Tuesday, March 10, 2020

Galaxies above! Space isn't empty! Plus it's dangerous!



The Science Faction


Between the stars we have the so-called interstellar medium, which contains mainly hydrogen (about 90%) and helium (about 9%). That leaves 1% for, in particular, so-called "metals" in a range of atomic weights from lithium to iron, including in particular carbon dust and oxygen and nitrogen as significant components among the gases. Dust grains account for most of this heavy fraction, with of the order of 0.1% being gases.

This doesn't only include the common istotypes, but in particular species of Hydrogen and Helium with a neutron or proton more or less than the common isotopes.

About 26 in a million Hydrogen (H) atoms across the universe seem to be Deuterium (D) with its extra neutron (this D is theorized to date from the big bang), while the fraction can be an order of magnitude bigger in some comets, and it is similarly prevalent in oceans (Earth water is theorized to come from comets).  But stars destroy Deuterium faster than they create it, so have lower fractions, while there are other places where nuclear processes must create it. Tritium (T) is also created by both natural and artificial nuclear reactions but is not stable – it has a half-life of 4500 days (12.32 years) and decays into Helium-3. Interestingly carbon dust grains preferentially adsorb Deuterium.

Within a star system, we have the so-called interplanetary medium, which is similar in composition but a highly conductive plasma, a gas of ions and electrons, and it incorporates particles from cosmic rays, solar wind and other stellar ejecta, in particular corononal mass ejections – ending at the heliopause where the solar wind transitions into the interstellar medium.

Cosmic rays are again largely hydrogen ions, that is they are 85-90% protons, with again a good proportion of helium in ionic form, known in this case rather as alpha particles, plus about 1% electrons, known also as beta radiation (although that also includes the much smaller fraction of positrons too. There is again about 1% "metals" plus a bit of antimatter (not just positrons but antiprotons, and potentially the full gamut of elementary particles – although most of them have very short lifespans). Of course, there's also true wavelike radiation, gamma rays, photons with energies and frequencies extending from X-rays down through UV, visible, IR and radio frequency ranges.

Of course radiation and rays have the idea of radiating out from somewhere whether in the form of waves or particles, massless or massive. And that somewhere is usually a star or some residual thereof, in particular supernovae and their remnants. The Fermi (1949) proposals relating to diffusive cosmic-ray acceleration and supernova remnant shockwaves has over the intervening decades become well established theory, although with still a long way to go in explain everything.

The focus of current theories is, like any good scientific theory, predictions that can be observed, and that means a focus on gamma rays given we are looking for indirect evidence of what is (or rather was) happening thousands or millions of light years away. 

But cosmic rays are here and now, and when the primary cosmic rays described above hit our atmosphere at close to the speed of light, things happen. The resulting secondary rays add scattering decay products including the full range of known particles - some of which were first detected in cosmic rays. Of particular note are the alpha particles and neutrons - the latter being one of the most penetrating and dangerous forms of radiation, but being neutral also some of the hardest to detect.

Of course, everything we think we know in science is really theory through the eyes of empirical evidence: manufactured faction rather than fact.

The Science Fiction


For science fiction authors, the focus is somewhat different: what would a space vessel experience when encountering these phenomena, what precautions would they routinely take, what new technologies need to be developed to allow zipping through the interstellar and interplanetary media at high fractions of c, within being ripped to threads by those tiny particles and dust grains - some of which are themselves traveling close to c.

These technologies include both active and passive measures, and in particular active and passive shielding - and just to make things interesting we might have ships powered by nuclear reactors and guided by quantum AIs, which might require their own special shielding. But for starters, we might focus on shielding for humans and other living entities, plus current generations of electronics and associated technologies.

For X-rays, or gamma rays across a broad spectrum, we'll probably look at heavy metal (high-Z) shielding. Those heavy shields will tend to work pretty well for protons and electrons, and the alpha particles and metal ions are fairly easy to stop. But for neutron shielding lighter elements (low-Z) tend to be indicated, in particular hydrogen, boron and carbon, as their interaction with heavier elements will tend to involve nuclear reactions and produce radioactive isotopes and secondary radiation. Even the absorption of gamma rays by a high-Z layer produces X-ray fluorescence.

A layered or graded-Z shield is typically used, where each layer deals with the secondary radiation from the previous one. Polyethelene (containing lots of H) is often used to deal with beta radiation while minimizing secondary radiation, while water of heavy water (with D or T instead of H) is often used to absorb, scatter and slow neutrons, then using a suitable isotope (such as lithium-6) to capture the slowed neutrons without triggering secondary radiation. On ships using hydrogen fuel, that can be stored so as to provide additional neutron protection.

In relation to alpha and beta radiation, as well as higher atomic weight ions (HZE), active electromagnetic shielding is currently being explored by NASA and others, including using superconducting magnets.  Permanent magnets are heavy, and conventional electromagnets are power hungry.

