Some time back, I had a post about dark matter.  So, I thought that I’d say a bit more about matter.  First of all, just what is matter?  After all, if we are going to talk about dark matter, we should have some idea what non-dark matter is, right?  This is going to be a bit more technical post than most that I have done.  Long time readers will note that I once in a while toss in things like this.

Ages ago, in a physical science class in high school, I learned a definition of matter that I still hear bandied about.  That definition went something like:  Matter is something that has mass and takes up space.  Well, that’s cute.  So, what is mass, and what is space?  After all, if you are going to use a definition, then the terms used in that definition should also be understood.  What I find, though, is that my physics students, much less the non majors, don’t understand the terms matter, mass, and space.  Heck, a number of physicists seem unclear on the concepts, too.

Already, in a previous post, I talked about space not really being empty.  This is a clue that there is something more going on in space than simply dimensional measure.  In cosmology, we learn that space can expand.  Two galaxies can each be sitting stationary in space, and yet be getting farther away from each other because the space between them is expanding.  This is hard to understand if you are stuck with the idea that space is simply a dimensional measure.  It may be more appropriate to think of space as an interaction realm.  Things can interact with one another, and the realm and range of that interaction is space.  This is still not quite right, but I don’t think that anyone really understands space completely, and this way of looking at it is a bit better than most people do.  The dimensions (length, width, height) are simply a way of quantifying how much space there is.  Space itself is something beyond just the dimensions. 

Now, this throws a monkey wrench into our definition of matter.   Implicit in the definition is that two separate pieces of matter can not occupy the same space.  But, if two objects do not interact, what keeps them from occupying the same space?  After all, if they don’t interact at all, they for all practical purposes can’t occupy the same space, since one object’s space is not the same as the other object’s space.  That’s a tough concept, I know.  Furthermore, it is not at all clear that some particles, such as electrons, actually take up any space at all, at least in terms of dimensions.  Electrons, and some other particles, are essentially point particles.  Now, we can discuss the interaction range of an electron with a probability distribution described by a wave function, but that isn’t really the same as saying that the electron is occupying space.  Now, there does exist a limit to how small a piece of space we can describe in physics, something called the Planck Length, which is about 1.6x10^-35 meters.  At this distance scale, the normal rules of physics start to break down, and something new is needed.  So, we can’t really say if such point particles are really point particles, or just tiny things the size of the Planck Length, but for all practical purposes, the statements are equivalent.

But, no one argues that electrons are not matter.  So, we are left with matter being something that has mass.  But, don’t be mislead.  Matter is not mass.  These are separate concepts.  However, mass is a property of matter.  All matter has mass.  But, doesn’t everything have mass?  No.  Photons, particles of light, don’t have mass, and neither do gravitons, the particles postulated to carry the gravitational force.  So, that makes photons and gravitons, and numerous other particles not matter.

So, let’s talk about mass for a moment.  What is mass?  Actually, this is a deep question.  If mass is that makes something matter, then we should understand what we mean by the term mass.  There are two basic types of properties of matter that we call mass.  This means that there are two separate masses.  These are the Inertial Mass and the Gravitational Mass.  These are different properties, but interestingly, they have the same value. 

The inertial mass is that property at the heart of Newton’s Second Law of Motion: 

F = m a.  Or, to put in another way, m = F/a.  You apply a force to some object, and it accelerates.  The ration of the applied force to the acceleration is the object’s inertial mass.  The more mass it has, the harder it is for the force to accelerate it.  Once moving, though, it takes force to slow it down or to stop it.  Again the ratio of the force to the deceleration is the mass, and this quantity is the very same as the mass that was determined in speeding the object up in the first place.  There are various reasons to expect this to be the case, but we don’t want to get even more technical than this is already.

Gravitational mass, though, is a different beast altogether.  Two pieces of matter interact with one another through the gravitational force.  They may also interact via the electromagnetic force, the weak force, or the strong force.  These four forces are the only forces in nature that we know about.  Everything that you can conceptualize as a force is really one or more of these forces at work (most commonly experienced forces in the macroscopic world other than gravity are electromagnetic in nature).  Now, not all pieces of matter interact via all of these forces.  Some don’t interact at all through one or more of these forces, but all interact via the gravitational force.  The magnitude of the interaction between matter objects depends upon a quantity associated with the objects called their gravitational masses.  The magnitude of the interaction is given by Newton’s Universal Law of Gravitation:  F = (G m₁ m₂) / r².  The m’s are the masses, the r is the separation of the masses, and G is a quantity known as the universal gravitational constant.  Its value is determined by the strength properties of the gravitational force and the particular choice of units that we select.  People who work on this sort of thing even talk about two types of gravitation mass:  active mass and passive mass.  The passive mass is the mass that interacts with a gravitational field, and the active mass is the mass that makes the gravitational field.  There are excellent reasons that the two are the same thing, but again we won’t go into that.

Now, the interesting thing about the inertial mass and the gravitational mass is that they seem to be the exact same quantity.  It isn’t altogether clear why they should be the same quantity, after all the gravitational mass is related to the gravitational field, and unless you are dealing with a gravitational force, then the mass has nothing to do with the force, so why should acceleration be dependent upon it?  There is actually still work being done on this to try to explain why things work this way.  But, as I said, it is not clear why this must be the case, so theorists have discussed what might happen if the two were different, even by a very tiny bit.  The things that would happen if the gravitational mass and the inertial mass are not the same quantity are called the Nordtvedt Effect, after the theorist who proposed it.  So far, every experiment to detect the Nordtvedt effect has failed to yield any measure of it.  If the Nordtvedt effect did exist, it would be most apparent in very large masses, such as those of galaxies or galaxy clusters.  So, this is the realm of the astrophysicist.  Again, we see absolutely no evidence of any Nordtvedt effect.  So, the matter goes back to the theorists to explain why the inertial mass and the gravitational mass must be the same.  One theory that I have heard is that perhaps a body moving through space interacts with the virtual particles there (see my earlier space post) and that might cause the quantities to be the same.  I don’t know.  That seems to perhaps be stretching it, but I am not a theorist.

Interestingly, Einstein has shown us that matter and energy are interrelated.  His famous equation E = mc² shows that energy and matter can be interchanged.  Photons, which are not matter, can become matter.  In fact, high energy physicists sometimes quote the mass of a particle as how much energy it takes to make one.  For example, they may way that an electron has a mass of 511 keV, meaning that it takes 511,000 electron volts of energy to make an electron.  However, a problem does arise when you create matter from energy.  There seems to be a conservation law in effect.  Matter has some property other than just mass.   Some particles of matter also have electric charge.  The conservation laws of charge tell us that with any interaction, you have the same electric charge before the interaction that you did after the interaction.  So, if you make a proton, having charge +e, out of a photon, which has no charge, then you have a problem.  Likewise, if you made an electron, having charge –e, from an uncharged photon, you have a problem.  So, you might suggest that you simply need an energetic enough photon to make both and electron and a proton, thus their charges of –e and +e balance each other out.  Well, that is correct, their charges would balance out, and if conservation of charge were the only thing going on, that would be fine.  But, there is more that goes on here.  As it turns out, you have to conserve all kinds of other properties besides electric charge.  Furthermore, it appears that there is some other hidden variable that must be conserved, so you can’t just balance a proton with an electron.  The only thing that can balance the equation for these other properties for a proton is something exactly opposite of a proton, or to balance an electron would require something exactly opposite of an electron.  As it turns out, such things exist.  There are particles exactly opposite protons that we call antiprotons.  They have the same mass, the same spin, the same everything else, except that they have charge –e.  Likewise, there exist anti-electrons (which we call positrons), which are just like electrons, but with charge +e.  These antiparticles, we call antimatter.  You can even have a positron encircling an antiproton to make antihydrogen.  Having mass, you’d think that we’d just call these antiparticles matter.  Well, the sort of are matter, in that they do interact with matter particles and other antiparticles with gravitational forces.  However, there is something different about them, because if a particle and its corresponding antiparticle meet, they annihilate each other, and both are converted into pure energy.  Likewise, energy, say from photons, for example, always creates a particle and its corresponding antiparticle if you make matter out of that energy.  There have been all sorts of interesting ideas about what antiparticles really are, including a suggestion that they might be particles moving backwards in time.  So, a creation or annihilation event is simply the particle changing direction in its time path.

