Here in the United States, we declare that winter starts at the Winter Solstice and ends at the Vernal Equinox.  Autumn starts at the autumnal equinox and ends at the winter solstice.  I’ve written about the seasons before, and I think that this is perhaps not the best way to declare the seasons, but I suppose that it works well enough.  Given that the solstices and equinoxes are well defined points in time, they make convenient dates to mark on the calendar.  That gives the television weather people something to talk about.  Really, of course, the weather is not significantly different on average on December 20 from what it is on December 22.  But, as I said, these are convenient dates to mark.  But, there is one point that they often get wrong.  Almost every weather forecast on the Winter Solstice says that the solstice marks the shortest day of the year.  This year, that was actually true, because the solstice occurred at 17:45 UT (that is 11:45 AM Central Standard Time).  Sometimes, though, when the solstice occurs during the night, early morning, or late afternoon, it is actually the calendar day before or the day after the date of solstice that is actually the shortest day.  The calendar date of the solstice is the shortest if the actual moment of the solstice is near the middle of the day.  If you want to determine how long the day is at your location, the US Naval Observatory has a really nice online utility to give sunrise and sunset data for one day at a time or even for one year.  If you look at the data using that utility, one finds that for where I live, the Sun rose on December 21 at 7:28 AM and set at 5:27 PM.  But, you find the same sunrise and sunset times for today, December 22.  But, at this time of year, the rate at which the length of daylight changes is very slow.  In fact, the slowest changes in the length of daylight occur near the solstices, and the fastest changes in the length of daylight occur near the equinoxes.

Frequently, when the weather reporters are talking about the solstices, they make a mistake.  Almost every year I hear one of them saying that the solstice marks the shortest day of the year and the latest sunrise.  Hey, that would just seem to make sense, right?  After all, if it is the shortest day of the year, that would seam mean that the sun rises latest and sets earliest.  Unfortunately, that is not correct.  If you look at the sunrise/sunset utility that I mentioned before, you’ll find that the sun rises tomorrow at 7:29 AM and sets at 5:28 PM.  The day is still just about as long, but the sun is rising later than today or the day of the solstice! In fact, you find that time of sunrise slips a bit until it is rising at 7:33 AM on January 7.  Sunset on January 7 occurs at 5:38 PM, so that means that the day is longer on January 7 than it is now, but the sun still rises later.  What gives?

The answer to this mystery resides in the motion of the Earth around the Sun.

In the diagram above, the Earth is rotating as it moves around the Sun.  Imagine the Sun at some point well below the bottom of the computer screen.  The Earth moves a little less than one degree around its orbit each day.  That means that it must rotate a little more than one degree in order for a point on the Earth facing the Sun to face the Sun once again.  This is the difference between what we call the sidereal day (the time that it takes to make one complete rotation) and the synodic day (the time that it takes to go from the Sun highest in the sky until the Sun is again at its highest in the sky).   The sidereal day is just over 23 hours 56 minutes long.  The synodic day is closer to 24 hours.    So, the Earth must rotate almost 361 degrees in order for one solar day to occur.  The stars, on the other hand, rise every 23 hours 56 minutes.  That means that the stars will appear to rise and set about 4 minutes earlier every day.

But, it gets a little more complicated than that.  Earth’s orbit is a little bit elliptical.  Earth was farthest from the Sun on July 4 of this year (2009), and it will be closest to the Sun January 3, 2010.  Note that it is the tilt of the Earth’s axis, not its distance from the Sun that causes the seasons!  But, as a planet moves around the Sun in an elliptical orbit, it speeds up and slows down.  The planet moves quickest at perihelion (closest to the Sun) and slowest at aphelion (farthest from the Sun).  So, Earth is currently moving a bit faster in its orbit than average.  That means, however, that in order to complete one synodic day, or solar day (a point facing the Sun to once again face the Sun), the Earth will have to turn a little more near perihelion than it would near aphelion.  That makes the days longer.  Indeed, the length of the synodic day changes over the year, being longest near perihelion and shortest near aphelion.  The synodic day (solar day) can range from about 23 hours 59 minutes 38 seconds to about 24 hours 29 seconds.  That isn’t much difference, but it adds up.  It would be a bit confusing if the clocks had to run at different speeds at different times of the year. Thus, clocks run at a constant rate set by the average length of the solar day over the year:  24 hours.  The actual day isn’t far off of that, but with a difference of nearly half a minute per day, the discrepancy adds up.  After a couple days, the clock will be a minute off.  That is one reason that the sun doesn’t always appear highest in the sky (local noon) at the same time of day every day.  Clock time and solar time are off a bit.  To correct between solar time and clock time, you need to know the equation of time, which will tell you how much to add or subtract from a sundial’s time to find the clock time.  With the solar (synodic) days out of sync with clock time, then solar events like sunrise, sunset, solar noon, etc, tend to be out of sync with the clock.  They slip a little each day at this time of year.  That is the reason that the latest sunrise does not occur at the Winter Solstice.  If the Earth had a perfectly circular orbit, then the latest sunrise and the solstice would occur at the same time, but Earth’s elliptical orbit skews things a bit.

-Astroprof

In years past, my Christmas presentation has sometimes been well attended, but most of the time the attendance is light.  Those who do attend it seem to be pleased, so I guess that I’ll continue doing it.  I don’t have to have a huge crowd for me to think that it was a successful event.

