# Practical Interstellar Mission to Alpha Centauri?

1. May 5, 2012

### chill_factor

Can this be done feasibly (with current/near term emerging technologies and sub-trillion dollar budget)?

Assuming that the human race survives the 21st century, I think we'll have the resources for a multi-decade unmanned flyby mission to Alpha Centauri even if technology shows little progress in these 100 years.

To get information back to earth, we need a very powerful transceiver so that the signal/noise ratio would be high enough to pick up at interstellar distances.

The Cassini probe had a 20 W transmitter and 4 m parabolic antenna and could do 166 kbps from Saturn at 9 AU. At Alpha Centauri, we'd be at 4*63*10^3 AU. This is clearly not feasible. Let's do lasers instead. A lightyear is about 10^12 km. With a 300 nm laser, the divergence of this laser, using the rough estimate new radius = distance*wavelength/original diameter, assuming a 1m across transmitter, would mean a 2.e+10 m radius beam by the time it got to earth from 1 lightyear. Quadruple that for 4 lightyears. Let's say we E=nhf with a 1000 kW laser. 1.51 e+24 photons emitted. We have roughly 10^22 square meters of the beam. There's going to be 151 photons per square meter.

This is detectable! We can communicate with the probe and if we engineer the mission such that a gravitational swingaround can be achieved, we'll be able to pick up the signals much easier on the return voyage; if we do a pure flyby, the signals will get weaker and by the time we can communicate with the probe it would be even further away. To prevent the satellite from destroying the receiver, within the solar system we need a backup radio system, and only after we're close to the lightyear level will the high powered laser be used.

To get to Alpha Centauri within 5 decades we need 0.1c propulsion. Let's set a 10 ton mass budget. All instruments, sensors, flight actuators, communications and mission computers can be thought of as roughly 1 ton. The power source can be a small 10 MW pebblebed reactor+heat engine weighing in at 5 tons. To change direction, the probe can expel used pebbles in a selected direction. The rest of the tonnage should be a very large light sail.

I'll use a Wikipedia estimate http://en.wikipedia.org/wiki/Interstellar_travel and say that we'll hit 0.11c at 0.17 lightyears with a 10 ton probe for a 650 GW laser on the moon (to keep maximum line of sight at all times). To get the probe to a distance where the 650 GW laser doesn't melt the sails, we'll assemble the whole thing in space and use booster rockets and gravity assists at first. The funding for the whole mission would be 1 trillion 2012 USD equivalents in 2100, paid with a global tax on the investment banks, with the moon based laser propulsion/receiver taking up the bulk of the 1 trillion. The actual probe itself would probably be only a few hundred billion dollars.

What do you guys think? Alpha Centauri mission doable or not?

2. May 5, 2012

### Ryan_m_b

Staff Emeritus
A few back of the envelope calculations:

Lower bound the energy required to accelerate a 10 tonne object to 0.1c is 4.5EJ. That's about 3.5 days of the entire planet's energy production. A bit of googling tells me that price per KWh in energy is roughly $0.1 dollar so the energy cost for the lower bound alone is roughly$125 billion.

I'm not sure how much solar radiation the Moon get's per m2 but assuming it's somewhere close to what space based solar panels receive that puts it on the order of 0.5KW. Granting 100% efficiency and 24 hour sunlight (which you wouldn't get on the Moon) you would need to build enough panels to cover 2km2 per GW, 1300km2 (>85% the area of London) for the whole set up you've proposed. I can't find the production cost for solar panels but assuming something cheap like $1 per m2 your cost becomes$1 million per km2 or $1.3 billion for the whole set up. Further googling tells me that the weight of solar panels per m2 is around 10-20kg. Being generous and saying 10kg/m2 that means you have 13 million tonnes of solar panels to ship to the Moon. Being very very speculative and saying that the cost per kg to the Moon could be bought down to$1000/kg you would still need $13 trillion to get it all up there. Obviously everything here is just a few quick figures found on google plugged into excel and should be taken as such. The point though is that even being generous you aren't going to get the set up described (unless you funnel large amounts of money into automated solar panel factories on the Moon and cross your fingers for a viable breakthrough). Two last points for your consideration: 1) What would give a greater ROI in terms of scientific discovery;$1 trillion for a probe mission or \$1 trillion for the development and deployment of future telescope technologies?