In an antimatter particle is the same as a particle, except for opposite charge, what about electrically neutral particles?  Particles such as neutrinos, actually seem to act as their own antiparticles.  That can add a whole layer of weirdness to this.  But, neutrons are not the same thing as antineutrons.  It turns out that neutrons are actually composed of smaller things, called quarks, and those quarks have electric charge.  The quarks composing a neutron simply have charge that adds up to zero.  So, antineutrons are composed of antiquarks, with each antiquark being the same as its corresponding quark, but with opposite charge.

It turns out that subatomic particles have a property that we call spin.  The name comes from the fact that a number of their behavioral characteristics can be accounted for by pretending that they actually are little spinning things.  Really, they aren’t.  They just have angular momentum.  Don’t ask me how they can do that if they are point particles and don’t really spin, like an electron.  It turns out that there are only certain spins that a particle can have, and an exchange of spin always occurs in units of a fundamental unit of angular momentum.  Particles can be classified by their spin behavior.  Particles that always have angular momentum that is an even multiple of this fundamental angular momentum are called bosons.  All the particles that mediate, or transmit, force between things are bosons.  However, particles that have spins that are always odd multiples of ½ of the fundamental angular momentum (such as +1/2, or -1/2), are called fermions.  Electrons, protons, and neutrons are fermions.  Particle physicists generally only consider fermions to be matter, even if some of the bosons have mass.  Fermions have an interesting property, one that without which the universe as we know it would not function the way that it does, and we would not be here.  That property is that no two fermions that can interact can be quite identical in every way.  They must have one property or another (angular momentum, spin, energy, etc) different from one another.  This is called the Pauli Exclusion Principle.  Without it, chemistry, nuclear physics, and all sorts of other things would not work.  You would not have stars, planets, galaxies, or people.

Fermions, themselves, can be grouped into two categories:  haydrons and leptons.  Haydrons are particles that can experience the strong force, and leptons do not experience the strong force.  Neutrons and Protons are haydrons, and electrons and neutrinos are leptons.  Haydrons are actually composed of smaller particles called quarks.  Haydrons can be classified as two types, baryons (composed of three quarks, and are fermions) and mesons (composed of two quarks, and are bosons).  As I indicated, many would not include mesons in matter, though I rather think of them that way.  Quarks are the only particles that interact using all four fundamental forces.  Interestingly, the current models suggest that haydrons are composed of a sea of quarks of all types, with only two or three of these quarks (called valence quarks) lending their properties to the haydron.  That they can be composed of a sea of quarks, allows one haydron to transform into another one by a change of valence quarks.  Thus, though neutrons have zero charge, the approximate mass of a neutron and proton combined, and they decay into a proton, electron, and neutrino if left alone for a while, they are most definitely NOT a combination of electron and proton, but are entirely separate particles from each. 

Dark matter, whatever it is, is not simply matter that is not lit up.  It is composed of something that does not interact via the electromagnetic force, and thus we are unable to detect it except by its gravitational effects (light is electromagnetic in character).  This suggests that it is most definitely not made of haydrons, because haydrons are composed of quarks, which do have electric charge and will interact with the electromagnetic force.

Ah, the joys of subatomic physics.  I get to confuse my general physics students with this stuff at the end of their second semester class.  I hope that I haven’t confused you too much.

-Astroprof

In the northern sky this time of year is the constellation Ursa Major. For those of us in the northern hemisphere, it is high in the northern sky, and up pretty much all night.  A bit south of the equator is is low in the northern sky, and up only for a while during the night.  If you are too far south, then you don't see it at all. 

Ursa Major is Latin for "Big Bear," and that is what this constellation is supposed to be:  a big bear.  However, I, like most people that I know, look at Ursa Major and immediately think mainly of the seven brightest stars in the constellation.  These stars make an unmistakable pattern in the sky that is one of the first patterns that anyone learns, and it is one of the few that most everyone that I meet knows.  Here in North America, these stars are almost universally known as the Big Dipper.  Four stars make a box shape that is the bowl of the dipper, and the remaining three are its handle.  However, this is only part of the whole constellation of the bear.  In particular, it is the back end of the bear.  Unofficial groupings of stars like these are called asterisms, rather than constellations.  The term constellation only refers to 88 official constellations recognized by astronomers.  The official list was decided in 1933 by the International Astronomical Union.  66 of these date back to the Romans.  Ursa Major is one of those 66.  However, all of the constellations, not just the 66 Roman ones, are named in Latin.  Though in the past, constellations were mental pictures made of the stars, modern astronomers don't think of constellations as patterns in the sky, anymore.  Instead, we think of the constellations as regions of the sky.  The entire sky is divided up into the 88 official constellations.  Now, before this time, every group of astronomers had their own set of constellations, most being the same, but some not.  So, this cleared up a lot of confusion.  Also, all sorts of cultures around the world have made constellations out of stars in the sky.  They are generally not the same, and sometimes not even the same groups of stars that make up the constellations.  There are lots of ways of taking the same set of stars and imagining pictures out of them!

As I said, the Big Dipper is only part of Ursa Major.  This group of seven stars is really very easy to spot, and so many groups of people over the years have made it a constellation of its own.  For example, many of the people of sub-Saharan Africa regarded these seven stars as a Drinking Gourd.  In parts of Africa, a hollowed gourd could be used to scoop water for drinking.  These areas of Africa where these stars were considered a gourd were where many people were taken to be slaves in the Americas, a sad note in our history.  These Africans brought with them many aspects of their culture, including some of their lore.  Much of that was lost, but the seven stars of the Drinking Gourd were so easy to spot and recognize, that it was no wonder that they remained.  As I said, even today, this is one of the first patterns that people learn, and one that sticks with them.  Well, the youngsters learning the stars from their elders had never seen a gourd.  However, they had seen metal dippers used to scoop water from barrels to drink.  So, the Drinking Gourd became the Big Dipper.

But, from the tale, it is obviously going to be an American asterism.  Indeed, in parts of Europe those seven stars had been seen as a Wagon or a Cart.  The four stars making the bowl of the dipper were the actual vehicle, with the other three being the handle with which it is pulled.  Other parts of Europe, particularly near Britain, saw this as the Plow, or more correctly, I suppose, the Plough (the British spelling).  The four stars that we regard as the bowl of the dipper were the blade of the plough and the other three were it's handle.  Think of a horse-drawn plow to get the picture of the plough.  (Now if that isn't a weird looking sentence, with both the American and British spellings of a word in it!  Probably those of you with a better literary sense than I are cringing right now!)

