Pluto down, next stop...interstellar?

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sevenperforce
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Pluto down, next stop...interstellar?

Postby sevenperforce » Thu Jul 16, 2015 6:20 pm UTC

Now that we've sent a probe past the (former) final planet in our own solar system, why not look beyond?

The nearest potentially habitable extrasolar planet, Gliese 581d, is really not that far away. Just a paltry 20.2 lightyears.

NASA's SLS Block 2 is proposed to be able to deliver 130 tonnes to LEO, and it's likely that SpaceX's Raptor rockets for the Mars Colonial Transporter would have a comparable payload. It's not unlikely that our species could get a 200-tonne-capacity lift vehicle operational within the next decade.

With a 200-tonne spacecraft intended for extended operation in deep interstellar space, what's the maximum attainable delta-v using current tech? Assume that the ship is operating outside of our solar system (a separate 200-tonne booster craft could be responsible for lifting it out of LEO and putting it on a hyperbolic solar exit trajectory). It has to contain all its fuel and power supplies. The weight of the actual probe will likely be negligible compared to the weight of the engines, fuel tanks, and power supplies.

Would it be possible to get a picture of Gliese 581d sent back to Earth before the end of the 21st century?

The Dual-Stage 4-Grid ion thruster has an exhaust velocity of 210 km/s. It is limited, unfortunately, by its extraordinarily high power consumption, requiring 250 kW of power to produce just 2.5 N. A series of thermoelectric nuclear reactors could do the trick, but they'd need quite a bit of fuel. There's probably some exact balance at which the maximum delta-v for a given spacecraft mass is achieved (reaction mass vs power supply vs nuclear fuel vs engine weight), taking into account the duration of burn time.

Let's say we could launch in 2025. The ship would need to get to Gliese 581d in under 55 years, meaning it would need to exceed 0.37c. Using the modified Tsiolkovsky equation relating rapidity to mass ratio, a rapidity of 0.4 is required. Since 210 km/s is 0.07% of c, we'd need a mass fraction of 1:9.9e247. Nope, not gonna work.

I should have known, anyway, given that the relativistic kinetic energy of a 1-tonne probe moving at 0.37c is 6.8e19 J, roughly the same as the annual electric energy production of the entire globe. You'd need 120 tonnes of D-T fusion fuel just to store that much energy.

So, using any combination of modern tech, what's the best-case scenario for transit time to Gliese 581d?

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Re: Pluto down, next stop...interstellar?

Postby thoughtfully » Thu Jul 16, 2015 6:43 pm UTC

Far away from habitation or anything else you don't want irradiated, a nuclear salt water rocket is probably a good bet for combining ridiculous amounts of thrust and specific impulse. Cheers!

http://www.projectrho.com/public_html/r ... Salt_Water
http://www.npl.washington.edu/AV/altvw56.html

The tl;dr on these is a continuously detonating Project Orion!
The creator of this design also describes plans for a 120 year mission to Alpha Centauri. I suppose getting to the Gliese system would take rather longer.

The nuclear light bulb is rather amazing and aims to keep all the icky stuff inside, but would require quite a lot of R&D and looks a bit dubious when the engineering challenges are confronted.
http://www.projectrho.com/public_html/r ... rgasclosed

There was actually ground testing of nuclear rocket engines in the 60s and 70s.
https://www.youtube.com/watch?v=GmxPRCyR-Co
https://en.wikipedia.org/wiki/NERVA


Anyway, Pluto isn't the last planet (Neptune is!) and it has lots of Kuiper Belt dwarf-planet siblings to explore, and there's the Oort cloud beyond that. Oh my!
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Re: Pluto down, next stop...interstellar?

Postby sevenperforce » Thu Jul 16, 2015 9:17 pm UTC

thoughtfully wrote:Pluto isn't the last planet (Neptune is!) and it has lots of Kuiper Belt dwarf-planet siblings to explore, and there's the Oort cloud beyond that. Oh my!

Well, that's why I said former final planet. Yes, we do have a lot of other Kuiper Belt dwarf-planet siblings to explore, but none of them were ever seriously considered to be planets. And since the entire mass of the Oort Cloud is probably less than five Earth masses, the chances of us spotting something way out there is pretty slim.

If we assume a launch date of 2025 and require a "phone home" to arrive on December 31, 2099, then the transit figures within the local neighborhood look something like this:

  • Alpha Centauri System (nearest stars), 4.3 ly: requires 0.06c, a mass fraction of 5e37 for ion thrusters, and would have an arrival kinetic energy of 1.7e17 J/tonne
  • Barnard's Star (nearest sun-like star), 5.98 ly: requires 0.09c, a mass fraction of 6e53 for ion thrusters, and would have an arrival kinetic energy of 1.7e17 J/tonne
  • Luhman 16 (nearest brown dwarf), 6.59 ly: requires 0.1c, a mass fraction of 6e59 for ion thrusters, and would have an arrival kinetic energy of 3.4e17 J/tonne
  • Epsilon Eridani b (nearest nearly-confirmed exoplanet), 10.5 ly: requires 0.16c, a mass fraction of 1e101 for ion thrusters, and would have an arrival kinetic energy of 1.2e18 J/tonne
  • EZ Aquarrii (nearest spectroscopic binary), 11.3 ly: requires 0.18c, a mass fraction of 1e110 for ion thrusters, and would have an arrival kinetic energy of 1.4e18 J/tonne
  • Kapteyn b (closest potentially habitable exoplanet), 12.8 ly: requires 0.06c, a mass fraction of 5e37 for ion thrusters, and would have an arrival kinetic energy of 1.2e18 J/tonne
  • Gliese 832 c (nearest most Earth-like exoplanet), 16.1 ly: requires 0.21c, a mass fraction of 5e127 for ion thrusters, and would have an arrival kinetic energy of 1.9e18 J/tonne
  • Fomalhaut b (nearest directly-imaged exoplanet), 25 ly: requires 0.5c, an infinite mass fraction for ion thrusters, and would have an arrival kinetic energy of 1.1e19 J/tonne
I think we're going to need a bigger ship.

I wonder how much additional speed could be picked up from a hyperbolic solar pass with maximal use of the Oberth effect and riding a solar sail out of the system.

Far away from habitation or anything else you don't want irradiated, a nuclear salt water rocket is probably a good bet for combining ridiculous amounts of thrust and specific impulse. Cheers!

According to Wikipedia, there's a design capable of 4,700 km/s exhaust velocity. That's mind-boggling...but if it would actually work, then that would be able to get us a phone-home signal from Alpha Centauri using a 200-tonne ship by the year 2091 (assuming a mass fraction for the 100-tonne stage of 1:85). Of course that would take 197.6 tonnes of fuel, and I'm not sure how much plutonium we have on hand.

Does the Oberth effect accumulate over long periods of continuous thrust or is it only really useful during gravity assists?

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Re: Pluto down, next stop...interstellar?

Postby SDK » Fri Jul 17, 2015 2:35 pm UTC

sevenperforce wrote:Does the Oberth effect accumulate over long periods of continuous thrust or is it only really useful during gravity assists?

The Oberth Effect is specifically for use while in a gravity well, so I'm not sure what you're asking.

0.06c is so far beyond what we've ever done, 18 000 km/s, that I think if you're looking to interstellar travel with current technology you're going to have to accept that this is going to take more than a century or two. Materials to provide shielding at those speeds is something that we have no real data on (or even a method to get data?), and we have no idea how much dust and ice is really out there, so I'd suggest baby steps here so we don't end up just shredding this craft after spending all that effort getting it up to speed.

Looks like a couple of numbers in your list are jumbled, by the way.
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Re: Pluto down, next stop...interstellar?

Postby Whizbang » Fri Jul 17, 2015 2:44 pm UTC

SDK wrote:
sevenperforce wrote:baby steps


Step one, collect underwear.

:D

So what do you imagine the first extra-solar mission to be? Just send a probe out into deep space and have it send back data on it's own condition, with maybe some data on it's nearby (realtively) surroundings?

What can be done besides just making the attempt? If the probe gets shredded, that at least will be some data to use for the next mission. If it somehow makes it, then gravy. Or are you proposing we explore (and develop?) our solar system much more before we attempt to go outside with any probes?

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Re: Pluto down, next stop...interstellar?

Postby PolakoVoador » Fri Jul 17, 2015 4:42 pm UTC

There's so much stuff we can look closer in our own neighbourhood before looking for extra-solar targets. I mean, we can try to throw something at Titan lakes and see what's in there! Or maybe go under the ice layer of Europa and check that potencial juicy ocean?

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Re: Pluto down, next stop...interstellar?

Postby SDK » Fri Jul 17, 2015 6:16 pm UTC

Quick calculation of a 1 milligram dust particle (about 0.4 mm in diameter assuming 2 g/cm^3 density - a large piece of dust, but still super tiny) hitting you at 0.1c: 1/2 mv^2 = 1/2*(0.000001 kg)*(29 979 245.8 m/s)^2 = 449,377,589 J = approximately 200 pounds of TNT. Lord help you if you happen to hit a 1 cm diameter particle (8 grams) - that's a thousand tons of TNT. For comparison with today's speeds, that same 1 cm pebble hits the space shuttle with only 256 kJ of energy... about twice as much as a baseball pitch. This is a problem on a completely different scale than we've ever dealt with before.

Thanks to this, I think figuring out how to surivive the trip is going to be even harder than propulsion. I want to explore other stars too, but throwing a craft out there at a thousand times faster than we've ever moved before is very likely just going to be a waste of fuel. Maybe we'll get lucky and the craft will hit nothing but microscopic interstellar dust - enough to damage the hull, but not destroy it. We'll have to solve this problem eventually, though, because we don't want the survival of potential colonists to be dependant on "luck".