But, for anyone else planning star parties this time of year, don’t be surprised if you don’t get a huge attendance.  There are a lot of competing factors.  For one thing, this was the weekend before Christmas.  For me, I would think that it would be the ideal time for this program, but I tend to be a bit out of sync with a lot of people.  Most other people are pretty busy with last minute Christmas shopping.  I have found star parties the weekend before Christmas to frequently be very poorly attended.  Adding to the timing, it was clear and cold.  For astronomers, that is perfect weather!  But, for the public, cold is a deterrent.  Most people don’t know how to dress for the cold.  Another factor that bit into our attendance is that last night the Dallas Cowboys played an important football game.  They are a big deal locally, so attendance at anything tends to drop when they are playing a game, and attendance drops a lot when they play a big game.

But, it doesn’t matter to me if there were a lot of people or not.  I enjoyed getting out of town and seeing a dark sky for myself.  And, the rangers seemed to find my presentation interesting.  As long as someone got something from the event, then it was worthwhile.

Here’s a big “Merry Christmas!” to everyone from Astroprof’s Page!

-Astroprof

According to a report on Science Daily, the Large Hadron Collider has set a new energy record.  The twin beams were at an energy of 1.18 TeV, beating the 0.98 TeV energy of Fermilab’s Tevatron.  That now makes the LHC the worlds most energetic particle collider.  Despite all of the cries to the contrary, we are still here after this milestone.  However, don’t expect all of the people crying that it is the end of the world to quiet down just yet.  Next year, the LHC will work its way up to an energy of 3.5 TeV per beam, for a total collision energy of 7.0 TeV.  Until it reaches that milestone, then the end-of-the-worlders will simply be telling us that the end is postponed.  Then, once 7 TeV collisions become common, they’ll come up with some other explanation for why we are still here.  The vast majority of physicists don’t have a problem with this collider.  In fact, there are collisions at FAR higher energies than this occuring all of the time in the air over our heads.  Galactic cosmic rays rain down on us at far higher energies than the LHC will ever be able to produce.  In fact, it is not unheard of for galactic cosmic rays to have energies in excess of 100000000 TeV.  Granted, the average cosmic ray energy is far lower than that, but the very high energy ones still occur, and they have done so for billions of years.  None have destroyed Earth yet, so I don’t see any compelling reason to expect the LHC to do so, either.

Part of what makes this particle collider, and high energy physics in general, so confusing to the general public is the terminology.   For one thing, the beam energy is often given in terms of electron volts (eV).  An electron volt is the energy that it takes to move a charged particle having charge e (the fundamental charge, 1.602×10^-19 coulombs) through a potential of one volt.  That turns out to be equal to 1.602×10^-19 Joules, a very tiny bit of energy.  But, we are talking TeV here.  What is that?  TeV stands for tera electron volt.  That is one trillion (10¹²) electron volts.  That’s got to be a lot, right?  Well for a subatomic particle, one TeV really is a lot.  But, to give you the sort of idea of what kind of energy this is, imagine figuring out the amount of energy that it takes to lift a penny.  If you lift the penny a distance of 0.045 mm (about 1/100 of an inch), then you have used about 7 TeV of energy.

It is hard to imagine how energy so small could scare anyone.  Granted, some are worried about possibly the energy being confined to a small enough volume to create a miniature black hole.  However, black holes that small would not be stable, so there would be little to worry about.  I am not going to go into all of the reasons why such things are nothing to worry about.  That’s been done before by many other writers on the internet.  Black holes themselves are grossly misunderstood, and are not nearly as scary as science fiction writer portray them.  Indeed, if miniature black holes really were a hazard to us, then we’d be doomed long before now due to the collisions from much higher energy cosmic rays.  So, I am not worried that the LHC is going to destroy the world when it reaches full power.

-Astroprof

Image Credit:  CERN

-Astroprof

The first picture, above, is of the rocket sitting on the launch pad on Monday night.  Below is a picture of me standing next to the countdown clock.  (Hey, I had to get a tourist-type photo, OK?).

But, alas, the countdown only proceeded to T minus 4 minutes.  It then held.  On Tuesday, they said that they were just a bit behind on tasks.  However, the weather played a factor, then a ship got in the way, and then more weather was a problem.  Future flights will not have the same constraint on triboelectrification that this flight had.  Part of the issue was that this was a developmental flight, so there were a lot of sensors on board looking at just how much triboelectrification this rocket will have anyway, so they needed pretty tight weather constraints.  So, the next day, we were back at the space center at 5:30am again, and the countdown ticked down to T minus 4 minutes again.  The picture above was during that second day (I got one of me on the first day, too).  The initial problem on the second day was checking out the rocket after lightning strikes in the vicinity overnight.  Everything checked out, but the weather was still not good.  The following picture was what we kept seeing, for nearly seven hours over the two days.

Eventually, though, the weather was within tolerances for a few minutes.  Fortunately, it was just long enough of a window for the rocket to get off the ground before the weather deteriorated.   I have a lot of pictures of that, but the next three images show the rocket right after engine ignition, shortly after clearing the pad structures, and in flight.

It was quite impressive.  I realize that there is some controversy over the Ares rocket design, but I am glad that I got to be there for the launch.  I would very much like to see other launches.  I’ll be investigating some more about the Ares rockets, including some of the criticisms, and I’ll be writing more about it later.  Whether the Ares project goes forward or not, the data collected on this launch may be useful for engineers designing future rockets.  Already, we know that there was a problem with the parachutes on the rocket.  Finding what doesn’t work right, of course, is one of the reasons for test flights.  This rocket was not the final Ares rocket, but it should provide useful data for those engineers building that rocket.  Think of this as a huge wind tunnel test and a test of the recovery system.