But, as I said, this part of the sky has been seen since ancient times as a bear.  How the Romans got a bear out of the stars of Ursa Major is sort of interesting.  What is more interesting is that these stars also seem to be a bear in a number of North American Indian nations.  This is baffling to me, since there is nothing even bear-like in these stars!  I seldom see a bear, even though I know that one is there.  The bowl of the dipper is the hind quarters of the bear.  His hind legs extend from what we'd think of the bottom of the bowl, and his front extends well past the front of the bowl.  The handle of the dipper is often portrayed as the bear's tail.  Huh?  Bears don't have long tails. Well, the Romans were notoriously anatomically inaccurate with their constellations.  H. A. Rey (author of Curious George) wrote a book some years back in which he formed new, easier to see patterns of the constellations.  He turned the bear around, and made the handle the nose of the bear.  It really looks more like a bear that way.  (Actually, Rey's book is a fantastic introduction to the sky for anyone wanting to learn enough to be able to go out and recognize some of the things that you see.)

But what do we make of the American Indians seeing a bear in the same set of non bear-like stars?  There have been several theories.  One idea is that this implies some interaction between Europe and the Americas.  We know that the Vikings made it to the American mainland, so perhaps they brought stories of the sky with them.  Or perhaps the earliest post Columbus explorers and settlers brought European constellations with them.  Native Americans could have picked up on these stars being a bear then, and then they could have incorporated that into their own tales.  Most of these early Europeans coming to America didn't really care what the people already here thought.  By the time that people started to take not of Native American star lore, the idea of these stars being a bear might have already become part of that lore.  And, naturally, there is always the way out there weird ideas that people come up with, like a group of aliens that look like bears visted both Europe and North America from stars in that part of the sky.  OK, I just had to toss that one into the mix.  As hard as it is to believe, I have actually heard that!  Of course, I've heard lots of crazy things.  (I might do a blog entry sometime on some of the wilder things that I've heard.  Hmm, I haven't done one like that in a while...).  But, trying to figure out how these stars got to be a bear in North America is a bit beyond my expertise.  I'll leave it to others.

Not all Native Americans have the same bear stories for these stars.  One that I like is a story that has it being a bear pretty much like the Roman one, with a long tail.  According to this tale, the bear was watching a fox one winter day.  This was before bears hibernated in the winter, and when they had long fluffy tails.  The bear observed that the fox would carefully walk out onto a frozen lake, and then use his teeth and claws to chip a hole in the ice.  He would then lower his long fluffy fox tail into the water.  A fish would come along and nibble on the tail thinking that it were some sort of strange weed.  The fox would quickly pull his tail out of the water, and the fish would come along with it.  The fox then would eat the fish.  Well, the bear saw all of this and thought to himself, "If Fox can do this, then so can I, for I am Bear, and I am much better than Fox."  So the bear went out onto the ice and punched a hole in the ice.  He then lowered his long fluffy tail into the water.  Sure enough fish came to nibble.  But, the bear thought to himself, "I am much bigger and hungrier than Fox.  One fish won't do for me.  I had better leave my tail in the water until several fish come along.  Then, I'll pull it out and eat them all!"  Well, soon, the bear lost feeling in his tail.  When he tried to sit up to find out what was wrong, he found to his horror that the water had frozen back, and now his tail was trapped in the ice!  He pulled and pulled, until finally he pulled loose, but his tail had come off and was still embedded in ice.  That is why bears don't have tails, and after this horrible incident, bears now hibernate all winter so that they don't have to go through this again.  (Of course, I am not sure how they could, since they don't have tails any more.)  Anyway, this tale tells a moral lesson to youngsters:  don't get greedy.  Or perhaps, it just tells them not to sit in freezing water.

Not all Native Americans had the same stories.  Another tale is that of three Indians hunting a bear.  The stars of the Dipper's bowl are the bear.  The other three are the braves hunting the bear.  As the sky rotates through the night, the stars of the bear always leads the hunters.  They are always chasing the bear.  But, every year in the Fall, they get close enough to the bear to wound him with a spear.  The bear then bleeds, and his blood spills from the sky onto the trees, turning the leaves red and orange.  But the bear always escapes, and the braves always keep after him.  Of course, the relative spacing and positions of the stars don't really change, but I guess that might take the fun out of the story.

And, while I am at it, I may as well relay a bear tale told to me by a park ranger with the Texas Parks and Wildlife Department.  This has nothing to do with stars, but it appealed to my warped sense of humor.  You see, my ranger friend was telling me of a friend of his who was a park ranger up in Colorado.  This Colorado ranger frequently led park visitors on a nature hike through the park.  Well, this park had bears in it.  These bears really didn't want much to deal with people, and if they heard people coming then they'd just walk away, so you never saw them.  However, you didn't want to walk up on one of them and surprise it.  So, they issued little bells to the hikers.  They made noise as they walked, and the bears would hear that and go somewhere else.  I guess that the nature walk must have focused on plants, since pretty much any wild animal would go somewhere else when a group of tourists came through.  The plants were stuck.  They couldn't run away.  Anyway, as they were doing their tour, they came upon some bear scat (a polite way of referring to the material that had passed through their digestive track).  Anyway, the ranger pointed it out, and told the visitors that this was bear scat, and it looked fairly fresh, so a bear must have been in that immediate area not long ago.  Well, as often happens with my students, there just had to be one of the tourists who would catch some notion and not let it go, making a pest of himself.  Well, on this tour, there was this one guy who wouldn't let the ranger get away with just saying that it was bear scat so a bear had been there.  He wanted to know what kind of bear.  Well, the ranger explained that you couldn't really tell from the scat.  There were two main types of bears in the park, though, black bears and grizzly bears, so it was probably one of them.  Well, the tourist wouldn't let it go.  He insisted that the ranger tell him what kind of bear it was.  The ranger patiently explained that both grizzly bears and black bears eat pretty much the same things, so you can't tell from the scat what kind of bear it was.  The size of the scat might give a clue, but that wasn't reliable either.  You simply can't tell from the scat what kind of bear left it.  Well, the tourist still wouldn't let it go, and the ranger really wanted to get on with the tour and not stand around discussing bear droppings!  So, he finally gave in and said that he'd investigate.  So, he took as stick and poked at the scat, stirred it up a bit, and then pronounced that it was a black bear.  As expected, the tourist still wasn't satisfied, and he wanted to know how the ranger knew that it was black bear and not grizzly bear.  "Well, " the ranger said.  "It is easy.  There are no bells in it."

-Astroprof

For a long time, astronomers have believed that planets form as a consequence of star formation.  The accretion disk feeding the protostar becomes a disk of material orbiting the young star.  This disk, called a proplyd, is the site of forming planets.  Otto Struve even proposed that planets are a natural consequence of star formation, not just an occasional thing.  So, an intensive search was on to try to find planets around other stars.  Several claims of extrasolar planets came forth, but were eventually disproved.  Finally,  in 1995, Michel Mayor and Didier Queloz, of Switzerland’s Geneva Observatory, presented the first conclusive evidence of an extrasolar planet orbiting the star 51 Pegasii.  But, this wasn’t really the first extrasolar planet found.  It was merely the first normal extrasolar planet.  What was an utter and complete shock to astronomers, and really blew my socks off, was the discovery in 1992 of two planets around the pulsar PSR 1257+12.  Now, let me explain why this was so shocking.