I guess what I'd like to do is throw a probe out there at something like 0.001c (300 km/s - still much faster than we've ever gone!). I would say "towards Alpha Centaui", except that this trip would take 4000 years, so there might be another target that makes more sense. In any case, the exact speed we're travelling would depend on fuel considerations, because I want this probe to have substantial energy reserves so that it can take measurements of the interstellar medium. That might mean just measuring vibrations caused by impacts, or maybe even taking samples over a few thousand square kilometers as it travels. Whatever the case, I want to know what we'd need to deal with as far as shielding goes so we can model and develop a way to shield a craft moving substantially faster.

... aaaand, after writing this I remember that NASA's already on the job! Stardust's Sample Return Capsule returned to Earth with a sample of interplanetary dust as well as dust from a comet back in 2006. Reading more on that, it sounds like we might be in trouble here. "Ten particles were found to be at least 100 micrometers (0.1 mm) and the largest approximately 1000 micrometers (1 mm)". That single 1 mm particle would be equivalent to a bomb's full explosion being concentrated to a 1 mm point on your hull if you're moving at 0.1c. Time to get to work on force fields, 'cause no material is going to withstand that...
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Re: Pluto down, next stop...interstellar?

Postby sevenperforce » Fri Jul 17, 2015 8:08 pm UTC

A little more calculation on the possibility of heading to Alpha Centauri by the turn of the 22nd century...

The Alpha Centauri system is inclined at -61 degrees from the invariable plane, so we'd want to send our craft on a hyperbolic trajectory around the sun from the opposite direction (119 degrees) to maximize eccentricity. We could probably use a spent fuel tank (the one used for intrasystem staging) as a heat shield, allowing the craft to come within a million miles of the sun's surface. Rather than trying to burn into a 61-degree inclination, it would be wise to use a series of gravitational slingshots in our solar approach. The high orbital inclination of the two innermost planets would appear to allow us to use a wider-opening hyperbola, but their gravities are insufficient to provide a tight enough turn at the high speeds we'll be dealing with.

Our best bet is to take gravity assists on a wide loop out around Saturn, then accelerate inward toward Jupiter for a final gravity assist toward the Sun at the proper trajectory. Jupiter has a negligible inclination from the invariable plane, so our hyperbola around the sun will be that 119 degrees. This means our periapse trajectory at the sun will be a sizzling 414 km/s. This yields an excess hyperbolic velocity of 237 km/s, which is a great start for our trip to Alpha Centauri but might prove difficult to achieve inside the solar system.

A "Grand Tour" trajectory yields the maximum delta-v from each planet, but requires a precise alignment -- basically, we need to describe a consistent spiral from the inner planets to the outer planets:

grand tour.png
grand tour.png (5.78 KiB) Viewed 5979 times


If such an arrangement were possible in the desired timeframe, we could ostensibly pick up the orbital velocities of all the planets out to Saturn, with a double dip on spry Mercury, to give us 207 km/s for "free". Now, that precise arrangement isn't terribly likely, but we might well come up with something close to it in the next two decades simply because all the inner planets orbit so fast. If we could get half of this speed on gravity assists alone, for a total of 104 km/s, we'd only need to come up with 134 km/s on our own. However, because we get to add in Jupiter again at the end when we're on our way back, this drops to 121 km/s.

Thankfully, we don't have to come up with all that on our own. We would definitely want to deploy a massive solar sail immediately after our hyperbolic exit in order to gain as much speed from the sun as possible. However, that makes for a tricky catch-22; should we fire our nuclear engines immediately after periapse? The Oberth effect says that's when they'll get the best results, but if we move away from the sun too quickly then we'll lose the chance to accelerate using the solar sail. The solution is to make the solar sail large enough to maintain our periapse velocity out past the solar system.

Gravity and light pressure both fall off as the inverse square of the distance from the sun. A lattice sailer supposedly has an acceleration ratio on the order of 20, enabling it to "lift" 20 times its own mass against the sun's gravity wherever it is.

I'm thinking two 200-tonne payloads, assembled in LEO. Payload 2 is merely the non-rigid fuel tank intended for interstellar travel. Payload 1 comprises the engines with an ablative propulsive heat shield, the science payload, the solar sail, and rigid fuel tanks for in-system maneuvers which double as a heat shields during final periapse approach and impact shields thereafter:

interstellar.png


To offset the pull of gravity and maintain speed following the hyperbolic exit, the sail needs to be able to lift the larger fuel tank, the science package, the structure, and the engines, plus itself. We were going to go with a 1:85 mass fraction, which means we're allowed 2.3 tonnes for the science package, the engines, and the structure. Lifting 202.3 tonnes with a lattice sailer will require a sail weighing 10.1 tonnes; it will be able to provide acceleration initially equal to 0.9 times that of gravity during the Grand Tour stage.

