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## Should we send interstellar probe to Alpha Centauri?

 Quote by Urvabara Ok. I just calculated that about 99.78% of the mass of the probe will be fuel, if assuming $$\Delta v = 29979245.8 + 29979245.8 = 59958491.6$$ m/s and maximum exhaust velocity of hydrogen nuke Orion of $$v_{e} = 9800000$$ m/s. That's a big fraction of fuel, I know...
An Orion-type rocket will need a *lot* of mass to protect the payload from the explosions.

 Quote by Urvabara I just used the Tsiolkovsky's rocket equation and put the numbers ($$\Delta v$$ and $$v_{e}$$) in.
You are hypothesizing relativistic velocities, so you need to use the relativistic rocket equation:

$$\frac{\Delta v}{c} = \tanh\left(\frac {v_e}{c}\,\ln\frac {m_0}{m_1}\right)$$

You chose an extremely unrealistic rocket. A more realistic choice of rocket technology will yield a very different mass ratio. For example, a VASIMR engine has an effective exhaust velocity of up to 300,000 m/s. The fuel mass ratio for a VASIMR engine is 99.999...%: 87 nines! There is no room for vehicle structure here. We need something a bit more exotic than VASIMR engine but something that has a ghost of a chance. How about injecting small amounts of antimatter into a plasma, forcing some plasma to escape at a high velocity, and just making a fly-by? Ths gets us down to 99.999....% fuel mass (13 nines). There's still no room for structure, but an improvement.

Note well that both of these rockets are way out there in terms of specific impulse. Nonetheless, neither a 200 fold or 600 fold increase in Isp will enable us to send a payload to Alpha Centauri.

 Quote by Oberst Villa 10g is muuuuuuuuch to high. First thing, for the probe to tolerate such an acceleration without breaking apart, you would need a massive support structure, which would add a lot of mass to your probe => Kiss your 99,78% goodbye. Second thing, there is no need for such a high acceleration. You need 40 years as a minimum, so would it hurt much to add e.g. 850 days for the acceleration phase ?
1] I am fairly certain a probe could be built to withstand much 10g's or more.
2] You don't need to add mass, you just need a good shock absorbing system.

 Quote by Oberst Villa 10g is muuuuuuuuch to high. First thing, for the probe to tolerate such an acceleration without breaking apart, you would need a massive support structure, which would add a lot of mass to your probe => Kiss your 99,78% goodbye. Second thing, there is no need for such a high acceleration. You need 40 years as a minimum, so would it hurt much to add e.g. 850 days for the acceleration phase ?
I just chose a number (10 g) I could imagine a machine could withstand. Ok, < 10 g will do...

 It bothers me how some can be so quick to shoot down optimistic ideas about space exploration. And not even just space exploration all kinds of sceintific ideas and engineering. Without optimistic and thinking "outside the box" we woulndt be where we are today. Throughout human history there was been many profound discoverys that have comepletly changed the way we see our universe. Many people believed Copernicus was crazy but he right to some extent. Back to the main idea.... I dont know much about using H-bombs as propulsion device but these things release a great deal of energy that could probaly be used as something bette then just a way to destroy life as they did in the past. As somone said early in the post using H-bombs to propel a craft is still science fiction but nukes havent been tested in like 30 years or something like that I believe China was the last country to stop testing. What if we were to try a test a safe distance from out planet, Is there a safe but not too far distance from earth where a nuke could actually be detonated without casuing harm to the planet. If this was going to be tested I would suggest the probe and nuke be strapped to a rocket and fired a safe distance from earth before the nuke is detonated. Even if the probe doesnt make it there the first time it could still end up being a milestone as Urvabara said. As for using an H-bomb as a way to slow down...I dont think ET would be to happy if we came flying into his solar system and detonated a nuke haha, they might not want much to do with us after that.

 Quote by blimkie.k It bothers me how some can be so quick to shoot down optimistic ideas about space exploration.
We have no power to stop anyone from doing anything. We're all just talking here; I don't imagine anyone's attempts to build their own rocket are hinging on the postive feedback of PFers.

 Quote by blimkie.k I would suggest the probe and nuke be strapped to a rocket and fired a safe distance from earth before the nuke is detonated. Even if the probe doesnt make it there the first time it could still end up being a milestone as Urvabara said.
Probes that don't make it to places have a way of coming back to where they started.