First, let’s discuss pulsars.  When a massive star, about eight or so times the mass of the Sun or more, dies, it does so in an amazingly violent manner.  The core of this star collapses from something about the size of the Earth to an object about the size of a large city.  This is an object so compressed that the atoms themselves are destroyed, and even most of the protons are transformed into neutrons.  In fact, most of the object is neutrons, so we call it a neutron star.  The energy released in forming this object then blows the rest of the star into space in a monstrous explosion.  This explosion, called a supernova is unbelievably violent.  A supernova momentary is so bright that it is as bright as an entire galaxy worth of stars.  The radiation and blast is pretty much enough to blow up anything nearby.  About all that can survive is the core of another star.  I might say more about supernovae in a later post.  For now, we are interested in the thing left behind:  the neutron star.  This neutron star has a massive magnetic field, and this magnetic field will likely be offset somewhat from the rotational axis, like so many rotating bodies with magnetic fields.  The magnetic field’s poles then serve as radiation sources, as radiation and particles stream outward.  As the rotating neutron star spins, these beams sweep around the sky.  If you happen to be somewhere in space where these beam sweep, then you will see a flash of radiation (mostly radio) at regular intervals.  This is a pulsar.  In other words, a pulsar is a neutron star whose rotation carries these beams of radiation past Earth on a regular basis.

So, a pulsar is a neutron star.  A neutron star is the byproduct of a supernova.  A supernova is an amazingly powerful and violent explosion.  Such an explosion should totally disrupt any planets that might be orbiting a star.  So what the heck is a planet doing orbiting a neutron star????  Well, to make it even more confusing, Aleksander Wolszczan and Dale Frail, the discoverers of the pulsar planets around PSR 1257+12 announced the discover of two planets orbiting the pulsar!  Adding even more confusion, in 1994, the same team of astronomers announced a third planet around this pulsar.  Then, in 2002, they announced yet another object orbiting PSR 1257+12.  All of these objects were found by the tiny gravitational effect that they have on the pulsar.  As the objects orbit the pulsar, they cause it to shift back and forth slightly.  The pulsar pulses at very regular intervals.  But, if the pulsar moves back and forth by a little bit, careful measurements can detect the tiny shift in arrival times of the pulses.  These pulses move at the speed of light.  The speed of light is FAST, but is it not infinite.  So, if the pulsar is slightly farther away, then the pulses arrive slightly late, and if it is slightly closer, then they arrive earlier.  Carefully studying the timing of these pulses shows the slight regular shift back and forth, revealing the presence of the planets.

The third planet found is actually the closest to the pulsar, and has a mass of only 2% that of Earth (about double that of our Moon).  Such a tiny mass makes for a tiny shift in the pulsar, making it a very difficult thing to detect.  The second and third planets are the ones first discovered.  These planets are similar in size, the second one being 4.3 times the mass of the Earth, and the third one being 3.9 times the mass of the Earth.  They are big enough and orbit close enough together that they have effects on each other’s orbits as the pass (the second planets orbits once every 66.5 days, and the third one once every 98.2 days).  This provides proof positive that they are really there, and that they are not an artifact of measurement.  The fourth object is really tough to call a planet, since it is a truly tiny thing, only 0.04% the mass of the Earth.  That makes it even smaller the Pluto, probably closer to the size of a large asteroid like Ceres.  This fourth object is also much farther out than the others.   It is 2.7 AU from the pulsar, whereas the other planets are located at 0.19 AU, 0.36 AU, and 0.47 AU (AU stands for Astronomical Unit, the average distance between the Earth and the Sun).  It is possible the this fourth object is the larger of a host of similar things located at this greater distance from the pulsar.  If so, it may signify the presence of an asteroid belt or a comet cloud, similar to our Kuiper belt.

Finding one object would be remarkable, as it is hard to imagine how such an object could survive a supernova explosion.  Four, or more, is just plain bizarre.  So, is this simply an anomaly?  After all, the universe is pretty big, and there are a lot of things in the universe.  So much so, in fact, that if something can happen, even if it is highly improbable, there are enough chances for it to happen that somewhere it likely will happen.  So, is PSR 1257+12 such an anomaly?  Perhaps.  But, perhaps not.

The problem with that interpretation is that another pulsar planet was found associated with pulsar PSR B1620-26 by Stephen Thorsett in 1993.  This one is another weird system.  It turns out that pulsar PSR B1620-26 was already known to be a binary system consisting of a pulsar and a white dwarf orbiting one another.  A white dwarf is what you get when a star smaller than 8 solar masses dies.  It is the collapsed core of a star, but it is only collapsed to about the size of the Earth.  The outer parts of the star are pushed off into space, but in a rather gentle manner compared with a supernova.  Thorsett’s finding was of a third body, about 2.5 times the mass of Jupiter, orbiting the pair of stellar remnants.  It is believed that this strange system results from the merger of two prior stellar systems, one of a neutron star and ordinary star, and a second system consisting of an ordinary star with a gas giant planet.  When the stellar systems passed close to one another, the original companion of the neutron star was ejected, and the new star and neutron star settled into orbit around one another while the gas giant went into orbit around the pair of them.  Eventually, the new star died and formed a white dwarf.

But, what about PSR 1257+12?  The same thing didn’t happen there.  Several theories have been proposed.  One is that perhaps the neutron star “stole” the planets from a passing normal star system.  But, that can’t really work, since it would be virtually impossible to steal a whole planetary system!  Another proposal has been that the original star had originally had several gas giant planets with large rocky cores.  The supernova stripped these gas giants of all but their cores, and these cores then spiraled close to the neutron star.  That might work for one or two such objects.  However, it doesn’t explain the smallest objects like the one found in 2002.   It definitely doesn’t explain how a belt of such things might exist, which is what this smallest object might signify.  This leads to the possibility that these are secondary planets, formed out of fallback from the debris cloud created by the supernova.

The suggestion that pulsar planets might be formed as secondary planets is strengthened by the announcement just a couple months ago by a team of astronomers let by MIT’s Deepto Chakrabarty of a dusty disk of material, similar to a proplyd, surrounding the pulsar 4U 0142+61.  This disk is believed to hold about 10 or so times the mass of the Earth worth of material.  If left alone, it may form one or more planets. 

So, this then leads to a new question:  How common are pulsar planets?  They are hard to detect, because they will only be detectable if they orbit neutron stars whose radiation beams pass by Earth.  But, most of the neutron stars will have radiation beams not too far from their rotational poles, so most motion of the pulsar due to planets will be side-to-side, which will not produce any detectable shift in pulse times.  Only back-and-forth motion is readily detectable, and that only if the beams of radiation originate from points on the neutron star fairly far from the poles.  So, this limits the number of possible cases where we could even hope to detect planets.  That we’ve found planets this way with PSR 1257+12 is amazing.  PSR B1620-26 is another strange case.  But, 4U 0142+61 is interesting because the dust disk was not detected by the shift of the pulsars, but rather by using the orbiting Spitzer infrared telescope.  That we have found this disk suggests that perhaps planets may form fairly regularly around pulsars.  Clearly, this is an area that needs lots more work.

-Astroprof

A number of years ago, I gave a public lecture on the Leonid meteor shower, and I entitled the talk “The Sky is Falling!”  It had crossed my mind to title this post the same, but even my rather warped sense of humor finds little humorous in the current situation in the Middle East.  I’ve posted several postings about rockets here.  Given that, people ask me about the rockets currently falling on Israel.  These I know a LOT less about.  Most of the rockets that I know more about are the bigger ones useful for putting things into space.  Still, rockets are rockets.  The same physics works for them all.  So, here goes.

