The recent debate between Jean Schneider (Paris Observatory) and Ian Crawford (University of London) is the sort of dialogue I’d like to see more of in public forums. When I began researching Centauri Dreams (the book) back in 2002, I was deeply surprised by the sheer energy flowing into interstellar flight research. True, it lacked focus and tended to be done by researchers in their spare time, as opposed to being funded by universities or government agencies, but I had not realized that the topic itself was under such serious investigation by so many scientists.
All those fascinating concepts, from laser sails to fusion runways, were the catalyst for this site, where keeping an eye on the ongoing discussion is the order of the day. In an era of short-term thinking and instant gratification through one gadget or another, taking a longer look at the human enterprise and where it is going is an imperative. One way to do that is to consider whether our species has a future in deep space, and just what the shape of that future might be. Discussions like Schneider’s and Crawford’s look long-term, at what we might one day accomplish with our technologies, and whether or not interstellar missions really are feasible.
We can surely say this much: Nothing in physics rules out interstellar flight, even though to accomplish it in the relatively near-term would require extremely long mission times for robotic vehicles whose expense would dwarf their potential utility even if we had the patience to wait out their journey. Let’s hope that by continued research we can learn to do better, and that means taking existing concepts and reworking them in light of new technologies to see what the possibilities are. All of which is fascinating stuff, and necessary as foundation-building even if we are, as seems most likely, one or more centuries away from the launch of any such mission.
Measurements of the Medium
First steps need to be taken in any enterprise. On that score, I remind you of the interstellar dust issue that Jean Schneider first raised in his original story in Astrobiology. We’re worried about a fast-moving vehicle (Schneider talks about 0.3c, though he and Crawford later drop the number to 0.1c) and the prospects of its encountering dust grains that could produce lethal damage. We’ll eventually need precursor missions to the edge of the Solar System and beyond to understand how the interstellar medium differs from interplanetary space in terms of dust.
In the afterword to his novel Flying to Valhalla, Charles Pellegrino makes a vivid case for potential disaster:
“Flying through space at significant fractions of lightspeed is like looking down the barrel of a super particle collider. Even an isolated proton has a sting, and grains of sand begin to look like torpedoes.”
Much data gathering is ahead, but the process has already begun. For now, we have New Horizons on its way to Pluto/Charon and the Kuiper Belt, taking useful readings through the mission’s dust counter, which is, by a happy choice, named after Venetia Burney, the English girl who named Pluto (Michael Byers’ account of the Pluto naming discussion in his novel Percival’s Planet is wonderful). New Horizons is now just beyond the orbit of Uranus and, as I write this morning, is 2 hours and 32 minutes light time (just over 18 AU, or 2,738,437,000 kilometers) from the Earth.
Image: An artist’s view of New Horizons approaching the Pluto/Charon flyby. Credit: SwRI.
Dust detecting instruments that have measured dust beyond the orbit of Jupiter have been rare, beginning with those aboard the Pioneer 10 and Pioneer 11 spacecraft, which were followed up by the dust analyzer aboard Cassini (more about this, and Voyager 2’s contribution , in a moment). The New Horizons dust counter is fairly straightforward, a thin plastic film on a honeycombed aluminum structure about the size of a cake pan mounted on the outside of the spacecraft. Each dust particle that strikes the detector sets off a unique signal, allowing its mass to be inferred. The fact that this is a student-built project (though with NASA engineering standards and professionally built flight instruments) brings home the excitement of this deep space mission in a way that, let’s hope, will galvanize future scientists and engineers.
Voyager 2 was also useful in dust measurements, though in a more indirect way. While the Pioneers carried dust counters, Voyager 2 measured the effects of space-borne dust by its effect on the spacecraft’s plasma wave instrument. The latter was designed to measure charged particles inside the magnetic field of gas giant planets, but usefully enough, it also registered a hit when the spacecraft encountered dust, picking up the plasma the vaporized particle created. Cassini carried a Cosmic Dust Analyzer to measure interplanetary dust grains beginning with its gravity assist at Venus in 1999 and lasting until arrival at Saturn in 2004.