2) A laser capable of continually focusing hundreds of gigawatts onto a point a few metres across is a classic example of a death ray. I'd hate to be in town if a rogue nation/terrorist cell/paranoid savant manage to hack into the controls and turn this Moon laser on Earth...

3. May 5, 2012

### Staff: Mentor

That is based on the rather ridiculous assumption that solar panels have to be produced on the Earth. Such things should be produced on site from the materials available there.

4. May 5, 2012

### Ryan_m_b

Staff Emeritus
Indeed, which is why I said:

5. May 5, 2012

### DrStupid

That sounds quite optimistic. Technology showed much more than a little progress compared to 1970 but there were almost no progress in space flight. Maybe this will change in the near future or maybe not. Nobody knows.

6. May 5, 2012

### Astronuc

Staff Emeritus
It's not so simple.

With respect to the propulsion system, is one planning to decelerate on approach to alpha Centauri, or simply pass through the system at 0.1c.

One needs to consider the propulsive efficiency of the propulsion system, including the specific impulse of the thrusters, and the power conversion system of the nuclear plant, and energy produced per mass of fuel.

I believe the communications part is underestimated.

Here is one estimate of a mission similar to what one has described: Project Longshot - An Unmanned Probe to Alpha Centauri.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890007533_1989007533.pdf

Fusion is not an enabling technology at present. A more conventional nuclear fission system and Brayton or Stirling cycle would be more appropriate.

7. May 5, 2012

### chill_factor

agreed, 1 trillion dollars here is flushed down the toilet.

you don't have to ship the panels there. Just build automated factories on the moon, which is currently not feasible but may be within 100 years and doesn't require any truly new breakthroughs that push the laws of physics. the costs for shipping the factory components would of course be astronomical, but maybe only needs 500,000 tons shipped. still hoping for a breakthrough and crossing the fingers, yes, but 100 years later, I'm hoping that computer costs would've decreased to this point, assuming the human race survives (which is no sure thing).

the 3 days of planet energy production can be spread out over a few years since we're using a light sail.

The thing is there's no way to get 24 hour sunlight, have cheap shipping, and be able to keep a line of sight to the probe simultaneously. Lagrange points would mean expensive shipping and nothing can give us line of sight to the probe at all times. That's a major limitation and would probably slow the mission down for years.

a problem is indeed that both the power laser and the communication laser can be used as weapons, moreso the power laser. maybe a global dictatorship would be able to do something like this and have absolute security. i'm sure that if any aliens have achieved interstellar travel, their system of government has got to be the equivalent of a global Nazi Germany as individual nation-states will never be able to pull something like this off.

Well, a 1000 kW pulsed laser can be made, and 151 photons can probably be detected per square meter at lightyear level, (its more photons than a typical lunar ranging experiment). The problem is again with line of sight. For communications TO the probe, we might be able to get the power laser to function also as a communication laser at high distances. and just use radio for within-solar system.

Last edited: May 5, 2012
8. May 5, 2012

### Ryan_m_b

Staff Emeritus
The problem is now we're veering into science fiction territory. Not because what we're suggesting is magic but because it's so far away from what we can currently accomplish there is no real science to discuss. Sure we can pull 500,000 tonnes out of the air as a figure but it's largely meaningless because we have no idea if that's a reasonable figure or an over estimate. Do you understand what I mean here?
Of course, my point was more to illustrate the astronomical amount of energy being discussed.
I disagree, a tyrannical regime produces resistance and revolution. A Moon based death ray ripe for the taking would be a dream come true for an underground resistance or guerilla army in the struggle for freedom. Not to mention what a fascist regime could accomplish with such a device.