This might help with the charged cosmic rays or interplanetary medium, but what of the neutral interstellar medium and dust grains? What of supernova remnants and shockwaves? What about space debri? What about neutrons?

One solution that comes up in SF is the idea of shooting the neutral particles to ionize them or annihilate them. 

Yes, that's theoretically possible, but is it practical? We could use a laser to ionize our targets so that our electromagnetic shields will guide them safely around the ship - but at a substantial fraction of light speed there's not a lot of time to detect and deflect, and there would be a lot of energy, mass and reverse thrust used to achieve this continuously and speculatively if we tried to avoid the detect part of this.

Ionizing with a laser would avoid direct expulsion of mass, and could ionize with some dispersion. A laser would also provide a kind of ranging detector too. We could follow up with accurately targeted antimatter of an appropriate kind - but that would be pretty hard to do head on, not to mention the antimatter being very expensive and difficult to produce, and antineutrons would be particularly hard to control too.

Shooting matched mass and momentum particles is interesting, the matched momentum implying a particular frame of reference in which the resultant particles (of elastic collision) would shoot of at right angles. Streams of particles would produce and explosive disk that wouldn't have much time to move out the way, but if ionized that might be enough for the magnetic shielding to work.

In my stories, I'm assuming the existence of an ionization and magnetic shielding system, and the ability to produce streams of particles of different kinds, including antimatter - but not as a routine path clearing solution.

But now I want to come back to the hazard of ion storms, particularly those that relate to supernova remnants, and the particular dangers they present - which would be way off the charts in terms relative to the interstellar and interplanetary media (ISM and IPM) that these radiation and dust protection devices are designed to protect against.

The Missing Link


Supernova, shockwaves, cosmic rays and neutron blasts


 

Neutrons have a half life of 10 minutes 10 seconds - so if faced by a cloud of neutrons half of them would be gone in 10 minutes, with only around a millionth of the left after an hour.  The mean time to decay is 14 minutes 40 seconds.

So how could they possibly be a problem?

Part of the interest in reading and writing hard science fiction is exploring the real science problems that get thrown up, whether in artificial intelligence, astrophysics, genetic engineering or particle physics - or all of the above at once.

Remember the Fermi shockwaves ?

Let's start with a supernova. As observers, we observe and classify taxonomically by which lines appear in the spectum, and whether they indicate presence or absence of key elements - in particular Hydrogen and Helium as the two most abundant elements. Type I supernovae don't show a hydrogen line, while type II does. Type 1a doesn't show helium either, but shows a strong silicon absorption line, while types 1b and 1c lack that, with type 1b showing strong helium lines and type 1c lacking them.

That's just looking at the spectra of the actual explosion. The next step is theoretical models to explain the spectra. Type 1a are hypothesized to involve a common mechanism with carbon-oxygen white dwarf progenitor stars that accrete matter beyond a 1.44 M☉ limit (40% bigger than our sun). Much of the carbon and dioxide then fuse into heavier elements within a few seconds, internal temperatures shooting up to billions of degrees, releasing an explosive shockwave at around 5% of the speed of light, and lighting up as 5 billion times brighter than our sun. Some theories suggest the associated graviton effects go forward and backward in time...

The other main type of supernova involves core collapse (CC). This includes both Type II and other subclasses of Type I. My stories are set in Andromeda when a supernova of Type I was observed in 1885 (SN1885A) but does not fit the type I model. The remnants of this supernova (amognst others) are currently being tracked. Other supernovae would no doubt have occurred in Andromeda (M31) since SN1885A, there might have been one today - but we won't know about it for another 2.5 million years.

So what are the implications for a wormhole from here to Andromeda given these wormholes and their shockwaves. How could we know if a shockwave from an event 2.5 million years ago, was going to affect our missions to Andromeda.  My stories assume they there are such events and the do affect our missions.

But for the present discussion the key question is how they will affect our ship with its ISM and IPM shielding. In terms of current observations, the SN1885A remnant has a diameter of around 2.5 parsec and is 725 parsec away, traveling at 11000 km/s, with absorption lines for hydrogen, potassium, calcium and iron, with the average velocity of the original ejecta being around 13000 km/s.

These ejecta travel faster than the speed of sound in the interplanetary and interstellar media.  They are ionified and so have strong magnetic fields, and in the first few hundred years they sweep up their own weight in circumstellar and interstellar matter, creating not just a single shockwave as the ultrasonic mass invades the medium, but additional shockwave as kinetic energy is transferred to new particles and deionizes and accelerates them to ultrasonic velocities.

