SETI always makes us ask what human-centered assumptions we are making about extraterrestrial civilizations. When it comes to detecting an actual technology, like the starships we’ve been talking about in the last two posts, we’ve largely been forced to study concepts that fit our understanding of physics. Thus Robert Zubrin talks about how we might detect a magsail, or an antimatter engine, or a fusion-powered spacecraft, but he’s careful to note that the kind of concepts once studied by the Breakthrough Propulsion Physics Project at NASA may be undetectable, since we really don’t know what’s possible and what its signature might be.
I mentioned zero-point energy in a previous post because Zubrin likewise mentions it, an idea that would draw from the energy of the vacuum at the quantum level. Would a craft using such energies — if it’s even possible — leave a detectable signal? I’ve never seen a paper on this, but it’s true that one classic paper has looked at another truly exotic mechanism for interstellar travel, the wormhole. These shortcuts through spacetime make space travel a snap. Because they connect one part of the universe to another, you go in one end and come out the other, emerging into another place and, for all we know, another time.
The fact that we don’t know whether wormholes exist doesn’t mean we can’t think about how to detect one, although the authors of the classic paper on wormhole detection make no assumptions about whether or not any intelligent species would actually be using a wormhole. The paper is “Natural Wormholes as Gravitational Lenses,” and it’s no surprise to find that its authors are not only wormhole specialists like Matt Visser and Michael Morris, but physicists with a science fiction connection like John Cramer, Geoffrey Landis, Gregory Benford and the formidable Robert Forward.
Image: A wormhole presents a shortcut through spacetime. Can one be detected? Credit: Wikimedia Commons.
The analysis assumes that the mouth of a wormhole would accrete mass, which would give the other mouth a net negative mass that would behave in gravitationally unusual ways. Thus the GNACHO (gravitationally negative anomalous compact halo object), which playfully echoes the acronym for massive compact halo objects (MACHOs). Observationally, we can look for a gravitational lensing signature that will enhance background stars by bending light in a fundamentally different way than what a MACHO would do. And because we have MACHO search data available, the authors propose checking them for a GNACHO signature.
In conventional gravitational lensing, when a massive object moves between you and a much more distant object, a greatly magnified and distorted image of the distant object can be seen. Gravitational lensing like this has proven a useful tool for astrophysicists and has also been a means of exoplanet detection. But when a wormhole moves in front of another star, it should de-focus the light and dim it. And as the wormhole continues to move in relation to the background star, it should create a sudden spike of light. The signature, then, is two spikes with a steep lowering of light between them.
The authors think we might find the first solid evidence for the existence of a wormhole in our data by looking for such an event, saying “…the negative gravitational lensing presented here, if observed, would provide distinctive and unambiguous evidence for the existence of a foreground object of negative mass.” And it goes without saying that today’s astronomy, which collects information at a rate far faster than it can be analyzed, might have such evidence tucked away in computer data waiting to be discovered by the right search algorithms.
Would a wormhole be a transportation device? Nobody knows. Assuming we discover a wormhole one day, it would likely be so far away that we wouldn’t be able to get to it to examine its possibilities. But it’s not inconceivable that a sufficiently advanced civilization might be able to create an artificial wormhole, creating a network of spacetime shortcuts for instantaneous travel. Matt Visser has discussed a wormhole whose mouth would be held open by negative energy, ‘…a flat-space wormhole mouth framed by a single continuous loop of exotic cosmic string.’ A primordial wormhole might survive from the early universe. Could one also be created by technology?
Civilizations on the Brink
More conventional means of transport like solar or laser-powered sails present serious problems for detection. In Jerry Pournelle and Larry Niven’s The Mote in God’s Eye, an alien lightsail is detected moving at seven percent of the speed of light, its spectrum the same as the star that it is approaching but blueshifted, which is how analysts have determined it is a sail. The novel’s detection occurs with far more sophisticated observatories than we have in our day, when finding a solar or lightsail in transit would be a tricky thing indeed. A fusion rocket, for example, would emit largely in the X-ray range and could be detectable for several light years, but a lightsail is a highly mutable catch.
I remembered reading something about this in Gregory Matloff’s Deep Space Probes (Springer, 2005) and checked the book to extract this:
If ET prefers non-nuclear travel, he might utilise a laser or maser light sail. If the starship is near enough and the laser/maser is powerful enough, reflections from the sail might be observable as a fast-moving and accelerating monochromatic ‘star.’ However, detection will depend on sail shape and orientation as well as other physical factors.
Therefore, it is not as easy to model the spectral signature of these craft as it is energetic nuclear craft. A starship accelerated using lasers or masers may be easier to detect during deceleration if a magsail is used.
Writing in the comments to yesterday’s post, Centauri Dreams reader James Jason Wentworth recalls Larry Niven’s short story “The Fourth Profession,” which has a lightsail detection something like the one in The Mote in God’s Eye:
“All right. The astronomers were studying a nearby nova, so they caught the intruder a little sooner. It showed a strange spectrum, radically different from a nova and much more constant. It got even stranger. The light was growing brighter at the same time the spectral lines were shifting toward the red.
“It was months before anyone identified the spectrum.
“Then one Jerome Finney finally caught wise. He showed that the spectrum was the light of our own sun, drastically blue-shifted. Some kind of mirror was coming at us, moving at a hell of a clip, but slowing as it came.”
Some sails could be truly gigantic, and we can imagine worldships large enough to require sails the size of a planetary radius, which could be detected when near their home or destination stars, but would be hard to find when in cruise. Matloff goes on to suggest that any search for this kind of ship should look near stars from which an entire civilization might be emigrating. A star like Beta Hydri is a possibility, a nearby (21 light years) solar-type star now expanding from the main sequence. This is the longest shot of all, but finding unusual signatures in visible light near a star leaving the main sequence would at least compel a second look.
The wormhole paper is John Cramer, Robert L. Forward, Gregory Benford et al., “Natural Wormholes as Gravitational Lenses,” Physical Review D (March 15, 1995): pp. 3124-27 (available online). See also Matloff and Pazmino, “Detecting Interstellar Migrations,” in Astronomical and Biochemical Origins and the Search for Life in the Universe, ed. C. B. Cosmovici, S. Bowyer and D. Werthimer, Editrici Compositori, Bologna, Italy (1997), pp. 757-759.
