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Dgennero
2009-Apr-04, 08:18 PM
I'd like to open a debate on how we can send a probe to the stars that arrives within an acceptable timeframe. By acceptable I mean within a human lifetime. Few people would finance a project the outcome of which neither they nor their children will ever see.
I know the subject has been discussed numerous times, but I'd just like to give it another try :)

What we need is a nice, unmanned, lightweight probe (maybe about the weight of a car) that gets to a close star within a human lifetime.
That means acceleration to 0.1c at least.

1.) No chems here
Chemical rockets of course won't get us to the stars - the amount of propellant is just to big, and if you want to brake in mid-journey, it gets ludicrously big.

2.) The stubborn RAM won't eat his SCRAMbled eggs
Ramjets/scramjets need to be huge, cause a lot of drag and if you need to decelerate they are utterly useless.

3.) Warped minds don't function well
I'd dismiss outlandish ideas like a Warp drive or drawing energy from "subspace" etc. The solution has to be practical.

4.) What about the drunken sailor?
Solar sails might give the right type of acceleration when laser-powered from Earth.
But how do we decelerate? Unless there's already a laser at the target system, the sailor will just zip through the system in a few days and coast on.

That leaves me with an ion/plasma drive, and THAT is IMO a feasible solution.
Current ion drives (which have been tested and worked) give us only a thrust of a few ounces, but over a long time - months. And there will be progress.

Simple Newtonian physics and basic algebra (F=m*a, v=a*t) reveils that with a star probe that weighs, say, 1000kg (yes, I only use metric ;) ), a force of 1000 Newton (about as much as a strong person pushing a car would come up with) gives us an acceleration of 1 m/s2, or 0.1g.
Applied over 30 million seconds or 347 days, we arrive at 0.1c.
A year before arrival, the probe turns around and decelerates.
Et voila, we've just reached Alpha Centauri in 45 years and our "Centauri Dreams" have come true.
If you give it 65 years instead and 10 years of acceleration, 100 Newton will do.

Big question is: How soon can we achieve this (or any other solution)?

These are the questions I'd have concerning a star probe with a plasma drive/ ion drive:

1.) How far along are we with a plasma drive/how much thrust can an ion drive provide?

2.) How long until we have developed nuclear fusion? I know there is some progress here, but they have been saying for 50 years that it will be ready in 50 years. We need an energy source, and nuclear fusion with a conversion of 1% of matter into energy would provide us with the necessary energy source for acceleration - the amount of fuel we'd have to carry with us would be tiny, so not much excess weight.

3.) Now that could be the biggest problem: How much propellant do we need to be expelled as plasma? If this figure is too high, my suggestion goes the way of the Titanic.

4.) The computer that guides the probe has to be semi-autonomous. Are we, and if not, WHEN might we be ready to design a computer system that will be able to get his bearings in a foreign star system, like a rover on Mars has to deal with unknown territory?

Alpha Centauri would give us not just one but three stars to explore, and rocky planets circling either might actually have stable orbits within the Goldilocks zone.
So what are we waiting for?
I'd love to hear the president announce that "before this century is out, a probe will be sent to the stars.". And for science, not prestige :)

JonClarke
2009-Apr-04, 10:22 PM
Project Daedalus

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

http://www.bisbos.com/rocketscience/spacecraft/daedalus/daedalus.html

cjameshuff
2009-Apr-05, 12:05 AM
Ramjets/scramjets need to be huge, cause a lot of drag and if you need to decelerate they are utterly useless.

Actually, they are likely to be more useful for braking than for acceleration. Their drag gives them a terminal velocity (an unfortunately low one) relative to the interstellar medium, even when operating to produce forward thrust...if they are moving faster than that terminal velocity, they will decelerate. You don't even need to operate them as fusion drives. The solar wind from the destination will assist further here.

3.) Warped minds don't function well
I'd dismiss outlandish ideas like a Warp drive or drawing energy from "subspace" etc. The solution has to be practical.

Considering the complete lack of any demonstrated ability to manipulate space-time in the ways required...this seems like a quite reasonable position.

4.) What about the drunken sailor?
Solar sails might give the right type of acceleration when laser-powered from Earth.
But how do we decelerate? Unless there's already a laser at the target system, the sailor will just zip through the system in a few days and coast on.

Robert Forward proposed a solution for this...drop the larger portion of your sail and use it as a reflector focused on the remainder. It continues to accelerate on ahead, but the reflected beam provides deceleration to the craft.

