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The Hubble Space Telescope will enter a decaying orbit in 2024.

This is a physics based question, so I don't want to get into the engineering details more than necessary.

Just for background, the last Shuttle mission fitted a soft capture mechanism to the back end of the spacecraft. This ring will give future robotic spacecraft an easy place to grab onto and possibly enable significant orbital alterations to be made.

Lagrangian Points

My question is: would the HST be of any practical benefit if it was transferred to a Lagrangian L4 or L5 point?

These points would require the least propellant in maintaining a "fixed" position.

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    $\begingroup$ No optical interferometer has flown in space. I cannot imagine that what you suggest would be easier than launching a completely new purpose-built interferometer. Or is it you re suggesting the baseline would be Earth to L5?!! The current largest optical/IR interferometers are able to work over tens of metres. $\endgroup$ – Rob Jeffries Jun 3 '15 at 20:03
  • $\begingroup$ @RobJeffries point taken, was confusing it with radio VLBI idea..will delete question...fibre optic cable would have to be a bit longish $\endgroup$ – user81619 Jun 3 '15 at 20:17
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    $\begingroup$ Surely the capacity to move the device to would be equally up to long-term station keeping where it is. Against the possibility that we get a service capability again. Also cheaper would be putting into the Earth-moon L4 or L5. $\endgroup$ – dmckee Jun 3 '15 at 21:03
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There is one big advantage. The Earth would (virtually) never get in the way of the observing target, allowing for continuous long exposures on any target and greatly simplifying scheduling.

The only other advantage I can think of is the possibility of getting parallax measurements slightly faster by using re-located-HST + a telescope on/around Earth imaging about 4 months apart to get maximum separation (rather than 6 months using only telescopes located at Earth). But this would require an HST-calibre instrument at Earth. In some cases ground-based AO-assisted imaging would probably work.

These seems outweighed by the drawbacks:

  • Need to get HST to L4/L5, which will cost a fair bit (it's a space mission after all).
  • Any further desired servicing would be much more complicated and expensive.
  • Data retrieval would be substantially complicated. The HST transmitter was not designed with that sort of range in mind. It's probable you could still get data off the spacecraft with sensitive receiving equipment, but transfer rate would probably suffer. This could lead to forced downtime between exposures to clear onboard storage for the next exposure.
  • Possible negative effects of not being shaded by the Earth. This might have implications for keeping the spacecraft cool? And for imaging faint objects (Earth shade reduces light scattered into the telescope)?
  • You could no longer point at a shiny dot in the sky and say "That's HST!" and be correct.

Besides, if HST is still operational and deemed desirable in 2024, I'm sure it will be nudged into or left in a stable orbit around Earth.

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  • $\begingroup$ thanks for that Kyle, it was initally as an interferometer I was thinking of...still kicking myself over being that stupid...I think you are right about your last point...NASA didn't put that ring on for nothing, but in the worst/pragmatic case I suppose it will just assist in getting it into the Pacific...ah well, thanks again $\endgroup$ – user81619 Jun 3 '15 at 21:16
  • $\begingroup$ Point #4 would be a major advantage. It would increase the observing efficiency considerably and make continuous observations possible. $\endgroup$ – Rob Jeffries Jun 3 '15 at 21:24
  • $\begingroup$ NASA's budget and the success/fail of the JWT if/when it gets operational are probably going to decide things for the HST $\endgroup$ – user81619 Jun 3 '15 at 21:33
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    $\begingroup$ @AcidJazz JWST is only sort of a replacement for HST - the wavelength overlap is only partial. You lose the UV end of the spectrum that HST can observe, but go further into the IR. That's nice for high redshift galaxies work, but some other niches that HST currently fills are just gone (no UV from the ground) until someone decides to put a high-resolution UV observatory up there. $\endgroup$ – Kyle Oman Jun 3 '15 at 23:57
  • $\begingroup$ @RobJeffries Oh yeah I totally missed that! $\endgroup$ – Kyle Oman Jun 3 '15 at 23:57

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