Can you see why we are reluctant to strap bombs to ... well, other bombs ... and lob them around our neighborhood?

 Quote by blimkie.k As for using an H-bomb as a way to slow down...I dont think ET would be to happy if we came flying into his solar system and detonated a nuke haha, they might not want much to do with us after that.
Why does everybody assume that the extraterrestrials would be like ET ? Think of the evil creatures in "Aliens". Blowing up some fat nukes in front of their doorstep might be a nice way of saying "Don't you mess with us !" (let alone thinking of eating us...)

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 Quote by blimkie.k It bothers me how some can be so quick to shoot down optimistic ideas about space exploration.
This is a scientific forum, not a complete fantasy and bad science fiction forum. The ideas proposed in this thread are pure science fiction. If we ever do send a probe to another star system, it will not be with any of the propulsion systems discussed in this thread.

I'll starting with what we know now (i.e., chemical rockets). Using chemical rockets to send even a small payload on a fly-by mission to another star in any reasonable time would require a mass of propellant that vastly exceeds the mass of the universe. Chemical rockets are obviously a non-starter. How about more advanced propulsion techniques?

The best propulsive technique developed to date is the Variable Specific Impulse Magnetoplasma Rocket (VASIMR). In one mode it has a very high specific impulse but a very low thrust. Suppose we develop something that maintains this high specific impulse but also generates a reasonable amount of thrust. We want to get up to 1/10 of the speed of light so a fly-by mission to a nearby star will occur in a reasonable amount of time. The relativistic rocket equation dictates that the vehicle be 99.99999999999999999999999999999999999999999999999999999999999999999999 99999999999999999% fuel. In other words, the mass of the fuel must be 1087 times the tare mass of the vehicle, including the fuel tank, vehicle structure, propulsion system, and payload. Getting a one gram payload with an otherwise massless vehicle would require a fuel mass equal to that of 1030 universes. Still a no-starter.

Claims for the specific impulse for an Project Orion-based vehicle (nobody has built one) are up to 30 times that of the VASIMR engine. This drops the fuel requirements considerably. For a fly-by mission at 0.1 c, the fuel mass need be a mere 19 times that of the tare mass of the vehicle. One big problem with a Project Orion-type vehicle: The fuel is nuclear bombs. Using hydrogen bombs is ludicrous. A Tsar Bomba would destroy even a city-sized spacecraft. A much, much smaller explosion is needed.

The obvious thing is to use fusionable material in the form of very, very small bombs. A huge spacecraft will still be needed to shield the payload from the explosions and to maintain structural integrity of the vehicle itself. The vehicle itself will need to mass several tons. Now another problem arises: We need twenty times that much mass in fuel. We don't know how to make deuterium fuse with deuterium. Deuterium and tritium? We don't have tons of tritium. Fusion bombs use a very small amount of tritium. Project Daedelus proposed deuterium/helium 3. They would get the helium 3 by mining Jupiter's atmosphere.

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 Quote by blimkie.k It bothers me how some can be so quick to shoot down optimistic ideas about space exploration. And not even just space exploration all kinds of sceintific ideas and engineering. Without optimistic and thinking "outside the box" we woulndt be where we are today.
My standard response to that is that in order to properly think outside the box, you first have to understand where the box is and what is in it.
 Throughout human history there was been many profound discoverys that have comepletly changed the way we see our universe. Many people believed Copernicus was crazy but he right to some extent.
Things like that are commonly cited, but they are incorrect analogies. Copernicus lived before science, so the reason people thought he was wrong was that no one had a way to properly analyze his ideas. That isn't the case today. We can properly analyze these ideas.

Also, please note that while the quality of people varies on any site, we're not just armchair scientists and engineers here - virtually every PF regular involved in this conversation is a professional at a related science or engineering discipline. So we really know what is and isn't likely, technologically, for the near future.

 Quote by Oberst Villa Why does everybody assume that the extraterrestrials would be like ET ? Think of the evil creatures in "Aliens". Blowing up some fat nukes in front of their doorstep might be a nice way of saying "Don't you mess with us !" (let alone thinking of eating us...)
ET = Extraterrestrial life form.
ET = The Extra Terrestrial film.

Many people may use the letters "ET" without referencing to the famous ET film.