I feel quite privileged to have asked to see the launch.  No matter what happens to the Ares project, there will only be one first launch of the configuration (granted, the Ares I-Y launch in several years will more closely match the final configuration, but this is a rocket of about the same size and shape), and I got to see it!  I would very much like to see some of the other launches, too.  I would particularly like to be present at some of the launches of the commercial rockets that are being developed.  This is a very exciting time that we are living in.  Decades ago, the only people launching rockets were government space agencies and military of those governments.  Now, there are a number of private companies that have gotten into the space launch business.  This is very important.  Aviation did not really take off as a major business, with all of the benefits that it provides, until private industry began to become heavily involved.  I can envision a future in which private companies are almost continually launching rockets carrying satellites, lifting people to space stations, supplying those space stations, and perhaps even sending missions to the Moon and beyond.  We are not there, yet.  And, there is still a role for the government (NASA) in space exploration.  But, these are exciting times.

-Astroprof

(Images of this post copyright Astroprof’s Page)

First of all, they were running a bit late.  The announcer said that there were no particular problems.  However, running late delayed the launch until clouds arrived.  Weather aircraft were studying the clouds and balloons took a look at higher levels.  Then, there was a delay due to high altitude winds.  Next, a cover over a sensor near the top of the rocket got stuck.  Finally, that came free.  Then, the aircraft found the clouds just a bit too much.  Finally, the weather cooperated.  The winds were OK, there was a break in the clouds.  The countdown started again.  Suddenly, with about two and a half minutes to launch, the countdown stopped.  The call was that a freighter had entered the exclusion zone offshore!  (I may have my delays confused.  I wrote this off of memory.  See Ed’s comment.)  At first, they were saying that it would take 90 minutes to clear the zone.  It took far less time than that, but they had to reset the countdown.  By the time they were ready to try again, the clouds had come back.  Then, there was a window in the clouds but it was too windy.  Finally, the wind died down, but the clouds were back.  Finally, they scrubbed the launc for the day.  We’ll go back in the morning to try again.  I would definitely like to see the launch, but it appears that trying to change travel plans to stay another day would push the cost of the trip far beyond my travel allowance.

So, what’s the deal with the clouds?  Naturally, those of us observing would like to have few clouds so that we could see the rocket arch across the sky.  But, the bigger problem is something called triboelectrification.  That’s a really fancy word that comes from Greek roots meaning electrification from rubbing.  You may be familiar with it as “static electricity.”  In fact, that is more like what they would have said years ago.  Of course, it isn’t really static, since the rocket is moving, and it is the motion through the clouds that causes the problem.

What happens is that the rocket, as it passes through the clouds, pushes water droplets out of the way.  The interaction between the rocket and the droplets results in charge being transfered between the two, leaving one positive and the other negative.  It is like rubbing your feet along the carpet on dry days.  You become electrically charged.  I remember always being told that it was friction doing the work to build the static charge.  But, as I understand it, chemical reactions, rather than friction, do the dirty work.  The rubbing past one another simply exposes more surface area to the action.

The end result, of course, is that the rocket becomes eletrically charged, and so does a tube of vapor along the path of the rocket.  That electric charge can interfere with signals between the rocket and the ground.  For an unmanned test flight like this, where the whole idea of the flight is to provide flight data for analysis, then interfering with telemetry is bad.  But, the interference also goes the other way.  It can interfere with signals from the ground to the rocket.  If the rocket were to go out of control, then the range safety officer has to be able to send a signal to destroy the rocket.  So, if there are too thick of clouds over the launch pad, then the launch doesn’t happen.  That was the problem most of this morning.

I got some good photos of the rocket on the pad, but my netbook lacks the tools to resize the to upload, so you’ll have to wait until I get home.  Hopefully, I’ll have photos of the launch tomorrow!

Astroprof

The Ares rockets and Orion crew module are supposed to eventually replace the Space Shuttle in NASA’s inventory of craft to ferry astronauts to and from space.   The Ares has been the target of quite a lot of criticism, too.  There are calls for the project to be scrapped.  However, most of those calling for scrapping the Ares project are outside of the space community, and almost all are outside of the field of aviation and rocketry.  The most common complaint is, “Why can’t we just use one of the other big rockets that already exist?”  Well, the reason is that those rockets are not designed for manned missions.  In order to carry humans, rockets and aircraft have to go through a rigorous test procedure, and each and every part has to be separately certified as compliant with those tests.  Going back and doing that for an existing rocket would be at least as expensive as building a new one from already certified parts.  That is what the Ares is:  a rocket built mostly from already certified parts.  So, I am not convinced that it would really save money to scrap Ares, and it might even cost more money and more time in the end.  Already, we are facing a period of time in which NASA will have no vehicle capable of getting astronauts to and from space.  We’ll have to either purchase flights from other nations (Russia) or hope that private companies come up with a space taxi of some sort to get to and from the space station.  That is why Ares is important.  I only wish that the Constellation Program (the overall program that includes Ares, Orion, the Altair lunar lander, and more) had more money and resources to develop the program faster.