A little integration reveals that this is equivalent to 48.69 km/s gained by the solar sail on the way out through the Grand Tour (moving from Mercury to Saturn). We now only need a delta-v of 72.31 km/s from our own engines. We need to add about 6 km/s to lift us from LEO out of Earth's gravity well, but we can also pick up 17.58 km/s from an in-flying gravity assist from Venus and Mercury, so we end up only needing 61 km/s from our fuel stores. Using that same 4,700 km/s saltwater nuclear rocket fuel, we'd only consume 5.3 tonnes of fuel to get that much delta-v.

We're left with 182 tonnes for the ablative propulsive heat shield, rigid fuel tanks, and fuel. If only half of that is fuel, then we've increased our effective final-stage mass fraction to 1:127, which gives us a final delta-V of 22,790 km/s. This, of course, is on top of our hyperbolic exit velocity of 414 km/s, which combine to launch us out toward Alpha Centauri at a dizzying 7.7% of lightspeed for a travel time of only 55 years.

SDK wrote:
sevenperforce wrote:Does the Oberth effect accumulate over long periods of continuous thrust or is it only really useful during gravity assists?

The Oberth Effect is specifically for use while in a gravity well, so I'm not sure what you're asking.

I was trying to think about whether the increase in kinetic energy of the remaining fuel has an effect, but then I realized that's the whole point of the rocket equation.

0.06c is so far beyond what we've ever done, 18 000 km/s, that I think if you're looking to interstellar travel with current technology you're going to have to accept that this is going to take more than a century or two.

I refuse!

Materials to provide shielding at those speeds is something that we have no real data on...

Whipple shields plus more whipple shields?

PolakoVoador wrote:There's so much stuff we can look closer in our own neighbourhood before looking for extra-solar targets. I mean, we can try to throw something at Titan lakes and see what's in there! Or maybe go under the ice layer of Europa and check that potencial juicy ocean?

Yes, definitely! And also go send a probe to Alpha Centauri too!

SDK wrote:Thanks to this, I think figuring out how to surivive the trip is going to be even harder than propulsion. I want to explore other stars too, but throwing a craft out there at a thousand times faster than we've ever moved before is very likely just going to be a waste of fuel. Maybe we'll get lucky and the craft will hit nothing but microscopic interstellar dust - enough to damage the hull, but not destroy it. We'll have to solve this problem eventually, though, because we don't want the survival of potential colonists to be dependant on "luck".

"Ten particles were found to be at least 100 micrometers (0.1 mm) and the largest approximately 1000 micrometers (1 mm)". That single 1 mm particle would be equivalent to a bomb's full explosion being concentrated to a 1 mm point on your hull if you're moving at 0.1c. Time to get to work on force fields, 'cause no material is going to withstand that...

I think that as long as you can avoid penetration, you're fine. Thanks to Newton's approximation for the penetration depth of a kinetic impactor, we know that penetration depth depends almost solely on the relative densities of the impactor and the target. As long as your foreshield is made of many space-separated layers of something that's denser and thicker than dust in the interstellar medium, we should be okay.

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Re: Pluto down, next stop...interstellar?

Postby SDK » Fri Jul 17, 2015 9:52 pm UTC

Does Newton's approximation hold up to 18000 km/s? No one has ever observed a macroscale impact at anywhere near those speeds - we're talking about bombs of kinetic energy here. Standard testing of whipple shields appears to be in the neighbourhood of 10 km/s, which is about right for what it's going to see during operation. That's pretty super fast, but not even close to this case. This is what I meant when I said in my first post that there's not even a good way to get data on material performance at these speeds. We'll be relying on models and hoping that what shielding we could come up with was modelled well enough to protect our trillion dollar investment.
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Re: Pluto down, next stop...interstellar?

Postby Zamfir » Sat Jul 18, 2015 8:29 am UTC

The 4700 km/s nuclear rocket is rather theoretical. It's really a bait-and-switch idea. First, the NSWR is supposed to avoid the craziness of project Orion, where the spaceship needs to windstand full nuclesr explosions. To do so, the salt water idea is presented at 'mild' settings. That works out to a concept that resembles chemical rockets: a contiuous controlled explosion with a nozzle to direct and expand the exhaust flow in the desired direction.With an exhaust velocity of 66km/s. More intense than chemical rockets, but not absolutely incredibly so. Handwavily, it's plausible that you could build a nozzle to withstand that intensity.

Then in step 2, he does away with all mild assuptions, and goes for near perfect fission of near pure uranium U233. That gives the 4700km/s. But obviously, we're now back at the project orion problem. The 'nozzle' now has to withstand the full circumstances of a maxed out nuclear explosion. It's supposed to take those ~4700km/s explosion products, and direct them backwards. For 10,000 bombs in succesion.

I see you've budgetted 2.3 tonnes for that nozzle ;-)

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Re: Pluto down, next stop...interstellar?

