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 2012-10-23, 07:24 #34 retina Undefined     "The unspeakable one" Jun 2006 My evil lair 10101110111112 Posts Random idea. Will it be practical? Build some (really) large mirrors and put them in orbit around the Sun. The angle of reflection will be adjusted by small thrusters to keep the reflected sunlight pointed towards AC Bb. Put a light-sail, and the associated payload, in the path of the sunbeams and surf your way to AC Bb. You can also use the same sunbeams to return to Earth if desired and/or required. Last fiddled with by retina on 2012-10-23 at 07:35
2012-10-23, 07:53   #35
xilman
Bamboozled!

May 2003
Down not across

1015710 Posts

Quote:
 Originally Posted by fivemack An RTG of the ASRG type has a half-life of 80 years, weighs 20kg and produces 140 watts; unfortunately it has moving parts. If we use something with a half-life of 8,000 years instead (americium-243 or curium-245), it would weigh two tons for the same power output; and after 40,000 years we'd still have three watts.
To be absolutely clear, I fully support your arguments about the feasibility of building something which has a usable lifetime of 40K years, especially something which contains moving parts.

However, the communications problem need not be so severe as you imply.

A 3W power supply could communicate over a few light years using feasible antennae, perhaps in the 100m class. The bandwidth would be very low but one bit per annum would be plenty during most of the cruise phase. The message conveyed, of course, is "I am still alive". The probe saves up its 3Wy (100MJ at this level of approximation) and sends it out in a modulated 1 second burst at an appropriate wavelength. A 100MW signal can be seen with ease.

When the probe reaches its destination it could collect as much sunlight as it needs to phone home. I doubt that anyone would be listening.

2012-10-23, 08:03   #36
xilman
Bamboozled!

May 2003
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7×1,451 Posts

Quote:
 Originally Posted by retina Build some (really) large mirrors and put them in orbit around the Sun. The angle of reflection will be adjusted by small thrusters to keep the reflected sunlight pointed towards AC Bb. Put a light-sail, and the associated payload, in the path of the sunbeams and surf your way to AC Bb. You can also use the same sunbeams to return to Earth if desired and/or required.
It will work. It won't meet the ludicrously tight time and financial constraints. We have enough trouble putting small mirrors into LEO.

You don't need the thrusters. The mirrors are themselves sails and can be steered appropriately.

Unfortunately, the thrust at the probe will be very small indeed, not much greater than using the raw sunlight directly.

Better would be to use the solar collectors to power lasers which are then used to propel the probe's sail. Coherent radiation is a much better way of transferring energy and momentum than incoherent sunlight, even after allowing for inefficiency in the laser generation.

As everyone doubtless now realises, people have been doing very serious thinking about a mission to nearby stars for several decades. I've been following their activity in some detail for over 30 years. Most of the obvious solutions using near-future technology have probably been investigated by now.

Paul

2012-10-23, 08:15   #37
fivemack
(loop (#_fork))

Feb 2006
Cambridge, England

18D116 Posts

Quote:
 Originally Posted by Andi47 Just two questions: * What was the cost of Apollo 11 in today's money? * What was the cost of the Curiosity mission (let's say until the end of primary mission, not taking in account any extension of the mission)?
"the cost of Apollo 11" isn't terribly well-defined, because so much of the programme was devoted to technology development and there was some degree of series production: the unit is the Apollo programme, which was $20 billion then-dollars. Assuming all the cost was incurred in 1966, that's 140 billion now-dollars. Developing the rockets which weren't used for anything post-Apollo was about a third of that spending. (an interesting point is that the$20 billion was about half of NASA's total spending 1960-1972)

Curiosity is \$2.5 billion now-dollars total programme cost from initiation through the end of year two on Mars, though obviously it was launched on a rocket of a kind that didn't have to be developed specially.

Last fiddled with by fivemack on 2012-10-23 at 08:26

2012-10-23, 08:15   #38
xilman
Bamboozled!

May 2003
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7·1,451 Posts

Quote:
 Originally Posted by fivemack So this is a probe consisting of two tons of ludicrously over-specced radioisotope generator and let's say two tons of huge antenna
A 100m antenna has an area of \pi (100*100)/4 square metres; call it 10^4 m^2. Make it out of metallized mylar 1 micron (10^{-6} m) thick. Total volume 0.01 m^3, with mass of 20kg. Ok, so you need struts and guy ropes but you should be able to keep the antenna mass down well below two tons.