I can’t help thinking we might be looking at completely the wrong size scale when it comes to interstellar travel. Given the colossal energy required to get up to a decent speed, it would make sense to send the smallest robotic craft you can. I suspect the first spacecraft to reach other stars will be the size of grains of sand, relaying data home via a long chain of such miniaturised craft.
Or maybe not, since even putting them at 1000 km intervals would need about a billion of them per light year, which could end up weighing a few tons anyway . . . But I think it’s at least conceivable that what we should be looking for is radio-transmitting micrometeors, probably transmitting at a wavelength consistent with their size.
I’ve really enjoyed the trio of excellent posts on this topic. While doing a bit of background research into this subject on the back of these posts I came across this recent paper by Juan Carlos Garcia-Escartin and Pedro Chamorro-Posada of the Universidad de Valladolid, Spain, who propose searching for ‘relativistic mirrors’ using the same wavelength-shifted stellar spectra method described in Niven’s short story. They also propose a possible survey that would narrow down where to look for them and even how a solar sail could be used as a beacon.
http://arxiv.org/abs/1203.3980
The biggest obstacle to detecting a starship would be the origin star, lying on the same line of sight as the ship. The farther a ship has come and the further away that star, the easier the detection. Without prior knowledge of the starship and its signature, however, detection would need a better signal-to-noise ratio for the launch laser’s narrow waveband or the fusion rocket’s x-ray excess. Isn’t fusion much hotter than the typical stellar corona? If so, then its x-rays would be at shorter and hopefully less noisy wavelengths. Also, the fusion signal might be flickering, as with the Icarus rocket, so a temporal filter on the receiver could search for a chopper frequency that enhances S/N.
Love the idea of thinking that we have already detected an ETI and the data is just waiting to be looked at in the right way. So, I am fascinated to learn that we could find GNACHO’s in the MACHO data we already have!
After the discussions in the comments of your first article on detect ETI I did some searching around and came across this paper: http://arxiv.org/abs/1203.3980
The authors discuss the idea of a relativistic ship acting as a mirror and what that might look like to a viewer. They also discuss launching a mirror away from the Sun and angling it to reflect a cosmological event, say a pulsar, at an alien world. Thought it was relevant to the second part of this article so I thought I would share.
Interesting posts lately. If we are talking about various different attitudes to SETI, it is also worth pointing out search for Dyson sphere-which surprisingly did result in some candidates.
http://home.fnal.gov/~carrigan/infrared_astronomy/Other_searches.htm
http://home.fnal.gov/~carrigan/infrared_astronomy/Fermilab_search.htm
John Cramer has collected most of his “The Alternate View” columns from Analog magazine on his web page; some are dated, but they’re all good reads. He has an excellent series on wormholes:
http://www.npl.washington.edu/AV/av_index.html#6
With the vastly improved capabilities that new instruments eg. SKA, LOFAR, LSST, ALMA, TMT, E-ELT, if these instruments are funded and completed,
(LOFAR and ALMA nearing completion), there is a chance that something artificial could be stumbled upon in the course of observing runs. Perhaps we might be graced with a serendipitous detection of ETI, whether a starship or some other activity. Just maybe that’s what the Wow! signal was.
I think it might be rather difficult to obtain support and funding for any of the search stategies I’ve read here over the last 3 days. Especialy considering how difficult it is to define precisely what we should be looking for. I think a good approach would be to be prepared to recognize anything that is not clearly a natural astronomical object to the best of our knowledge. So perhaps piggybacking SETI on existing and future astronomical programs would be the most practical approach.
There will be enormous data-mining opportunities in the data bases produced by these new instruments for conducting searches for anomalous objects. Any strange detections would then be vetted. If all possible known natural objects can’t describe an anomalous detection then maybe we’ve discovered a new class of astronomical object or we stumbled onto ETI.
I’m certain that there will be groups conducting these kind of searches when the data becomes available.
I think that Matloff’s idea of searching Beta Hydri (or similar stars) for signs of a mass migration misses the ultimate by a long chalk. In that case it would make no difference to that ETI (other than preference) if that migration was over a thousand years or 10 million. Under those circumstances, optimising for travel velocity and reducing costs would be more likely to give a flux of travellers on the lower side, and to make the migration time longer. To me, much better is the case of an O or B star 10,000 years before the supernova. Here all must leave suddenly and use the resources at their disposal. This highly unusual situation would then make it optimal redirecting most light of their star rather than produce a narrow laser beam, and shine it over their massive and disbursed migrating fleet. They now have true impetus to leave as one cohort.
I feel that people think too often about the sort of star that a K2 ETI is likely to evolve on, rather than the sort that it would chose as a home. Let me put it this way. If you could be suspended without aging, and you could pick, would you rather wake up in a hundred years time at Proxima Centauri, or in ten thousand years at Rigel knowing that your descendants would then go on to create a civilisation that would eventually be a hundred million times more powerful and significant.
Rob,
in your latest comment (August 22, 2012 at 20:14) you touch a very significant topic which reminded me of a similar issue, with regard to survival strategy, in biology (ecology): long-lived but slowly reproducing and slowly dispersing (called K-select) climax species, versus short-lived but fast reproducing and fast dispersing (called r-select) pioneer species.
So, eventually, this would all depend on whether an ETI would, based on its abilities and inabilities, prefer a very long and stable stay in one location, or relatively short stays at various locations followed by rapid migrations, as a survival strategy.
But then, the combination may also be possible and even preferable: rapid dispersal to, plus long stay at many suitable locations. These are the true winners. Reminds me of redwoods: fast-growing and fast-dispersing, but also very long-lived.
Back to Fermi.