Two Ares rockets are planned.  The smaller rocket (and even it is huge!) is the Ares I.  The Ares V rocket will be taller, but also much wider.  If built, it will be the largest operational rocket ever constructed.  The Ares I will stand over 300 feet high.  The Ares 1-X currently sitting at Launch Pad 39B at the Kennedy Space Center is 327 feet high.  That makes it the tallest rocket launched from the Cape since the early 1970s, when the last Saturn V rocket launched.  Incidentally, the last Saturn V rocket was launched in May 1973 from Launch Pad 39A.  Over a week later, a Saturn I-B rocket lifted off from Launch Pad 39B with a crew of three astronauts to man the space station.  I think that may have been the last time that two different rockets sat at the launch pads at Launch Complex 39 (Kennedy Space Center).  Now, over 36 years later, two different rockets are sitting at launch pads at Launch Complex 39 once again:  Ares I-X at Launch Pad 39B and the Space Shuttle Atlantis on Launch Pad 39A for the STS-129 mission.  The following photograph catches this historic moment.  Click on the photo to get a larger version.

The Ares I-X  is the first test flight of the Ares configuration.  The first stage of the Ares uses the same type of solid rocket motors that the Space Shuttle uses for its Solid Rocket Boosters.  The Space Shuttle uses a stack of 4 solid rocket segments.  The Ares uses a stack of 5 solid rocket segments.  The second stage uses a J-2X engine, derived from the famous J-2 engines used on the upper stages of the Saturn V rockets.  The body of the upper stage is derived from the Space Shuttle external tank.  This is a new stage, and though it is derived from existing technology, there are modifications that need to be make, so it is not simply a matter of sticking an engine onto the end of a shuttle external tank.  Atop the second stage would be the completely new components:  the Orion Crew Exploration Vehicle and its service module and the new Launch Abort System, designed to pull the crew capsule away from the rocket in the event of a catastrophic failure of the rocket.  These upper stages are not yet ready for flight.

So, why is the rocket flying on Tuesday if the upper stages are not ready?  This first developmental flight of the Ares stack is primarily a test of the first stage and the design of the stack.  Remember, the first stage is derived from the shuttle’s solid rocket boosters.  These rockets are strapped onto the side of the shuttle’s external tanks.  They were not originally designed to fly alone.  So, one of the goals of this rocket flight is to test the solid rocket first stage of the stack.  Also, the shuttle’s solid rockets only have a nose cone on them.  The first stage of the Ares has another rocket on top of it (the second stage) and even more hardware on top of that!  So, there is a very real engineering concern here.  There should be no serious problem.  The rocket should be easy to control, and the rocket segments should have the strength to hold all of this extra weight.  Remember, the shuttle’s rockets have helped lift the much heavier Space Shuttle into orbit, and that was strapped to the side of the rockets (an even tougher problem from the point of view of engineering).  Still, the rockets have not flown in this configuration before, and so there is always the potential for unforeseen problems.  That is why we need this test.

For the Ares I-X flight, the first stage will be a bit scaled down from the full five segment first stage of later Ares flights.  This will be essentially a four stage rocket that is a modified shuttle solid rocket booster with an inert fifth segment.  The upper stages will consist of dummy stages (though there will be active guidance and thrusters on the dummy second stage).  The rocket will take off, reach an altitude of about 130,000 feet and the second stage will deploy as if on an actual orbital mission.  Since the second stage rocket will not fire, the stage will then fall into the Atlantic Ocean.  After stage separation, momentum will carry the rocket about another 20,000 feet higher before it falls back to Earth.  Parachutes will deploy, and the first stage will splash down into the Atlantic Ocean, where a surface ship will retrieve it.  There will be sensors on the rocket recording every facet of the rocket operation.  The Ares developmental flights will have far more sensors than will fly on the operational missions.  Data collected on the flight (upper stage sensors as well as first stage sensors) will then be studied over the next couple of years.  With any new engineering design, even a modification of an existing design, there will almost always be something unexpected to turn up.  Having the first operational test so early in the project (the full unmanned test stack won’t likely fly for at least five years) allows engineers to study the data from the launch in order to modify the upper stages as needed before they are finished being built.  That is the smart way of doing things:  test out each part as it is ready.  The next Ares launch, the Ares I-Y, is expected in four years, around November of 2013.  The Ares I-Y will test the high altitude emergency Launch Abort System.  In the mean time, there will be separate tests of the upper components, just not on an Ares stack.  A full blown Ares/Orion launch, called the Orion 1 mission, may launch sometime early 2014.  That would be the first test including a fully functioning (but unmanned) Orion capsule atop the stack.

Though there will be only a four segment first stage in the Ares I-X flight, the experience for those of us watching the launch should be the same as for a full five segment first stage.  The biggest difference will be in how far and high the vehicle flies.  I am hoping that everything goes well.  I will be leaving for the Cape in a few hours.  The rocket has passed its test review, and everything seems go for launch so far from the engineering aspect.  Weather, though, may be a problem.  The forecast is for clouds.  Since this is a test flight, they must have clear weather to observe all aspects of the mission.  However, they only need about 15 minutes of clear weather, so there is a decent change of getting that.  Yesterday, the mission team estimated Tuesday’s weather as only 40% go.  However, Tuesday is still several days away, and things might look up.  As I look at the weather, it looks like it may be a bit better, perhaps 50%, but I am not the one making the call.  Still, I’ll be there Tuesday and Wednesday to see the launch.

-Astroprof

Images courtesy NASA

The basic idea is that like electric charges repel.  So, an object with a very large positive charge would repel incoming positively charged particles, like protons.   The idea sounds really good at first glance.  However, as with many ideas, further study shows that there are some considerable difficulties that need to be worked out before it is truly a viable shield.  Working out those difficulties, though, means a lot of time, effort, and hours from scientists and engineers.  They need to be paid a salary.  It also means experimentation, building simulations, computing, etc.  All that requires money, too.  Unfortunately, I think that far too little money is being spent at this time on working out all of the problems any time soon.  Of course, so little money is being spent on extending space exploration in the US that it will be a very long time before we even need an effective radiation shield.

