Interstellar Flight: The Dust Conundrum

by Paul Gilster on February 6, 2009

It would be helpful if space were a bit more empty. A key problem facing an interstellar probe would be encounters with dust in the planetary system it leaves and, as it reaches cruising speed, dust impact in space between the stars. Although our Solar System seems to be in an unusually sparse pocket of space, the galaxy-wide distribution of hydrogen is roughly one atom per cubic centimeter. Dust — bits of carbon, ice, iron compounds, and silicates — is far rarer still, but enough of a factor to a ship moving at a significant fraction of the speed of light that the designers of the Project Daedalus craft built in a payload shield 32-meters in radius to protect their starship.

Then again, much depends on your location. Have a look at the image below. It’s an area called the Red Rectangle some 2300 light years from Earth in the constellation Monoceros. Although the center of the image seems to be a single star, it’s actually the double star system HD 44179. The Red Rectangle is a nebula, a cloud of gas and dust that shows what can happen when we move into areas of intense dust concentration. You can imagine that movement through an environment like this one would demand serious shielding requirements for any craft on a mission of interstellar exploration. And although we can avoid nebulae, any interstellar flyby probe has to reckon on the gas and dust within its destination system as it screams through to study the inner planets.

red_rectangle

Obviously, we’d like to know more about dust, and that’s a problem. Donald York (University of Chicago) states the matter baldly, saying of interstellar dust “We not only do not know what the stuff is, but we do not know where it is made or how it gets into space.” York and collaborators have been studying the Red Rectangle looking for clues, and they’ve turned up a workable hypothesis.

Image: A Hubble Space Telescope image of the Red Rectangle. What appears to be the central star is actually a pair of closely orbiting stars. Particle outflow from the stars interacts with a surrounding disk of dust, possibly accounting for the X shape. This image spans approximately a third of a light year at the distance of the Red Rectangle. Credit: Credit: NASA; ESA; Hans Van Winckel (Catholic University of Leuven, Belgium); and Martin Cohen (University of California, Berkeley).

One of the stars at the heart of the Red Rectangle nebula has burned through its initial hydrogen, collapsing upon itself until it could generate the heat to burn helium. Such stars go through a period of transition that, in a matter of tens of thousands of years, causes the star to lose an outer layer of its atmosphere. The thinking is that dust forms in this cooling layer and is then pushed out from the star by radiation pressure, along with large amounts of gas. The larger star in the Red Rectangle is too hot to concentrate dust in its atmosphere, but double-star systems like this one often show a disk of material forming around the second star, creating a jet that blows dust out into the interstellar medium. Here’s Adolf Witt (University of Toledo) with more detail:

“Our observations have shown that it is most likely the gravitational or tidal interaction between our Red Rectangle giant star and a close sun-like companion star that causes material to leave the envelope of the giant. The heavy elements like iron, nickel, silicon, calcium and carbon condense out into solid grains, which we see as interstellar dust, once they leave the system.”

Take the Red Rectangle process and make it ubiquitous and you’ve located one source for the dust that is such a factor in interstellar space. Back to Daedalus, which was designed to move at 12 percent of lightspeed for the fifty year journey to Barnard’s Star. Along with its beryllium shield for the cruise phase, Daedalus would have needed additional protection for the stellar encounter, which designer Alan Bond suggested could take the form of a cloud of dust deployed from the main vehicle, heating and vaporizing any larger particles before they could damage the payload. And because Daedalus would deploy smaller probes within the system, each would need a cloud of its own.

Gregory Matloff and Eugene Mallove once suggested that a starship could use, in addition to a shield, a high-powered beamed energy device to destroy or deflect any larger objects in its path. So the options for interstellar protection are slowly being placed on the table. But first we have to learn more about the nature of the problem, which means studies like these that tell us how dust forms in the first place. The paper is Witt et al., “The Red Rectangle: Its Shaping Mechanism and its Source of Ultraviolet Photons,” accepted for publication in the Astrophysical Journal and available online. A University of Chicago news release is available.

