If this thread's in the wrong place, please relocate...thanks!

Assuming for the sake of argument here that someday we'll have propulsion systems capable of propelling vehicles at a significant fraction of the speed of light, what kinds of technical challenges will be presented by the interstellar medium?

Right out of the gate, I can't see how anything we might build could survive hitting so much as a dust grain at even 0.01C. Heavy forward shields have been proposed, but the jolts from such collisions even if the spacecraft isn't vaporized would seem a bit unsettling to the payload.

Assuming that issue can be overcome and that we can actually go even faster (<0.5C), at what point would interstellar hydrogen become aerodynamically (or even hydrodynamically!) significant as far as drag? Would true starships actually have to look something like hypersonic aircraft, or even subs depending on relative hydrogen density?

Just to give you some simplistic ballpark numbers. Assuming that drag in this case is entirely caused by momentum transfer, no elastic collission complications etc,

The density of the interstellar vacuum is awfully low - one of the estimates I've see puts forward 1 atom of hydrogen per cubic centimeter. I've no idea if that is accurate but lets just work with that for a minute.
Kinetic energy of 1 atom of Hydrogen impacting at 10%c ~ 7.5e-13 Joules.
Each square meter of frontal surface of the spacecraft sweeps over 100x100x(10%c) CC's of space every second. That works out at 22 Joules/sec/m^2.

So you have to be able to overcome that amount of drag at least.

As far as shielding your spacecraft against erosion caused by this sort of drag is concerned I can't say but it is worthwhile realising that even at 10%c the quantity of atoms that are encountered is minute when compared to solid material. There are about 3E25 molecules of water in a 10cm cube of water. It would take your spacecraft around 300million years of travel at 10%c to encounter a similar quantity of atomic hydrogen over an equal area. Admittedly each impacting hydrogen atom might dislodge very many water molecules but it seems unlikley to me that it would actively erode away a shield at anything more than a few tens or hundreds of atoms per impact and so I think that the Interstellar gas atoms wouldn't be a huge physical problem. A relatively light shield of a a few cm's of material should do fine.

Actual dust particles would be a very different thing though. A single 1 gramme dust grain impact would have a KE equivalent to about 100 kilotons of TNT. you'd need one hell of a shield to deal with that sort of impact. On earth that sort of energy release would result in a cater in the 100-150m diameter range,

Thanks for the numbers, Helvick; interesting!

Yeah, the dust particles are scary. I know that they've got to be extremely rare, but all it takes is one for a very bad day. As long as we're postulating a high-energy drive, how about replacing the shield with a really mean forward IR laser with a "muzzle" beam diameter equal to the longest longitudinal cross-section of the ship? The thing would continuously blast during high-speed flight & hopefully vaporize threatening particles.

I don't think it would do any good on anything bigger than your 1g object, but I would be surprised if encountering bigger rocks is a statistically significant risk.

That would make much more sense than lugging tons of armour up to a percentage of c but having one huge laser seems very power intensive, and more power means more fuel and more mass. It might make more sense to have a very accurate radar system sweeping the flightpath ahead and linking this to an array of smaller lasers to pick off debris one peice at a time. Actually destroying a large peice of matter could result in it fragmenting in any case, which might actually compound the problem. Is there some means by which a particle of dangerous size could be 'nudged' out of the probes path? im thinking either a directed laser again or perhaps some kind of reverse bussard ramscoop using powerful electric or magnetic fields.

Edit: A single large laser would clearly overcome the problem of fragmentation; if it can breakup/ vaporize a big peice it could do the same for smaller fragments, but considering how widely spaced large grains are in interstellar space it would still be a very inefficient approach IMHO.

Inefficient to be sure, but I was assuming that there was a completely hypothetical & unreasonably large energy source available; this whole house of cards couldn't stand without it!

Yeah, I thought about targeting individual specks & zapping them as needed, but the closure velocity is so extreme that the system might not be able to cope with multiple targets if by any chance such things might exist in loose gravitational associations; hard to rule that possibility out, IMHO.