Given the very large sail sizes that are postulated for light sail starships (Robert Forward’s Epsilon Eridani expedition starship design had a 1,000 km diameter sail, and the sail of the alien vessel in Larry Niven’s “The Fourth Profession” was “…a few molecules thick and nearly five hundred miles across when it’s extended”), maybe some temporary “luff” in the edges of the sail (perhaps occurring if the launching laser’s beam aim was re-adjusted during the acceleration phase) would reflect detectable amounts of light from its launching laser in our direction, even if such a ship was traveling as much as 90 degrees to our line of sight? Also:
Assuming that such a huge sail would be spin-rigidized, any movable light pressure vanes or movable sail panels that would be used for maneuvering might also reflect laser light toward Earth. A “multi-stage” light sail like Robert Forward’s design might also cause such stray reflections when the outer ring sail detached from the inner deceleration stage and then changed its shape to focus laser light on the deceleration stage (which would flip 180 degrees to reflect light from the ring sail in the deceleration stage’s sail’s direction of motion, like a retro-rocket). Such powerful flashes of monochromatic light might look very much like deliberate ETI optical interstellar laser signaling.
Check out “extreme scattering events” (Mark Walker is a champion of these in the literature). You will see that the profile of an ESE is as described in the article (but in the radio, not the optical).
Tim:
As I think you have started to realize yourself, there is a formidable array of problems with scaling down spacecraft, concerning propulsion, communication, and shielding. Small devices cannot produce strong magnetic fields, or contain plasma, or capture beamed energy. Small antennas radiate much less power than large ones, and are less sensitive in reception. The mass of the spacecraft goes down as the cube of size, while the amount of ISM it encounters goes down as the square. Coupled with the ineffectiveness of magnetic shielding at small size, this spells doom for small, near-relativistic craft as they quickly erode in the oncoming ISM.
Tim J: I’ve had similar thoughts and described such a scheme within a comment to a post a year or so ago. I speculated on using quantum entanglement to (hopefully!) get around the communications challenges. I believe I suggested something more pebble-sized (1 cubic cm) than grain-sized, as that would allow cell-phone quality optics and a larger power budget for other sensors and on-board equipment.
Eniac: You raise very good points. If quantum entanglement is not feasible for long-distance communication, I agree the communications challenge is very difficult, even with the “bucket brigade” aproach that Tim and I (earlier) suggested. Friction with the interstellar medium might be an even bigger challenge, perhaps unmanageable at such a small size probe, given known physics. I still think it may be useful to carefully consider this class of craft and determine if the challenges they present are easier to overcome than the challenges faced by other types.
Eniac: “Small antennas radiate much less power than large ones, and are less sensitive in reception.”
I think you mean that they are less directive. It is theoretically possible to make small antennas highly directive and capable of large power rating, but only with exotic materials, superconductors in particular. There are also some fabrication issues to deal with.
Since these materials and techniques are not out of the question in the foreseeable future, smallness becomes a problem for power; that is, how do you pack a lot of usable energy into a tiny spacecraft? Without it there will be no effective transmission capability.
Eniac:
“Coupled with the ineffectiveness of magnetic shielding at small size, this spells doom for small, near-relativistic craft as they quickly erode in the oncoming ISM.”
But consider Brin’s compact, smart vessels in EXISTENCE…rugged and interactive.
Wojciech said on August 22, 2012 at 14:59:
“Interesting posts lately. If we are talking about various different attitudes to SETI, it is also worth pointing out search for Dyson sphere-which surprisingly did result in some candidates.”
So are these candidates being followed up on, and by whom? Nice to see that someone is trying something other than the usual SETI route.
And those who have the capability should really be checking out NGC 5907, speaking of Dyson Shells and Kardashev Type 2/3 civilizations.
While this article is largely about life aboard a Worldship and how it could be arranged, this will naturally affect the external appearance of the vessel and even in turn its method of propulsion. Of course this is assuming that ETI will be sending their species around the galaxy in such a manner and have some biological resemblance to us.
http://news.discovery.com/space/icarus-interstellar-project-hyperion-worldship-120710.html
Speaking of other “exotic” methods of doing METI or at least creating attention on an interstellar scale, check out this fellow’s ideas here:
http://www.obs-hp.fr/~larnold/homepage.html
Especially this from there:
http://www.obs-hp.fr/~larnold/news_0504.html
Tim J, Eniac, Mike Lockmoore, Ron S and Gregory Benford: I was thinking about ‘pebble-sized’ (indeed also 1 cubic centimetre) craft in my story “Connoisseurs of the Eccentric“, which just went live on Escape Pod about two weeks ago.
Fired like a bullet from a very long gun (44 kilometres? 440 kilometres?), I imagine a *very* elongated cylinder (say: 1000 or more times longer than its diameter) : both the back and front end could function as a shock/heat absorber annex impact shield.
Firing it, and drilling itself into a large body (asteroid, moon) to come to a standstill might be the biggest problem, but one could conceivably test this by firing probes into asteroids of the asteroid belt, before going interstellar.
Crazy? I suppose weirder things have been thought of. And now I need to get a copy of David Brin’s EXISTENCE.
Jetse de Vries:
I think the formula is L=v^2/2a, where L is the length, v is the final velocity, and a the acceleration. For v anywhere near c this becomes an enormous number. Your cannon probably has to be more like 440,000 km long, while still requiring an insanely high acceleration.
Eniac:
Are you sure you’re not missing a metres to kilometres conversion somewhere?
With an acceleration of (as my story mentioned) a billion G, with a 44 kilometre cannon you get 10% of lightspeed. Admittedly, a billion G sounds insane.
However, current long range guns have acceleration forces upwards of 100,000 G (I saw one claimed to have a 170,000 G, on average).
So, if your formula is correct (and I think it is: it’s Saturday night and I will check it later), then the cannon length and acceleration have an inverse proportional relation ship.
Hence, a 440 km barrel needs 100 million G; a 440,000 km barrel (OK: a third over the Earth-Moon distance) needs ‘only’ 100,000 G: this is what long range guns already deliver.
Jetse de Vries, let’s forget the strength necessary for anything to retain some sort of internal structure. At a billion g, release would produce pressure waves through the material stronger than any molecular bonding, so it would explode if made of ordinary matter. Is your probe made of degenerate matter?
Jetse: I think your analysis is correct. The only difference between our opinions is what acceleration we consider “insane”. Note that we are talking a functional device rather than a slug of lead. It also has to be big enough to withstand the ISM and send signals. Supportable acceleration is inversely proportional to size, so we have a very tough trade-off, here.
Besides the constraints that we do not want our projectile to be flattened, producing acceleration gets more difficult as the relative velocity between projectile and gun increases.