The Israel Defense Force (IDF) reports that that rockets falling on Israel are Fajr-3 rockets.  Fajr means “dawn” in Farsi, I am told.  Most likely, these are Syrian made copies of the Fajr-3 rockets exported by Iran to the region over the last few years.  While they can be set up and fired individually, the Fajr-3 is normally fitted in banks of truck mounted launchers.  This makes the Fajr-3 a descendent of the Soviet Katusha rockets fielded during World War II.  The Katusha rocket system was nicknamed “Salin’s Organ” because it looked sort of like a pipe organ, and as the rockets left their launcher, they made a vaguely organ-like whoosezuuooo sound.  This type of weapon is more correctly characterized as an artillery rocket.  It is used much like an artillery piece, but with longer range.  The Fajr-3 is apparently based upon missile systems sold to Iran by North Korea in the 1980’s.

The rockets being fired into Israel are being launched by Hezbollah forces.  According to reports that I read, Hezbollah has been acquiring Fajr-3 systems since Israel pulled out of southern Lebanon.  This was a number of years ago, so the current situation has been building since then.  Hezbollah has apparently fitted banks of Fajr-3 rockets to Isuzu trucks.  They have likely three dozen or more of these systems.  This makes them very mobile, and thus hard to defend against.  I have no idea how many actual rockets are available, since the launchers can be reloaded.  Clearly, from the number that have already been fired, they have far more than one rocket per launcher.  And, they are shooting them without apparent concern for running out.  This suggests either a very large supply of rockets, or very poor military planning.  From what I’ve seen, either or both are plausible explanations.

The Fajr-3 itself is a 5.2 meter (17 foot) long, 240mm (9.5 inch) diameter solid fueled rocket capable of a 45 kilogram (100 pound) warhead for about 45 kilometers (28 miles).  Iran has recently test fired a new improved Fajr-3 that is supposed to be rather stealthy, that is, it is hard to detect on radar.  That would make it difficult to try to intercept by Israeli Patriot missile batteries.  That is especially true, since the Israeli Patriots are not the upgraded ones deployed by the US Army in the second Gulf War.  The difficulty in intercepting the rocket means that pretty much any Fajr-3 rockets fired will strike targets in Israel (I don’t have data on launch failures, but the technology is pretty robust).  While a 45 kilogram warhead is bad if it is landing right near you, it isn’t a particularly powerful bomb.  Car bombs used in the area have had substantially bigger explosives.  This warhead is only a bit larger than the explosives of two or three human suicide terrorists.  However, the Fajr-3 rocket’s accuracy drops dramatically with range.  At extreme range, they typically land only within a circle of radius 1 kilometer of their target.  That makes them low precision weapon.  They are being used at extreme range in the current conflict, so they not likely to hit a target unless that target were something like a port or refinery.  Even then, they are unlikely to hit exactly where aimed at those facilities.  They are primarily weapons of terror.  They are fired in the vicinity of a target, and everyone drops what they are doing and scrambles for shelter.  The warhead is small enough that a substantial portion of the damage done is simply by the impact of the rocket structure itself.  However, the warhead can be fitted with antipersonnel fragmentation warheads.  Still, they are unlikely to hit in just the right place to do major damage.  The suicide bombers are more likely to actually do lots of damage and kill lots of people.  Still, there is a matter of just shear terror of death raining down from the sky.  And, if you happen to be where one of these things is landing, it really doesn’t matter if it had actually been aimed there or not.  Some uprated Fajr-3 rockets may have a range up to 60 kilometers (37 miles).

A very close cousin to the Fajr-3 rocket is the Fajr-5 rocket.  The Fajr-5 5 is about the same length, but is wider, at 333mm (13.1 inch) diameter.  This gives it the capability of lifting a heavier warhead, or going a farther range, perhaps over 75 kilometers (47 miles).  Uprated versions are likely available.  However, the Fajr-5, being wider, can not be fired from the same launch tubes as the Fajr-3.  I don’t know if Hezbollah has either Fajr-5 rockets or their launchers.

At any rate, the current situation is very sad, and disturbing.

-Astroprof

I had an idea to write about cosmic rays and cosmic ray exposure, but this post turned out to be a much larger project than I thought.  Below you will find information about cosmic rays themselves, as well as the exposure that airline crews experience due to cosmic rays.

A century ago, scientists were just learning about radiation.  Different substances were shown to emit radiation of various types and energies.  Other substances were seen to act as shields to radiation.  How effective a shield something turned out to be was shown to depend in part of the type of radiation and in part upon the intensity and energy of the radiation.  But, there soon appeared a mystery.  No matter how much shielding was used, there was always some radiation.  Believing that perhaps the source of this residual radiation was from terrestrial background sources, radiation experiments were conducted on the surface of lakes, but this had no effect on the residual intensity.  But, when the experiments were conducted deep underground, the residual radiation decreased.  This did not seem to make sense if the source was terrestrial.  Following a hunch, Victor Hess lifted radiation meters several kilometers into the air aboard balloons.  He found that the radiation level dramatically increased with altitude.  Furthermore, the intensity seemed to increase exponentially with altitude.  This is what you expect if the atmosphere were acting as a radiation shield.  So, this mystery radiation was apparently coming from outside the Earth’s atmosphere.  He coined the term cosmic rays to describe this extraterrestrial radiation.  Victor Hess won the Nobel Prize in 1936 for his work with cosmic rays.  Later experiments showed that the radiation level actually peaks at an altitude of about 15 kilometers, somewhat higher than commercial aircraft fly.  Though the radiation level peaks higher than commercial aircraft fly, they fly close enough to the peak that cosmic radiation levels can be hundreds of times higher than experienced at the surface of the Earth.

So, what are these cosmic rays?  Well, we need to differentiate between what Hess discovered and what we find at very high altitudes.  It turns out that that cosmic rays are extremely high energy atomic and subatomic particles moving at high speeds through space.  When they enter Earth’s atmosphere, they collide and interact with atoms in the atmosphere.  These interactions can result in the creation of new particles that then can either interact with more atoms or decay directly into even more particles.  The original cosmic ray particle might even go blasting through interacting with many air atoms before slowing to the point that it doesn’t have much effect.  Each interaction can create more particles or radiation.  So, a single such particle can create a shower of other things.  This multiplying factor is why radiation levels peak at about 15 kilometers.  So, we differentiate between the original particles, called primary cosmic rays, and the created particles, called secondary cosmic rays.

So, what are these cosmic rays?  Where do they come from?  What effect do they have on us?  In particular, since commercial aircraft fly at altitudes where the radiation level is much higher than on the ground, what effect does it have on the passengers and crew of those aircraft?

To start with, let’s think about the origin of the cosmic rays.  That means looking at the primary cosmic rays to start with.  These can be classified as three general types:  galactic cosmic radiation, solar radiation, and anomalous cosmic rays. Each have a different origin, and they have a different effect when they strike the atmosphere.

