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kamaz
2010-Nov-27, 10:01 PM
Recently, we heard news about the launch of NROL-32, the "largest spysat ever". Allegedly, this is a Mentor type bird, which, allegedly, look like this: http://reseau.echelon.free.fr/reseau.echelon/satellites.htm

...So it is essentially a 100m radio telescope. According to the same site, the cost is one billion dollars. So, theoretically, it would be possible to build one for scientific use.

Questions:

1. What would 100m radio telescope in orbit be good for? How would it compare to terrestrial arrays?

2. If we launched two of these to geosync orbit 180 degrees apart, would it enable interferometry with a baseline of 84'000km? What would that be useful for?

3. If we launched two of these into a solar orbit, would we have an interferometer with a baseline of 2 AU? What could such instrument be used for?

ngc3314
2010-Nov-27, 10:46 PM
Some uses that pop to my mind are -

- Low-frequency observations orbiting over the lunar farside, where it is hidden from the strong interference of emission from the auroral ovals and van Allen belts

- Interferometry with a constellation of these, combining considerable sensitivity with angular resolution. One of these dishes has roughly the same collecting area as the VLA (25m * sqrt(27) = 130 m effective equivalent). Interferometry between ground-based and orbiting stations has been done with the Japanese HALCA antenna in high Earth orbit; there are specialized timing requirements that were one of the most challenging technical issues. As a result, the standard software for aperture synthesis now has the possibility of stations' being specified not by geodetic coordinates but with orbital elements. The most suitable spacing depends on the application - an array is effectively blind to structures too large for its spacing, so going straight to 2 AU might bot be the most informative thing to do. On the other hand, a roughly logarithmic spacing of baselines (a la the VLA) retains sensitivity to a wide range of angular scales. Collimation and acceleration regions of quasar jets, gravitational lensing by the Galactic Center black hole, neutral hydrogen structures and dynamics of galaxies in the early Universe, galaxy distances via proper motions of maser regions in their arms' star-forming regions, are some things that come to mind right away.

antoniseb
2010-Nov-27, 11:31 PM
...So it is essentially a 100m radio telescope. According to the same site, the cost is one billion dollars. So, theoretically, it would be possible to build one for scientific use.

Questions:

1. What would 100m radio telescope in orbit be good for? How would it compare to terrestrial arrays?

2. If we launched two of these to geosync orbit 180 degrees apart, would it enable interferometry with a baseline of 84'000km? What would that be useful for?

3. If we launched two of these into a solar orbit, would we have an interferometer with a baseline of 2 AU? What could such instrument be used for?

- You'd probably want more than two for nice interferometry imaging.
- It's not impossible but the computation for interferometry gets tougher if the dishes are moving relative to each other... even worse if they are accelerating.
- You'd also probably want lots more surface area. You can get good resolution, but the number of photons coming from small areas is pretty small, even in radio ranges, so a couple 100 meter dishes 2AU apart might have to stare at a source longer than the duration of a transient event (maybe weeks or years at full resolution) to get a respectable image.