 Quote by Urvabara ET = Extraterrestrial life form. ET = The Extra Terrestrial film. Many people may use the letters "ET" without referencing to the famous ET film.
At first I thought that's what he meant too, but it isn't.

He doesn't mean 'why is everyone talking about E.T.'?

He's reading our posts and we keep talking about the alien race we'll encounter and he's hearing us assume it will be friendly and non-belligerent. He's wondering why we don't think it'll be a race that wants to eat us.

 Quote by DaveC426913 At first I thought that's what he meant too, but it isn't. He doesn't mean 'why is everyone talking about E.T.'? He's reading our posts and we keep talking about the alien race we'll encounter and he's hearing us assume it will be friendly and non-belligerent. He's wondering why we don't think it'll be a race that wants to eat us.
Yes, exactly, thanks for explaining it more clearly. And it is not only in this thread, take for example a look at the Pioneer plaque. The decision to include a drawing of a man and a woman... "Hmm, look how yummy we are !"

On a more serious note, no I don't really think there is someone out there who wants to eat us. However, if we look at our own civilization and its history, the assumption that all extraterrestrials (should they exist) are friendly and non-belligerent is certainly not justified.

 Quote by Oberst Villa On a more serious note, no I don't really think there is someone out there who wants to eat us. However, if we look at our own civilization and its history, the assumption that all extraterrestrials (should they exist) are friendly and non-belligerent is certainly not justified.
I do not think so either. If an ET civilization is to able to do interstellar travel, then they should also have enough food and be able to produce artificial meat. No need to eat Earth meat which in fact could be very poisonous for them.

Almost all _intelligent_ people are friendly. I can hardly imagine a scientist who would like to do any harm to ET civilization. Of course, there are people who would like to do harm to them, but this minor group of our civilization is not intelligent. They belong to a group called "brutal animals".

 Quote by DaveC426913 Probes that don't make it to places have a way of coming back to where they started. Can you see why we are reluctant to strap bombs to ... well, other bombs ... and lob them around our neighborhood?
Very good point that could be deadly, sounds like risky buisiness.

 Quote by Oberst Villa Why does everybody assume that the extraterrestrials would be like ET ? Think of the evil creatures in "Aliens". Blowing up some fat nukes in front of their doorstep might be a nice way of saying "Don't you mess with us !" (let alone thinking of eating us...)
Haha and even if they did retaliate it would be a long time before we have to worry about something gettin back to us.

 Quote by russ_watters Also, please note that while the quality of people varies on any site, we're not just armchair scientists and engineers here - virtually every PF regular involved in this conversation is a professional at a related science or engineering discipline. So we really know what is and isn't likely, technologically, for the near future.
Russ, I dont doubt your or any other PF's qualifications, im just young and the idea of something like this happening in my lifetime would be amazing, especially if we did encounter intelligent life. And all this talk about exoplanets these days is really quite exciting

 Quote by Urvabara I do not think so either. If an ET civilization is to able to do interstellar travel, then they should also have enough food and be able to produce artificial meat. No need to eat Earth meat which in fact could be very poisonous for them. Almost all _intelligent_ people are friendly. I can hardly imagine a scientist who would like to do any harm to ET civilization. Of course, there are people who would like to do harm to them, but this minor group of our civilization is not intelligent. They belong to a group called "brutal animals".
You mean like george bush and other political leaders, lets see if they have any oil we can take from them!

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 Quote by blimkie.k You mean like george bush and other political leaders, lets see if they have any oil we can take from them!
Oh joy, what a wonderful turn you're taking this thread on. Let's get back to powering probes with hydrogen bombs, at least its related to the topic at hand and not mindless political commentary.

 Quote by Mech_Engineer Oh joy, what a wonderful turn you're taking this thread on. Let's get back to powering probes with hydrogen bombs, at least its related to the topic at hand and not mindless political commentary.
I don't even see a point for you to post on this thread. Every post you have made has been negative contains a excessive amount of sarcasm, takes a stab at belittling someone or all of the above.

 Quote by blimkie.k I don't even see a point for you to post on this thread. Every post you have made has been negative contains a excessive amount of sarcasm, takes a stab at belittling someone or all of the above.
The occasional cold, hard reality check is his point. This thread is the better for it.

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