What "weed need" is a space mission!

[science_man_88]

Originally Posted by xilman

Where do you get those numbers from? I understand the 3652 as being hours per ten years. Though if you insist on using ludicrously precise results, you should use 3652.425 instead. Personally I'd say 3650 is more than adequate precision and your conclusion should be 19G miles.

In particular, where does that "60"? come from?

19G miles is 30G km in more conventional units and 20 AU is even more conventional in this area. It's about the right order of magnitude in that Uranus is about that distance from us and the Voyager probes took about that length of time to get there, though not in a straight line of course. The distance they actually travelled to reach Uranus was significantly greater.

If you want to go as far as possible outwards you should be able to do significantly better by bouncing off at least two of the Moon, Venus, the Earth, Jupiter and Saturn en route, especially if you use a long-duration low-thrust ion engine during the cruise phases. Voyager was constrained by having to visit Uranus after its earlier and compulsory gravity assists from Jupiter and Saturn. A long distance mission to the middle of nowhere doesn't have that limitation.

The science from such a package would, in my view, best be devoted to observing charged particles, magnetic fields and ultra low frequency radio. Put cameras on board if you wish for the planetary fly-bys, but heading out into interstellar space is likely to be better served by using a significant fraction of the payload for electrical power, reaction mass (presumably Xe or Hg0, a very large comms antenna and even longer ULF radio antennae. If the comms antenna is large enough, in the 20 to 100m class, it could do double duty for radio astronomy and, in particular, enable mult-AU baseline interferometry. I haven't yet worked out the numbers but it may be possible to detect parallax shifts for the nearest quasars which would get the cosmologists excited.

I found a source that said dawn's fastest acceleration (and apparently the fastest) in space was at 60 mph/ day as for the formula it's just my way of working with d=vi*t+.5at^2, admittedly I didn't quite think of gravity assist in this scenario.

[chalsall]

Originally Posted by fivemack

NASA put about a billion dollars into space interferometry development, to discover that building an exciting space interferometer cost more than they would be prepared to spend ... planet-hunting by interferometric astrometry is so much more expensive than Kepler.

Yeah. Funny. It worked great here on Earth (where everything is relatively fixed). Didn't work so well is space (where everything is relatively moving)...

Originally Posted by fivemack

So you probably do need a billion-kilometre baseline for direct parallax to quasars, but the water-maser work might well be possible with sufficiently large dishes at Earth-Sun L4 and L5.

Synthetic stereo vision and reconstruction is OTC now-a-days in the Computer Vision space.

[fivemack]

Originally Posted by Uncwilly

Yes, but they are FREE.

They are free white elephants - the missions they'd be really useful for are ones where they're pointed at Earth, and that's explicitly forbidden by the terms of the deal handing them over. Building a spacecraft to put them on, and launching it, will run into high hundreds of millions of dollars ... for mirrors that big you need pointing to 100-milliarcsecond over the course of multi-hour exposures, and whilst entirely possible that's not a cheap thing to design into a spacecraft.

They're not capable of being run cold enough to work in the cosmologically-interesting bits of the infra-red; they could do awesome sky-survey work in the optical because of the size of the field of view, if you could afford the focal plane array (focal length is about 20 metres, field of view about one square degree so 30cm on a side, tiling that with 10um pixels is 64 4k x 4k arrays at about a million dollars each for space-qualified hardware), but the data rates are getting high enough to be inconvenient.

I expect they'll get launched because it would be such awful PR not to launch them.

[fivemack]

Originally Posted by chalsall

Yeah. Funny. It worked great here on Earth (where everything is relatively fixed). Didn't work so well is space (where everything is relatively moving)...

I think you need to read some of the SIM white papers; the space interferometry proposals that got developed were ones in which the spacecraft consisted of various pieces mounted very solidly to an extremely solid optical bench. The ESO's Messenger magazine has some very good articles about the difficulties of getting the VLTI working, in an environment where you can work on the real hardware after it was designed and where you can bolt things to foundations sunk into Andean bedrock.

Nobody's got close to a design which would get wavefront combining between free-flying spacecraft; microwave ranging between free-fliers has been done quite effectively for GRACE and GRAIL, but that's a hundred times worse precision than would be needed to get fringes.

Synthetic stereo vision and reconstruction is OTC now-a-days in the Computer Vision space.

Interferometry is very, very different from synthetic stereo vision.

[chalsall]

Originally Posted by fivemack

I think you need to read some of the SIM white papers; the space interferometry proposals that got developed were ones in which the spacecraft consisted of various pieces mounted very solidly to an extremely solid optical bench. The ESO's Messenger magazine has some very good articles about the difficulties of getting the VLTI working, in an environment where you can work on the real hardware after it was designed and where you can bolt things to foundations sunk into Andean bedrock.