So, what are the problems with an electrostatic shield idea?  Well, the first problem is that it would do nothing to shield against X-rays or gamma rays.  Also, if positive charged bodies are used, they would only repel positive particles, like protons.  They would even pull in negative particles, such as electrons!  So, one strategy would be to have a multiple layer shield.  One layer of the shield would deflect protons, and the other would deal with the electrons.   Depending upon who you ask, the outer layer might be negative to repel electrons (they are easier to deal with than the heavier protons), and the inner layer would repel the protons.  Conversely, the outer layer could be a strong positive charge to repel the protons, and the inner layer negative to handle the electrons that were accelerated by the outer shield.  There are advantages and disadvantages to each approach, of course, and I don’t want to go into all of the technical details of each.

But, there are some more serious matters to contend with.  First, what sort of charge and voltage would be needed to repel the charged particles?  Well, that depends upon the energy of the incoming particles.  Solar particles are much slower than galactic cosmic rays, and are thus easier to deal with.  Also, solar radiation is almost entirely protons and electrons, while galactic cosmic rays have a higher percentage of heavier nuclei.  The voltage required to shield against the particle increases roughly as the weight of the particle.  But, the really big problem is that galactic cosmic rays move much faster than solar radiation particles.  Galactic cosmic ray particles often move at very near the speed of light.  The voltage needed increases roughly as the square of the velocity, so it would take a very high voltage to block the galactic cosmic rays.  The charge on the deflectors is related to the voltage, so that means a very large charge would be needed, and that requires a very big power source.  The power requirements of the shield are another issue that has to be dealt with.  It may be that electrostatic shielding simply won’t be effective with galactic cosmic rays.  But, I think that it could certainly be made to work for solar radiation.  It may even work for the most common forms of galactic cosmic rays, but if so, then that’s OK.  The heavier particles are far less common, and radiation exposure due to them alone during a mission might be within acceptable limits.  Further work is needed.

-Astroprof

Image Credit:  NASA/ASRD Aerospace Corp./  Charles Buhler

In an earlier installment, I said that radiation is a process where energy travels from one body to another in essentially straight lines.  Since particles (and even light) can be deflected, that is not entirely correct, but it is close enough for our purposes.  The radiation when absorbed can heat things, cause electrical currents, or if enough energy is absorbed by an individual atom, ionize that atom (remove an electron).  Ions (charged atoms) behave different chemically than neutral atoms.  So, ionizing radiation has potential to do more harm than non-ionizing radiation.  In the human body, there are vast numbers of atoms and molecules.  Most of the atoms are, in fact, part of molecules.  It is the chemistry of how these molecules interact with one another, forming new molecules, releasing molecular energy, absorbing energy, etc, that is the basis of life.  Modern biology is not just memorization of species classifications or anatomical terms.  Rather, modern biologists are deep into organic chemistry and biochemistry.  That is sometimes a shock for students in first year biology at my college.  Students are surprised to find that their biology professor is talking about the laws of thermodynamics, electrical transport, chemical potentials, and a host of topics that used to be the domain of physicists and chemists.  But, it is here, in the realm of molecular biology, that radiation does its damage:  by upsetting the chemistry of life.  Ionizing radiation can keep atoms and molecules from behaving as they should in a cell, or they can break molecular bonds, altering a molecule.

Most of the time, the damage done by radiation is simply altering a rather minor molecule.  There are plenty of other molecules in the cell to take the place of the disrupted one.  In rare cases, the altered molecule bonds to some other molecule(s) that it wasn’t supposed to, taking two or more molecules out of commission.   But, there are so many molecules in the cell that this is normally of little importance.  However, once in a while, the radiation damages a somewhat rarer and more important molecule.  Sometimes an amino acid, a protein, or some other important molecule that has an important function in the cell is damaged.  It doesn’t work right, and so the cell doesn’t behave properly.  New proteins are being made though, so unless the cell is very unlucky, the damaged protein is eventually replaced by a correct one.  But, on very rare occasions, the most important of all molecules in a cell, one of the cell’s strands of DNA (deoxyribonucleic acid) is damaged.  DNA controls almost everything about how the cell works.  When the DNA is damaged, then the cell no longer works properly.  In most cases, that means that the cell dies.  In some cases, the cell lives but no longer performs the function in the body that it is meant to perform.  If such an altered cell reproduces, and its daughters reproduce, and so on, then a tumor can be formed.  If the altered cells are particularly invasive and the body’s immune system doesn’t recognize them as a danger, then the altered cells develop into a cancer.

As I said in an earlier post of this series, some forms of radiation are more damaging than others.  Some, such as X-rays, may damage one atom, and thus one tiny bit of one base nucleotide of the DNA.  Other, more energetic radiation may damage a cluster of atoms, damaging more thane one base pair.  The more damage is done, the more likely it will be that the cell will not function properly. In addition to direct damage to DNA, the radiation sometimes can alter some other molecule that can then damage the DNA.