What are the galactic cosmic rays?  Most of them, about 89% are pretty ordinary protons, only moving at extremely high speeds.  About 10% are helium nuclei, and about 1% are heavier particles, mostly nuclei.  In fact, the primary cosmic rays can consist of just about every imaginable type of stable nucleus, though nuclei heavier than iron are exceedingly rare.  Most of the galactic cosmic rays are particles moving as speeds ranging from a little under half the speed of light up to speeds of about 99.6% the speed of light.  A few, though, have much higher energies.  One particle was found with an energy corresponding to a proton moving at 99.99999…995% the speed of light (the … there signifies that it is 99 followed by twenty three nines and then a five).  This particle is nicknamed the Oh-My-God particle.  Imagine a baseball moving at about 60mph.  That is about the same energy as this super energetic particle, except that it is a subatomic particle having that same energy!  It and a few others that have been found have energies that exceed what theoretical astrophysicists had believed was the maximum energy that a particle could carry without interacting with the cosmic background radiation, slowing down the particle and producing more subatomic particles.  These super high energy cosmic rays are a complete mystery.  But what about the ordinary galactic cosmic rays?

We don’t really know for sure the source of cosmic rays.  However, we suspect that most come from supernovae explosions.  Actually, they aren’t likely accelerated to such energies directly in the stellar explosion, but rather they are accelerated in the expanding supernova remnant that follows.  As the cloud of gas expands for thousands of years after the supernova, particles within it collide with one another.  These collisions randomly move energy from particle to particle.  Some speed up and some slow down.  A tiny few just by change speed up far more often than they slow down, and these become cosmic rays.  This explains most of the ordinary galactic cosmic rays, but not the high energy ones, and certainly not the super high energy ones.  Various models have been proposed for the high energy cosmic rays, including particles from jets from black holes, from hypernovae explosion jets, from particles given off by neutron star spin offs, and various other more exotic hypotheses (like those aren’t exotic enough!).

Solar cosmic rays, as the name suggests, come from the Sun.  The Sun is always shedding particles into space, what we call the solar wind.  Most solar wind particles are not really all that energetic, and so they are not a concern except to astronauts.  Even solar cosmic rays that strike the atmosphere fail to produce sufficient secondary cosmic rays to be a major concern.  Normally, that is.  The Sun occasionally has a very violent explosive release of energy called a solar flare.  These solar flares can bathe the Solar System in X-rays, and they can accelerate large chunks of the Sun’s corona into space, in what we call a Coronal Mass Ejection (CME).  The more flares, the more CME’s.  The bigger the flare, the bigger and more energetic the CME and the more other particles are flung around the solar system.  If sufficient particles run into Earth’s near space environment, we have what is called a Solar Radiation Storm.  NOAA’s Space Environment Center monitors such solar activity, and issues radiation storm watches and warnings.  Normal solar wind isn’t much of an issue for Earthbound or aircrews.  However, during and energetic solar radiation storm, the intensity and energy of these solar particles, mostly protons, drastically increases.  For those of us on the ground, even solar radiation storms are not a problem.  But astronauts can be exposed to very dangerous levels of radiation, and aircrew can experience major radiation dosage.  During major solar radiation storms in 2003, aircraft flying at about 40,000 feet in North America and northern Europe experience a radiation exposure equivalent to about 2 or 3 chest X-rays, per hour of flight.  Transatlantic flights, which flew quite far north on the so-called Great Circles, could be exposed to even higher radiation levels, with a single flight producing upwards of 25 chest X-rays worth of radiation.  Just 4 such flights would expose passengers and crew to as much radiation as is generally allowed for an entire year for the general population from all sources. Just one flight would nearly reach the maximum radiation exposure that a pregnant woman should experience during a single trimester of pregnancy.  That’s one flight, not a round trip.  Fortunately, such radiation storms are quite rare.  Most radiation storms occur near solar maximum.  The Sun goes through a cycle of activity lasting about 11 years on average.  However, statistically, the most powerful solar flares often occur on the downside of the cycle.  We are just at the cusp of starting a new solar cycle, which should peak in about 2012, give or take a year.  But, solar cosmic rays have an interesting interaction with the galactic cosmic rays.  The interaction between solar cosmic rays and galactic cosmic rays tend to rob the galactic rays of their energy.  So, when the solar cosmic rays go up in intensity, the galactic cosmic rays tend to decrease.  This effect is called the Forbush decrease, after the physicist Scott Forbush, who first discovered this effect.  At peak solar activity, the galactic cosmic rays can decrease by up to 30%.  Since most solar cosmic rays are low energy, this has the strange effect that aircrew actually experience lower cosmic ray exposure during solar max than they do at solar min (we are now at solar minimum), assuming that they are not flying during the occasional radiation storm. 

As mysterious as the super high energy galactic cosmic rays are the anomalous cosmic rays.  We really don’t know where they come from.  We suspect that they come from the edge of our Solar System, what we call the heliopause.  This is the boundary between the region of space dominated by the Sun and the interstellar medium.  It is believed that neutral particles may enter the heliosheath (the region around the magnetic boundary of the heliopause) and become ionized.  They are then accelerated and become low energy cosmic rays.  An alternate hypothesis is that some galactic cosmic rays interact with the buildup of particles along the heliosheath and slow down to be anomalous cosmic rays.  As my post of a couple of days ago said, there are four spacecraft leaving the Solar System right now.  Two are still working.  Voyager 1 passed into the heliosheath a bit over a year and a half ago.  Voyager 2 should do so in about two or three years.  Unfortunately, the data from Voyager 1 is not really consistent with our models of anomalous cosmic rays.  We don’t know if that is because of something strange in the particular part of the heliopause that Voyager 1 went through, or if perhaps we are totally off base on where these things come from.  Hopefully, Voyager 2 will clear up some of the confusion.

But, unless you are an astronaut, or you own or operate a satellite, primary cosmic rays are not really a direct concern.  They rarely survive to the ground.  Rather, they interact with the atmosphere to produce secondary cosmic rays.  These are the ones that you will normally have an interaction with.  As I said earlier, a single cosmic ray particle, if energetic enough, can produce a large number of secondary particles.  In fact, a single cosmic ray particle hitting the atmosphere with sufficient energy can sometimes shower nearly a square kilometer of the ground with particles.  At sea level, a single square meter gets about 8 cosmic ray showers per second.  You can see this effect with very high efficiency cooled digital cameras, such as astronomers use.  Taking a image of nothing, with the cap on the CCD camera, you should see nothing.  Instead, you get an image that looks like a sparse star field.  There are little dots here and there across the image.  These are pixels that are charged by interactions with these secondary cosmic rays.  It is something that has to be taken into consideration if you are doing real science, and not just taking pretty pictures.

But, what are these secondary cosmic rays?  Some are things like protons, electrons, and neutrons.  However, these typically don’t go very far through the atmosphere, and comparatively few reach the ground.  Most are a short lived particle called a muon.  The muons that make it to the ground are very high energy particles, moving at very near the speed of light.  They are very penetrating particles, too.  You can’t really effectively shield against them.  When I was an undergraduate physics major, I did a senior lab project in which I measured cosmic ray muons.  I did the experiment in the basement of the physics building.  So, these muons went through many miles of air, through two floors of a concrete and steel building, and into the basement where I measured them.  Now these things are very short lived, so about the best way to shield against them is to go deep underground;  that way they’ll have longer to reach you and will have a chance to decay.  But, if you go higher, you are closer to the source, and so more will reach you. 