Postby Neil_Boekend » Sat Jul 18, 2015 10:50 am UTC

SDK wrote:
Spoiler:
Quick calculation of a 1 milligram dust particle (about 0.4 mm in diameter assuming 2 g/cm^3 density - a large piece of dust, but still super tiny) hitting you at 0.1c: 1/2 mv^2 = 1/2*(0.000001 kg)*(29 979 245.8 m/s)^2 = 449,377,589 J = approximately 200 pounds of TNT. Lord help you if you happen to hit a 1 cm diameter particle (8 grams) - that's a thousand tons of TNT. For comparison with today's speeds, that same 1 cm pebble hits the space shuttle with only 256 kJ of energy... about twice as much as a baseball pitch. This is a problem on a completely different scale than we've ever dealt with before.

Thanks to this, I think figuring out how to surivive the trip is going to be even harder than propulsion. I want to explore other stars too, but throwing a craft out there at a thousand times faster than we've ever moved before is very likely just going to be a waste of fuel. Maybe we'll get lucky and the craft will hit nothing but microscopic interstellar dust - enough to damage the hull, but not destroy it. We'll have to solve this problem eventually, though, because we don't want the survival of potential colonists to be dependant on "luck".

I guess what I'd like to do is throw a probe out there at something like 0.001c (300 km/s - still much faster than we've ever gone!). I would say "towards Alpha Centaui", except that this trip would take 4000 years, so there might be another target that makes more sense. In any case, the exact speed we're travelling would depend on fuel considerations, because I want this probe to have substantial energy reserves so that it can take measurements of the interstellar medium. That might mean just measuring vibrations caused by impacts, or maybe even taking samples over a few thousand square kilometers as it travels. Whatever the case, I want to know what we'd need to deal with as far as shielding goes so we can model and develop a way to shield a craft moving substantially faster.

... aaaand, after writing this I remember that NASA's already on the job! Stardust's Sample Return Capsule returned to Earth with a sample of interplanetary dust as well as dust from a comet back in 2006. Reading more on that, it sounds like we might be in trouble here. "Ten particles were found to be at least 100 micrometers (0.1 mm) and the largest approximately 1000 micrometers (1 mm)". That single 1 mm particle would be equivalent to a bomb's full explosion being concentrated to a 1 mm point on your hull if you're moving at 0.1c. Time to get to work on force fields, 'cause no material is going to withstand that...

That 1 mm dust particle was in the aerogel. That sample system was used inside the coma of a comet. Normal space is more empty. Sufficiently so that there is no problem with New Horizons while a 1mm particle would have damaged the craft significantly (that gold foil is not up to it) despite the relatively low speed (23 km/sec). A 1 mg particle would have had a kinetic energy of 264.5 J, or double the energy of a .22 LR pistol round (can be lethal). No critical part has been hit by such a particle.

SDK wrote:Does Newton's approximation hold up to 18000 km/s?

No. Even high strength steel behaves like a liquid in those types of impacts. NASA calls it hypervelocity impacts. They aren't even close to 18000 km/s but even at a paltry 8.5 km/s the materials behave very differently. According to the wikipedia page the strength of the material is very small compared to the stresses induced by the kinetic energy dumped into it.
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Re: Pluto down, next stop...interstellar?

Postby Whizbang » Sat Jul 18, 2015 1:20 pm UTC

You just won't believe how vastly, hugely, mind-bogglingly empty it is. I mean, you may think your beer glass is empty, but that's just peanuts to space.

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Re: Pluto down, next stop...interstellar?

Postby nadando » Sun Jul 19, 2015 6:46 am UTC

How do you plan to get a signal back to earth from however many lightyears away? It's hard enough talking to Pluto.

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Re: Pluto down, next stop...interstellar?

Postby elasto » Sun Jul 19, 2015 2:49 pm UTC

nadando wrote:How do you plan to get a signal back to earth from however many lightyears away? It's hard enough talking to Pluto.

That was my first thought too: Signals coming back from Pluto have the deep black of space behind them and that's still tough enough; Signals coming from so close to a foreign star - well, if the distance alone doesn't stimie you, won't the light from that star drown out your signal?

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Re: Pluto down, next stop...interstellar?

Postby Neil_Boekend » Sun Jul 19, 2015 4:30 pm UTC

In time we may become able to use the light of that star as a means. Place a cloud of sail like objects in the path between the star and Earth. Either rotate them to block 0.00001% of the light for a zero or rotate them to let the light trough for a 1. Communication will be slow but over the years a useful amount of data can be send.
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Re: Pluto down, next stop...interstellar?

Postby >-) » Sun Jul 19, 2015 6:14 pm UTC

cant we just send signals as radio waves, or anything not in the center of the blackbody spectrum. the spectral radiance of a 5790K blackbody is 1.4E-100 W/m^2/sr/m at 10km wavelength, so we shouldn't get any interference from the stars

at 4 lightyears and with a 1kW transmitter, the aercibo telescope receives about 4E-27 W of flux. A photon with wavelength 10km is 2E-29 joules, which means we would count 200 photons/s, enough to send a signal

given the huge amount of space we have, i suppose dropping a 10 km wire to do the transmitting wont be too hard.

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Re: Pluto down, next stop...interstellar?