A combination of techniques may be used as well. Nuclear pulse propulsion in a powered flyby of the sun (getting the most of the Oberth effect by briefly performing high acceleration maneuvers deep in the biggest gravity well around), followed by beam propulsion...a laser sail or a particle beam rider (again, a relative of the Bussard ramjet that takes advantage of the drag), followed by reflected laser sailing or braking against the solar wind of the target, and nuclear pulse propulsion during another close flyby of the target sun.

The nuclear pulse system needn't be the typical Orion system with a big pusher plate, it may be possible to couple to the explosions with the same magnetic field you use for particle beam launch and for braking against the target's solar wind. You could possibly cut a significant amount of time off this way, making more of the trip at higher velocity.

KaiYeves
2009-Apr-05, 05:54 PM
Diversity in combination- I like it!

sohh_fly
2009-Apr-05, 08:29 PM
Dgen:

I don't think we could get to the star's in a human lifetime(not even to the A centauri system).
even though that would be cool.

Siguy
2009-Apr-06, 12:24 PM
The only engine which can give that kind of performance is a fission-fragment drive. From what I've researched, it would be possible for interstellar travel within a human lifespan. The problem is, nobody's built a fission-fragment rocket, and they're a relatively new concept.

samkent
2009-Apr-06, 04:25 PM
While speed and travel time may be the first consideration. Don’t forget the smaller and just as important things like communications. Forget the round trip time delay, will we be able to receive the transmissions from a craft with limited power output from those distances?

Currently Voyager 1 is the farthest craft from the sun (108 AU). That’s less than 4 tenths of one percent of the distance needed. It uses a 23 watt transmitter feeding a 14 foot dish. You need a 100 foot dish to just receive the signal.

Lets assume a lifetime to be about 50 years of cognitive usefulness. I doubt you can get a transmitter much beyond a few thousand watts if you expect it to last that long. Even with a larger dish on the probe, I question whether we can receive a signal that weak at those distances.

Dgennero
2009-Apr-06, 06:41 PM
@samkent: Shoot, I knew I forgot something ;)
Actually, two things:

1.) Lifespan of the probe. Since the voyagers have been underway and doing fine for more than 30 years, I see no problem here.

2.) Data transmission:
We need exact figures to calculate how many watts we need for a data transmission to be received on Earth from that distance - and how big a dish will do.
However, the transmissions only have to occur in bursts - we need no open channel all the time!
So the "battery" is not going to be depleted since the probe sends data only in intermittent bursts.
And of course if we plan to let the probe accelerate, decelerate and after its mission to return, there's no transmission problem - the data will be stored locally :)

samkent
2009-Apr-06, 07:28 PM
Well if you double the distance you have to quadruple the power. Since Alhpa Centari is about 271000 AU distant that would mean poor old Voyage would have to carry a 375 million watt transmitter. Imagine the battery bank to run it.

Scamp
2009-Apr-06, 07:54 PM
What do you want to bet that the first probe launched to the stars won't be the first to get there?

In fact given the progress of technology, I would not be surprised if the first probe launched arrives to a formal welcome party and lunch at McDonald's.

joema
2009-Apr-06, 08:13 PM
Well if you double the distance you have to quadruple the power. Since Alhpa Centari is about 271000 AU distant that would mean poor old Voyage would have to carry a 375 million watt transmitter. Imagine the battery bank to run it.
As you'd expect, those planning interstellar probes are aware of this and thus likely wouldn't use a conventional radio transmitter.

The general plan seems to be using a laser. For example Project Longshot was a proposed unmanned flyby probe to Alpha Centauri. Using a 250 kw laser, it could transmit 1 kilobit per second from 4.3 light years away:

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

rommel543
2009-Apr-06, 08:19 PM
Thinking on the idea of the laser powered acceleration, what kind of strength are we talking about here? Could the idea be used as a communication device as well? A laser mounted pointing back towards earth (or another craft in synchronous orbit). I know we're talking having to aim a laser across stellar distances, but wouldn't it be the same for the laser powered acceleration?

rommel543
2009-Apr-06, 08:20 PM
Well there's my answer, posted while I was typing....

Dgennero
2009-Apr-06, 08:28 PM
@Scamp: Yes, there will probably a succession of probes, and the later ones will arrive first.
Still, let's give it a first shot ASAP :)

JonClarke
2009-Apr-06, 10:15 PM
Presumably the first probe would not be sent until there was a good chance that it would be the first! :)

Ilya
2009-Apr-07, 11:59 AM
I'd like to open a debate on how we can send a probe to the stars that arrives within an acceptable timeframe. By acceptable I mean within a human lifetime. Few people would finance a project the outcome of which neither they nor their children will ever see.
First and foremost -- greatly extend human lifetime.