This is stumbling onto a big issue rarely addressed in interstellar space travel discussions; how functionally indistinguishable the technology is from doomsday weapons. Solar sail laser = death ray. Interstellar space ship = biosphere devastating RKV. Self replicating factory = self replicating weaponry. Etcetera etcetera

9. May 5, 2012

### Staff: Mentor

Oh come on, securing the laser system is easy: Build it on the far side of the moon. Build the solar cells nearby (useful anyway) and there is no easy way to destroy anything on earth with it. Sure, you could use large mirrors in space, but that is way more complicated than just reprogramming the software.

Within the error margins for technology in 2100, a factor of 2 is quite small, so having a line of sight at 50% of the time should be fine.

However, what about the propulsion? 650GW provide a maximal force of ~4kN, assuming a perfect mirror. This can accelerate 10 tons to .1c within ~2 years and a distance of .1 light years.
Using the numbers from the data transmission and assuming a sender of 50m diameter (like E-ELT, and moon has lower surface gravity), the beam has a radius of ~4*10^7m at .1 light years. I am quite sure that the probe is not designed to have a sail of this size. Even with 1mg/m^2 (some atom layers), this would weight ~10^6 tons.

I would use other methods of propulsion. Or even more power.

10. May 5, 2012

### Astronuc

Staff Emeritus
650 GW is more than the entire electrical generation of the US! The 650 GW of laser power requires much more (4 or 5 or more times) power to be input. This is certainly not practical.

Regarding, "Oh come on," I expect one is speaking with a thorough lack of experience in any capital project or engineering discipline.

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11. May 5, 2012

### Oldfart

12. May 5, 2012

### Chronos

The energy required to accelerate just a single ton of payload to .1c is a staggering
4.5E17 Joules. A like amount is required to slow it down at the destination then do it all over again for a return trip. Even at that speed the total travel time is still, at best, an entire lifetime. How much air, food and water is needed per human per century? Increasing velocity to near light speed is no real help. The already unbelievably huge energy demands quickly spiral outside the realm of possibility.

13. May 5, 2012

### D H

Staff Emeritus
Only just now? This thread has been solidly ensconced in science fiction land since the opening post. A continuously operating ultraviolet megawatt laser with a 0.1 microradian dispersion angle. Such a device doesn't exist. (And this ignores that the this small a dispersion requires a corresponding pointing accuracy and stability.) A pebble bed thermonuclear propulsion system (that ejects pebbles??). We haven't built a pebble bed thermonuclear propulsion system that ejects hydrogen. A continuously operating gigawatt lunar laser for solar sail thrust -- Please!

This is all 100% pure science fiction.

14. May 5, 2012

### D H

Staff Emeritus
And that's ignoring the brutal nastiness of the rocket equation. Suppose we could make a rocket with a pure fantasy exhaust velocity of 106 m/s. The launch mass of this rocket would have to be 1013 tons to get that one ton payload to 0.1 c. Reduce the exhaust velocity to something more in line with science fiction rather than fantasy and the launch mass quickly exceeds the mass of the solar system.

15. May 5, 2012

### Chronos

Yes, the numbers get very ugly in a hurry - which is a reasonable explanation for why ET is not here. To put the numbers into perspective, 1 kg of anti matter could theoretically produce about 1E17 joules - less than a quarter of the energy needed to accelerate 1 ton to .1c. Directing that magnitude of energy output is merely a technological issue. So basically, a round trip to alpha centauri would only require about 18 kg of anti matter per ton of payload [with the right technology]. And the round trip would still take over 100 years. Space is big.

16. May 6, 2012

### chill_factor

yes, yes, i know, it doesn't exist. For communication purposes, it doesn't need to be continuously operating though. Pulse is fine for communication. Just 1 bit per second every few years is good enough for a 50 year mission.

This was mostly thinking about how insanely hard it is to even get to Alpha Centauri or even to communicate with a probe that's already there.

There's nothing in the universe (seriously there isn't) that's an effective gamma ray mirror carryable on a ship. Antimatter isn't going to do it since most of the reaction energy escapes as wasted gamma rays. Light sail is the only thing, seriously, that can do it without takeoff mass being solar system level.