But, biology can be amazing.  DNA is very important to cell operation.  As a consequence, it is not really surprising that there are biological mechanisms that are at work to limit damage to the DNA.  The first of these is simply the structure of the DNA itself.  DNA is described in textbooks as a “double helix.”  What that means is that it is composed of two matched strands arranged in a helical pattern.  By matched strands, each nucleotide on one strand is matched with one on the other strand.  There are only four nucleotides:  adenine, thymine, cytosine, and guanine.  Adenine matches with thymine, and cytosine matches with guanine.  So, if one of these molecules is damaged, then it is no longer the correct molecule.  It doesn’t fit in the DNA.  The cell has enzymes that will detect the miss-matched base pair and substitute the damaged nucleotide with the correct match (adenine, thymine, cytosine, or guanine) that corresponds with the other base nucleotide on the undamaged helical strand.  This works wonders, and it happens all of the time in our bodies.  The problem comes, though, if both nucleotides in a pair are damaged.  In that case, the cellular repair mechanism has problems.  It can either fill in the gap with random nucleotides, it can splice a fragment of DNA into the chromosome (viruses can do this), or it can simply spice the undamaged ends of the DNA back together, effectively eliminating whatever the damaged portion of the DNA was.  In all cases, the cell will no longer work the way that it did before.  That is why the heavier and higher energy particles in galactic cosmic rays are so dangerous.

But, during part of a cell’s cycle, particularly during mitosis when it is replicating, the DNA is more vulnerable to damage.  At that time, as the DNA is replicating, damage is difficult to repair.  Some of the DNA base pairs are involved in separating and forming new base pairs, and so the enzymes that repair damage are unable to find the correct match for base pairs.  The damage thus often is not repaired properly.  Cells that are in mitosis a lot are thus more easily damaged by radiation.  In a normal human body, these include the hair follicles, the linings of the gut, and bone marrow cells.  Thus, radiation exposure can cause someone to lose their hair, get nauseated, and become anemic.   The anemia is often the first clinical symptom of extreme radiation exposure.  If the exposure is at a low enough level, the nausea may not present, but the anemia will often still occur.  Low level chronic exposure often causes the hair to gray rather than completely fall out.  Incidentally, cancer cells are often multiplying out of control (which is why they are invasive and eventually get in the way of the body functioning, thus killing the patient), so they are also susceptible to radiation damage.   That is how radiation therapy works with cancer patients.

So, the effects of radiation occur at the molecular level.  Any sort of realistic strategy for protecting astronauts on long duration space missions requires an understanding of radiation at this level.  Unfortunately for astronauts, cosmic rays, particularly the galactic cosmic rays, contain some of the most damaging forms of radiation possible.  Thus, very low doses of cosmic rays can act like much larger doses of other forms of radiation.  It is quite fortunate that such radiation occurs in such low levels in space that astronauts can be exposed to it for many months before the risk of cancer due to the radiation is significant.  But, as I said before, long duration missions, such as an extended stay on the Moon or a mission to Mars, could easily reach or exceed NASA imposed limits on cancer risk.  But, even an extended mission to Mars would not expose astronauts to a clinically lethal dosage of radiation.  Rather, it may create some health issues and would most certainly create a risk for development of cancer that exceeds the level to which NASA is willing to subject astronauts.  Thus, some strategy must be developed to protect astronauts from the danger of space radiation.

Researchers at NASA, in industry, in the military, and in universities are working on the problem of dealing with radiation.  The simplest solution is to shield against the radiation.  Most of the time, such shielding is accomplished by putting enough absorbing material between the source of the radiation and the person being shielded from the radiation.  Sometimes, though, that is not practical.  After all, the shielding can be expensive, and it is often very heavy.  For aircraft or spacecraft, heavy shielding is not realistic.  That means that some other strategy is needed.  One possibility is to simply move some of the spacecraft systems so that they shield the occupants.  Water turns out to be a good radiation shield.  So, putting the water storage and waste water handing systems surrounding the crew compartment provides some shielding.  But, there is only so much water carried on board a spacecraft, so water shielding can not be the sole solution.  Another idea is to perhaps use some other form of shielding than just a physical shield that absorbs the radiation.  One strategy being tossed around is an electromagnetic shield that would deflect the radiation.  At present, though, electromagnetic shields exist primarily just on paper.  Another strategy, though, is biological.  The body does have repair mechanisms for DNA.  So, perhaps there is some sort of biological approach that may protect astronauts.  This could be in the form of drugs that limit the damage from radiation or that boost the cellular repair mechanisms.  Some progress has been made on this front.  Perhaps the best solution for an extended space mission would be a combination of different types of shielding and drugs.  While current technology and spacecraft construction do not provide much radiation shielding for astronauts, it appears that real progress is being made to come up with strategies for protecting astronauts from radiation exposure.

-Astroprof

Image Credit:  NASA

About a century ago, when physicists were first seriously studying radiation and seeking to know the nature of this phenomenon, these researchers noticed that even when all known sources of radiation were removed from the vicinity of a radiation detection device that device still recorded a low level of radioactivity.  Eventually, researchers began to realize that radioactive elements and isotopes were all around them.  Even the materials used to shield against radiation, such as concrete blocks or lead plates, contained some radioactive isotopes.  Some researches then began to wonder if air itself might contain a very small level of radioactivity.   To isolate the radiation of just the air and not terrestrial surroundings, researchers put radiation detectors in balloons.  As expected, the background radiation level dropped in the balloon flights at low altitudes.  However, as the balloons went higher, the radiation level began to increase with altitude.  Victor Hess won the 1936 Nobel Prize in physics for his work in radiation studies, including his explanation for this mysterious increase in radiation with altitude:  cosmic radiation.  In short, Hess realized that the source of some of the background radiation was from above, having its origins in space.   At first, the nature of this cosmic radiation was unknown, only that it originated from beyond the Earth.  Today, we know a  lot more about cosmic radiation.