It will be fascinating to see how New Horizons’ dust data compare with earlier missions (thus far what is being seen is in agreement with data from the Galileo and Ulysses missions in Jupiter space). What we learn about dust in the Kuiper Belt will be entirely new information as New Horizons speeds through this vast region, giving us clues about what an interstellar precursor mission might one day encounter. Some scientists, Schneider among them, believe dust could be a showstopper for probes moving at 10 percent or more of the speed of light. If that proves to be the case, we’re in for serious rethinking of interstellar mission concepts.
What about all the concepts put forward, some of them even by NASA, to either magnetically or electromagnetically repel dust in the path of the probe/craft? A similar magnetic mechanism was proposed a while back to protect astronauts from cosmic radiation and solar flares, by bending the particles around the generated magnetic field(s), thereby protecting the craft and any occupants. Another theorteical proposal is to negatively charge the hull of the craft, thereby repelling dust and other particles electromagnetically. Granted, at 0.1c, the requisite field strength would probably need to be quite intense to disuade particles approaching at relativistic velocities. However, if the hull charge was directly extracted from the propulsion system, it should also increase proportionately over time.
Electromagnetic shielding won’t work because these dust particles are (I think) electrically neutral and non-magnetic. Both the Scheider and the other paper suggest that 10% light speed is doable. But anything over is not possible due to the kinetic energy of the collisions with the dust particles. It all depends on the upper size limits of the dust particles. If its 5 microns, then 30% light-speed is doable, assuming you have a propulsion system that can get you that speed. If the upper size limit is 100 microns, then 10% light-speed is probably the upper limit.
Only ways I can see that electromagnetic/electric shielding would work is if there was away of to create an electric charge on the dust particles themselves. If they don’t already have a charge already, being bombarded by ionizing radiation and electrons, I’m thinking some dust particles may already have a charge. If not, either a high energy UV laser or a stream of electron’s sent ahead of the vessel would do it. As the shield itself, I would probably go with a form of plasma entrapped in a makeshift magnetosphere around the spacecraft, this would form some what of an ablative shield as well as using electromagnetic/electric fields to move the dust particles to one side, forming a wake from the vessel.
A potentially efective system for deflecting dust particles is to ‘fire’ a stream of particles from the front of your ship.
What type of particles?
First, you should fire a laser, in order to ionise the dust particles.
Then, you should release a stream of positively charged particles (protons, for example) in order to deflect the dust particles. These particles will not have to be greately accelerated, relative to the ship. The density of the charged particle beam depends on the dust density in the interstellar space region you fly through.
Such a system would have 2 major advantages:
1 – It should work both at 10% lightspeed and at 90% lightspeed.
At 10% lightspeed, the particles you fired, relative to the dust, will move at ~10% lightspeed; if they are fired in sufficient density, your charged particles will deflect the dust.
At 90% lightspeed, the deflecting particles will move at ~90% lightspeed relative to the dust – yes, the dust will pack a much stronger punch at this speed, but your particles will do likewise; the proportionality will be preserved.
2 – The charged particles will fly far ahead of your ship – if they deflect the course of dust particles in the slightest, this dust will miss your ship (unlike skin-tight magnetic fields of physical shields).
Has anybody calculated or estimated how much various methods of dust protection will slow down a vehicle? All interactions — impacts by uncharged particles, electromagnetic deflections of charged particles, and firing of a particle beam — *will* effect a slow down.
Just thinking outloud….the interstellar ramjet which has been talked about over many decades was to use gas particles in the cosmic medium as a fuel to accelerate or decelerate a star probe.The idea was dropped to to discoveries that the gases in our area of space are not dense enough to be used for this purpose. Ok so how about using magetized dust particles instead of gas particles as fuel. A fusion power source would be needed to charge, direct and speed up the dust particles. With the greater mass over gas atoms dust could even be a better fuel for the ramjet, instead of just protecting the star craft from dust, use it?
This whole debate over the hazards of interstellar dust seems a bit surreal to me given that a thin (only a few atoms thick) stand off shield would vapourise and disperse dust in the probes path long before the probe encountered it.
Carrying on from the point I made in the other thread about the advantage of probe velocities closer to c, if we’re prepared to wait a century until we get the results from a voyage, at 0.1c we can visit stars up to 9 light years away, at 0.5c it’s 33 light years, that’s about 50 times as many stars within range at the cost of a 25 fold increase in the probes kinetic energy.
