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With the Hubble telescope to be decommissioned any time now, and the James Webb Space Telescope at least in 2018 and unlikely to be active while Hubble still remains, will there be a gap when we have no space telescope at all? Or is there (planned, or present) another one to fill in?

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The the United States National Reconnaissance Office has donated two identical declassified reconnaissance telescopes to NASA in 2012. They say the telescopes are more powerful than Hubble.

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    $\begingroup$ The donations are not complete systems; and can't be used as is. The article you linked to states that the candidate mission that one is being considered for use in won't fly until the 2020's; several years after the Webb is planned to be operational. $\endgroup$ – Dan Neely Aug 1 '13 at 14:12
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To answer your original question regarding the state of affairs of space telescopes, from the US (NASA), besides JWST, there are two large space telescope projects. Work has started on the WFIRST (wide field infrared survey telescope). The LUVOIR (large ultraviolet/optical/infrared survey) telescope is in the study phase.

WFIRST was originally a dark energy project that morphed into a facility that will enable research in many fields, including dark energy. WFIRST will have a 2.4 meter diameter primary mirror with a field of view of 0.79 by 0.43 degrees. The primary is one of the donated ones mentioned in the answer above. Although the primary is the same diameter as the Hubble primary, the optical systems are different. The Hubble is a two mirror Ritchey-Chretien, while the WFIRST will be a three mirror anastigmat to provide a much wider useful field of view.

LUVOIR is intended to combine an enhanced version of the capabilities of Hubble plus a planetary coronagraph. Its primary mirror will be between 8 and 16 meters in diameter. It will include instruments working from the far UV to the near infrared (about 100 nm to 1700 nm) and be diffraction limited at 500 nm. However, to be chosen by the decadal survey, LUVOIR will have to compete with other NASA missions. Likely competitors include a dedicated planetary coronagraph, a major X-Ray telescope, and a space-based gravitational wave sensor.

I've provided links to the official sites for WFIRST and LUVOIR. I've also provided a link to the AURA Report which you may not find so easily on your own. The AURA (the Association of Universities for Research in Astronomy) Report provides a detailed list of the areas of astronomical research that astronomers hope to explore in the next decades and the requirements for telescope facilities, like WFIRST and LUVOIR, that are necessary to perform this research. Lastly, I've provided a link to the ATLAST entry on Wikipedia. "ATLAST" (Advanced Technology Large-Aperture Space Telescope) was the previous name for what is now mostly called LUVOIR.

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  • $\begingroup$ Could you add links to sources please. Even if the info is easy to find it improves answers a lot $\endgroup$ – OrangePeel52 Oct 21 '16 at 16:55
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    $\begingroup$ Hi OrangePeel52. The answer above is based on personal experience. I am, or have been, directly involved in both projects. $\endgroup$ – Vince 49 Oct 21 '16 at 17:36
  • $\begingroup$ Even if you can't find a citation for everything in your answer, links would be helpful. Some details of LUVOIR are mentioned here for example. space.com/31778-nasa-next-great-space-telescope.html $\endgroup$ – OrangePeel52 Oct 21 '16 at 18:13
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While Hubble and the James Webb Space Telescope are not the only telescopes in space, let me make a statement about "another one to fill in". Please note the complexity of space telescopes and the time needed for their development. The ideas and first concepts for Hubble came up at the same time the development of the Space Shuttle started. This was essentially around 1970. It took two decades to build and launch it and even a few more years to bring it to its full capacity. The time required for the development of the James Webb Space Telescope is at similar orders of magnitude. With decades for building such stuff, it is hard to talk about another one just "filling" in.

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    $\begingroup$ Why? Take any of hundreds of existing spy satellites, replace optics with generic space observation ones, rotate 180 degrees, done ;) $\endgroup$ – SF. Jul 17 '13 at 16:14
  • $\begingroup$ This idea does in fact exist, in some way ... space.com/21064-nasa-donated-spy-telescope-mars.html $\endgroup$ – s-m-e Jul 17 '13 at 16:17
  • $\begingroup$ @SF "replace optics with generic space observation ones". This can't be done in space. Only Hubble was designed to have that kind of servicibility. So it's not as simple as this plus rotating 180 degrees. $\endgroup$ – DrZ214 Jun 11 '15 at 1:03
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    $\begingroup$ The optics are the easy part. You can't use the CCDs from a spy satellite, you have to replace them with much more sensitive astronomy-grade CCDs. Those might need cooling. You'd also need to add instruments (spectrometer, for example). Those instruments need more space than the single camera in a spy satellite. The mirror may need to be improved (a spy satellite is limited by the atmosphere anyway, so less need to make a perfect mirror). $\endgroup$ – Hobbes Nov 11 '16 at 12:15

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