Because these are shockwaves of charged particles, they do not even need to collide to transfer this energy, but rather can be turned back by the magnetic fields, a "magnetic mirror" effect. In the first order Fermi process, particles get trapped between a faster main wave and a slower secondary shockwave, bouncing back and forward and gaining energy each time they bounce off the faster primarily shockwave (much like the energy gains in a slingshot effect). This accelerates the particles to close to the speed of light, and only at very high fractions of c can they break through the secondary wave - and they will all have to do this by the time the faster primary wave catches up and merges in. This was elaborated detail by Parker (1958) and analysed in further detail by Wentzel (1962) so that this is also known as Fermi-Parker Wentzel diffusive shock acceleration (DSA). DSA in Supernova shockwaves is seen as a primary source and mechanism for cosmic rays. This stage can last for thousands of years.

In the second order Fermi affect, the "magnetic mirrors" are the more randomly moving "magnetic mirrors" in magnetized gas clouds of interstellar material.

In these processes, the initial taxonomic type and inceptive origins of supernova become blurred as the remnants and their associated shockwaves develop their own character (Reynolds, 2008). The DSA traps are very energetic sources in their own right, ejecting highly energetic particles at close to c, with TeV energies, and spawning the full gamut of secondary radiation as pairs of shockwaves close and merge. The primary shockwave comes close to an "effectively infinite-mass scatterer" (Reynolds, 2008, p97).

What happens when a ship is caught in a DSA trap? Or is within an AU or two of the merging shockwaves?  There is no time for the neutrons to decay, so we cannot ignore them.

If we consider a wormhole between galaxies, then these shockwaves will necessarily intersect it at multiple points, and at times our traversing ship can expect to meet a DSA trap.

Bibliography


Chevalier, R.A., and Platt, P.C. (1988), "On the nature of F. Andomedae (SN 1885A)",
Astrophys. J.  331: L109

Fermi, E. (1949), "On the origin of cosmic radiation", Phys. Rev. 75: 1169– 1174.

Fermi, E. (1954), "Galactic magnetic fields and the origin of cosmic radiation",  Astrophys. J .119: 1–6.

Parker E.N. (1958a) "Origin and Dynamics of Cosmic Rays". Phys Rev 109:1328–
1344, DOI 10.1103/PhysRev.109.1328

Parker E.N. (1958b) "Suprathermal Particle Generation in the Solar Corona".
Astrophys. J. 128: 677, DOI 10.1086/146580

Parker E.N. (1992) Fast dynamos, cosmic rays, and the Galactic magnetic field.
Astrophys. J. 401: 137–145, DOI 10.1086/172046

Reynolds S.P. (1998) "Models of Synchrotron X-Rays from Shell Supernova Remnants". Astrophysics J. 493: 375–396, DOI 10.1086/305103

Reynolds, S.P. (2008), "Supernova Remnants at High Energy", Ann. Rev. Astron. Astrophys. 47: 79-126

Wentzel, D.G. (1963), "Fermi acceleration of charged particles", Astrophys. J., 137: 135–146.

Wentzel, D.G. (1964), "Motion across magnetic discontinuities and Fermi acceleration of charged particles", Astrophys. J., v. 140, No. 3, 1013–1024.



My Paradisi Lost stories

Encounters with wormholes and asteroids, exploited, benign and catastrophically dangerous feature in the Paradisi Chronicles stories, including my Casindra Lost subseries, which also feature genetic engineering, an emergent AI 'Al' and a captain who is reluctantly crewed with him on a rather long journey to another galaxy - just the two of them, and some cats... There's another AI, 'Alice' that emerges more gradually in the Moraturi arc. It is not space opera, stories that could be set anywhere, or space fantasy, stories that are more magic than science, but stories where the science drives the story, and engineering provides the solutions. The stories also aim to help us to think about our own planet, and to develop science and engineering that will conserve rather than destroy.

The Paradisi colonization aims to preserve the pristine ecosystems of New Eden, restrict mining to the other planets and asteroids of the system, and genetically modify people to suit the ecosystem rather than overwhelm it with introduced species: https://paradisichronicles.wordpress.com/

Casindra Lost
Kindle ebook (mobi) edition ASIN: B07ZB3VCW9 — tiny.cc/AmazonCL
Kindle paperback edition ISBN-13: 978-1696380911 justified Iowan OS
Kindle enlarged print edn ISBN-13: 978-1708810108 justified Times NR 16
Kindle large print edition ISBN-13: 978-1708299453 ragged Trebuchet 18

Moraturi Lost
Kindle ebook (mobi) edition ASIN: B0834Z8PP8 – tiny.cc/AmazonML
Kindle paperback edition ISBN-13: 978-1679850080 justified Iowan OS 

Moraturi Ring
Kindle ebook (mobi) edition ASIN: B087PJY7G3 – tiny.cc/AmazonMR
Kindle paperback edition ISBN-13: 979-8640426106 justified Iowan OS 

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