Muons are not the only secondary cosmic ray, though.  There’s a whole zoo of things that can be produced.  You can get a shower of protons, neutrons, pions, and muons.  The pions don’t last long before they interact with another atom or decay into a muon and a neutrino.  You also get high energy gamma rays.  These are very high energy gamma rays, in fact, and were among the first type of secondary cosmic ray identified.  When I was a child, the textbooks listed “cosmic rays” as the shortest type of electromagnetic wave.  Now, we realize that these are just very high energy gamma rays.  Something else interesting is what we call Cherenkov radiation.  Many of these cosmic ray particles are moving at very close to the speed of light.  That is, they are moving very close to the speed of light in a vacuum.  Light moves a bit slower when passing through a medium.  When a particle moves faster than light in a medium, it slows down and produces a form of light that we call Cherenkov radiation.  This produces the blue glow around water moderated nuclear reactors.  It also makes the entire sky glow a little bit.  Astronomers are now using telescopes specially designed to study this Cherenkov radiation.

So, when I started this (rather long) entry, I indicated that I would talk about the effect of cosmic rays on aircrew and passengers.  I’ve got several really good friends who are flight attendants or pilots.  Naturally I am interested in their health if they are going to be irradiated like this.  So, how much radiation do they actually get?  A lot.

I did a little research to prepare this posting.  I asked some of my flight attendant friends for some information on average flying time for their airlines, and I looked at documents from several government agencies, airlines, and aircrew associations.  I found numbers all over the place for radiation exposure.  I think that I will continue to research the topic even after this posting.  To make the topic even more confusing, everyone was using different units.  As my poor physics students figure out, this whole radiation business is confusing, since there are a multitude of ways of measuring radiation exposure.  They don’t always give the same answer, even if you are measuring the same radiation.  That is because they have different effects that they are looking at.  Most of the figures I found from the airlines or the FAA, used a measure of radiation energy absorbed.  I found some other data from scientists using relative exposure (this is a measure that includes the radiobiological differences due to different types of radiation).  I converted from one to another to try to compare the results.  So, the numbers that I give here should be taken with a grain of salt.  They are definitely a back-of-the-envelope type calculation, but they should be in the ballpark.  The FAA actually has devised a very good program to compute likely actual exposures per flight.  This is really the best thing to use, since there are so many factors involved.  Radiation exposure depends upon altitude flown, solar activity, and flight path.  The flight path matters because the cosmic rays are charged particles, and so they are affected by Earth’s magnetic field.  On average, the closer that you are to the Earth’s magnetic poles, the more the cosmic radiation you are experience. The FAA program to do the calculations is called CARI-6, and it can be found here.  It isn’t terribly user friendly, but it is supposed to be very good.  Another, web-based, calculator for determining radiation exposure per flight can be found here.

So, what did I get from all my research?  Well, it seems that the average pilot or flight attendant gets a lot of radiation exposure.  A whole lot, that is.  As I said, the amount of radiation depends significantly upon the actual flights flown (altitude, flight path, and solar activity).  However, using sort of average numbers, and assuming an average number of flying hours per month from my friends that I asked, I came up with some interesting numbers.  Assuming that an average flight attendant flies about 85 hours per month, and about 1/3 of those hours are transatlantic flights, 1/3 are transcontinental flights, and 1/3 are medium range flights, and assuming typical altitudes for those flights, I came up with a figure of a little under 600 mrem per year for radiation exposure.  More transatlantic flights drastically increases that figure, since those are the highest radiation flights.  More flights in the New England, Canada, or South America also significantly increase radiation exposure, since those areas receive more cosmic radiation due to anisotropy in Earth’s magnetic field.  Incidentally, this figure is near the upper range of what United Airlines has published as “typical” for a flight attendant, once all unit conversions are made.  It is also almost 6 times the level at which someone is considered to be occupationally exposed to radiation.  It is about 5 times the level that a typical nuclear reactor technician experiences.  It is about what nuclear waste handlers get.  This value is also just under 1/3 of the latest value recommended as the maximum recommended exposure for nuclear workers.  The difference is that those people get a lot more education and a lot more monitoring of radiation exposure.  And this is typical exposure, excluding solar radiation storms.  When I compute a somewhat higher percentage of transcontinental and transatlantic flights, I get numbers of about 1 rem per year.   This is the threshold value established by the EPA for radiation exposure to the general public for requiring emergency measures following a nuclear accident.  Hmm.  I guess this is why all my flight attendant friends have such “glowing” personalities!  Lol!

So, what effect does all this radiation have?  Health Canada reports that radiation exposure due to cosmic rays can produce about a 1% chance of a fatal cancer after 30 years of flying about 1000 hours per year.  This is about what my friends report as average flight attendant flying hours.  But, the human body has a remarkable repair mechanism for single gene radiation damage.  However, certain cells are more susceptible to radiation damage than others.  Bone marrow, intestinal linings, and hair follicles (and incidentally cancer cells, too) are particularly susceptible.  That is why radiation sickness, or whole body radiation treatment for cancer, produce nausea, anemia, and hair loss.  I once spoke with astronaut Susan Helms, who reported that after extensive time aboard the International Space Station, her hair had gray streaks in it (which gradually went away after returning to Earth).  However, radiation can also damage women’s eggs or men’s sperm.  This can lead to all sorts of reproductive problems.  Also, the developing fetus is highly susceptible to radiation damage.  The FAA Office of Aviation Medicine has released a document that is suggested reading for female aircrew who either are or expect to become pregnant.  It can be found here.  The International Commission on Radiological Protection recommends a maximum exposure during the entire course of pregnancy of about 05. milliSievert, which corresponds to 50 mrem.  Using my typical data, this corresponds to under 10 transatlantic flights, or under 40 transcontinental flights.  Short haul flights produce less exposure, so up to about 80 of them are OK, and it would take nearly 400 very short haul regional flights to yield that sort of exposure.  Health Canada recommends less than 200 hours of flying for pregnant women.  That figure obviously does not consider high radiation transatlantic flights.

So, with all this radiation exposure, what are we doing about it?  Well, European airlines seem to be taking measures to limit radiation exposure to their employees and passengers.  They are even assigning pregnant employees to ground jobs.  During the major solar radiation storms of 2003, some transatlantic flights were held on the ground for several hours until the storms subsided.  Flights in the air flew lower so that they’d get less radiation exposure.  So, what are US airlines doing?  Well, I guess that they are of the opinion that if you can’t see or feel radiation then it must not be anything to worry about.  They don’t seem to be doing much.  In fact, they seem to actively lobby the FAA to keep them form imposing the sort of restrictions that Europe uses.  Some US airlines that fly to and from Europe, or who have code sharing agreements with European airlines are being forced by those agreements to do at some minimal radiation education activities.  Looking around the internet, I find that the Association of Flight Attendants had been active in trying to get something done to protect aircrews.  They have a web page here with some links on radiation issues for aircrew.  The International Federation of Air Line Pilots is trying to promote a policy limiting radiation limits for flight crews to 20mSv per year.  That sounds good, until you realize that 20mSv per year corresponds to 2 rem per year, the recommended maximum radiation exposure for commercial radiation workers and double the level that requires emergency response from a nuclear accident. 

So, why so little response?  It has been suggested by more than one person that perhaps the airlines are afraid of how the general public might respond to the news of radiation exposure on airplanes.  They are already hurting economically, so they might fear anything that might reduce paying passenger flights.  The reality, though, is that while the radiation levels due to cosmic rays are substantially higher in high flying aircraft than on the ground, we get a lot of radiation exposure due to various other sources all the time.  For most passengers, the exposure on a single flight isn’t really all that much.  They’d get about as much sitting too close to their TV.  But, even most frequent fliers don’t fly even remotely as much as pilots and flight attendants.  So, the aircrew are the ones that really get the high doses of radiation.  A passenger would have to fly something like 60,000 miles to start to get significant radiation exposure.  Most flyers don’t fly that much.