We've already seen effects back in the 1980s which are in the right ballpark for low-relativistic collisions with dust particles - and that was the Giotto Halley encounter. Bigger lumps, and more of them, going slower certainly sandblasted the front of the vehicle, and killed the optical path to the camera - not to mention attitude control issues. The good news, though, is that the o-o-o-o-o-ld idea of a 'Whipple Shield' worked as advertised.

These days, a comparable situation would be Stardust's aerogel collectors (a material which was not available in the days of Giotto).

I imagine that some sort of multi-layer shield and aerogel layers would sort out practically all small impacts, allied with an active RCS system to get the vehicle pointed once more. Perhaps a long thin vehicle would be the answer, with redundant mass at the front - think of 'Discovery' from 2001, but flying backwards...


Bob Shaw

Points noted, Bob. Problem is, as Helvick so ably articulated, that the kinetic energy release from a collision with even minute objects at these speeds is so large that it would almost certainly destroy a spacecraft even if it had a shield that could absorb the impact.

I'd think that a jolt of hundreds of thousands of Gs would result from such a collision. While the structure itself might survive if designed appropriately, I can't begin to imagine all the things that would happen to, for example, solder joints on the circuit boards. Unless the bus & payload electronics physically consisted of something like a giant diamond crystal with few (if any) mechanical interfaces like connectors, I just don't see the thing operating at nominal levels after the event.

Give it all of the above, lasers, layerd areogel shields, small cross section, and include huge shock absorbers.

Perhaps trying to make one probe proof against all possible disasters is the wrong approach. If we have our unfeasibly huge power supply why not split it between a couple of dozen probes, take some basic precautions with each (aerogel shields, small cross section ect) and accept that at least some of them won't make it?
If IEE comes to fruition it might open the door for missions to the interstellar medium that would tell us exactly what precautions are needed, and in what amount.

Edit: nprev point about the power supply is well made, clearly it is this more than engineering issues which will be a showstopper for any mission to another star. Although not forever I hope.

Remember that Giotto didn't just encounter smoke-particle sized material, but larger lumps too - and that the mass of objects scales as the cube of their diameter, so the upper end of the Giotto scale of impacts with the largest particles was probably at the lower end of interstellar flight impacts with tiny particles.

As for larger vehicles, the BIS Project Daedalus looked long and hard at such matters, and concluded that vehicle survival was more than likely!


Bob Shaw

That would be an excellent approach if it's affordable by the time that high-energy technology's available. IMHO, we sure don't want to launch our first interstellar Flagship-class equivalent mission without a few smaller, cheaper forays first. Pioneers necessarily must precede Galileos & Cassinis!

EDIT: Here's a worst-case scenario thought. How about two spacecraft for the first interstellar mission? The first (leading) one is big & dumb. It consists of as much durable mass as we can fly, and its only real function is to act as a cosmic bulldozer, paving the way for spacecraft #2, which is smaller & smarter (the payload).

#1 doesn't even have to decelerate as it approaches the target star; it's no longer needed when #2 begins deceleration, since the drive exhaust in this scenario would probably be energetic enough to zorch incoming dust particles along the line of flight.

Another thing I like about this is that if #1 actually hits something any secondary debris is still probably traveling at or near the transit velocity (in the same inertial reference frame, anyhow), so #2's shielding probably doesn't have to be extremely robust.

Good idea but I think this will lead to a very challenging positioning/navigation problem as S/C #2 must follow a straight line left behind by #1 at extremely high speed.

There's another idea to send a giant spider web-like spacecraft for the first interstellar mission, driven by powerful laze beam from Earth orbit. Its light-weight miniatured payload will be distributed at the web's many nodes to counter against the problem of hitting dust particles along the way. If some nodes fail, the remaining will continue to operate just like the way the Internet works.