As it turns out, much of what Hess and others in his day were measuring was not the radiation that astronauts would experience.  In fact, they did not even measure what we now think of as cosmic rays.  What they measured were secondary cosmic radiation or particle showers created by the space radiation’s impact with the upper atmosphere.  This is the radiation that airline passengers and crew are exposed to.   I’ve written about that before.  What I want to concentrate on this time is the radiation in space itself.

Remember, from my previous posting, the term radiation actually applies to a wide range of phenomena.  All of these different uses of the term are at work in space, so there are multiple sources of radiation.  Not only are there multiple sources of radiation, but there are also multiple kinds of radiation and multiple different ranges of energy for that radiation.  This makes any kind of effective shield against the radiation for astronauts problematic.  However, being difficult to shield against does not mean being impossible to shield against.  It is an engineering problem, albeit a rather serious one.  Given sufficient time and effort, a solution is likely possible.  That will be the topic in a subsequent posting.  Now, I wish to talk about the different types of radiation that are in space.

First, I’d like to mention that space radiation can fall into two major categories:  electromagnetic radiation and particle radiation.  The electromagnetic radiation consists of the whole range of the electromagnetic spectrum, from gamma rays to radio waves.  The shorter wavelength (and higher frequency) radiation such as gamma rays and X-rays are considered ionizing radiation, and that is what is a worry to astronauts.  There are many different sources of this radiation, just as there are on Earth.  However, there are some processes that create ultra-high energy gamma rays.  Some of these gamma rays are produced in astrophysical processes far from the Solar System.   Gamma ray bursts have been recorded by satellites since the early days of spaceflight.  Other ultra-high energy gamma rays can result from interactions of some of the ultra-high energy particles with the Earth’s upper atmosphere.  These gamma rays are one type of secondary cosmic rays.  As a rule, gamma rays of that energy level are seldom (if ever) produced on Earth in any natural terrestrial process.  So, when I was growing up, charts of the electromagnetic spectrum often labeled this ultra-high energy electromagnetic radiation “cosmic rays.”  That is how I learned it.  Years later, when I was in college, electromagnetic spectrum charts no longer had that label.  That ultra-high energy radiation is now included in the overall category of “gamma rays,” unless you look at a old chart or one prepared by someone not familiar with the current usage of the terminology.

Most of what we consider “cosmic rays,” however, rather than being electromagnetic radiation, are high energy particles.  Many of these particles are charged particles.  There are a few neutral particles in the mix, but they are a very tiny percentage of the cosmic rays.  There are three basic sources of cosmic rays.  Some are particles originating from the Sun.  Others originate from beyond the solar system.  And some are believed to originate at the boundary between the solar system and the interstellar medium.

The first that I’ll talk about are the solar cosmic rays.  There are always particles streaming away from the Sun.  This is the solar wind.  Most of these particles are protons (hydrogen nuclei) and electrons.  These particles are moving at several hundred kilometers per second.  The energy is low enough that ordinary materials of the spacecraft would be effective shielding from the particles themselves.  Some of these particles, though, would be problematic for astronauts in spacesuits.  However, when charged particles rapidly decelerate, as they would do when they hit the spacecraft, they emit radiation called bremsstrahlung radiation (braking radiation).  For most of the solar wind particles, this bremsstrahlung radiation would be soft X-rays — fairly easy to shield against.  However, the real danger lies in solar storms.  During these storms, generated by a massive explosive release of magnetic energy in the Sun due to a solar flare event, particles can be blasted from the Sun at far higher energies.  The radiation storms generated by solar flares can be far more dangerous to astronauts outside of Earth’s magnetosphere.  In fact, astronauts exposed outside of a spacecraft would not be sufficient protected by any reasonable spacesuit, and they may experience a lethal dose of radiation during a severe solar radiation storm.  Even astronauts inside a spacecraft would experience dangerously high levels of radiation.  These solar storms are difficult to predict far in advance.  Solar scientists are getting better, but we still don’t do much better than a few days of warning that conditions are right for such storms.  That is not sufficient to protect astronauts on an extended mission.

The good news, though, for astronauts in low Earth orbit, though, is that the Earth’s magnetosphere deflects the brunt of these solar radiation storms.  However, the blast of particles from the Sun does shift the magnetosphere quite a bit.  This has the effect of accelerating charged particles trapped in the magnetosphere (such as in the van Allen radiation belts).  These now fast moving particles then cause auroral displays when they crash into Earth’s atmosphere and can pose some radiation problems for astronauts in Earth orbit.  For a spacecraft that could be a problem.  However, long duration missions, such as on a space station, astronauts will have a lot of water on board.  This water makes a decent shield against the trapped particles, thus providing some shielding for astronauts on board the space station during a geomagnetic radiation storm.  Still, astronauts will unavoidably be exposed to excess levels of radiation during such an event.  Nonetheless, inside of Earth’s magnetosphere, astronauts inside a spacecraft should be sufficiently shielded so that they will not suffer any immediate health problems.

The most important thing about solar radiation storms is that they don’t last long.  It may be too expensive to shield a spacecraft outside of Earth’s magnetosphere completely from solar radiation, but it may be possible to make small shielded areas where astronauts can retreat to in the event of a radiation storm.  They’d just have to wait out the storm in what may be an uncomfortable setting.  But, the storm will subside.  A greater threat, though, are the higher energy cosmic rays that originate outside of the solar system.  These are much tougher to shield against, and they provide a steady background level of radiation.

