Sometimes when I talk to my kids about energy-related issues, they may propose a solution that seems quite simple and obvious to them. The problem is that the real issues may be more complex than they appear on the surface and may require some foundational knowledge to fully understand. One such potential solution to our energy needs is hydrogen-powered cars. Hydrogen is a clean burning fuel and is certainly plentiful given all the hydrogen that exists in Earth's oceans. Why do we not just use hydrogen to solve our energy problems?
To understand why we do not use hydrogen more as a fuel, we need some basic chemistry knowledge. Burning is a chemical reaction where energy is released. When hydrogen is burned, oxygen atoms from the atmosphere each combine with two hydrogen atoms to form water as a waste product while giving off heat in the process. Other chemical reactions consume energy rather than release energy. The problem with hydrogen is that it is very light and very reactive. All of the hydrogen on earth has either floated away into space because it is so light, or it has bonded with other atoms and requires at least as much energy to release as what we get from burning it.
Hydrogen then can be a convenient way to store energy, like a battery, but it is not a source of energy. Extracting hydrogen from water takes as much energy as we recover from burning it. It is like using boulders rolling down a hill as a source of energy. The problem is they are all currently lying at the bottom of the hill. We could push them to the top of the hill and then use them, but then we might as well just use the energy directly that we used to push them up the hill.
The only reason we might prefer the rolling boulders is that they give us a lot of energy fast while the process of pushing them up is longer and slower even though the total energy is equivalent (minus whatever is lost through inefficiency). This is why hydrogen is sometimes used as rocket fuel. The real source of energy was the coal that generated the electricity that was used to extract the hydrogen from water. But the hydrogen works much better as rocket fuel because it can release its energy much faster.
Backing up even further, the energy in the coal ultimately came from the sun. Prehistoric plants used photosynthesis to convert sunlight into chemical energy that was eventually buried, becoming the fossil fuels we use as our primary energy source today. The energy from the sun comes from hydrogen, so in essence we are already running everything on hydrogen. The difference is that the process that generates energy in the sun is nuclear rather then chemical. Chemical processes only rearrange atoms. Nuclear reactions change one kind of atom into another either by combining nuclei (fusion) or splitting nuclei (fission). For elements lighter than iron, fusion releases energy, while elements heavier than iron release energy with fission. The amounts of energy are several orders of magnitude greater than those involved with chemical reactions.
Unlike regular burning of hydrogen, nuclear fusion could provide us with all the energy we would ever need. The problem is that extremely high temperatures and close proximity are required to start and sustain a reaction. Stars do this with gravity, but it is not so easy on earth. So far the only way we have been able to use the energy of nuclear fusion is in thermonuclear (hydrogen) bombs. These blow themselves apart by design. To sustain the reaction and produce usable energy we would have to hold it together. This is a challenge we have yet to resolve.
Thursday, September 27, 2012
Thursday, September 20, 2012
Orbital Dynamics
As a follow up to some of my previous posts about space flight, I have decided to discuss orbital dynamics. The general public is grossly misinformed about real space flight thanks to so much misinformation in movies and on TV. The Jet Propulsion Laboratory has an excellent tutorial here for those who want to get into even more detail about space flight.
Why discuss orbital dynamics? Because every body in the universe is in orbit around something. Furthermore, orbital dynamics are interesting because they are so non-intuitive when it comes to maneuvering a space ship. Understanding orbital dynamics is essential to real space flight.
Johannes Kepler discovered by observation that planets follow elliptical orbits around the sun. Isaac Newton showed that elliptical orbits are a consequence of the inverse square law of gravity. More generally, orbits are conic sections. When you throw a baseball, its path is a parabola. A comet that only approaches the sun once, and then heads for interstellar space is following a hyperbolic curve.
If two space craft are orbiting the earth in the same orbit, but separated by some distance, how can they rendezvous or send items back and forth. MIT Professor Walter Lewin discusses this problem in his ham sandwich lecture, which is posted online here. As Lewin states it, how do you throw a ham sandwich to the hungry astronaut in the space ship that is in orbit ahead of you? You might want to just throw it toward him, but that would put the ham sandwich into a higher orbit, thus ultimately slowing it down and sending it further away from the hungry astronaut. The right way to send him the ham sandwich is to throw it backwards, away from his space ship. This would initially slow down the ham sandwich, but then it would pick up speed as it fell into a lower orbit. If done just right, the elliptical orbit of the ham sandwich would intersect the more circular orbit of the hungry astronaut's space ship at just the right time on the next orbit.