Henry G.: is there a calculation to show at which point acceleration will produce pressure waves stronger than molecular bonding? Not disputing your point: I’m interested at which point acceleration becomes “insane”.
With an upper bound to acceleration we could conceivably extrapolate the cannon length needed (which might also be inconceivable or impractical). Knowing it’s either impossible or immensely impractical is also progress.
Eniac: what I wasn’t clear enough on is that I’m imagining the probe itself to be a very elongated cylinder, as well (like the cannon, even if not that long). Anything travelling with a large portion of the speed of light will not have any glancing impacts from the ISM on the cylindrical side: it’s the circular front end of the–very elongated–cylinder that will impact with the ISM.
As such, most of the front end of the elongated cylinder might function as both a (disposable as it wears down throughout the journey) shield and shock absorber.
Send signals from a small probe: again if its an elongated cylinder, the relatively small front end is your resistance/shield against the ISM, while the long back end (say the actual probe is in the middle) might function as an antenna.
Otherwise I will have to resort to the magic handwaving of ‘quantum entanglement’ for communication (which I indeed did in the story).
Do feel free to tell me what I’m overlooking: I think it’s a fascinating thought experiment.
Very few materials have strength beyond 10GPa, and for optimal weight per strength beyond 50 GPa we are only really left with combinations of boron, carbon and nitrogen.
If your one cm probe had the rather light solid density of 1kg/litre it would suffer compression of 100GPa, so it is almost possible for a perfect theoretical super technology now, so if that is your purpose you might only have to dial it down an order of magnitude.
As for what is possible for our technology in just a century or two, many others here are better placed too comment than me, but I note that you are pumping the equivalent kinetic energy into that probe of about 9 orders of magnitude more than its internal chemical bonding energy over a very short period. Somehow only that probe must be isolated from all but that compressional energy of the acceleration – and I think that that would be an engineering marvel.
Jetse: Giving the projectile a needle shape is a great move. Not that I think it is enough, but it goes some ways towards the ISM and communication arguments. Quantum entanglement is a non-starter for communication, as you have pretty much acknowledged.
As Rob correctly points out, when the relative velocity we are talking about reaches kinetic energies greater than that of molecular cohesion, we reach all sorts of limits, not only of strength but also of the ability to further accelerate a projectile. The rate of change of electromagnetic fields, for example, is limited by molecular cohesion of the device producing them, and that has implications for any electromagnetic acceleration mechanism. Coil guns, rail guns, and such tend to max out at a few km/s, for this reason.
To get around this, you’d have to have a long series of nested devices, each a few km/s faster than the previous, each accelerating the next. This starts looking much like a rocket, which I think is the only realistic way to go. Of course, a rocket needs some kind of “combustion chamber” and those are hard to miniaturize. The cyclotron radius of ions in magnetic fields is of particular interest here. Any magnetic nozzle would have to be large enough to contain it.
When using a mass driver to accelerate a payload, the energy you expend grows quadratically with velocity.
When using a rocket (on-board fuel), the energy you expend grows exponentially with velocity.
Needless to say, an exponential increase is far steeper than a quadratic increase.
And, of course, the necessary infrastructure tends to grow proportionally to the energy you must master.
Whatever problems may arise when trying to accelerate a payload to 0,1c by mass driver pale in comparison to the difficulties encountered when trying to accelerate the same payload by rocket.
Avatar2.0:
I disagree, vehemently.
We can readily imagine a nuclear rocket that provides the necessary exhaust velocity to get it done. At a decent mass fraction, exponential rocket equation and all.
On the other hand:
1) We are nowhere near a stationary device that can accelerate a projectile whizzing by at 0.1c. It completely, utterly boggles the mind. Conventional electromagnetic mass drivers cannot do it. Calculate the electromotive force associated with the required rate of change in magnetic fields if you do not believe me. It would short out ANY material, and the vacuum, too.
2) Even if you did find a way, what would it do to your mass fraction if you had to include all several hundred thousand kilometers of the driver? How heavy would it be, compared with the payload? It does not help if it goes with the square (only?) if it ends out being a billion to one.
3) Lastly, the rocket has the distinct advantage that anything larger than a microchip will not be crushed by tens of thousands of g of acceleration. And we know we will need something quite a bit larger, in order to avoid being vaporized in the ISM headwind.
Avatar2.0:
What you are missing, I think, is that only nuclear processes could produce the energy density needed to render either the square or exponential increases managable. A non-nuclear mass driver has no chance, even if the law is square only. Look at it this way: The square of 100,000 is much larger than the exponential of one.
0.1 c is comparable with nuclear energy (i.e. the fragments of a fission reaction happen to have about that velocity). On the other hand, it is 100,000 times faster, or 100,000^2 times more energetic than energy that is produced or transmitted by molecular interactions.
So, yes, your statements would be absolutely correct if we were talking chemical rockets, only. Luckily, we are not restricted to that. The key is in fission (or fusion) fragment rockets
“What you are missing, I think, is that only nuclear processes could produce the energy density needed to render either the square or exponential increases managable. A non-nuclear mass driver has no chance, even if the law is square only.”
Who said anything about a non-nuclear mass driver? The mass driver will be powered by the same fusion/fission reaction that would power the rocket. It’s just that, with a mass driver, a LOT less energy will be needed for 0,1c; and at this point of the quadratic/exponential curves, the slightest increase in velocity translates into enormous disparities for the expended energy.
“1) We are nowhere near a stationary device that can accelerate a projectile whizzing by at 0.1c. It completely, utterly boggles the mind. Conventional electromagnetic mass drivers cannot do it. Calculate the electromotive force associated with the required rate of change in magnetic fields if you do not believe me. It would short out ANY material, and the vacuum, too.”
No really.
You only need to calculate the rate of change in magnetic fields that your best materials can withstand. When the payload is stationary, it will impart a high acceleration; when the payload has gained velocity, the imparted acceleration will drop quadratically (as per e=mv^2).*
Trying to impart the same acceleration to a payload flying at different speeds is a fool’s errand.
The mass driver will need to be long. As any current interstellar strategy that involves sending a respectable payload involves mega-engineering. Except you will be able to reuse the mass driver, as opposed to a rocket.