I've always liked the idea of propelling a space-sail using lasers, and you could even decelerate at the target star by sending a secound sail/mirror ahead to reflect the laser back at the first.
However I have read a laser sail study (soz I can't remember the title or author) that the laser would need a titanic muzzle apeture to drive the sail across interstellar distances, something like 10,000 meters. If anyone else knows what I'm talking about could you direct me toward the study as I can't remember why the laser needed to be so large. After all even a ten km laser would look like a point source after travelling from earth to mars, never mind a fraction of the distance to alpha centauri.

Aha! A sort of 'A' Ark and 'B' Ark approach, with all the telephone sanitisers going first? Excellent!


Bob Shaw

Well, Bob, if they were manned vessels then #1 would definitely be where all the cheap seats are located...

Thu, that's an interesting approach. The only thing I wonder about is whether your web still might be excessively vulnerable to debris damage to the degree that the structural integrity of the entire vehicle could be compromised, regardless of payload redundancy. Per Helvick's calculations, remember that a 1g impactor carries as much punch as a tactical nuclear weapon at these speeds...ouch!

The nav problem in the two-vehicle scenario actually isn't that challenging. Both ships would be essentially at rest with respect to each other (same inertial reference frame), and there wouldn't be any radical lateral maneuvering to worry about; any basic autopilot could perform this function easily. In fact, the two ships part company permanently at deceleration time, since #1 doesn't have to stop & #2 is shielded by its own engine exhaust during that phase. Secondly, whatever magic high-energy drive employed (been notionally thinking of a matter/antimatter reaction rocket, here) probably would have a MEAN exhaust, so #2 would have to be a few million km behind #1 anyhow. Therefore, there's some implicit time available for emergency maneuvering just in case #1 hits something it can't handle.

At EOM, when approaching the target solar system, you could flip your vehicle and start retro-thrusting as soon as you get within the local Oort Cloud, with your exhaust acting as an icebreaker ahead of you...

...all this, of course, powered by Unobtanium!

Oh, and BTW: in base 13, 9 times 6 *does* equal 42!


Bob Shaw

No, Thu has it absolutely right. The craft is thin so the impactors go straight through with only a minimal transfer of kinetic energy to cause damage.

You know, I was thinking that too, but I also kept thinking that the entire vehicle would have to be pretty mechanically robust just to survive continuous acceleration up to cruise speed via the magic high-energy drive, so this would imply some very firm (and therefore kinetic-energy-transmissive) connections between the various sections of the Web.

However, what if the payload elements were very light & small themselves and therefore did not require a robust physical support structure? For example, we're really not far from nanoprocessors based on quantum principles...the only catch seems to be that optical & RF sensors have to have comparatively large surface areas, unless there are a lot of them working in a coordinated fashion al a TPF or the VLA. Catch # 2 then is that the whole Web would have to be functional upon arrival for nominal performance...

I think we are neglecting a VERY important issue here. The cosmic microwave background radiation! If I calculated right (which is questionable!) at just 10% of C the CMBR would be blueshifted into the deep ultraviolet range. CMBR photons outnumber photons from all other sources in the universe by an enourmous ratio, I suspect you would be absolutely ROASTED alive by the UV flux.

If it can withstand 1G, continuous acceleration for about 35 days will get it to 0.1c. The mechanical robustness would come from a structure that can store the volume and accelerate the mass of fuel required to run the engine for 35 days.

...and 35 days of deceleration plus terminal manuevering as well. Thanks, Mchan.

Deglr, could you please post your source equations? That sure sounds like a scary possibility...this whole mission isn't gonna be easy for somebody, someday...

EDIT: The CMBR is non-directional, right? Therefore, the only "enrichment" of the radiation would be directly along the flightpath of the spacecraft with some sort of probabilistic distribution (normal, Gaussian, etc.); is the relative "density" of it along an interstellar trajectory significant enough to induce undesirable effects?

AFAIK, the same argument would also apply to cosmic rays, though those that the ship would encounter with flight paths directly opposite to the ship's vector would be fearsome in terms of energy; maybe these deserve some thought!

Agree with you on this, I don't think CMBR will be a danger for the ship.

I'd like to summarize the idea for the first interstellar spacecraft as below.