The astronauts in the Gemini program in the 60s, the first to execute a rendezvous in earth orbit, had to learn to go contrary to their intuition. The first required maneuver was to match the plane of the target. Next, it was necessary for the perusing ship to be in a lower orbit so it could gain on the target. Once the target was in sight, the astronauts had to fight their natural tendency to fire thrusters to move their space ship toward the target. Instead, they needed to maintain their lower orbit, only pulling up into the same orbit as the target at exactly the right moment. Of course, this is all old hat now for real astronauts, but the complex and intricate details are not always fully appreciated by a general public fed with a steady diet of Star Trek and Star Wars.
Orbital dynamics also come into play in planning trajectories for interplanetary travel, but that is a subject for another post. If you can't wait until then, check out the JPL Space Flight Tutorial I mentioned previously.
Why discuss orbital dynamics? Because every body in the universe is in orbit around something. Furthermore, orbital dynamics are interesting because they are so non-intuitive when it comes to maneuvering a space ship. Understanding orbital dynamics is essential to real space flight.
Johannes Kepler discovered by observation that planets follow elliptical orbits around the sun. Isaac Newton showed that elliptical orbits are a consequence of the inverse square law of gravity. More generally, orbits are conic sections. When you throw a baseball, its path is a parabola. A comet that only approaches the sun once, and then heads for interstellar space is following a hyperbolic curve.
If two space craft are orbiting the earth in the same orbit, but separated by some distance, how can they rendezvous or send items back and forth. MIT Professor Walter Lewin discusses this problem in his ham sandwich lecture, which is posted online here. As Lewin states it, how do you throw a ham sandwich to the hungry astronaut in the space ship that is in orbit ahead of you? You might want to just throw it toward him, but that would put the ham sandwich into a higher orbit, thus ultimately slowing it down and sending it further away from the hungry astronaut. The right way to send him the ham sandwich is to throw it backwards, away from his space ship. This would initially slow down the ham sandwich, but then it would pick up speed as it fell into a lower orbit. If done just right, the elliptical orbit of the ham sandwich would intersect the more circular orbit of the hungry astronaut's space ship at just the right time on the next orbit.
The astronauts in the Gemini program in the 60s, the first to execute a rendezvous in earth orbit, had to learn to go contrary to their intuition. The first required maneuver was to match the plane of the target. Next, it was necessary for the perusing ship to be in a lower orbit so it could gain on the target. Once the target was in sight, the astronauts had to fight their natural tendency to fire thrusters to move their space ship toward the target. Instead, they needed to maintain their lower orbit, only pulling up into the same orbit as the target at exactly the right moment. Of course, this is all old hat now for real astronauts, but the complex and intricate details are not always fully appreciated by a general public fed with a steady diet of Star Trek and Star Wars.
Orbital dynamics also come into play in planning trajectories for interplanetary travel, but that is a subject for another post. If you can't wait until then, check out the JPL Space Flight Tutorial I mentioned previously.
Tuesday, September 18, 2012
Common Sense
Everyone seems to know what common sense is, but no one has ever been able to fully explain to me exactly what they mean by it. One idea about common sense that I have gleaned from the various opinions I have surveyed is that it is something that cannot be taught. You either have it or you don't. Another is that it is somehow inversely related to formal education. Often people who are lauded as having common sense do not have much formal education while the highly educated are sometimes thought to lack it. Common sense seems to be a type of knowledge, but which specific knowledge constitutes common sense seems to vary from one individual to another.
My ex-father-in-law was someone who was said to have a lot of common sense. He did not have formal education beyond high school, but he ran successful businesses and made wise investments, and thus accumulated enough money to retire very early and be quite comfortable. He also had the fortune to invest in real estate during a time when values were always rising, a condition that no longer always holds true. My impression of him was that he was very intelligent, hard working, and constantly learning about things that were important to him. I don't think he was born with the knowledge he had. Perhaps common sense in this case means learning from life experience and from paying attention to things rather than in a formal classroom setting. I am not sure why we need to make this distinction. To me, knowledge is knowledge regardless of how we acquire it.
When my oldest son was preparing to take the written driving exam to get his learner's permit, his friend dissuaded him from studying by telling him that it is all just common sense. My son took the test and failed. Then he studied the material and passed it. So does he lack common sense because could not pass the test without studying? Did he acquire common sense after he studied, or does that not count? Maybe only those who pass the test without the need for study are the ones with common sense. I suspect that this friend had already acquired the knowledge in other ways. Maybe his parents talked to him about things and he paid attention. This same son is very good at strategy games. No one can ever beat him. He seems to have an intuitive knack for it. This probably just seems like common sense for him.