Links would be appreciated.

Originally Posted by fivemack

Nobody's got close to a design which would get wavefront combining between free-flying spacecraft; microwave ranging between free-fliers has been done quite effectively for GRACE and GRAIL, but that's a hundred times worse precision than would be needed to get fringes.

But, as you say, NASA spent a billion dollars trying to do this.

WTF?

Originally Posted by fivemack

Interferometry is very, very different from synthetic stereo vision.

I'm very aware of that.

What has produced the better results?

[fivemack]

Originally Posted by xilman

Another mission: fly a soft-gamma ray observatory with a high-resolution (arcsec or better) telescope optimized for the 1.809MeV gammas emitted by the decay of ²⁶Al

Does anyone know how to make optics for that line? NuSTAR is state of the art with depth-graded multilayers, and manages 40 arc-second resolution at 80keV; the papers about unexpectedly high refractive indices of silicon lenses at 2MeV suggest that the 'unexpectedly high' value is still something like n=1+1e-6, at which point you're talking separate lens and detector spacecraft at the very least.

[fivemack]

Originally Posted by chalsall

Links would be appreciated

http://arxiv.org/pdf/0807.1668.pdf is quite a nice overview of the SIM-Lite mission concept design. http://arxiv.org/pdf/0708.3953v2.pdf is more a summary of the things that astronomers would be able to do given SIM-Lite or equivalent.

The 'PIONIER' article in http://www.eso.org/sci/publications/...nger-no146.pdf, the GRAVITY article at http://www.eso.org/sci/publications/...o143-16-24.pdf and the earlier article at http://www.eso.org/sci/publications/...o137-25-29.pdf give some idea of the technical difficulties of getting the VLTI to work; VLTI interferometry has
resolved the disc of the red-giant component in symbiotic binary systems. Probably the most visually appealing optical-interferometry example is the CHARA Array image of the eclipsing disc at Epsilon Aurigae: http://www.gsu.edu/41129.html


NASA spent a lot of money (it was in fact nearer $200 million than $1 billion) doing technology development for microarcsecond astrometry from large optical-bench structures; interferometry between free-flying spacecraft was always running at much more of a wish-list level.

[chalsall]

Originally Posted by fivemack

http://arxiv.org/pdf/0807.1668.pdf is quite a nice overview of the SIM-Lite mission concept design. http://arxiv.org/pdf/0708.3953v2.pdf is more a summary of the things that astronomers would be able to do given SIM-Lite or equivalent.

NASA spent a lot of money (it was in fact nearer $200 million than $1 billion) doing technology development for microarcsecond astrometry from large optical-bench structures; interferometry between free-flying spacecraft was always running at much more of a wish-list level.

OK. Cool. Thanks.

Did anyone run an analysis as to what is more effective -- interferometry between closely spaced observers (the closer, the more error) or synthetic observations greatly distanced?

[Uncwilly]

Originally Posted by fivemack

They are free white elephants - the missions they'd be really useful for are ones where they're pointed at Earth, and that's explicitly forbidden by the terms of the deal handing them over.

How about putting one into lunar orbit and looking down there?
Take the other one, put it out in an orbit around E-M L2. Have it dedicated to Hubble deep field type projects. The data thus accumulated could integrated on-board before transmission.

Originally Posted by fivemack

NASA spent a lot of money (it was in fact nearer $200 million than $1 billion) doing technology development for microarcsecond astrometry from large optical-bench structures; interferometry between free-flying spacecraft was always running at much more of a wish-list level.

That is part of the reason that I suggested that they be physically linked together.

[xilman]

Originally Posted by fivemack

Does anyone know how to make optics for that line? NuSTAR is state of the art with depth-graded multilayers, and manages 40 arc-second resolution at 80keV; the papers about unexpectedly high refractive indices of silicon lenses at 2MeV suggest that the 'unexpectedly high' value is still something like n=1+1e-6, at which point you're talking separate lens and detector spacecraft at the very least.

Grazing incidence reflectors? Masks?

It's not something I've put any great resources into finding out the best technology so arcsec resolution may be impractical within budget. Even arcmin would be orders of magnitude better than what we have now.

[xilman]

Originally Posted by xilman

19G miles is 30G km in more conventional units and 20 AU is even more conventional in this area. It's about the right order of magnitude in that Uranus is about that distance from us and the Voyager probes took about that length of time to get there, though not in a straight line of course. The distance they actually travelled to reach Uranus was significantly greater.

Oops! Major cock-up

One AU is 0.15G km, so 30G km is 200AU, not 20AU as posted and used in subsequent burblings.

It doesn't make too much difference to the maximum solar distance as much of the velocity is tangential (as I did note for the Voyager missions) but it's still an embarrassing OoM error.