S/C design:
- Giant spider web-like space craft with payload distributed at the nodes (maybe a kilometer in diameter)
- Very light weight with miniatured instrument (less than 100kg for the whole s/c?)
- It carries no engine nor fuel but propelled by light pressure from a powerful laser beam from Earth orbit
- The material will be strong enough to withstand acceleration of 1g without affecting the web structure (is carbon nanotube suitable for that job?)

Pros:
- Because it is so light that the amount of energy is required to accelerate it to 0.1c is not unimaginable. A quick calculation with a s/c mass of 100kg showed an energy of 45*10e9 megajoules. I think we can achieve this amount of energy at the middle of this century.
- Low surface area -> reduced chance of dust hitting the s/c (but this is also bad for light propulsion from Earth)
- Large "virtual aperture" is good for radar communication with Earth (but I'm not sure whether it is good for optical remote sensing or not)
- Required technologies: nanotech, quantum computer, solar sail... is technologically feasible within this century.

Cons
- Chance of hitting by dust particle, although low but still can happen
- There's no way to decelerate the s/c once it gets to its destination (don't tell me that ET has prepared another laser beam station at their site to slow the craft down )

With this design, I believe we can see the first close-up images of the nearest star system by the end of this century

Thu; Very nice summary, although if we had ET at the other end with a deceleration laser at least we'd be gauranteed someone for the probe to take pictures of at the other end! The problem of decellerating without a co-operative space alien at the destination has been looked at in a couple of papers: T. Taylor, R.C. Anding et al., “Space Based Energy Beaming Requirements for Interstellar Laser Sailing,” CP664, Beamed Energy Propulsion: First International Symposium on Beamed Energy Propulsion, ed. By A.V. Pakhomov (2003), American Institute of Physics 0-7354-0126-8. The original Forward paper — now considered a classic — is “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets 21 (1984), pp. 187-195. I'm still working out how to post links, sorry im a bit of a technophobe to be honest, but if you put 'interstellar laser sail' into google you should get a good sweep of material on the subject. Hope this is of some use.

Lots of good ideas in this thread.

I also favor a free-flying shield, as well as a simple precursor IEE mission (think Pioneer 10 and 11) that could give us important data on what the ISM is like at 5-15% c. I'm guessing that as little as a decade or two of data would be enough for us to extrapolate the risks of more ambitious missions.

For the record, my dream target for a first mission would probably be the Sirius system; even though it's twice as far as AC, assuming we find no planets at the latter, the former will give us in situ data on two objects very different from the Sun.

Huh? At 10% of C, relativistic effects are still quite small, since y = 1 / (1 - v^2), where v is measured in C (half light speed = 0.5) is still only about 1.01 (Time dilation = T/y, Lorentz contraction = L/y), so the biggest effect by far would be the CMB directly ahead blue-shifted by 10% due to the vehicles velocity relative to the CMB. The radiation would remain in the microwave domain, undetactable except by special antennae.

Bill

Oops! I guess I did that wrong! Hey I was only 5 orders of magnitude off. Hmmm so I suppose we can neglect that issue until we get to the 99.999% C area (~16nm wavelength)...... Did I do THAT one right?

Don't feel bad, Deglr; I was too lazy to do the math at all! Thanks to you & Mongo for fleshing it out.

Thu, I understand you now. Been thinking that it would decelerate & remain in the target system, but a fast flyby mission would at first glance be much easier to execute from an engineering standpoint. Two problems, though:

1. Given the large relative speed, would a fast flyby yield enough scientifically worthwhile data to justify the trip?

2. Re the debris collision problem: There's a LOT more of it within a solar system; do you think the probe can survive the encounter phase?

Nprev, I'm sorry for not mentioning it's a flyby trip earlier.


Actually I'm thinking of this mission is somehow kind of Pioneer 10/Pioneer 11 mission when we were not sure whether a s/c can safely pass the asteroid belt or not. Technically speaking it is also feasible within the next some decades, I think. However for a mission to yield enough data to satisfy UMSF fans here I think a much better s/c design is obvious


You have a reason but let's think of sending multiple spacecrafts for redundant.