“2) Even if you did find a way, what would it do to your mass fraction if you had to include all several hundred thousand kilometers of the driver? How heavy would it be, compared with the payload? It does not help if it goes with the square (only?) if it ends out being a billion to one.”
The mass driver must not be accelerated to 0,1c; only the payload. It being heavier is actually an advantage, for it means it will stay more or less in one place, removing several engineering and logistical problems.
“3) Lastly, the rocket has the distinct advantage that anything larger than a microchip will not be crushed by tens of thousands of g of acceleration. And we know we will need something quite a bit larger, in order to avoid being vaporized in the ISM headwind.”
The acceleration problem, while serious, is overstated. The payload will only need to withstand the top acceleration for a few seconds. For 99.99% of the length of the mass driver, the acceleration will be significantly lower.
*What about the enormous nuclear energy expenditure needed for the first hours of a nuclear rocket’s flight? I doubt there are materials that can withstand so much heat/etc.
Unless you make the nuclear rocket start with a lower acceleration and increase this acceleration as the rocket grows lighter. In other words, the same philosophy as the solution I proposed for the mass driver.
Eniac, while I agree there are major problems with Avatar’s scheme I feel that you are setting up the straw man a little in the following regards
1) Limiting the (chain of linked?) accelerators to just 100,000 km length. Though I admit that this seems a very reasonable assumption to begin the debate, further calculation shows that this generates such difficulties as to indicate a real working model would be much longer.
2) Launching each probe as a single piece, rather than accelerating the shielding separately followed by assembly (by nanobots?), is more likely for the reasons you gave.
3) Deriving a mass ratio as if such a launcher would only launch one probe to one star. This would be strange as it would clearly have the potential to launch millions of these probes to thousands of stars at very little marginal extra cost.
Rob Henry
One failure of any current atomic rocket (including Icarus) is to be able to accelerate to 0,1c and then to decelerate to 0. The amount of fuel needed for deceleration spirals out of control. Indeed, such fusion/fusion rockets will, most likely, only be able accelerate to 0,05c and then decelerate to 0.
But, with a mass driver, one could accelerate a payload/ship to 0,1c and then decelerate this ship to 0.
HOW?:
By using the mass driver to accelerate only the fuel to 0,1c – the fuel being the fissionalbe or fusionable material.
It will be accelerated in pellets only a few grams heavy. Targeted precisely towards the ship by stations, situated a few light-minutes/hours away from the mass driver, that correct the trajectory of incorrectly-targeted pellets.
The ship, now en route, will give the arriving pellets an electrostatic charge. The ship itself will be equipped with an electrostatic field that decelerates/accelerates the pellets until they have the same velocity as the ship and does the reverse when the pellets are departing the ship.
Of course, the fuel will only depart the ship once it has undergone fission/fusion and accelerated/decelerated the ship.
The fundamental question is – how much fuel must you accelerate by mass driver?:
Now you don’t need to worry about adding fuel to accelerate the fuel already on-board – which gives the exponential curve of the rocket equation; there is little/no fuel on board the ship at any time of the journey.
If the ship is 100 tonnes heavy, now you must only accelerate the fuel to carry these 100 tonnes to 0,1c; NOT 100 tonnes + fuel + fuel to accelerate the fuel +fuel to accelerate this fuel, etc. Which translates into immensely less fuel than a standard nuclear rocket, slave to the rocket equation, needs to reach 0,1c.
And if you want to accelerate the payload/ship to 0,1c and decelerate it to 0, you only need to accelerate by mass driver TWICE as much fuel as you would need to only bring the ship to 0,1c.
That’s right – if the ship is and remains at all times 100 tonnes heavy and can take its fuel en route, the amount of fuel needed grows only LINEARLY with delta v. NOT exponentially.
In conclusion:
On the one hand, you have nuclear rockets and other schemes that either can only accelerate to 0,1c (deceleration implying astronomically high energies/amounts of fuel) or are very VERY slow.
On the other, you have a mass driver (for accelerating fuel: less fuel – by an enormous amount – than needed by the rocket equation for the same delta v) and a respectably heavy ship who can both accelerate to 0,1c and decelerate to 0 with this fuel.
Which is the better option?
Avatar:
But, presumably, the power will have to be transmitted by non-nuclear means, which limits its density.
This is incorrect. A fission fragment rocket with a 0.1c exhaust velocity requires a very reasonable mass ratio, so it will require maybe a few times more energy. Not a LOT, compared with the other problems.
This was not part of the original length estimate. That was based on constant acceleration. This new equation of yours will require VASTLY larger accelerators, still. And the rate of change limit for magnetic fields provides a maximum velocity at which acceleration is possible at all, and this is around a few km/s, far, far from the goal.
Not true. The rocket would not require mega-engineering at all, which is exactly what makes it so attractive.
It does not matter if it is being accelerated or not. Using an ideal fission fragment rocket, you can accelerate a 1 ton payload to 0.1c using just a few tons of fuel. I’d take that over a billion ton mega-accelerator any day.
No, I do not think it is overstated. Let me try to restate it clearly and without bias: At a constant acceleration of 10,000 g or so you will need a length of 1/(2a) v^2 ~ 4.5 million km. For a more “reasonable” length of 45,000 km, you would need to support an acceleration of 1,000,000 g. This is assuming acceleration is constant throughout the entire process, even at the muzzle velocity of 0.1c. If it is not, as you seem to envision, all bets are off on length, except that it certainly won’t be shorter.
Well, first of all, the energy expended by the rocket is stretched out over several years of relatively gentle acceleration, rather than just a few seconds. Second, you are correct that no material can support this much power, which is why fission/fusion fragments have to be exhausted directly, never hitting any materials. A magnetic nozzle is a reasonable engineering solution for that. AFAIK, there is no reasonable solution for channeling the much greater power needed for your accelerator using any type of materials.
Rob: You make interesting points, but 1) assuming a shorter than necessary accelerator is hardly a strawman, especially since anything of that size is already much too large. 2) Assembling a probe in flight from separately launched pieces is nearly laughable in its impracticality. The mother of all docking problems, I’d say. 3) Even if a billion ton accelerator could send millions of teacup sized probes, it would still be a mass ratio of 100,000 or so, while the fission rocket would make do with 3-5. And rocket fuel is much easier to process than a complicated mega-structure with an impossible power channeling capacity. You could send a million rockets for much cheaper than building that mega-accelerator.