The cosmic rays that Hess discovered were largely the byproducts of what we now call galactic cosmic rays.  These are extremely high energy particles, most moving at near the speed of light.  At such high velocities, when they slam into atoms near the top of Earth’s atmosphere, they create a cascade of subatomic particles that rain down towards Earth.  Some of these particles make it to the ground, and others slam into other things creating yet more particles to rain downward.  A single high energy cosmic ray can thus create an incredible number of secondary particles that create a shower of particles across hundreds or thousands of square meters on the surface of the Earth.  Not all of the particles make it all the way to the ground, so the higher one goes into the atmosphere, the more of these secondary particles that you encounter.  That is what airline passengers and crew experience:  secondary cosmic rays.  Cosmic rays of this energy striking the wall of a spacecraft would likewise create a shower of particles that would hit astronauts inside the spacecraft.  Even some of the primary cosmic rays would pass through the wall of the spacecraft to hit the astronauts directly, creating a shower of particles inside the astronaut.  As you can see, these primary galactic cosmic rays can be incredibly dangerous. About 89 to 90% of the galactic cosmic rays are high energy protons.  About 9 to 10% are helium nuclei.  About 1% of the particles are heavier nuclei.  High energy electrons are also likely generated in the mechanism that produces galactic cosmic rays, but electrons are much lighter than atomic nuclei and are thus easier to deflect and slow down.  The nucleus of practically any fairly stable known element in the periodic table has been found in cosmic rays, though nuclei of elements heavier than iron are rare. The heavier the cosmic ray particle, the more damage it typically does when it hits something.

Even very low levels of galactic cosmic radiation can act like much higher levels of lower energy radiation in terms of biological damage.  Worse, how do you shield against these things?  A fairly dense and heavy (and thus expensive to launch) shield on a spacecraft would slow the particles, but create bremsstralung radiation of very high energy:  hard X-rays and gamma rays.  Any effective physical shield would be too heavy to launch.  To date, the most effect way of dealing with such high energy radiation is to simply limit exposure.  If you are exposed to such radiation for short periods of time, then it would be manageable.  NASA regulations limit astronaut radiation exposure to a level that is estimated to subject the astronauts to no more than about a 3% risk of cancer induced by the radiation exposure.  Since we don’t really know the exact relative biological effect for this radiation, then we don’t really know how long of an exposure that might be.  Estimates range from as short as 100 days exposure to as long at 300 days, but most falling in the 200 day exposure range.  That is a little over six months exposure to galactic cosmic rays.  That is far longer than the Apollo missions to the Moon, so they were not at great risk due to cosmic rays.  However, NASA is talking about long duration missions to the Moon to man a permanent or semi-permanent base there.  Those missions would last 4 to 6 months.  That puts the length of the proposed missions right in the middle of the exposure limit range.  A single mission would thus expose astronauts to their entire career maximum radiation limit, excluding any radiation from solar storms.  Worse, a mission to Mars would take 7 to 8 months just to get to Mars, and nearly 3 years round trip (Astronauts cannot simply come back from Mars whenever they want. They have to wait until the planets are aligned in such a way that the return trip is possible.  That forces them to wail over a year at Mars until Earth and Mars are back in the proper configuration, making any possible round trip almost 3 years in length).  So, just the trip to Mars would exceed the 3% cancer risk that is the NASA imposed limit to astronaut radiation exposure.  Then, you have the trip back.  And, the thin atmosphere of Mars would make the surface of Mars experience a radiation exposure from secondary cosmic rays similar to what high altitude aircraft experience on Earth, which is near the peak level of secondary radiation.  Thus, astronauts on a mission to Mars would experience a significant health risk, perhaps in excess of a 10% risk of cancer, far in excess to NASA’s radiation limits.   But, this is also a limit for astronauts working anywhere in space, even on the Moon or in Earth orbit.

Incidentally, you might wonder why Earth’s magnetosphere protects us from galactic cosmic rays.  Well, it does, a little.  When charged particles move in a magnetic field, they are deflected.  This tends to make them move in circular or helical paths.  The radius of the circular motion is dependent upon the velocity of the particles and their mass.  So the high energy (speed) and mass of the galactic cosmic ray particles makes their circular orbits so large that they still run into Earth.  Ironically, until sufficient studies had been done by unmanned spacecraft above Earth’s atmosphere, some scientists had speculated that the trend of increase in radiation levels with altitude observed in balloon and sounding rocket experiments might continue.  Had this trend in radiation levels indeed continued, then the environment of outer space would have been far too high in radiation levels for manned spaceflight to even be possible.  Fortunately, that trend does not hold.  Once you get above the level of peak secondary radiation, then the radiation levels begin to drop to background level that is manageable (though still much higher than experienced on the surface of Earth).

The third type of cosmic rays are what we call anomalous cosmic rays.  These anomalous cosmic rays are of mid-energy range between the solar particles and the galactic cosmic rays.  The origin of the anomalous cosmic rays are a bit of a mystery, but they are believed to come from the boundary region between the solar system and the interstellar medium, a region known as the heliomagnetosheath, of the heliosphere’s magnetosheath.  This is where the solar wind particles are slowing down and bunching up and where the interstellar medium particles are being deflected as they pass by the solar system.  It is the edge of the Sun’s magnetic influence.  It has been suggested that perhaps particles trapped in the magnetosheath get buffeted and accelerated to high enough energies to escape this region and become the anomalous cosmic rays, but we don’t know that for sure.  They are mostly protons.

So, that is a brief primer on cosmic rays.  Next, I’d like to talk about how radiation does damage to biological systems, but that will be the topic of my next post.

-Astroprof