 2012-10-23, 08:20 #39 fivemack (loop (#_fork))     Feb 2006 Cambridge, England 18D116 Posts As you noticed I wrote the note at 2am and it may not have been as well-argued as possible, I should have been clearer about where I was purely waving my hands and where I was googling wildly and then waving my hands. I got two tons as a reasonable payload fraction of the eleven tons that a Delta IV Heavy is specified as launching to GTO, since USA-202 went up on one of those and was advertised as being really big and heavy (pretty much in those words - NRO is not a loquacious organisation) (I'd have multiplied the weight of the Terrestar-1 antenna by (100/18)^2.mumble but I can't find that weight figure either) Last fiddled with by fivemack on 2012-10-23 at 08:21
2012-10-23, 16:41   #40
Andi47

Oct 2004
Austria

2×17×73 Posts

Quote:
 Originally Posted by xilman Oh, I can design one all right, well under budget and well within the time limit. The point is that the mission is then pointless. Throwing an upper stage rocket to Alpha Cen is straightforward and it will get there in a few tens of kiloyears. So?
Hmmmm... for me it seems that if we want to stay within the time limit of 15 years up to start (or let's say up to "ready for start and waiting for a good constellation for planetary slingshot"), then we have to use conventional rocket technology, i.e. such as Ariane 5 or maybe Saturn V. So it will be several kiloyears to Alpha Centauri... (It won't be much faster than the New Horizons probe)

The point is: If we set up a mission, we (or our grand-grand-grand³...grandchildren) want data, even if it lasts 40 kiloyears or so... So how to construct a probe which is still alive and operational when it arrives at Alpha Cen?
As already stated, we can't use any moving parts, because after kiloyears, they will be stuck/jammed/cold welded/whatever.

So the first problem is (let's assume that the energy supply is solved by a BIG radionuclide battery + solar panels*) ): How to keep the probe stable for THAT long time, i.e. keep the antenna pointing to earth? Will the probe stay in a stable position for THAT long time even if it's rotating itself around an axis pointing to earth? You can't use Gyro-stabilisation, because that's moving parts, and you don't have infinite fuel for thruster-stabilisation...

Second: When the probe arrives, you want at least to take a few pictures of the Alpha Centauri System and count the big dots or even better to take photos of a planet. So how to adjust and operate a camera (or other scientific instruments) after tens of kiloyears?

*) BTW: Will a solar panel still be in good operating condition after tens of kiloyears? You can't deploy it after tens of kiloyears because after that time any moving parts are probably dead, so you have to deploy it when the probe is still in (or close to) our solar system. And what will cosmic radiation do to a solar panel in kiloyears? What will it do to a computer within that time?

2012-10-23, 16:56   #41
science_man_88

"Forget I exist"
Jul 2009
Dumbassville

26×131 Posts

Quote:
 Originally Posted by Andi47 Hmmmm... for me it seems that if we want to stay within the time limit of 15 years up to start (or let's say up to "ready for start and waiting for a good constellation for planetary slingshot"), then we have to use conventional rocket technology, i.e. such as Ariane 5 or maybe Saturn V. So it will be several kiloyears to Alpha Centauri... (It won't be much faster than the New Horizons probe) The point is: If we set up a mission, we (or our grand-grand-grand³...grandchildren) want data, even if it lasts 40 kiloyears or so... So how to construct a probe which is still alive and operational when it arrives at Alpha Cen? As already stated, we can't use any moving parts, because after kiloyears, they will be stuck/jammed/cold welded/whatever. So the first problem is (let's assume that the energy supply is solved by a BIG radionuclide battery + solar panels*) ): How to keep the probe stable for THAT long time, i.e. keep the antenna pointing to earth? Will the probe stay in a stable position for THAT long time even if it's rotating itself around an axis pointing to earth? You can't use Gyro-stabilisation, because that's moving parts, and you don't have infinite fuel for thruster-stabilisation... Second: When the probe arrives, you want at least to take a few pictures of the Alpha Centauri System and count the big dots or even better to take photos of a planet. So how to adjust and operate a camera (or other scientific instruments) after tens of kiloyears? *) BTW: Will a solar panel still be in good operating condition after tens of kiloyears? You can't deploy it after tens of kiloyears because after that time any moving parts are probably dead, so you have to deploy it when the probe is still in (or close to) our solar system. And what will cosmic radiation do to a solar panel in kiloyears? What will it do to a computer within that time?
only possibility I see is a unmanned or very high speed (talking like .9c), as to the moving parts problem could self replicating machines work long enough to replicate machine parts ?