Avatar: You seem to driven by mindless fear of the rocket equation. All the rocket equation really does is limit the terminal (cut-off) speed of any rocket to a small multiple of the exhaust velocity. For chemical rockets, the exhaust velocity is ~3-4 km/s, and rockets pretty much top out at 10-15 km/s. For nuclear rockets, on the other hand, exhaust velocity can be around 0.1c, limiting the cut-off speed to around 0.3-0.4 c. This very much allows for a 0.1c trip including deceleration with very reasonable quantities of fuel.
Of course, we do not know how to design an efficient nuclear rocket, but all in all it seems like a much more doable proposition than your impossible mega-accelerator. So, all things considered, in answer to your question:
I would have to answer with a resounding: The nuclear rocket, of course!
Eniac
I talked about a system capable of accelerating a payload to 0,1c and decelerating it to 0.
A mass driver energy consumption grows quadratically;
A rocket energy consumptions grows exponentially.
For a delta v of 0,2c:
1.
A rocket DOES involve megaengineering; you effectively send a not-so-small moon to the nearest star – fission fragment or whatever. Do the calculations for 0,2c yourself if you do not believe me, before claiming: “Not true. The rocket would not require mega-engineering at all, which is exactly what makes it so attractive.”.
A mass driver also involves megaengioneering – but not on the scale you assume.
2.
If we neglect thermodynamic losses, the difference in expended energy between a quadratic growth and an exponential growth are ENORMOUS – for delta v of 0,2c.
You essentially assume that thermodynamic losses give the rocket the edge, despite this. But your argument is not sufficient – by a large margin – for this:
“But, presumably, the power will have to be transmitted by non-nuclear means, which limits its density.”
For a mass driver, the energy will be generated in nuclear reactors; it will be transmitted by superconductors, made as thick as needed (enormous capacity, little losses).
As for the rocket – it starts with the enormous minimum of an exponential curve for a delta v of 0,2c. And you have all sorts of limits:
-a reactor built to expel its reaction mass is much more inconvenient than a conventional one – which translates into losses;
-you say “you are correct that no material can support this much power, which is why fission/fusion fragments have to be exhausted directly, never hitting any materials.”; but the rocket is propelled by the reaction of these fission/fusion that never touch any materials – aka fission/fusion fragments that DO touch the rocket. Magnetic fields to contain them? – we are talking about material limitations yet again.
3.
And you invoke limits that should make the mass driver not feasible:
a.I already said how the the limit of changing magnetic field can be overcome – by making the mass driver longer.
Is it megaengineering? Yes. But I will take megaengineering that can be used thousands of times over megaengineering that can only be used once (the 0,2c rocket).
b. “Using an ideal fission fragment rocket, you can accelerate a 1 ton payload to 0.1c using just a few tons of fuel. I’d take that over a billion ton mega-accelerator any day.”
An ideal fission fragment rocket still gives you a small moon for 0,2c.
And my billion ton mass driver can be used thousands of times, not only once, as your rocket.
c. The acceleration problem – “No, I do not think it is overstated.”:
The mass driver will only accelerate fuel pellets – one weighing a few grams and being at most, 1 cm long.
Because it is homogenous fuel, it can withstand enormous acceleration. Because is is at most 1 cm long, the acceleration differential must be gigantic before breaking the fuel pellet apart.
Because it weighs a few grams, its acceleration – initial and otherwise – will be FAR larger, shortening the mass driver.
Eniac
Talking about fuel pellets – we can go far further than a few grams or 1 cm.
We can make than of molecular size before diffraction becomes from negligible, an obstacle.
And, if we have molecular size, we can accelerate these fuel molecule-sized pellets with electric fields – LHC does this, reaching speeds of 0,9c.
We can make the accelerator circular and use the Lorentz force to bend the trajectory of the fuel pellets – thereby transforming the accelerator from megaengineering to a relatively small-sized construction (for a solar system civilization, that is).
PS
“Avatar: You seem to driven by mindless fear of the rocket equation”
Really? There is nothing irrational about the proven fact that a rocket with a delta v of 0,2c is a megaengineering structure that can only be used one time.
Eniac, you are the one that seems driven by an irrational fear of mass-drivers – accelerators in general.
Eniac
“For nuclear rockets, on the other hand, exhaust velocity can be around 0.1c, limiting the cut-off speed to around 0.3-0.4 c. This very much allows for a 0.1c trip including deceleration with very reasonable quantities of fuel”
What efficiency of the fusion/fission reaction are you using for these calculations? To think you are actually reproaching me that I am using absolutes and disregard practical limitations.
Redo these calculations by using fusion/fission reactions at efficiencies we know – or, at least we a have a doable idea on – how to achieve. And then tell me how large the rocket must be for a delta v of 0,2c.
Avatar:
I am assuming a device that can direct fission fragments, which move at roughly 0.1c, out the back of a nozzle without slowing them down too much or losing a large fraction of them. See here for a description and links of several avenues of research that may be pursued: http://en.wikipedia.org/wiki/Fission-fragment_rocket. I like this one, in particular: http://www.rbsp.info/rbs/RbS/PDF/aiaa05.pdf.
If an efficiency of only 20% can achieved, we still have a reasonable mass ratio to get to 0.2 c. As a reference, also study the Daedalus design, which is generally known as conservative, as interstellar designs go. It has a burnout velocity of 0.3 c, if I am not mistaken, and is not moon-sized at all. Note also that I am not saying it is easy, just that it is a lot less phantastic than your mega-accelerator.
While the conductors can be as thick as needed (which would really jack up the weight of the thing, though), in the end all of that power has to be concentrated on the projectile. Enormous power, itty-bitty space. Aka impossible power density.
Calculate the power density needed for continuous 1,000,000 g acceleration, and then tell me the materials that you think can withstand it, either in the projectile or in the accelerator.
Calculate the magnetic field strengths necessary to accelerate an object of your choice (other than an individual charged particle) at 1,000,000 g, and tell me what type of magnet you’d be using to generate them.
I could go on and on with these, but I’d be happy if you could just do one of these to convince yourself.