This is not a joke. To people who do not age, a 10,000 year journey at modest 0.5% c may not be a big deal.

dgavin
2009-Apr-08, 02:02 AM
First and foremost -- greatly extend human lifetime.

This is not a joke. To people who do not age, a 10,000 year journey at modest 0.5% c may not be a big deal.

i would be awefully boring though. Holodecks perhaps?

Dgennero
2009-Apr-08, 01:45 PM
Well, in the 1990s there was the first anti-aging craze, but it might be more difficult than the optimists thought.
Taking your antioxidants won't be enough, but if we can delay aging soon until we find a cure for it, maybe.

I'd like to say one more thing about that Robert Forward proposition.
Maybe I misunderstand the idea, but that seems to be impossible due to the actio-reactio principle:
If the dropped off reflective solar sail is still tethered to the ship, it is like trying to propel a sailboat by sitting inside, blowing at the sail.
If it is not tethered, while the ship starts to decelerate, the reflective sail will accelerate in the opposite direction and the two get quickly separated.

Ilya
2009-Apr-08, 04:18 PM
If it is not tethered, while the ship starts to decelerate, the reflective sail will accelerate in the opposite direction and the two get quickly separated.
Yes, that's how Forward envisioned it. Frankly, I do not see how the beam would remain sufficiently collimated.

loglo
2009-Apr-11, 01:21 AM
The idea was that the larger sail would have over 10 times the mass of the spacecraft and braking sail so move off slowly while the payload decelerates much more quickly. The sails had robotic helpers to keep them trimmed and the beam tight. His paper is here (http://www.aiaa.org/content.cfm?pageid=406&gTable=JAPaperImportPre97&gid=8632) (if anyone has a subscription to the Journal of Spacecraft and Rockets. The first page is available for free.

Interestingly he credits Tsiolkovsky as a possible creator of the idea. Is there nothing in space science that guy didn't dream up first?

KaiYeves
2009-Apr-11, 12:52 PM
Well, he wanted to make a space station out of bricks, so perhaps not a metal space station.

Ilya
2009-Apr-11, 05:33 PM
Well, he wanted to make a space station out of bricks
I never heard of Tsiolkovsky saying that. The Brick Moon (http://en.wikipedia.org/wiki/The_Brick_Moon) is by Edward Everett Hale.

KaiYeves
2009-Apr-11, 08:25 PM
Joe the Councilor told us that at Space Academy. Of course, he could have been wrong.

loglo
2009-Apr-12, 04:42 AM
It appears Joe might have been wrong about the bricks but it seems Tsiolkovsky took the idea and ran with it:-

Among Tsiolkovsky's innovations, only thinly veiled in the guise of fiction, were the idea of generating artificial gravity through spinning the station on its axis, hydroponic "space greenhouses" and solar power.

Noclevername
2009-Apr-19, 12:45 AM
Project Orion. (http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)) Kickin' it Old-School with an Atomic Rocket. Of course an interstellar jaunt would take the majority of nukes on Earth, if not far more. You could even (if you don't mind a little fallout) use smaller Orions to launch nuclear stockpiles into space (Superman did it the hard way, there's no need to send the missiles when the warheads are the important part), and assemble the big mother in orbit.

Re: Ramjet, if you mean Bussard ramjets, the drag only outweighs the thrust if using a magnetic field (which also makes it easy to decelerate, you just increase the drag) but an electrostatic attractor would not have nearly as much drag.

publiusr
2009-Apr-27, 09:06 PM
I might try NTR-Solid/DUMBO with an AIMSTAR payload.

http://www.projectrho.com/rocket/rocket3c2.html#ntrsoliddumbo 3.5 million newtons thrust
http://www.engr.psu.edu/antimatter/Papers/AIMStar_99.pdf Aim gives you little thrust but the exhaust velocity is just short of 600,000 m/s if only 55 newtons thrust

I don't think Pulse-Orion or NSWR will be ready anytime soon, but the combo listed above gives you a good one-two punch that doesn't seem too unbelievable. Its a good place to start, and gives you a REAL payload, unlike starwisp.

Comperable to the enterprise with a damaged if still fuctioning impulse system.