High-Altitude Floating Space Telescope

[sanman]

Would it be feasible/practical to construct a large floating space telescope that perhaps would be incorporated into the structure of a large dirigible, blimp or balloon. Or perhaps it would simply be carried aloft by the said buoyant platform?

Has anyone ever before proposed the construction of such an observation platform? Couldn't this enable better-than-Hubble observation on the cheap?

I realize that adaptive optics has greatly advanced the field of astronomy and reduced the handicaps that ground-based observatories have traditionally faced. But space telescopes like Hubble still provide unique and important information.

Additionally, a floating telescope would be mobile, and able to shift locations and orientation in a way that fixed ground-based telescopes can't.

So tell me, is this a useful idea worth consideration? Comments?

 

[chroot]

There are so many problems with this idea it's hard to know where to begin.

1) The atmosphere has wind. Especially at high altitudes, winds can be very intense. Telescopes like Hubble need to be able to be pointed precisely at their targets for many hours or even days at a time. A floating telescope is completely at the mercy of the wind, and one small gust can destroy days of imaging. It's very hard to imagine a feasible thrust and control system that could keep the telescope's orientation correct down to a couple of thousands of a degree when the entire platform is being buffeted by wind.

2) The atmosphere actually swells and shrinks from night time to day time, meaning the altitude of the dirigible will be constantly changing throughout the day. This has the same obvious problems as noted in (1).

3) It's very difficult to keep helium and other light gases inside a membrane that's still light enough to fly. You'd need to be constantly replenishing its gas, and this would be expensive and difficult.

4) It's hard to imagine a positioning system that could permit pointing anywhere in the sky.

5) One of the enormous advtanges of spaceflight is that you're out of the atmosphere's scattering, so you can perform imaging during the daytime. Your telescope lacks this.

6) Another enormous advantage of spaceflight is that the weight of the telescope is only important for launch considerations; once in space it's weightless, and only needs very occassional engine burns to maintain its orbit. It doesn't have to deal with any of the mechanical engineering challenges of weight-bearing superstructures and so on.

- Warren

 

[neutrino]

http://chile1.physics.upenn.edu/blastpublic/ (a recent example)

http://www.astro.caltech.edu/~lgg/boomerang_front.htm (the more famous example)

But these can never replace space telescopes.

 

[chroot]

I should mention that neither of the 'scopes posted by neutrino operate in the visible spectrum, and thus are able to circumvent some of the issues I note.

- Warren

 

[berkeman]

I think NASA or somebody already has a fairly large telescope mounted in a high-flying large jet plane. It gets above enough of the atmosphere that it is able to take pretty good pictures. I'll see if I can find it...

 

[berkeman]

Well, that was easy. I googled NASA telescope plane, and bingo:

http://www.space.com/searchforlife/seti_devore_sofia_020509.html

 

[chroot]

There have been numerous studies done aboard long-range aircraft, including a couple of IR surveys and some CMBR surveys. Most were relatively low-resolution, as are the 'scopes linked by neutrino.

I don't think it's possible to get better-than-Hubble visible-spectrum results from a floating telescope, even if you had infinite resources to design and build it.

- Warren

 

[sanman]

chroot, my understanding is that if you float high enough, you are above the atmospheric turbulence and are for all practical purposes near the edge of space. Furthermore, the vestigial atmosphere at that very high altitude has a higher ionic content. Your big blimp could use solar panels to grab energy, and then maybe run some ion-wind propulsors or electric fan-motors to maneuver and re-orient the craft. But the atmosphere is calm up there, and things are supposed to be quite still. Also, wouldn't it be thin enough that the scattering effects would be negligible?

I was suggesting the floating giant blimp/balloon/dirigible idea because the lifting capacity would be greater than a plane, and it would be able to remain more stationary than a plane. So wouldn't one be able to get a really big telescope up there?

http://en.wikipedia.org/wiki/WALRUS_HULA

http://64.233.187.104/search?q=cache:PYieaIZ0xyYJ:sspp.gsfc.nasa.gov/news/press_releases/04-091.pdf+heavy+lift+balloon&hl=en&gl=us&ct=clnk&cd=10 Wasn't the original Telstar communications "satellite" just a high-floating balloon?

http://www.elec.york.ac.uk/comms/haps.html

Comments?

 

[chroot]

Well, your blimp depends on buoyancy, right? When the density of the gas inside it is the same as the density of the air outside, it will stop rising. You're going to have a hard time designing a balloon that can reach "the edge of space!"

I also don't see how the upper atmosphere can be "quite still" when the entire atmosphere is shrinking and swelling throughout the diurnal cycle. I would have to do some more research on the very outer atmosphere before I'd be willing to accept the claim that the outer atmosphere is very still -- it goes against all of my pilot training, though, of course, airplanes don't go that high.

- Warren

 

[sanman]

Oooh, look, here's the latest:

http://www.space.com/searchforlife/070104_sofia_update.html

 

[sanman]

Hmm, I dunno, hydrogen and helium are considerably less dense than our atmosphere. I have heard about research being done into "vacuum lift" (ie. rigid vacuum enclosures used for lighter-than-air buoyancy). Aerogel is supposed to be an interesting lightweight structural material for that purpose.

 

[chroot]

From what I can tell, the best high-altitude balloons can get to about 25 or 35 kilometers altitude. Space is usually considered to begin at about twice this altitude.

- Warren

 

[sanman]

Here's another one:

http://adsabs.harvard.edu/abs/2003SPIE.4857..227G

And more:

http://www.spie.org/Conferences/Programs/02/as/confs/4857.html

 

[russ_watters]

Hmm, I dunno, hydrogen and helium are considerably less dense than our atmosphere.

Density decreases with altitude. Yes, the density of helium decreases at altitude too, but that means that to fly higher, you need a bigger balloon.

 

[chroot]

I'm still trying to find data on the practical exposure limits of these experiments. I suspect the exposures are relatively short, on the order of an hour or less. Hubble excels specifically because of its ability to take extremely long exposures, sometimes days long. I'm not sure that exposures of such lengths are even theoretically possible with a balloon-based telescope.

- Warren

 

[sanman]

I would think that Telstar was up there for a long time. I don't see why a balloon with a suitably impermeable sheath wouldn't be able to stay up there for long. There are companies looking to deploy very high altitude balloons even to host broadband services.

Here's one company that calls theirs a "stratellite":

http://en.wikipedia.org/wiki/Stratellite

I don't think the size of a balloon has to be the showstopper. After all, what's up there to bump into?