Incidentally, as I said, I asked some of my flight attendant friends for some information in preparing this.  One of them, Flygurlual, actually got interested in the topic, and posted a blog entry of her own on the topic.

-Astroprof

In the evening of March 2, 1972, an Atlas-Centaur rocket flamed into life and lifted off from Cape Canaveral (If you look at international launch data, they will say that the rocket launched on March 3, 1972, since it was after midnight Greenwich Mean Time, but it was still March 2 in Florida).  Atop the rocket was a small hexagonal piece of equipment, 36cm tall, with sides 71cm long.  This was the heart of the Pioneer 10 spacecraft.  Affixed to the central body were antennae, radiothermal generators, magnetometers, and cameras.  Within moments the craft was hurling through space at over 51,000 kph.  After only about 11 hours, it was farther from Earth than our Moon.  Pioneer 10 continued onward.  Unlike previous spacecraft, it was not headed for anything as nearby as the Moon, nor to Venus, our nearest planetary neighbor, nor even the red planet Mars.  Pioneer 10 was the first spacecraft aimed at the outer Solar System.  It passed through the asteroid belt and entered the Jupiter system in November 1973.  It didn’t stop there, though.  The little spacecraft passed closest to Jupiter December 3, 1973.  Jupiter’s gravity pulled the craft inward and faster.  As it slung past Jupiter, it left faster than it had approached.  This gravitational boost, called a slingshot effect, hurled the little vessel outwards towards interstellar space.

Pioneer 10 is remarkable because it was the very first interstellar spacecraft ever launched from Earth.  It continued onwards, and in fact continues to this day, though it no longer operates.  It is now nearly 91 astronomical units (AU) from the Sun.  An astronomical unit is the average distance that Earth is from the Sun.  Pioneer 10 is so far away that it takes nearly 13 hours for light to reach it from the Sun, or for radio signals to reach it from Earth.  It is currently moving at a bit over 12 km/s outward in the general direction of the constellation Taurus.  But it isn’t alone.

One month after Pioneer 10 launched, Pioneer 11 launched to follow it.  Though launched only a month behind Pioneer 10, Pioneer 11 did not arrive at Jupiter until one year later.  Now, Jupiter was in position to hurl Pioneer 11 outward with a slingshot effect, as it did to Pioneer 10, but this time towards the great ringed planet Saturn.  Pioneer 11 passed closest to Saturn on September 1, 1987.  Saturn’s gravity hurled Pioneer 11 onward out of the Solar System.  Currently, Pioneer 11 is a little less than 72 AU from the Sun, moving at about 11.5 km/s.  Being closer, it takes light only a bit under 10 hours to reach Pioneer 11 from the Sun.  Pioneer 11, like its sibling Pioneer 10, is no longer functioning.   From Earth, Pioneer 11 would appear to be in the general direction of the constellation Scutum.

The Pioneer 10 and 11 spacecraft were the first man-made objects that were sent out of the Solar System.  As a sort of public relations gimmick, NASA affixed a plaque to the side of each vessel bearing a figure of a human man and woman, a schematic of the Solar System, and a map showing the Sun’s location relative to several pulsars.  The Pioneers are not heading towards any other star system, and if they were it would take tens of thousands of years for them to arrive.  Then, they would be a tiny piece of metal flotsam flying through the system.  It would be an incredible miracle if it were to be found, even if it passed through a star system filled with space faring aliens.  And, if it were found, the plaque would likely be completely illegible, having been eroded by micrometeorites, interstellar dust collisions, etching from interstellar medium, and general chemical degradation.  And, even if it were still able to be read, the map would be useless.  Pulsars are rotating neutron stars.  As they rotate, “hot spots” beaming radiation into space sweep across space light the beams of a lighthouse.  When these beams pass by our line of sight, they give a burst of radio static --- the pulses.  If the beam doesn’t sweep past, you don’t see the pulsar.  So, it would be pretty unlikely that anyone finding the plaque would be in a location to see the pulsars so described, unless they were fairly near the Solar System anyway.

But, the Pioneers are not the only craft leaving the Solar System.  In late summer, 1977, two larger, much more costly, and substantially more sophisticated spacecraft were launched on similar missions:  Voyager 1 and Voyager 2.  Interestingly, Voyager 2 was launched first, on August 20, 1977, and Voyager 1 followed on September 5, 1977.  Though Voyager 2 launched first, Earth and Jupiter were better placed just two weeks later, so Voyager 1 actually arrived at Jupiter first, on March 3, 1979.  Voyager 2 followed, arriving August 9, 1979.  Jupiter hurled both craft onwards towards Saturn.  Voyager 1 passed Saturn November 13, 1980, and Voyager 2 passed August 27, 1981.  I was in college when these events occurred.  Amazing amounts of data began flowing back.  So much of what we knew about Saturn changed from when I began college until when I left.  (Hmm.  OK, so I never actually left.  Now, I am just at the front of the classroom instead of being a student.)    Voyager 1 passed by Saturn with a trajectory that allowed the best study of the rings, but this meant that Saturn’s gravity hurled it away from the plane of the planets’ orbits, so it would never again pass by another of the Sun’s planets.  It is currently the most distant man-made object, at a distance of just under 100 AU from the Sun.  It is so far that it takes nearly 14 hours for signals to reach Earth from the spacecraft, which is still functioning, and heading in the general direction of the constellation Ophiuchus 17.1 km/s.  However, using Saturn to slingshot itself onward, Voyager 2 continued on to Uranus, passing that planet January 30, 1986.  I was a graduate student then.  Uranus then propelled Voyager 2 on to Neptune, which it passed August 15, 1989.  By that point, I was teaching astronomy.  The first textbook that I used teaching had very little to say about Neptune, since it was written before the first spacecraft had even been there!  Currently, Voyager 2 is about 80 AU from the Sun, and it takes about 11 hours for signals to go between Earth and the spacecraft.  Voyager 2 is heading outwards at 15.6 km/s in the general direction of the constellation Telescopium. 

Though the idea of the plaque on the Pioneers seemed cute, and it was a major public relations thing for NASA, it drew serious criticism.  The problem was that the artists, not knowing if aliens would recognize the concept of clothing, had drawn the people on the plaque naked, and anatomically correct.  This turned out to be a serious public relations blunder for NASA.  So, the Voyagers had a different public relations gimmick.  A gold plated long playing record was affixed to the sides of the craft.  That way, any aliens finding it would simply take the record and put it on their turntable in their flying saucer, spinning it at 33 1/3 rpm and listen to the sounds of Earth.  The president will greet them, there will be representative music, the sound of rain, birds chirping, and whales singing.  Yeah, right.  Better hope the aliens have turntables with the right size stylus.

All four of these craft are our interstellar ambassadors, leaving the Solar System.  But they now have a new cousin.  Earlier this year, the New Horizons spacecraft blasted off on a mission to Pluto.  Eventually, it will pass Pluto and continue onward through the Kuiper Belt, becoming the fifth spacecraft to leave the Solar System.  It has a way to go, though, as it is currently less than 2.75 AU from the Sun (as of this writing), but it is moving fast.  It is moving at nearly 26 km/s, meaning that it is moving at the incredible clip of almost 5.5 AU per year!  Gravity is pulling on it, slowing it down, but it will remain the fastest craft ever launched from Earth.

-Astroprof