Postby elasto » Mon Jul 20, 2015 1:09 am UTC

>-) wrote:cant we just send signals as radio waves, or anything not in the center of the blackbody spectrum. the spectral radiance of a 5790K blackbody is 1.4E-100 W/m^2/sr/m at 10km wavelength, so we shouldn't get any interference from the stars

What about from all the bodies in that solar system though? Planets, asteroids and so on. Those could also provide random interference across frequencies.

Neil's idea seems best, but either way we're going to need a compression algorithm with a lot of redundancy to make up for the fact that there's going to be noise and lost packets, and that it'd take decades for us to instruct the probe "Sorry, didn't catch that, please resend"..!

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Re: Pluto down, next stop...interstellar?

Postby >-) » Mon Jul 20, 2015 2:19 am UTC

How would planets create interference? I know Jupiter produces radio waves but their wavelength isn't anywhere near 10 km. And if the planets in the destination system do produce radiowaves at that wavelength, we can examine it and pick a different frequency beforehand.

If it's really neccessary, with a bit of work, we could also send a bunch of large radio telescopes into solar orbit as far as jupiter, which would work as an interferometer with an angular resolution around a microradian, enough to resolve distances a fraction of an AU at 4 light-years. this should let us separate spacecraft signal from planet interference.
At that point, it might be better to just drop the radio and pulse a gigawatt maser.

---

assuming you have a thousand tons of mass to play with, and a square meter of foil weighs just a gram, and assuming you can get as close to the star as the surface, and ignoring the need for any way to control the foil, you can blanket 1E9 square meters of star.
alpha centauri has a surface area of 1.6E18 square meters (when projected onto a plane orthogonal to the line between earth and alpha centauri).
you'd have to detect a light difference of less than one part in one billion for this to work, and it seems doubtful that the energy output of the star remains stable enough for the difference not to get entirely overwhelmed by noise

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Re: Pluto down, next stop...interstellar?

Postby Neil_Boekend » Mon Jul 20, 2015 2:58 pm UTC

The reflecting system would indeed be too big.

And with modern composite radiotelescopes you could have a useful resolution, especially when combined with a couple of radiotelescopes in a large orbit around our planet. Especially when the frequency is fixed launching a few thousand cubesats with radio receivers and transmitters wouldn't put too much of a strain on the project budget, relatively speaking to the budget to get a probe 4 light years out there in 20 years. You'd need to send a lot of cubesats up to get the signal strength up to what's required.
To be more precise, you could combine the LOFAR concept and SVLBI (Space Very Long Baseline Inferometry). The LOFAR concept keeps the per sat cost low, the SVLBI provides the resolution.

An other solution is going optical. Getting a good angular resolution is easier in the optical range, although the energy required to send a strong enough signal is more difficult.
The OWL would have had an optical resolution of 1 arc milisecond, or 2.7778×10-7 degrees. That works out to a spot of approximately 1.11120 × 107 km, or about two sun diameters. If you build one of those and send the probe in an Earth sized orbit around the star you could have sufficient optical resolution to receive data for a large part of it's orbit. Granted: it would be too close to, in front of or behind the star for months at a time, but the data speed would be significantly higher. The light gathering capabilities of the OWL would allow for a relatively small laser.
You still need a massive laser and a huge power supply and a massive heat dump (massive lasers get hot), but less massive than with a composite telescope.
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Re: Pluto down, next stop...interstellar?

Postby sevenperforce » Tue Jul 21, 2015 6:05 pm UTC

SDK wrote:Does Newton's approximation hold up to 18000 km/s?

Neil_Boekend wrote:No. Even high strength steel behaves like a liquid in those types of impacts. NASA calls it hypervelocity impacts. They aren't even close to 18000 km/s but even at a paltry 8.5 km/s the materials behave very differently. According to the wikipedia page the strength of the material is very small compared to the stresses induced by the kinetic energy dumped into it.

Uhm...Newton's approximation presumes that the target behaves like a liquid -- e.g., that it deforms without cohesion based on momentum exchange alone. I'm not sure what the picture is intended to demonstrate; the depth of the crater is not necessarily the same as the penetration depth of the projectile.

Newton's approximation ought to hold until the point that the relative speed is on the order of the interaction timescale for Coulomb and degeneracy forces, at which point atoms will simply be passing through each other without interacting. But that won't happen until you get up to the high 90% end of c.

One nice (nice?) thing about a hypervelocity spacecraft is that you don't have to worry about where the projectiles are coming from. The Space Station needs shielding all over, because micrometeoroids could come from any direction, but for a hypervelocity spacecraft everything else is basically standing still. So you can design your shield with this in mind...perhaps with a momentum deflection arrangement.

Zamfir wrote:The 4700 km/s nuclear rocket is rather theoretical. It's really a bait-and-switch idea. First, the NSWR is supposed to avoid the craziness of project Orion, where the spaceship needs to withstand full nuclear explosions. Then in step 2, he does away with all mild assumptions, and goes for near perfect fission of near pure uranium U233. That gives the 4700km/s. But obviously, we're now back at the project orion problem.