David February 6, 2009 at 12:49

This is why I am not sold on dark energy. to Me this makes any supernova measurements questionable. There might be some other cause of Omeag 1.01 from the CMBR.There are a lot of mysteries

Adam February 6, 2009 at 20:58

Hi Paul

I’ve been re-reading the “Daedalus” discussion of the impact problem and note that the sub-probes had a very low probability of collision with substantial impactors so they only had solid erosion shields, not “dust-bug” projectors. The heating from protons is interesting, but I haven’t read it close enough yet to get a proper understanding. However it seems that most will interact with the electrons surrounding atoms rather than colliding with their nucleii. Nuclear interactions don’t become an issue until V > 0.9c apparently.

Funny how real starships will need ablation shielding, high-powered lasers, magnetic “deflectors” and constant proximity scanning – making them, essentially, warships looking for an excuse to fight. Wouldn’t want to get in their way. I wonder if their crews will be as wise and ethical as the crew of the “Enterprise”? Or will they be like the crew of the “Liberator” instead?

Paul F. Dietz February 7, 2009 at 9:45

I’ve wondered if the particles that compromise cold dark matter could be an issue. They normally interact only weakly with ordinary matter, but what if that normal matter is traveling at close to the speed of light relative to the CDM? Could the interaction cross section become high enough it could impose an unacceptable radiation load on the passengers/payload, yet not high enough for shielding to be practical?

andy February 7, 2009 at 12:58

If dark matter particles were a problem we’d have seen their effects in particle accelerators by now, surely? (Then again, particle physics isn’t really my forté)

Invader Xan February 7, 2009 at 19:46

“We not only do not know what the stuff is, but we do not know where it is made or how it gets into space.”

Well, in fairness while we don’t know the precise details, we do know a fair bit about interstellar dust.

For a start, a lot of it is silicate material. Glasses and simple minerals like olivine. Likewise, we’re fairly certain that a lot of the material out there is carbon rich. Things like the diffuse interstellar bandsare most likely from hydrocarbon molecules. Nanodiamonds have also been detected around some stars.

Perhaps a good means of dealing with these things would be to ionise them somehow (indeed, it’s likely many of them already are ionised) — then they could conceivably be deflected by an electric field. In principle, it would be the same as the solar particle shielding proposed for lunar missions, except with much heavier particles. Of course, at high speeds, I’m not sure how effective that might be…

Adam February 7, 2009 at 22:31

Hi

From the bit I know of DM physics the stuff is expected to emit radiation when it collides with DM and self-annihilates. Why high speeds would influence that is unknown presently, but it’s another thing to worry about.

andy February 8, 2009 at 10:37

In several science fiction novels, civilisations are depicted as spreading along the spiral arms and avoiding the regions between. In reality, it may be that the spiral arms are barriers to interstellar travel because of the larger amounts of interstellar material.

James M. Essig February 13, 2009 at 22:44

Hi Folks;

Regarding fusion rockets, with all of the work currently being done in the field of nanotechnology, carbon fullerines such as buckballs and other molecular cages, and also reports as current as 2004 in main stream press reports that the U.S. Airforce is studing the feasibility and applications of small quantities of antimatter, it is my opinion that essentially pure fission fusion bombs could by just around the corner. Although such fission fusion bombs would probably find defensive miitary applications, I see the result as a potential boone for Orion type pulse rockets.

In the event that a tiny amount of antimatter could be kept within a molecular cage or several proximate molecular cases within a mass of pure U-235, which could be surrounded by dueterium ice, or perhaps some sort of metalic dense deuterium material, then perhaps rupturing or opening the molecular cage could permit antiprotons contained within to leak into the nuclei of the U-235 atoms thus causing the atoms to fission, If enough fissions can be generated per unit of active cell within the overall U-235 mass, the whole U-235 sample could fission thus heating the deuterium to fusion temperatures with the result being the the whole pure thermonuclear bomb explodes.

Such a device with a yield on the order of 0.1 kt to 1.0 kt could make for an excellent Orion style space craft. With a high enough fuel to vehicle dry wieght ratio, I can see how perhaps 0.3 C or even 0.35 C could be reached.