Gotcha. But 0.1c translates into around 30,000 km/sec in-system relative velocity. I'm concerned that it would be difficult to target close planetary flybys (to say nothing of acquiring a useful data set).

You could certainly get some good particle & field data for the target system (note: this was the primary focus of the Pioneers) as well as detailed observations of the star itself, but I have my doubts that such a mission could acquire planetary imagery comparable to some of the more ambitious Terrestrial Planet Finder (TPF) proposals, and literally any of those would be less risky & more cost-effective. So, apparently the type of mission is fundamentally dependent upon its goals!

EDIT: Here's a thought: What if the Web's payload packages are equipped with retros? Specifically, if there are, say, 100 payload packages with onboard guidance, control & propulsion (notionally each of them an MRO-equivalent in terms of payload capability) & they each have a smaller version of our magic drive, why can't the probe drop them off just before it passes through the system? Each surviving probe would be targeted to a specific planet within range of the "root" trajectory (that of the Web), and capable of achieving at least a highly elliptical orbit around its target planet. (Yes, we're gonna need some good AI here...)

One concern would be returning data to Earth from these critters. Some of the payload elements would have to be relay comsats with laser tranceivers capable of reaching Earth, and multiplex RF links for the exploratory elements.

EDIT2: Heck, you could even equip the payload packages with solar sails to augment deceleration if you release them early enough...solar drogue chutes?

Have a look at this:

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

Project Daedalus was a BIS study carried out in the 1970s to look at a 'reasonable' interstellar mission, and most of the points in this thread were addressed by it (bar the light-sails, which hadn't been thought of at that point).

And:

http://ntrs.nasa.gov/archive/nasa/casi.ntr..._1989007533.pdf

The US Naval Academy/NASA Centuari probe study - unlike the BIS flyby study, it would go into orbit around the new star after it's 100-year journey.


Bob Shaw

Thanks, Bob. I had just barely heard of the Centauri study; looks like some good reading!

Well, we're certainly not re-inventing the wheel here; that would assume that the wheel already exists! Re enabling technologies for an interstellar mission, the only one that seems to be advancing at the desired rate is electronics; there seems to be little if any magic engine research in the offing...

There seems to be no shortage of ideas and enthusiasm out there, just a shortage of suitable energy sources.
Perhaps minaturization will progress to the point where an interstellar payload would be small enough that such fearsome fuel requirements wont be needed. Could a a 1kg or less (payload mass) mission ever be practical?

Sending a lot of tiny probes packed with self-replicating technology and designed to return the important stuff - information - may well be the way to go. Think of it as the race between sperm to fertilise an egg - millions fail, but one gets there in the end, with a result with which we are all familiar...


Bob Shaw

After the acceleration phase, one could simply fly through space with the pusher plate forward . . .

{Yeah, I read the Dyson book}

I find the "school of starwhisps" image quite endearing but I can't see anything that's being suggested here being even remotely possible.

For me the the best approach remains taking a very long term view and firing out some really slow things designed to function for 100's of years just in case we never figure out any way to go fast efficiently and reliably.

It's abundantly clear from engineering concept studies (like the BIS Daedalus Project) that interstellar travel is possible. It's also clear that the level of effort required pretty much requires the resources of a space-based industrial civilization. Without some low plausibility engineering/physics breakthrough, it's beyond any but the maximum coordinated effort of a global planetary civilization.

Several SETI related posts delete ( there is a thread for SETI discussion - but it is not encouraged - read the rules )

Further posts regarding time capsules and historical references also deleted. Certainly OT and virging on the political.

Thread moved from Voyager/Pioneer into an appropriate sub-forum.

Doug

also your bad to quote the whole of the preceeding post....read the rules on that one as well - doug

Sorry- my bad!