Duncan Ivry writes:
This is actually the biggest argument against the Bussard ramjet idea — Robert Zubrin and Dana Andrews have shown that drag would be a huge factor. Magsails for braking at the destination system seem more feasible.
Duncan & Paul,
The slow-down on a macroscopic probe without a magsail or ramscoop or other type of high cross-section device is negligible. Both momentum and energy of impact can be substantial, to be sure, but not compared with the enormous kinetic energy of the craft.
Perhaps an positron beam sent out at the bow might work. It could both ionize and repel incoming neutral matter, and it would not cost much in terms of lost mass. There is, of course, the matter of how to generate the positrons.
Another possibility would be to have multiple ships ride a dense proton beam. If there are enough ships in succession, each could deflect some of the beam back for propulsion, and focus the rest ahead for the next ship. This would avoid the fuel mass fraction problem, solve the beam dispersion problem and help with dust, as the proton beam should ionize and repel dust along the path. The beam would be generated by a high-current accelerator right here at home, using solar energy.
The trick would to come up with a magnetic lens that would refocus the beam with high efficiency, with most of the loss pointed to the rear to produce thrust.
Hi Guys
One interesting point is that interstellar dust *is* charged and the high charge-dust ratio of the smaller particles causes the Sun’s Heliosphere to exclude them from penetrating further into the Solar System – there’s an actual pile-up of dust caused by this effect. A bit more charge might allow deflection of most interstellar dust with high-powered LIDAR and laser-defenses taking care of the big stuff – though a precursor shield would probably do just as well.
It’s as Paul wrote, there is a need for more information from a Interstellar precursor, that will directly affect the method of dust protection, and its impact on velocity of the spacecraft. It’s good to know there are potential solutions.
I read yesterday’s TZ post on the potential problem of dust impacts limiting inertial travel through space to less than 0.1 C.
One possible remedy for such a situation might involve deploying some sort of extreme magnetic field around the craft that would induce a dipole moment within the atoms of the dust particles and thus cause the dust to be pushed out of the way of the space craft. The magnetic field might be emitted from a long spindly electromagnet or perhaps from some form of extreme yet to be developed permanent magnet. The intensity of the magnetic field would be greatly amplified with respect to the dust particles due to special relativistic effect, and so perhaps a 200 to 2,000 Tesla field might work at extreme gamma factors. Now if the magnetic field could pull the particles around to the back end of the ship, perhaps the particles could be made to collide with objects or a mass distribution that is at rest with respect to the ships frame thereby producing lots of heat and BB thermal radiation with could be reflected and thus used as a photon rocket exhaust.
I do not know how to produce permanent magnets with field intensities so high, but perhaps some form of yet to be developed materials would work such as exotic forms of somehow stabilized neutronium or quarkonium. Quarkonium might take the form of strange matter, charmed matter, or perhaps bottomonium, Another option might be higgsinium, or monopolium.
I throw these ideas out for anyone to bite on them and try to work with them. My gut feeling is that the dust problem can be remedied for extreme gamma factor craft. However, I would gladly be a crew member on a 0.1 C starship to Alpha Centauri if such a mission would be launched this decade. I don’t see this happening quite yet, though.
I don’t suppose there is any means to turn the dust into an advantage, for example by using it to brake when the destination is near?
If we have the technology to get a probe up to 0.1c, surely we could design it to be as close to 1-D as possible, and minimise the collisional cross section, and hence the shielding needed?
“I don’t suppose there is any means to turn the dust into an advantage, for example by using it to brake when the destination is near?”
There isn’t enough of it to be useful, or in my opinion to be harmful with even the simplest of defenses.
“If we have the technology to get a probe up to 0.1c, surely we could design it to be as close to 1-D as possible, and minimise the collisional cross section, and hence the shielding needed?”
Yeah that makes sense but there are always trade-offs, a probe launched by a rail gun would naturally be long and thin, a probe carrying huge quantities of liquid He and H fuel would have to be built with the realities of tank storage in mind.
This is an idea only. I am unsure if there have been any studies to back it up or refute it (what a drag!), but that may not matter.