Perhaps a massive thick shield could capture dust particle and interstellar atoms, molecules, and ions in such a manner that the shield would gradually transmut into fission and fusion fuel which would then be used for propulsion. A good heat energy dissipation mechanism could recycle the KE of dust and atomic scale particle impacts while the shield increased in rest mass and in avarage atomic number.

I would like to think that mission parameters that far out do the Project Daedalus craft might be developed in the few short decades from now.

Thanks;

Jim

James M. Essig February 14, 2009 at 18:59

Hi Folks;

What can we do with fusion rockets?

Take the relativistic rocket equation delta V = C tanh [(Isp/C) ln(Mo/M1)] where C is the speed of lght, Mo is the initial mass of the fueled weight of the vehicle, and M1 is the dry weight of the vehicle or final payload rest mass.

We will let M0/M1 = a hefty 1,000 which might be doable for vehicles having tanks made of metallic hydrogen that could be scavenged for use as fusion fuel. The maximum theoretical Isp of hydrogen to helium is .119 C. Note that these equations assume constant Isp output.

The result is delta V = C tanh [(.119C/C) ln 1,000] = .6762 C. For Mo/M1 = 10,000, we have delta V = C tanh [(.119C/C) ln 10,000] = .79907 C, and for M0/M1 = 100,000, we have delta V = C tanh [(.119C/C) ln 100,000] = .87870 C which corresponds to a gamma factor of about 2. This bad boy could put any star system within a 60 lightyears in range of Earth for a healthy young robust crew. Without medical life span enhancement, child birth and rearing could happen enroute. A medically enhanced life expectancy of 1,000 years could permit crews to arrive at stars that are 1,900 lightyears distant yet still allow the crew members an average of most of a century to live out the rest of their lives on the new home.

For a perhaps more reasonable, M0/M1 = 100, we have delta V = C tanh [(.119C/C) ln 100]= .49899 C, and for a very reasonable M0/M1, we have delta V = C tanh [(.119C/C) ln 10] = .2673 C.

Now for a maximum Isp of 0.04C for nuclear fission fuel, and a very reasonable M0/M1 = 10, we have delta V = C tanh [(.04C/C) ln 10] = 0.0918 C. For Mo/M1 = 100, we have delta V = C tanh [(0.04C/C) ln 100] = 1.821C.
For Mo/M1 = 1,000 we have delta V = C tanh [(0.04C/C) ln 1,000] = .2694 C.
For Mo/M1 = 10,000 we have delta V = C tanh [(0.04C/C) ln 10,000] = .35259 C, and for a still plausible Mo/M1 = 100,000 we have delta V = C tanh [(0.04C/C) ln 100,000] = .4305 C.

The point is, we can get to the stars under nuclear power and reactionary propulsion mechanisms that by far out do Project Daedalus. I would love to be the commander of such a nuclear vessel radioing back to Earth, “Underway on nuclear power”, just as did occur when the first nuclear powered submarine was launched by the U.S. Navy.

I really think that if we can develope the propulsion issues, the dust problem although not trivial, will be minor to engineer around.

Thanks;

Jim

James M. Essig February 14, 2009 at 21:51

Hi Folks;

Now lets try matter antimatter electron, positron annihilation rockets assumming no energy losses.

First will will assume an M0/M1 value of 10. Using the relativistic rocket equation delta V = C tanh [(Isp/C) ln (Mo/M1)] = 0.98 C. For M0/M1 = 100, we have delta V = C tanh [(Isp/C) ln (100)] = 0.9998 C. For M0/M1 = 1,000, we have delta V = C tanh [(Isp/C) ln (1,000)] = .999998 C. For Mo/M1 = 10,000, we have delta V = C tanh [(Isp/C) ln (1o,000)] = 0.99999998, and lastly for M0/M1 = 100,000, we have delta V = C tanh [(Isp/C) ln (100,000)] = 0.9999999998 C.