Reflecting on these and other experiences, I think I might be seeing a common thread to what people mean when they speak about common sense. The best I can figure out is that it means "what is obvious to me." Any two people will have some knowledge in common, and other knowledge that is unique. My wife was playing a game on a mobile device and called her daughter for help on one of the levels. Her daughter's response was, "You're having trouble with that one?!" She found one of the levels difficult that her daughter found easy. The rest of the story is that her daughter had already gone through a similar game consisting of maybe 100 levels and this was my wife's first one and she was perhaps on level 16. There are many other subjects where the roles would have been exactly reversed. We all know different things.
This leads me to the main reason for writing this post in the first place. People sometimes express incredulity that someone else does not know something that is obvious to them. This is often what gets labeled as common sense. I think that this reaction is unkind and unnecessary. In almost every case, the roles could be reversed if the subject were different. The concept of common sense seems to be very egocentrically defined. A better reaction would be kindness, patience, and the willingness to share our knowledge without making any value judgements about what others ought to know. The flip side of this is the willingness to learn something from every person we meet. Ralph Waldo Emerson summed up this idea very well. “In my walks, every man I meet is my superior in some way, and in that I learn from him."
My ex-father-in-law was someone who was said to have a lot of common sense. He did not have formal education beyond high school, but he ran successful businesses and made wise investments, and thus accumulated enough money to retire very early and be quite comfortable. He also had the fortune to invest in real estate during a time when values were always rising, a condition that no longer always holds true. My impression of him was that he was very intelligent, hard working, and constantly learning about things that were important to him. I don't think he was born with the knowledge he had. Perhaps common sense in this case means learning from life experience and from paying attention to things rather than in a formal classroom setting. I am not sure why we need to make this distinction. To me, knowledge is knowledge regardless of how we acquire it.
When my oldest son was preparing to take the written driving exam to get his learner's permit, his friend dissuaded him from studying by telling him that it is all just common sense. My son took the test and failed. Then he studied the material and passed it. So does he lack common sense because could not pass the test without studying? Did he acquire common sense after he studied, or does that not count? Maybe only those who pass the test without the need for study are the ones with common sense. I suspect that this friend had already acquired the knowledge in other ways. Maybe his parents talked to him about things and he paid attention. This same son is very good at strategy games. No one can ever beat him. He seems to have an intuitive knack for it. This probably just seems like common sense for him.
Reflecting on these and other experiences, I think I might be seeing a common thread to what people mean when they speak about common sense. The best I can figure out is that it means "what is obvious to me." Any two people will have some knowledge in common, and other knowledge that is unique. My wife was playing a game on a mobile device and called her daughter for help on one of the levels. Her daughter's response was, "You're having trouble with that one?!" She found one of the levels difficult that her daughter found easy. The rest of the story is that her daughter had already gone through a similar game consisting of maybe 100 levels and this was my wife's first one and she was perhaps on level 16. There are many other subjects where the roles would have been exactly reversed. We all know different things.
This leads me to the main reason for writing this post in the first place. People sometimes express incredulity that someone else does not know something that is obvious to them. This is often what gets labeled as common sense. I think that this reaction is unkind and unnecessary. In almost every case, the roles could be reversed if the subject were different. The concept of common sense seems to be very egocentrically defined. A better reaction would be kindness, patience, and the willingness to share our knowledge without making any value judgements about what others ought to know. The flip side of this is the willingness to learn something from every person we meet. Ralph Waldo Emerson summed up this idea very well. “In my walks, every man I meet is my superior in some way, and in that I learn from him."
Tuesday, September 4, 2012
Navigating an Asteroid Field
In "The Empire Strikes Back" Han Solo navigates his Millennium Falcon through an asteroid field to escape star destroyers. Since then, a number of other TV shows and movies have depicted similar crossings through asteroid fields where a space ship weaves through to avoid collisions. How likely is a scenario such as this?
We do not have first hand knowledge of any actual asteroid fields similar to the one from Star Wars. In our solar system, we have an asteroid belt between the orbits of Mars and Jupiter, but it is far more sparsely populated than the one depicted in Star Wars. The average distance between asteroids is about 1 million miles. All of our space probes have passed through it without incident and with no course corrections needed. The probability of encountering even a single asteroid is remote, unless we explicitly set out to do so.
