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Deploying really big radio dishes in space seems to be possible and "frequently" done with SIGINT satellites in geosynchronous orbit (like the assumed specs of the Orion satellite class). They are said to have a 100m parabolic antenna.

What would the challenges be to bring a radio telescope with such a nice dish out to L4 and or L5?

Benefits:

  • (relatively) radio silent
  • can keep communicating with planetary probes while the planet is in opposition with earth
  • can point at one object for prolonged periods of time
  • this dish/es used with earth bound dishes as an interferometer would provide an amazingly wide baseline (at least in one dimension)
  • L4 and L5 are "stable" so the device could operate for a very long time with limited fuel use
  • it's outside earths magnetosphere (maybe less distortion on signals)

Issues:

  • getting something big to L4 / L5 is neither easy nor cheap
  • data needs to be transmitted 150Gm (1AU) to earth, probably requiring DSN capacity (would laser communication be possible over that distance?)
  • the radio telescope hardware can't be upgraded
  • is outside of earths magnetosphere (fully exposed to stellar and interstellar particles)

Does anyone know if such a mission is

  • planed?
  • proposed?
  • would make sense?
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    $\begingroup$ There are some examples of what such big antennas might look like at What is the largest antenna deployed in space? I think in these cases the dish is not a large fraction of the spacecraft mass, so if they can get all the way to GEO then it doesn't take a heck of a lot more delta-v to get to heliocentric C3=0 and then just drift over to the triangular Lagrange point of your choosing. But if you want to do radio astronomy with one 100 meter dish and at only a few GHz (these aren't going to be that flat) there's no real advantage over what's $\endgroup$
    – uhoh
    Commented Feb 10, 2022 at 10:53
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    $\begingroup$ already available on Earth, and if you really need radio-quiet there's also the far side of the Moon. Getting out of Earth's absorbing and phase-shifting precipitable water vapor and wiggly ionosphere is really important for millimeter wavelengths and you will definitely need a much better design for an unfolding dish with that kind of surface figure accuracy. But then a dish is basically a single-pixel camera or at most a few dozen pixels. $\endgroup$
    – uhoh
    Commented Feb 10, 2022 at 10:58
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    $\begingroup$ What you would like even more is at least a few dishes with some distance in between to make an interferometric array. Of course Earth could be one point and this could be the other, like how Spektr-R worked. (note added in proof: the far side of the Moon is actually a lot harder for locating a big dish antenna than heliocentric orbit. It's a nice place to build an array though!) See also array on the Moon $\endgroup$
    – uhoh
    Commented Feb 10, 2022 at 11:00
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    $\begingroup$ @uhoh thanks. I also know about the idea of a telescope one the far side of the Moon on the moon surface itself. Using the Moon to protect from interference from earth (you'd just need a really long ethernet cable to the other side of the moon to transmit the data to earth). But 1AU to L4 and L5 would really provide you an amazingly long baseline for interferometry. We've also had the stereo mission in L4/L5 to watch the sun (optically). $\endgroup$
    – TrySCE2AUX
    Commented Feb 10, 2022 at 11:08
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    $\begingroup$ and if the dishes are looking towards the poles instead of along the ecliptic plane they would get a nice spread in 2D when used in conjunction with earth bound telescopes. Imagine what such an interferometer could get us in terms of resolution! $\endgroup$
    – TrySCE2AUX
    Commented Feb 10, 2022 at 11:11

1 Answer 1

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One significant issue is we cannot maintain a fixed baseline in this way. L4 and L5 are indeed stable, but in space (or rather, in the space that actually exists) "stable" does not mean perfectly stationary.

L4 and L5 would be stationary points in a reference frame that co-rotates with the orbital motion of the massive bodies and in which the massive bodies are stationary. But the real orbit of Earth around the Sun is elliptical, not circular, and so Earth will move (in a small figure-eight pattern) in the co-rotating frame. The Lagrange points will therefore be perturbed in response. With Mercury the effects of this eccentricity contributes to the lack of real stability in that planet's L4 and L5 points; with Earth the eccentricity is smaller but may still impact the accurate rendering of the baseline.

In addition, of course, the Lagrange points are affected by perturbations from other planets, which may again affect their stability as well as their relative locations. Earth's Lagrange points face perturbations from Jupiter (which is sufficiently massive to act significantly over 5 AU), the relatively close passes of Venus, and even our Moon (which pulls the barycenter of Earth's orbit around the Sun almost 5000 km off Earth's center).

We therefore need to reckon with the movements of the Lagrange points in response to these disturbances, whereas the baseline on conventional Earth-based radio interferoneters would be more stable. Without an accurate compensation for a complex motion that may arise from the perturbations, the favorable effects of interferometry would surely be lost.

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