2012-10-23, 17:00   #42
xilman
Bamboozled!

May 2003
Down not across

7×1,451 Posts

Quote:
 Originally Posted by Andi47 Hmmmm... for me it seems that if we want to stay within the time limit of 15 years up to start (or let's say up to "ready for start and waiting for a good constellation for planetary slingshot"), then we have to use conventional rocket technology, i.e. such as Ariane 5 or maybe Saturn V. So it will be several kiloyears to Alpha Centauri... (It won't be much faster than the New Horizons probe) The point is: If we set up a mission, we (or our grand-grand-grand³...grandchildren) want data, even if it lasts 40 kiloyears or so... So how to construct a probe which is still alive and operational when it arrives at Alpha Cen? As already stated, we can't use any moving parts, because after kiloyears, they will be stuck/jammed/cold welded/whatever. So the first problem is (let's assume that the energy supply is solved by a BIG radionuclide battery + solar panels*) ): How to keep the probe stable for THAT long time, i.e. keep the antenna pointing to earth? Will the probe stay in a stable position for THAT long time even if it's rotating itself around an axis pointing to earth? You can't use Gyro-stabilisation, because that's moving parts, and you don't have infinite fuel for thruster-stabilisation... Second: When the probe arrives, you want at least to take a few pictures of the Alpha Centauri System and count the big dots or even better to take photos of a planet. So how to adjust and operate a camera (or other scientific instruments) after tens of kiloyears? *) BTW: Will a solar panel still be in good operating condition after tens of kiloyears? You can't deploy it after tens of kiloyears because after that time any moving parts are probably dead, so you have to deploy it when the probe is still in (or close to) our solar system. And what will cosmic radiation do to a solar panel in kiloyears? What will it do to a computer within that time?
All your questions are good ones. AFAICT, none of them are solvable with current technology.

Some artefacts are still usable after tens of kiloyears. They invariably have at most one moving part (hammer and anvil, for example) and have not had any significant amount of use since manufacture. I own one, a dual-edged flint hand-axe and scraper, which still works just fine. I doubt it has otherwise been used in the last 5000 years. To be fair the primitive nature of the tools which still exist is a measure of the technological level of their creators but, even so, tens of millennia is way beyond what current engineers would expect to be possible for something capable of sending data a parsec or more.

In many ways the ISM is much more benign than the surface of the earth but it's still not harmless. Things evaporate in hard vacuum. Cosmic rays cause damage, only a small part of which is self-repairing. Thermal cycling (from at least 300K at departure and arrival, to 3K mid-voyage) has deleterious effects. Abrasion from dust impacts will cause damage to delicate components such as large but thin sails and antennae.

Paul

2012-10-23, 17:03   #43
xilman
Bamboozled!

May 2003
Down not across

7·1,451 Posts

Quote:
 Originally Posted by Andi47 So the first problem is (let's assume that the energy supply is solved by a BIG radionuclide battery + solar panels*) ): How to keep the probe stable for THAT long time, i.e. keep the antenna pointing to earth? Will the probe stay in a stable position for THAT long time even if it's rotating itself around an axis pointing to earth? You can't use Gyro-stabilisation, because that's moving parts, and you don't have infinite fuel for thruster-stabilisation...
In principle you could use the PSU to drive a magnetic tether working against the galactic magnetic field. No moving parts.

 2012-10-23, 17:26 #44 Andi47     Oct 2004 Austria 1001101100102 Posts Another thing: What about electromigration? Will any electric circuit be operable after tens of millennia, especially, if it is constantly in use, e.g. for magnetic tethers or phoning home?

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