Some years ago among nanotechnology circles there were discussions of “utility fog” (see http://en.wikipedia.org/wiki/Utility_fog), which would be used to fabricate objects from what might be considered a very strong arosal. The thought I had is that since this utility fog would be, I hesitate to say “intelligent,” but certainly highly networked within the structural elements and very lightweight, it could be ideal for applications in interstellar missions. Utility fog might enable shields to be generated and then continuously replenished to absorb the dust particles ahead. I am not considering anything besides dust, of course.
This idea might have movement value, along the lines of Edward de Bono’s “water logic”*, even if the calculations, do not pan out. Let’s face it, interstellar travel at velocities > 1% C is not going to happen without RAN (the word for chaos in Japanese, but here meaning Radically Advanced Nanotechnology.)
Interstellar ships will be the most complex and beautiful, in both aesthetic and engineering senses, objects ever built. We need to get wild in our ideas to even begin to understand how such craft could be constructed.
“*de Bono contends that traditional logic is static, based on the solid foundations of ‘is’ and identity. In contrast to the traditional ‘rock logic’, he proposes ‘water logic’ which is based on ‘to’ and the flow of the mind: ‘What does this lead to?’ as opposed to ‘What is…?’ This new logic is surprisingly easy to learn and to use, and results in a visual ‘flowscape’, which allows you to lay out and then look at your thinking. “
Relativistic velocity dust and gas shields for space craft might take the form of a cloud of smoke particles distributed in front of the space craft. The hardness, latent heat of vaporization, latent heat of fusion, latent heat of ionization and density of the particles are important factors to consider in deploying a smoke shield. Particles of more than one elemental, isotopic, and molecular species can be included in the smoke shield. The mass, density, and size distribution and spectrum of particles can be optimized for each range of gamma factors so as to most effective at block or divert the kinetic energy of interstellar dust grains. This is so because dust speck collisions producing differing explosive temperatures produce plasma and neutral particle jets with different characteristics and so the mass, density, and size spectrum of the smoke cloud particles can influence the attenuation and divergence of jets produced by the dust speck collisions.
At high gamma factors, say at about gamma from 100 or 300, the production of charmed, strange and bottom mesons and baryons becomes possible and any of the longer mean life time particles as such can travel macroscopic distances before colliding with other smoke particle atoms and atomic nuclei. Certain species of atomic nuclei are better at disrupting certain species of mesons and baryons, an heavy charged leptons, than others by transformer interactions.
Once we obtain a gamma factor much above 100, the pressure of dilated Cosmic Microwave Background Radiation can easily dislodge the smoke cloud and so an electrodynamic mechanism may become necessary to retain the smoke cloud. For a smoke cloud comprised of superconducting particles, the solution to this problem might be as simple as employing the Miesner effect to hold the smoke particles in place. For cases where the smoke particles are magnetic, or magnetizable, perhaps some sort of magnetic field generator can be used to control the distribution of the smoke cloud. The magnetic containment might take the form of a pulsed magnetic field that pulls the particles back into position when they start to drift apart but not to the extent where the particles would all be pulled to one location within magnetic field such as at one of the magnetic poles. If the smoke particles could be electrically charged, the use of electric fields to hold the particles in stable configuration might be possible.
For large superconducting portions of materials, the Miesner effect could be easily employed to hold superconducting pebbles fixed. Such a Miesner shield could contain several layers of permanent magnets that are distributed in a plate like manner ahead of the space craft where superconducting pebbles would be distributed between the magnetic sheets or magnetic plates and where the foremost magnetic sheet holds a distribution of pebbles at the very front of the shielding mechanism in order to act as an initial collision buffer. The atomic, isotopic, and chemical composition of the pebbles could be judiciously chosen in order to disrupt the incoming dust particles.
Another option involves deploying a large balloon or series of balloons ahead of the space craft that are filled with gas so as to ablate or ionize the incoming dust particles. The balloons might employ nanotech self assembly repair mechanisms or some other kind of self healing fabric to repair the small holes produced by dust particles that punch through the balloons. Alternatively, macroscale microbots could continually scan the balloons for holes and and make repairs with macroscale patches as necessary. The atomic and isotopic composition of the internal gas can be chosen so that the disruption of the kinetic energy jets produced by the colliding dust is maximized.
