations per year. Moreover, such a spacecraft doesn't need to know where it is. Those infrequent stationkeeping operations can be calculated on the ground and uploaded to the spacecraft as a delta-V maneuver to be performed at a specific time.

In comparison, a spacecraft operating at a Lagrange point would need to perform much extremely frequent stationkeeping operations, and its flight software would need to know where the spacecraft is in space. The orders of magnitude higher stationkeeping costs combined with the more complex (and hence more expensive) flight software preclude satellites from operating directly at any of the unstable Lagrange points.

  • $\begingroup$ A spacecraft within a couple of degrees of SEL1 would be "unable" to communicate with Earth!? Is it really that tough? Is a wide enough halo orbit required for working radio communication from SEL1 impractical for orbital mechanical reasons and station keeping demands, or does it just happens to work out fine? $\endgroup$ – LocalFluff Apr 12 '18 at 11:58
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    $\begingroup$ @LocalFluff - A couple of examples: SOHO and ACE, both of which have been operating for over 20 years. This would not have been possible had they operated at the Sun-Earth L1 point. That would entail fighting physics as opposed to taking advantage of physics. SOHO is in a roughly elliptical halo orbit (roughly elliptical from the perspective of the rotating Sun-Earth frame). The closest approach to the Sun-Earth line as seen from the Earth is 5°, with a design goal of never getting within 4.5° of the Sun. $\endgroup$ – David Hammen Apr 12 '18 at 12:26
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    $\begingroup$ ACE is in a Lissajous orbit that does sometimes take the spacecraft close too the Earth-Sun line of sight. The solution there is simple: The spacecraft has data recorders. The spacecraft needs those data recorders even when the spacecraft is far from the Earth-Sun line of sight because communications aren't continuous. Communications aren't scheduled when the spacecraft gets too close to the Earth-Sun line of sight. $\endgroup$ – David Hammen Apr 12 '18 at 12:29
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    $\begingroup$ Getting back to SOHO, its halo orbit takes SOHO from ±206448 km of the Sun-Earth L1 point in x (along the Sun-Earth line), ±120000 km of the Sun-Earth L1 point in y (normal to the Sun-Earth line but in the Sun-Earth orbital plane), and ±666672 km of the Sun-Earth L1 point in z (normal to the Sun-Earth orbital plane). SOHO is always at least 238790 km from the Sun-Earth L1 point. $\endgroup$ – David Hammen Apr 12 '18 at 12:43
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    $\begingroup$ @LocalFluff see SEZ (Solar Exclusion Zone) of 4° radius for DSCOVR here on page 4 for example. Also see links, pics, etc. in What exactly is the interaction that blocked Juno's data downlink near solar conjunction? $\endgroup$ – uhoh Apr 12 '18 at 12:50

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