Well that's not very nice.

Not quite as bad as the Project Orion problem because at least it's a continuous flow. Less shock and so forth. But ballpark, what would the pressure on that nozzle be?

elasto wrote:
nadando wrote:How do you plan to get a signal back to earth from however many lightyears away? It's hard enough talking to Pluto.

That was my first thought too: Signals coming back from Pluto have the deep black of space behind them and that's still tough enough; Signals coming from so close to a foreign star - well, if the distance alone doesn't stimie you, won't the light from that star drown out your signal?

As others have suggested, a powerful optical-spectrum laser is really the only way to have a ghost's chance of sending back communication reliably.

Neil_Boekend wrote:In time we may become able to use the light of that star as a means. Place a cloud of sail like objects in the path between the star and Earth. Either rotate them to block 0.00001% of the light for a zero or rotate them to let the light trough for a 1. Communication will be slow but over the years a useful amount of data can be send.

So basically binary/Morse Code stellar smoke signals? :)

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Re: Pluto down, next stop...interstellar?

Postby sevenperforce » Thu Jul 23, 2015 3:39 pm UTC

Rather than trying to take the "grand tour" out to Jupiter and back again, it might be better to do a single burn out to Jupiter and use a retrograde gravity assist to get rid of tangential velocity. This would allow the spacecraft to simply "fall" toward the sun, picking up a nice healthy 388 km/s without spending any propellant.

Getting the remaining delta-V to get out of our gravity well and boost up to 414 km/s with a 66 km/s ve requires a mass fraction of 38%. The NSWR is probably fairly heavy, so I'd bump that up to a solid 45%. If we do three 200-tonne LEO launches and use a solar sail to ride from Earth's Hill Radius out to Jupiter (the solar sail will need to be about 30 tonnes), then our mass as we approach periapse will be on the order of 330 tonnes.

New Horizons was a half-tonne, so if we budget for a two-tonne final probe, this gives us 298 tonnes for our propulsion system. If only around 250 of that is usable propellant (the rest being engine structure and so forth), this gives us a mass fraction of 4.125, yielding a delta-V post-periapse of 93.5 km/s, boosting our effective periapse velocity to 508 km/s, getting us up to a hyperbolic excess velocity of 315 km/s.

But let's not forget our solar sail. It was able to lift 600 tonnes, so adding its mass to the final 2-tonne payload means that its thrust will be 18.75 times the pull of gravity. By the time the sun is too far to exert significant pressure on it, it will have given a massive impulse of 7,275 km/s, bringing our total speed to 7,590 km/s or 2.5% of c.

That could get us to Alpha Centauri in just 170 years. We'd all be dead by then, of course, but it would be worth it for the sake of our kids and grandkids.

EDIT: By bumping up to 4 super heavy launches (which is pushing the bounds of feasibility, I know), we'd achieve 7,719 km/s. Tyranny of the rocket equation right here.

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Re: Pluto down, next stop...interstellar?

Postby Mutex » Tue Jul 28, 2015 7:23 pm UTC

Talking about theoretical propulsion methods. This has been doing the rounds again: http://www.iflscience.com/no-em-drive-w ... -time-soon

I tried searching for topics on this but the search tool ignores the "em" making it kinda impossible. So... just how plausible do people here think it is? Even vaguely likely? Or pure crank?

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Re: Pluto down, next stop...interstellar?

Postby gmalivuk » Wed Jul 29, 2015 3:11 am UTC

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Re: Pluto down, next stop...interstellar?

Postby Mutex » Wed Jul 29, 2015 6:22 pm UTC

Thanks.

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Re: Pluto down, next stop...interstellar?

Postby jewish_scientist » Fri Sep 18, 2015 1:52 am UTC

I know that this is compoletly off topic from what everyone is talking about, but I think that this is the next step in space travel. The basic idea is to base flight baths on orbits around Lagrange Points instead of gravitational slingshots. I will admit that I do not completely understand the idea, but 'cheap interplanetary travel' would come before 'visiting exoplanets' on my 'list of cool things to do in space'. Plus, we still have a bunch of stuff to do on the Moon. My favorite is number mA6.
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Re: Pluto down, next stop...interstellar?

Postby sevenperforce » Fri Sep 18, 2015 3:35 pm UTC

The Lagrange points are the saddle points in the gravitational potential around planets -- the places where the gravity of the planet is balanced against the gravity of the sun.

Imagine that you're riding a jet ski on the ocean and you want to get as far as you can with limited fuel. One option is to point yourself toward a passing ship and swing close enough that you get caught in the ship's wake. Once you've picked up the amount of speed that you want, you can rev your engine briefly to escape the wake and leave with the added momentum.

This works fairly well, but it requires you to be able to find a ship that is going in the direction you want, and it requires a non-negligible amount of fuel to get in and out.

