[X] My Assistant

[nprev]

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?

[helvick]

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,

[nprev]

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.

[marsbug]

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.

[nprev]

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.

[Bob Shaw]

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

[nprev]

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.

[marsbug]

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.

[Bob Shaw]

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

[nprev]

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?

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.

[Thu]

...
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).
...

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.

[marsbug]

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.

[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).

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


Bob Shaw

[nprev]

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.

[Bob Shaw]

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