Your fallacy here is really obvious by now, but I will try to make it as explicit as I can: The quantity in your dreaded exponential is not 0.2c, it is 0.2c/Ve, where Ve is the rocket exhaust velocity. As long as Ve is not much less than 0.2c, your vivid arguments about ENORMOUS rockets simply do not apply.
Now you are talking particle beams. Those are indeed interesting for external propulsion ideas, but they have generally been considered inferior to lasers or microwaves because of collimation issues.
You can only do this with particles, because you cannot charge macroscopic pellets to the degree and precision that would be needed to do the tricks we do in particle accelerators.
You could envision a giant linear accelerator composed of individual stations that refocus the beam and at the same time use its momentum to accelerate outwards. If you keep adding stations, you can in theory extend such an accelerator all the way to a nearby star. Hurdles here would be the minuscule momentum that currently feasible particle beams have, and the fact that even the tiniest losses during refocussing would add up to a total loss over the enormous number of refocus stations that would be needed for interstellar distances. The number and cost of the refocus stations would also be truly astronomical, and it is not clear at all whether the beam expansion can happen at anything close to 0.2c.
Note also, if you accelerate a particle to 0.9c, its kinetic energy far exceeds any nuclear energy it might generate if it were fuel. It therefore does not make sense to accelerate “fuel”, you would use the beam energy directly and save yourself the enormous difficulty of capturing and burning the fuel.
Of course, collimation is the elephant in this room, and I think the outlook here is quite hopeless.
Eniac
Mass driver with fuel pellets:
About efficiency:
“If an efficiency of only 20% can achieved, we still have a reasonable mass ratio to get to 0.2 c.”
And if the superconducting magnets the mass driver is made are made of a material with a – insert arbitrarily large critical current density – then one can make the mass driver a few thousand kilometers long.
That’s how you operate?
For the mass driver, only what we know today – with certainty – can be achieved.
For the rocket, discoveries that may or may not come – that may or may not even be feasible.
Is this the comparison you are setting up? It’s a straw/man, Eniac.
“As a reference, also study the Daedalus design, which is generally known as conservative, as interstellar designs go. It has a burnout velocity of 0.3 c, if I am not mistaken, and is not moon-sized at all. Note also that I am not saying it is easy, just that it is a lot less phantastic than your mega-accelerator.”
The Daedalus design can only ACCELERATE to ~0,1c. It cannot decelerate.
What don’t you compute the small-moon-size the ship must be to decelerate – delta v of 0,2c.
And use fission at today’s – or near future – achievable efficiencies.
“While the conductors can be as thick as needed (which would really jack up the weight of the thing, though), in the end all of that power has to be concentrated on the projectile. Enormous power, itty-bitty space. Aka impossible power density.”
Impossible? If you could accelerate the entire Earth to near-light speed, despite the enormous kinetic energy it will contain, it will be just fine. From the other POVs, it will have enormous kinetic energy. So what?
“Calculate the power density needed for continuous 1,000,000 g acceleration, and then tell me the materials that you think can withstand it, either in the projectile or in the accelerator.”
ANY material could contain the kinetic energy once he has the necessary speed.
As for the acceleration phase – what breaks the pellets apart would be the difference in acceleration between the ends of the pellet. If this pellet is short, the difference will be too small to vaporize the pellet regardless of the order of magnitude of the acceleration.
“Calculate the magnetic field strengths necessary to accelerate an object of your choice (other than an individual charged particle) at 1,000,000 g, and tell me what type of magnet you’d be using to generate them.”
I already answered this, Eniac. And you acknowledged it before suddenly forgetting about it – because it’s in the interest of your rocket pet idea.
Eniac
“Your fallacy here is really obvious by now, but I will try to make it as explicit as I can: The quantity in your dreaded exponential is not 0.2c, it is 0.2c/Ve, where Ve is the rocket exhaust velocity. As long as Ve is not much less than 0.2c, your vivid arguments about ENORMOUS rockets simply do not apply.”
Before you go on with this affirmation of yours, do read how Daedalus was built for a delta v of ~0,1c.
Now – take the rocket equation:
http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation
Calculate the effective exhaust velocity NOT at an ideal 0,1c – but by considering fission at efficiencies we can do today (see the specific impulse). You’ll find that the figure for effective exhaust velocity is..let’s say, somewhat below 0,1c.
And then honestly calculate the natural logarithm needed to have, on the other side of the equation, delta v of 0,2c.
Result – small moon in flight.
Eniac
Super LHC (NOT a mega-engineering construct) with molecular-sized fuel pellets:
“Now you are talking particle beams. Those are indeed interesting for external propulsion ideas, but they have generally been considered inferior to lasers or microwaves because of collimation issues.”
The molecular-sized fuel pellets will be a few thousand fuel atoms/molecules large (aka particles).
Small enough to be accelerated to 0,1c (and even beyond) by electric fields (after they are accelerate, they will, of course, be made electrically neutral).
Large enough that diffraction is negligible.
As for collimation.
Do name the limits of collimation as known today. When you have the distances available in space (light-minutes/light-hours) these limitations will evaporate.
A few solutions present themselves – are, indeed, obvious:
1
The deviation of the number of particles not targeted correctly can be corrected by stations situated a few light-minutes/a few light-hours away from the accelerator. These stations could use changing magnetic fields to correct the errand particles’ path – and their distance from the accelerator will make the stations effective (the vast majority of particles that are so precisely targeted, that after a few light-hours they have not considerably changed their path will remain on course for light-years).
I can even think of other, not really needed solutions:
2
A station (let’s say, in the form of a ring), now situated just outside the accelerator, will heat the fuel particles (with radiation hitting the fuel particles sideways aka not in their direction of motion) until said fuel particles reach their melting point.
By using changing magnetic fields, this station will push the now molten fuel particles (again, sideways aka not in their direction of motion), from all sides (note – it can do so with a lot of force, given that the particles have no sideways velocity), to the center of the particle beam.
The vast majority of the particles will come together, forming larger, more easily manageable fuel pellets.
3
etc
“Note also, if you accelerate a particle to 0.9c, its kinetic energy far exceeds any nuclear energy it might generate if it were fuel. It therefore does not make sense to accelerate “fuel”, you would use the beam energy directly and save yourself the enormous difficulty of capturing and burning the fuel.”