Your friend is also riding on a jet ski. But unlike you, he realizes that two ships are about to pass close by each other, and sees an opportunity. Instead of letting himself get caught in the wake, he aims his jet ski for a point halfway between the two ships. The wakes of the two ships cross, forming a larger wave by constructive interference, and he rides that wave until the two ships move apart. Because the interaction of the two waves will dissipate on its own as the ships move apart, he never has to rev his engine to escape.

That's an imperfect analogy, of course, but it is close enough for our purposes. We only have a handful planets that we can use for gravitational slingshots, which really limits our ability to get around. For example, our solar system currently looks like this (distances to scale, excluding the ice giants):

slingshot.png
slingshot.png (2.62 KiB) Viewed 4289 times

There aren't many ways to connect the dots here.

However, if you add in the Lagrange points of these six planets, things get much more interesting:

with lagrange.png
with lagrange.png (3.33 KiB) Viewed 4289 times

Although the shortest distance between two points is (presumably) a straight line, the cheapest distance between two points is often a circuitous, winding, looping route between available inflection points. But with roughly four times as many gravitational potential inflection points, it will be much easier to "coast" from place to place, which will enable shorter transit times and even less fuel.

In the former case, there are 32 possible paths that a spacecraft can take without repetition, starting at any one planet and ending by passing any other planet. However, it is very rare that any of those paths will be optimal at any given time. Jupiter and Saturn return to roughly the same starting positions every sixty years (basically the "reset" button for inner-solar-system celestial mechanics); if any given path becomes optimal once per cycle, then an optimized path is only available every 22 months or so.

In the latter case, however, there are 67 million paths that a spacecraft can take without repetition, meaning that there are thousands of optimized paths available every day.

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Re: Pluto down, next stop...interstellar?

Postby drachefly » Wed Sep 23, 2015 6:36 pm UTC

... locally optimized may be globally pessimized.

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Re: Pluto down, next stop...interstellar?

Postby jewish_scientist » Wed Sep 30, 2015 10:25 pm UTC

I know what Lagrange points are; I just do not see how you are suppose to get speed from them. Gravitational slingshots work via Kepler's Second Law; the closer a satellite is to the body it orbits the faster the satellite travels. Lagrange points are defined as points that are stable between two gravitationally interacting bodies; a satellite would lose speed as it approached a Lagrange point.
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Re: Pluto down, next stop...interstellar?

Postby sevenperforce » Thu Oct 01, 2015 1:39 pm UTC

You will lose speed if you are traveling from a planet to that planet's Lagrange point, but you will gain speed if you are traveling toward a Lagrange point from a completely separate planet. L1, L2, and L3 are saddle points in the gravitational potential field. If you come in along a "downhill slope" then you can "turn" toward either inflection with far less delta-V than you'd otherwise be spending, and use your new trajectory to either head for a gravity assist around a different planet or to head for another Lagrange point.

One thing to keep in mind: there are two kinds of gravitational slingshots. The first is based on the Oberth effect, where firing your thrusters at periapsis, when your velocity is highest, results in an advantage. The second is based on a momentum exchange, where the change in the planet's position as you make your hyperbolic pass will enable you to bleed off some of the planet's momentum and increase your speed without firing thrusters at all.

Suppose you're trying to pull two gravitational slingshots around Mars and Saturn, in sequence. Saturn has much higher gravity and thus will maximize your Oberth advantage, while Mars has a higher orbital speed and thus will give you a better momentum kick. You therefore plan to make a loop around Mars without burning any fuel, saving it for the burn around Saturn periapse (obviously not to scale):

mars to saturn.png
mars to saturn.png (8.66 KiB) Viewed 3885 times

All good so far, right?

The trick, however, is getting the tightest possible turn around Mars, because that's what's going to maximize your momentum assist. Unfortunately, if Mars and Saturn aren't lined up just right, you might have to settle for a very shallow turn around Mars, sacrificing most of the advantage.

However, if you can find a trajectory that will allow you to fly "downhill" through one of Jupiter's Lagrange saddle points, then you can execute a tighter turn around Mars and still hit Saturn:

L3 version.png
L3 version.png (8.36 KiB) Viewed 3885 times

This is also possible with the L4 or L5 points. They aren't saddle points, but you can still use them to pull a "free" turn and thus change your trajectory to a more optimal one without burning any fuel.

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Re: Pluto down, next stop...interstellar?

Postby jewish_scientist » Fri Oct 02, 2015 1:38 pm UTC

I want to make sure I understand what you are saying.

A satellite that approaches L1, L2, or L3 from the planet's orbital path will be pulled into the L Point; but if it approaches along the Sun-Planet Axis then it will be pushed away. This is why these points are unstable. The JPL found a way to reliable use this instability to change the direction and magnitude of a satellite's velocity vector. In addition, they found that the L4 and L5 points can be used to change the velocity vector's direction, but not its magnitude.

By 'saddle points' you mean that they have negative curvature a.k.a. are hyperbolic paraboloids.
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Re: Pluto down, next stop...interstellar?

Postby sevenperforce » Fri Oct 02, 2015 2:38 pm UTC

Essentially, yes.


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