17. May 6, 2012

### Ryan_m_b

Staff Emeritus
Point taken :tongue:

18. May 6, 2012

The only motivating factors in sending such a probe to the Alpha Centauri triple-star system would be:

1. Detection of a planet
2. A SETI signal

Barring these, the incentive for the expenditure of time, money and effort involved is missing. After all, if these are absent what would a probe do on arrival in that general vicinity that would justify the attempt?

Last edited: May 6, 2012
19. May 6, 2012

### Staff: Mentor

And to put that technological issue into perspective, 1E17 joules is about twenty megatons worth of thermonuclear bombs, something that's been within our reach for the last fifty years. It's a lot easier to release energy than to control it.

20. May 7, 2012

### Ryan_m_b

Staff Emeritus
Of course that 1E17 figure is the absolute lower bound i.e. it assumes 100% conversion of energy-to-momentum. As you point out it's not an easy case of control and I doubt that using nuclear explosions will get close to that 100% figure. Finally the inherent problem of fuel mass raises it's head. Say you had 20 megatons worth of bombs for a 1kg payload. Now you have to add on a few more bombs to take into account the mass of the infrastructure of the previous 20, and you'll have to have a few more for that, and for that etc. I think that the rocket equation is what one uses to work out problems like that but I don't know how to use it.

21. May 7, 2012

### hitchiker

i remember carl sagans documentry about this topic where he says " it won't be "us" who travel to alpha centauri or near by stars it will be a species evolved from us with more of our strengths and less of our weakness "

22. May 9, 2012

### qraal

After working on "Project Icarus", Icarus Interstellar and our new Project Forward, led by Jim Benford (Greg Benford's twin-brother) I tend to agree that the numbers seem daunting - at first. However most of the problem is thinking of it as a whole jump from present-day 100 kW lasers to multi-ton probes doing 0.1c.

A more reasonable approach is an evolution of beamed energy/propulsion systems, from satellite powering systems, to orbit-raising, to LEO-boosting, to interplanetary propulsion, and so forth. I've discussed using microwaves with Jim, and lasers with Young K. Bae, and the technology can be developed, over decades, with applications for all ranges out to the stars. It very definitely isn't science-fiction as there are no inherent physics issues with beamed propulsion. Incremental refinement of existing techniques will get there - if it's sustained. To power beamed propulsion will require developing large scale solar power systems in space, with obvious implications for powering life on Earth too.

As for the rocket equation... rockets have issues, thanks to the exponential growth of the mass-ratio for mere linear increases in the final velocity. But staging can improve things. There's a JPL study from 1963 which essentially proves that stage rockets can get arbitrarily close to the speed of light, with enough stages. The real problems arise engineering them to have sufficient engine power to accelerate quick enough to achieve the mission. Fission-fragment rockets, for example, can achieve exhaust velocities of ~4,500 km/s. This is sufficient to get a probe to Alpha Centauri in ~100 years with two stages - if it can produce sufficient power in the exhaust jet. Present concepts lose about ~90% of their energy as thermal emissions, which limits the maximum power achieveable. If this can be improved - an engineering NOT a physics issue - then ultimately fission fragment rockets could reach Alpha Centauri in a century.

Alternatively large thermonuclear devices with high burn-up fractions would allow pulse driven rockets to achieve the same mission, perhaps with twice the exhaust velocity. Either system is nuclear technology not too far removed from present know-how. Controlled fusion would make the job easier, and is assumed by "Project Icarus", but the preferred ignition system is currently still under investigation. Whether such can be engineered small is another question.

23. May 9, 2012

### Ryan_m_b

Staff Emeritus
A minor quibble but just because something isn't physically impossible doesn't mean it isn't science fiction. It may well be that there is a development pathway that can be followed towards the creation of multi-hundred-GW lasers but that's so far removed from what we can achieve today that it is more fiction than science. This isn't a bad thing but we should be honest with what we're discussion so as to avoid giving a false sense of ease.

24. May 9, 2012

### qraal

I agree that portraying it as easy is a mistake, but the "science fiction" tag sounds to me like saying it's unphysical, which is the worst kind of thing to say on Physics Forums IMO.