Note that the use of very thin balloons filled with very low pressure helium gas in space as nuclear warhead decoys has been suggested. If the balloons were thin enough, and contained a low enough pressure helium gas, the balloon might survive 100 meter proximity to the detonation of a one kiloton ABM nuclear warhead in space. According to the reasoning, the balloon’s metallic fabric would be so thin that it would radiate away the heat generated by the blast so quickly that it would not get hot enough to melt or vaporize. The gamma rays and neutrons generated by the blast would deposit only a very small amount of their energy within the balloon membrane and its internal very rarified gas. The point is the such dust speck disrupting balloons might survive high but modest gamma factors and the resulting cosmic rays if a self repairing feature could be employed.
Alternatively, a series of membranous sheets or plates may be distributed ahead of the craft, where the first sheets in the series would act to ionize the dust specks. Judicious spacing between the sheets can be employed to permit divergence of the plasma and neutral particle jets produced by the collisions. If necessary, any of the previously described smoke distributions could be used further disrupt the kinetic energy jets where the smoke particles would be instilled between the plates. The further option of including magnetic pebble distributions between the plates may also be of use here. Regardless, an electric and/or magnetic field distribution between the plates can be employed to cause plasma jets produced by particle collision to experience greater diversion based on the rest mass to charge ratio, the momentum to charge ratio, and the kinetic energy to charge ratio spectral distribution of the plasma particle produced by dust impact collisions. The divergence effect would be analogous to that of a mass spectrometer.
Then there is the possibility of using lasing stations or microwave beams directed in front of the space craft in order to ionized or vaporize dust particles while they are still far away from the space craft. For cases where the space craft is traveling in a very strait line, the beaming process is simple. For cases where the ship is experiences high degrees of angular acceleration, the beam angle has to be adjusted so that the dust particles are vaporized or ionized while they are safely away from the ship. Charged particle beams may also be of help in this regard. For highly relativistic space craft, microwave and IR beams can be used because the frequency of the radiation relative to the dust specks located in the path of the space craft will be blue-shifted to the soft U-V to hard U-V/soft x-ray frequencies.
Does anyone actually know what happens when a dust particle strikes matter at circa 10%c ? Andrew W says that a shield only a few atoms thick will vaporise the dust. My feeling is it will simply punch a hole straight through.
I don’t think there are any experimental or observational data on macroscopic particle interactions with matter exceeding a few 10’s km/s. There are a few orders of mag to go yet in the experiments.
In mass, there is much less dust than gas. Even that is scary, at 90%c the heat dissipation from relativistic proton interactions in a shield is many kW per sq cm I believe.
@kzb: I think you are right if the shield is a few atoms thick, although it is possible that the energy of impact will vaporize the dust particle even then. If the shield has a thickness comparable to the size of the dust particle (still just a few micrometers), it stands to reason that the particle will not survive. Multiple layers of shield should deal effectively with resulting debris. Sufficient forward distance will deal with the radiation. All of this should be amenable to fairly straightforward calculation.
You are also right about the heat, and the front shield will have to be very refractory and actively cooled. There would be no way to go faster than some moderate fraction of c with material shields, because of heat overload. A possible way out would be some sort of electric or magnetic nose-cone that would deflect the relativistic particles slightly, just enough to miss the ship, but not enough to deposit much energy. For example, a proton or positron beam might be good to both ionize and electrostatically deflect atoms, but it may be too costly in terms of mass and energy. A superconducting current loop may be used to deflect charged particles magnetically without loss of mass or energy, but it may be difficult to protect the loop itself from impact and overheating.
“Multiple layers of shield should deal effectively with resulting debris. Sufficient forward distance will deal with the radiation.”
Thanks Eniac, but also remember that the dust particle is going to absorb a lot of kinetic energy when it’s hit by something moving at 10 or 50%c, so it’s going to explode, with the debris moving at such high velocity that distance should also deal with most of the debris.
kzb; “at 90%c the heat dissipation from relativistic proton interactions in a shield is many kW per sq cm I believe.”
With a membranous shield most of the gas would pass right through, you would, as you’ve said, have multiple shields spaced out ahead of the probe, each deflecting a small portion of the gas, each dissipating a small portion of the heat.