Whether you use accelerator/mass driver or rocket, you NEED to accelerate the fuel (particle or not – but the same mass) to 0,1c (and back). Therefore, it makes a lot of sense to accelerate it.
Note also that the energy expended to accelerate this particle to 0,2c – or 0,9c – by mass-driver/accelerator, is far inferior to the energy to accelerate the same particle (now in a fuel tank) to the same velocity by exponential rocket.
Also note that you can reuse the accelerator for thousands of launches, but the rocket is single-use.
Therefore, it makes a lot of sense to use the accelerator/mass-driver.
Avatar: You are right Daedalus was designed for “only” 0.12 c burnout velocity. Still not too shabby. The effective Ve for Daedalus is ~0.03 c, which is well below the velocity of fission fragments. That one is actually much larger than I thought, around 0.34 c, according to here: http://beforeitsnews.com/space/2012/04/fission-fragment-rocket-engine-thrust-velocity-at-1-7-of-light-speed-2012668.html. That velocity is independent of any “efficiency”, it is a constant determined by the nature of nuclear fission reactions. Efficiency comes in when it comes to fissioning all the fuel molecules and sending all the fragments out back without losses. At 10% overall efficiency in this respect, we would still be more than 3 times better than Daedalus, i.e. at ~ 0.4 c given a mass ratio of 100:1
You should consider that anything other than single particles will have a charge-mass ratio that is 1) MUCH smaller than that of a particle, and 2) impossible to control to single charge precision. Either of these renders a standard particle accelerator ineffective. Fields in an accelerator are tuned to the charge-mass ratio, if it is off even by one charge unit, the projectile is lost. A small charge mass ratio requires correspondingly bigger accelerator, and we are talking several orders of magnitude here.
Sorry I was not clear. I did not mean to say that it made no sense to accelerate the pellet. I meant to say it is unnecessary for the pellet to be fuel. It could be anything at all, because its kinetic energy is as large as any nuclear energy it could produce.
A moot point, of course, because there is no known way to accelerate pellets to such velocities. We are struggling to design mass drivers that can even reach orbital velocity. In theory, even before talking about practice.
I believe you are talking about hitting a starship no more than a few km in size from a distance of several light years. That is an accuracy of about 10^-13, equivalent to hitting a penny from 100,000,000 km away. The only thing that will evaporate here is your pellet, as it is heated by the oncoming ISM.
Eniac
About the rocket:
“The effective Ve for Daedalus is ~0.03 c, which is well below the velocity of fission fragments. That one is actually much larger than I thought, around 0.34 c, according to here: http://beforeitsnews.com/space/2012/04/fission-fragment-rocket-engine-thrust-velocity-at-1-7-of-light-speed-2012668.html. That velocity is independent of any “efficiency”, it is a constant determined by the nature of nuclear fission reactions.”
In the paper you quoted, the initial velocity of the fission fragments is 3,4%c aka 0,034c, NOT 0.34c.
In the end, the speed of the escaping fission fragments is 0,017c.
The rocket equation – for an effective exhaust velocity of 0,017c, if you wish to reach a delta v of 0,2c, the propellant mass must be ~125000 larger than the payload!
In the quoted paper, the payload is 60 tonnes. That’s literally a toy starship – it’s doubtful 60 tonnes can sustain 12 astronauts for 80 years (a trip to Alpha Centauri).
Still, you must carry 7500000 tonnes of propellant for these 60 tonnes.
These strategy will insure that any travel to another star will be for scientific reasons. No colonization (unless we’re talking frozen embryos), no commerce, etc
About the mass driver:
“there is no known way to accelerate pellets to such velocities. We are struggling to design mass drivers that can even reach orbital velocity. In theory, even before talking about practice.”
Actually, a mass driver to orbit is already designed:
http://en.wikipedia.org/wiki/StarTram
http://www.startram.com/
As for 0,1c – no ‘known way’? It’s quite known – just not designed in detail or built.
“I believe you are talking about hitting a starship no more than a few km in size from a distance of several light years. That is an accuracy of about 10^-13, equivalent to hitting a penny from 100,000,000 km away.”
An achievable accuracy – as shown (with the necessary calculations) in the sailbeam concept:
http://en.wikipedia.org/wiki/Jordin_Kare#Sailbeam
http://www.niac.usra.edu/files/studies/final_report/597Kare.pdf
“The only thing that will evaporate here is your pellet, as it is heated by the oncoming ISM.”
Friction with the ISM gives you a temperature of 600K.
If you want fusion, use Lithium deuteride:
Lithium deuteride, in vacuum melts at 680°-697°C, with virtually no decomposition. 680-697C is FAR above 600K. Plus, it’s a solid fuel.
Fission fuels are at least as good.
Friction with the ISM at 0,1c is an eminently solvable problem.
“You should consider that anything other than single particles will have a charge-mass ratio that is 1) MUCH smaller than that of a particle, and 2) impossible to control to single charge precision. Either of these renders a standard particle accelerator ineffective. Fields in an accelerator are tuned to the charge-mass ratio, if it is off even by one charge unit, the projectile is lost. A small charge mass ratio requires correspondingly bigger accelerator, and we are talking several orders of magnitude here.”
1
The fields in the superLHC* will be tuned to the charge-mass ratio, of course.
The superLHC must only accelerate the particles (made of ~1000 atoms) to 0,1c, NOT 0,99c, as today’s accelerators do.
The superLHC will be 2-3 orders of magnitude larger than today’s LHC – still FAR too small to qualify as megaengineering.
2
I doubt it will be necessary, but one can also accelerate single atoms – it may even be the preferable option.
As said, while exiting the accelerator, they can be compressed into macroscopic pellets, if necessary.
And, if they are not compressed into pellets – even fissionable heavy atoms are so large, that DIFFRACTION is irrelevant – the wavelength of the atoms is in the range of pico-meters – a beam made of these atoms can be efficiently focused with a small structure (not moon-sized ones, as you would need for light).
As for collimation – I await your objections, after having read the sailbeam concept, Eniac.
*I chose the superLHC and changing electric field propulsion for the simple reason that we already have, in the LHC, experimental confirmation that it works.
Mass driver can, theoretically, work just as well. It may, indeed, have significant advantages.