Some nice ideas have been presented here, but dividing a probe into several miniature ones doesnt solve one of the most important problems. And that is communication back to Earth.
Would those 'starwhisps' by some magical trick somehow pull a terrawatt laser and nuclear reactor our of their sleeve?
(Yes the sail items might be modified to work as one dish for radio also, but again, that would add quite to the complexity of the sail and the number of things it should be able to do. Then the power needed for transmission would not be that much smaller and complex radio signals carrying megabytes of data will degrade more than laser in the interaction with gas, the stellar wind, galactic magnetic field etc etc.)

Another misunderstanding with lightsails is that they're unable to break their speed. They actually are!

One way would be to use the laser light itself and change to the opposite tack, so that the 'mirror' surface directs the laser light forward.
(Anyone who knows a bit of sailing here? Its more or less the same thing as when you arrange the sails of a ship to go against the wind.)

Its not as efficient as the acceleration but if you could charge the sail with quite a lot of electricity it would generate its own magnetic field and choosing the polarity the sail would be able to break. If the heliosphere around that star are as extensive as for Sol it could break for almost a lightyear in this fashion.

Last and most risky would be to go very near the destination star breaking both by the light and stellar wind.

One insterstellar probe might use not only one but two or all of these methods to either break enough to get a decent observation time at the target star or even come to a stop and so be able to explore interesting worlds one by one at closer range.

I would guess that this is where the self replicating probe idea comes into its own, as a probe that could build a copy of itself could build a reactor and transmitter on site as well- if it could find suitable resorces.

The big problem with 'tiny' probes is the relationship between surface area and mass. It's ideal for light-sailing vehicles, but you'll end up printing the payload onto the sail to get anywhere. In one sense, that's not such an issue - if you build your spacecraft the same way as you build CPUs in computers then you can do a helluva lot in a tiny space, and then distribute the processing/sensors etc so that things are reasonably failsafe. You could even tack against the laser light from earth by using the same sort of technology as is found in LCD projectors, with nanomechanisms which tilt as required. All these are barely beyond the state of the art. It's when you try to manipulate dumb matter, however, that things get difficult - or at the very least, expensive.


Bob Shaw

The other problem with tiny probes are diffraction limits and photon count limits. Diffraction limits what they can see and do, looking around, and transmitting a narrow angle data beam back to earth/solar-system. Photon count limits what you can see without a light bucket above the noise level of your detectors.

The BIS Daedalus payload, at least as a straw-man idea, consisted of a hubble class telescope. It would get orbits on planets and larger asteroids in a system as it approached with enough time to pick a "best" fly-through trajectory, then be able to do hubble class imaging and spectroscopy on all planets during the pass.

Sure would be nice to figure out a way to stay awhile in the system, though.

If the mission had suitably equipped flyby/orbiter-class sub-probes (programmed in advance by some pretty smart AI on the part of the main vehicle computers), they might be able to stay in the system via a combination of direct thrust (assuming that the carrier vehicle can decelerate enough while entering the system before releasing them) and sub-combinations of aerobraking through the atmosphere(s) of gas giants & subsequent gravity assists to reach targets of interest.

If something like TPF or better could survey the target system in sufficient detail beforehand, this might yield enough data to plot such a mission.. it's still a pretty risky & by no means manifestly feasible approach. And, as Myran pointed out, returning data at all would require at least that the carrier vehicle was equipped with a monster laser, and that the subs would be capable of sending data to it as it recedes @ some huge velocity, given the fact that it might take them years to reach their final targets.

Wild thought: How about hyper-aerobraking the carrier ship through the atmosphere of a hot Jupiter? (Additional assumptions: extraordinarily temperature-resistant materials and spacecraft components capable of withstanding perhaps a few million Gs). Idea here is to shed just enough speed at this point (right near inbound trajectory perihelion, which is good) after powered braking to get below the star's escape velocity, then release the subprobes to do their respective things. This way the carrier with its megalaser stays relatively nearby to act as a comm relay while the subprobes conduct their missions.

Twice this thread has entered tin-foil hat ground. Twice I've recieved complaints. It is now closed.