I’d envisage the probe launching shields forward at regular intervals to replace those worn away.
The fact that there is significant heat generation in the shield also means there is significant drag at relativistic velocities. There is no “coast” phase of the mission, thrust is needed to overcome drag and maintain speed.
If you launch a stack of lightweight membrane shields in front of the vessel -or come to that a cloud of gas or smoke particles- these will come back to hit you with the same velocity with which they were launched. Unless the shield is also under thrust or is supported in a structure extending forwards from the main vehicle.
Since thrust is needed throughout the flight, it’s not much of a stretch from that to having a forward-pointing engine to deflect gas and dust. This means you need a fusion/photon torch pointing backwards, and a weaker one pointing forwards, both operational throughout the flight.
“There is no “coast” phase of the mission, thrust is needed to overcome drag and maintain speed.”
While I have no doubt a probe or ship would slow a little it would not be by enough to warrant constant additional thrust throughout the voyage.
“these will come back to hit you with the same velocity with which they were launched.”
You design them to disintegrate when they reach their maximum distance forward, and spin them so that the debris is thrown out of the path of the following shields and the vessel.
@kzb: The drag from the ISM is minuscule compared with the momentum of the ship. You can see this clearly once you realize that the ISM column between here and Alpha Centauri, were it condensed into a solid or liquid, would only be a few micrometers thick. Not nearly massive enough to put any sort of dent in the relativistic momentum of our ship. “Coasting” is thus absolutely the appropriate term once the engine is shut off.
Eniac, I’ve checked your assertion about the amount of matter between here and alpha cent, and I agree, if you imagine a column 1cmx1cm extending for 4.3LY, there is something like only 7E-07 moles of gas per sq cm. Quite a surprising result -at our atmosphere temperature and pressure I make it the equivalent of about 15 microlitre of hydrogen per sq cm !
However we’ve still got to reconcile the megawatts of kinetic energy loss, I can’t believe that this rate of energy does not equate to significant braking. So given the tiny mass of ISM encountered, I am now wondering if there is a mistake in the shield energy dissipation calculations I have seen elsewhere….
Hi Folks;
I had noticed the discussion regarding the need for heat dissipation and just thought that I would chime in.
It would be great of some exotic form of stabilized neutronium or even quarkonium plates could be developed and fashioned into a long slender cone in order decrease the astrodynamic drag on the vessel and also to increase the black body radiative surface area of the craft.
If an at rest cone could have a length to maximum diameter as rest aspect ratio of say 10,000/1, then even at gamma = 1,000, the ratio as such would be 10/1.
If quarkonium has a density of say 10 EXP 21 kilograms per cubic meter, then a 10 EXP – 18 meter thick plate would have a rest mass of 1 metric ton per square meter, a 10 EXP – 17 meter thick plate, 10 metric tons per square meter, and a 10 EXP – 16 meter thick plate, 100 metric tons per square meter.
Quarkonium may exist in the form of strange, charmed, and/or bottom matter.
For much, much longer rest aspect ratio plates where the gamma factor is much more extreme, the quarkonium might be stably extended from bending moments by some form of electrical charge instillation within the cone which is repelled by an opposite electrical charge near the base of the cone.
My guess is that for extreme gamma factors, exotic forms of materials may be required such as neutroniums, quarkoniums, Higgsiniums, and monopoliums.
Regarding prospects for new materials, although perhaps of only periodical table elemental forms of stable superheavy nuclei, see the following Science News link for some encouraging discoveries that provide some support for the existence of stable superheavies.
http://www.sciencenews.org/view/generic/id/64519/title/Sailing_toward_the_island_of_stability
@kzb: In order to reconcile the large energy loss per surface area with negligible drag you only have to calculate the mass of the craft per surface area multiplied by velocity squared. You will find that for any good-sized relativistic craft the enormity of this kinetic energy, which is of the order of mc^2, will dwarf the (merely) substantial drag energy into utter insignificance.
Also, noting that drag is proportional to v, and energy to v^2, you can see that even though the energy is substantial, the drag/pressure is actually quite small, even before you compare it to the relativistic momentum of the ship.