The gamma factor of 0.98 C is 1/{1 – [(v/c) EXP 2]} EXP (1/2) = 1/{1 – [(.98C/C) EXP 2]} EXP (1/2) = 5.0252. For v = 0.9998 C, the gamma factor is 1/{1 – [(.9998 C/C) EXP 2]} EXP (1/2) = 50.0025. For v = 0.999998 C, the gamma factor is 1/{1 – [(.999998C/C) EXP 2]} EXP (1/2) = 500. For v = .99999998, the gamma factor is 1/{1 – [(.99999998C/C) EXP 2]} EXP (1/2) = 5,000,, and for v = 0.9999999998 C, the gamma factor is 1/{1 – [(.9999999998C/C) EXP 2]} EXP (1/2) = 50,000.

One can as a result of the numerical symmetry of the above series compute the gamma factor of any positive integer power of ten value of M0/M1. Some gamma factors for higher Mo/M1 values determined from inspection are, for Mo/M1 = 10 EXP 6, gamma = 500,000. For Mo/M1 = 10 EXP 7, gamma = 5,000,000. For Mo/M1 = 10 EXP 8, gamma = 50,000,000. For Mo/M1 = 10 EXP 9, gamma = 500,000,000, and for Mo/M1= 10 EXP 10, gamma equals 5,000,000,000. A ship accellerating at a constant 1 -G to 2-G value to a gamma factor of say 5,000,000,000 would permit the ship to curcumavigate the entire visible universe in under one contemporary human life span, ships reference frame.

Thanks;

Jim

James M. Essig February 17, 2009 at 0:18

Hi Folks;

I will go one further extreme on the relativistic rocket themes by suggesting that one can imagine antimatter rockets that derive their normal matter reactants from the interstellar medium wherein the propulsion system effeciency would approach 100 percent, exactly, and wherein the effective Isp therefor equals 2C.

For such as system wherein the effective Isp = 2C, first we will assume an M0/M1 value of 10. Using the relativistic rocket equation delta V = C tanh [(Isp/C) ln (Mo/M1)] = C tanh [(2C/C) ln (10)] = 0.999866 C. For M0/M1 = 100, we have delta V = C tanh [(Isp/C) ln (100)] = C tanh [(2C/C) ln (100)] = 0.99999998 C. For M0/M1 = 1,000, we have delta V = C tanh [(Isp/C) ln (1,000)] = C tanh [(2C/C) ln (1,000)] = C – [ 2 x 10 EXP - 12] C. For Mo/M1 = 10,000, we have delta V = C tanh [(Isp/C) ln (Mo/M1)] = C tanh [(2C/C) ln (10,000)] = C – [2 x 10 EXP - 16]C,, and lastly for M0/M1 = 100,000, we have delta V = C tanh [(Isp/C) ln (100,000)] = C tanh [(2C/C) ln (100,000)] = C – [2 x 10 EXP - 20]C.

For v = 0.9998 C, the gamma factor is 1/{1 – [(.9998 C/C) EXP 2]} EXP (1/2) = 50.0025. For v = .99999998, the gamma factor is 1/{1 – [(.99999998C/C) EXP 2]} EXP (1/2) = 5,000. For v = C – [ 2 x 10 EXP - 12] C, the gamma factor is 1/{1 – [[[C – [ 2 x 10 EXP - 12] C]/C] EXP 2]} EXP (1/2) = 500,000. For v = C – [ 2 x 10 EXP - 16] C, the gamma factor is 1/{1 – [[[C – [ 2 x 10 EXP - 16] C]/C] EXP 2]} EXP (1/2) = 50,000,000, and for v = C – [ 2 x 10 EXP - 20] C , the gamma factor is 1/{1 – [[[C – [ 2 x 10 EXP - 20] C]/C] EXP 2]} EXP (1/2) = 5,000,000,000.

Like wise, one can compute the gamma factors for such perfectly efficient craft where M0/M1 = 1,000,000, 10,000,000, 100,000,000 and so on to yield gamma factors of 500,000,000,000, 5 x 10 EXP 13, 5 x 10 EXP 15 etc. respectively assumming no losses including complete cancellation of astrodynamic drag.

The point once again is that even rockets, if only fusion rockets at first, can get us to the stars. Once government leaders and academics realize this, I think the issues of designing around the interstellar dust particle impacts can be effectively dealt with.

I think we are going to the stars, and I think the process of simply working to develop the hardware to get us there will prove to be lots of fun.

Regards;

Jim

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