The asteroid belt was denser in the early solar system, but still nothing like the asteroid field from Star Wars. Structures like that would be unstable. Collisions would tend to knock asteroids further apart until they were far enough apart to make collisions a much more rare occurrence. There are stable points of gravitational equilibrium called Lagrange points where debris could accumulate forming something as dense as that depicted in Star Wars, but these would not extend over a large area and would be very easy to avoid altogether. Lagrange points are in known locations relative to large celestial bodies.
Perhaps it is possible for a dense asteroid field to exist for a short time before repeated collisions drive the asteroids further apart. If so, it would always be much simpler to navigate around it rather than through it. If it was uncharted and you came upon it suddenly, the speed differential between you and the asteroids would likely be so great that there would be no time to react before your spaceship was obliterated. Bodies in space travel at extremely high velocities relative to each other unless they are in the same orbit around a planet or star.
The only way to produce the scene from Star Wars is to nearly match the speed of the asteroids with your spaceship before attempting to navigate through. However, it would not look anything like that depicted on Star Wars. The Star Wars spaceships fly more like airplanes than spaceships, which real spaceships cannot do because there is no air to push against (see blog post Spaceships That Fly Like Spaceships). It would be more like navigating the spaceship in the old arcade game, Asteroids, but more complex because you would be dealing with three dimensions rather than two, and without the option of shooting the asteroids.
Remember also that if you have to flee from an enemy in space, you have many more possibilities than you do on the surface of the earth because you can fly in three dimensions. It is difficult to get our minds wrapped around the possibility of traveling freely in three dimensions because we are too accustomed to traveling gravity-bound on the surface of the earth. It is true that we can go short distances in the third dimension, but our travel on earth is predominantly in only two. In space you could travel forever in a third dimension.
If you just need to get from point A to point B, it should be no problem to simply plan your course around the asteroid field. Because of the way these type of structures might form, they are likely to lie in the same plane, meaning that you could just adjust your plane of travel by a small amount to get around it.
So even though the scene from Star Wars would likely never need to occur, no matter how advanced our technology, it still makes for an exciting action sequence.
We do not have first hand knowledge of any actual asteroid fields similar to the one from Star Wars. In our solar system, we have an asteroid belt between the orbits of Mars and Jupiter, but it is far more sparsely populated than the one depicted in Star Wars. The average distance between asteroids is about 1 million miles. All of our space probes have passed through it without incident and with no course corrections needed. The probability of encountering even a single asteroid is remote, unless we explicitly set out to do so.
The asteroid belt was denser in the early solar system, but still nothing like the asteroid field from Star Wars. Structures like that would be unstable. Collisions would tend to knock asteroids further apart until they were far enough apart to make collisions a much more rare occurrence. There are stable points of gravitational equilibrium called Lagrange points where debris could accumulate forming something as dense as that depicted in Star Wars, but these would not extend over a large area and would be very easy to avoid altogether. Lagrange points are in known locations relative to large celestial bodies.
Perhaps it is possible for a dense asteroid field to exist for a short time before repeated collisions drive the asteroids further apart. If so, it would always be much simpler to navigate around it rather than through it. If it was uncharted and you came upon it suddenly, the speed differential between you and the asteroids would likely be so great that there would be no time to react before your spaceship was obliterated. Bodies in space travel at extremely high velocities relative to each other unless they are in the same orbit around a planet or star.
The only way to produce the scene from Star Wars is to nearly match the speed of the asteroids with your spaceship before attempting to navigate through. However, it would not look anything like that depicted on Star Wars. The Star Wars spaceships fly more like airplanes than spaceships, which real spaceships cannot do because there is no air to push against (see blog post Spaceships That Fly Like Spaceships). It would be more like navigating the spaceship in the old arcade game, Asteroids, but more complex because you would be dealing with three dimensions rather than two, and without the option of shooting the asteroids.
Remember also that if you have to flee from an enemy in space, you have many more possibilities than you do on the surface of the earth because you can fly in three dimensions. It is difficult to get our minds wrapped around the possibility of traveling freely in three dimensions because we are too accustomed to traveling gravity-bound on the surface of the earth. It is true that we can go short distances in the third dimension, but our travel on earth is predominantly in only two. In space you could travel forever in a third dimension.
If you just need to get from point A to point B, it should be no problem to simply plan your course around the asteroid field. Because of the way these type of structures might form, they are likely to lie in the same plane, meaning that you could just adjust your plane of travel by a small amount to get around it.
So even though the scene from Star Wars would likely never need to occur, no matter how advanced our technology, it still makes for an exciting action sequence.
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