I recently discovered an old hoax saying that Rolling Rock was going to turn the moon into a giant billboard by projecting their logo onto it with a laser. Even though this was a hoax, the idea of advertising in space is not new. It is a common enough idea that it even warrants its own wiki page. I wanted to consider, from a physics perspective, some of the logistics that would be necessary to project an advertisement onto the moon. Continue Reading
I was recently watching an episode of The Universe, the show from the History Channel, which detailed ten different ways to destroy the Earth (Season 4 Episode 6). One of the proposed destruction techniques was to instantly stop the rotation of the Earth. This would cause everything on the surface of the Earth to be violently thrown as the ground beneath it instantly stands still. It would be quite a violent stop. For some perspective, in New England, where I am located, you would be thrown at a velocity of about 800 miles an hour to the East. It is very unlikely that anybody could survive an impact with anything at that speed. That is not to mention that every building would be thrown at that velocity as well. However, the contributors to the show, professional astrophysicists, claimed that if you could survive the initial stop it is still unlikely that you could survive the aftermath. Specifically, the atmosphere would not be stopped, so there would be windspeeds of up to 1000 miles per hour as it continued rotating at its initial velocity. The show claims that this wind would create so much energy that it would heat up the atmosphere enough to melt rock. This claim seemed a bit too dramatic to be true, so I decided to do some calculations to see how credible it is. Brace yourselves, quite a bit of math is ahead as we fact-check…The Universe! Continue Reading
There has been quite a bit of press about construction of a space elevator by Obayashi Corp, a Japanese construction company, lately. As exciting as an elevator to space would be, let’s just look at some of the hurdles that such a project would have to overcome from a scientific point of view.
What may not be obvious to non-physicists is how you would keep an elevator to space from falling back down to Earth. It would, in some sense, be the tallest thing mankind would have ever constructed, but you can’t just build a really tall building into space. Instead, you would have to assemble a giant platform in geosynchronous orbit (altitude 36,000km) and then lower the elevator cable from space. This would allow you to build the elevator from the top down. However, the plans proposed by Obayashi propose extending the elevator to 96,000km. This means that if the top of the elevator were not connected to Earth in any way, it would orbit slower than the Earth rotates. When you connect the two, the Earth would actively pull on the top platform, speeding it up to match the rotational speed of the Earth. This makes sense from a construction point of view; having the platform want to fly away from the Earth means that you can pull on the cable in order to climb it without pulling the elevator down on top of you.
Even though the top of the elevator has to be pulling on the Earth to keep tension in the cable, it is a productive exercise to figure out just how much tension would be produced from something at that altitude. First, we need to make a few assumptions. Let’s assume the counterweight at the top of the elevator has about the same mass as the International Space Station (450,000kg) and for now let’s ignore the mass of the cable connecting the counterweight and the Earth.
It is easy enough to write down the force on the counterweight due to the gravity of the Earth. Newton figured it out centuries ago:
Fearth = -G M m/r2
Meanwhile, the force from the Earth accelerating the counterweight to match the rotation speed of the Earth is given by:
Frotation = m ω2 r
where ω is the rotation velocity of the Earth.
The sum of these two forces is what acts on the counterweight:
Ftotal = -G M m/r2 + m ω2 r
At an elevation of r=102,400km (the altitude plus the radius of the Earth), Ftotal = 227,799 Newtons. This is equivalent to the weight at Earths surface of a 25 ton object. In construction terms however, that is not too bad; many things weigh much more than that are suspended by cables. However, the cables used to make such an elevator would have to stretch almost 100,000 km; about 100 times longer than any current cable in existence! Not only that, but when you factor in the weight of the cable itself, it is not feasible unless the material it is made of has a much higher tensile strength/mass ratio than steel. Currently the only material that has such a high ratio is carbon nanotubes, but the longest continuous piece of carbon nanotubes was 2cm long made at MIT, not even close to useful lengths.
So, due to the limited manufacturing capability of carbon nanotubes, the space elevator is likely to remain a lofty dream for the foreseeable future. This doesn’t mean we can’t imagine what it would be like to visit the top of the space elevator. For instance, if you were way up at the counterweight, you would not be weightless like the astronauts in the ISS. Instead, you would actually feel a weak force pushing you away from the Earth, about 1/20th that of gravity on the surface of the Earth. This means that in order to not fly away and off into space, you would either have to stand with the top of your head facing the Earth or somehow tether yourself to the counterweight. This would have to be considered when building the elevator car. When you start your journey on the Earth, you would experience normal gravity. However, that gravity would slowly decrease until you reached and elevation of 36,000km where you would be weightless. Then, as you climbed more, you would experience a force pushing you into the ceiling. That means what started as the ceiling became the floor and when you travel back down to the planet, you would have to switch one more time. So, controls, storage, facilities, etc. in the elevator car would need to be able to operate regardless of the apparent direction of the force of gravity.
The next neighbor on the block we are setting out to meet can be, without a doubt, considered mayor of the celestial neighborhood: The Sun.
Just the Facts:
The Sun is by far the biggest thing in our solar system. It contains 99.8% of all of the mass in the solar system. The Sun is about 93 million miles away from the Earth, just the right distance away so that we are not too hot or too cold. However, if the sun were to suddenly stop shining, we would not know about it on Earth for about 8.3 minutes because that is how long it takes light to get here from the Sun. There are two things that are vital to keep in mind when discussing the Sun; it is MASSIVE and HOT. The radius of the sun is a bit over 430,000 miles. That is almost twice the distance between the Earth and the moon. Meanwhile, the temperature at the surface is a sweltering 10,000 degrees Fahrenheit (5800K) while at the core it is an unconceivable 28 million degrees Fahrenheit (15.6 million Kelvin).
Where did it come from?:
To understand where the Sun comes from, we need to know what is inside of it. The Sun contains about 73% Hydrogen and 25% Helium. However, there is still 2% of the Sun that is made up of heavier elements like iron, lithium, carbon, and basically any other element you can think of. The Sun creates it’s energy by taking Hydrogen atoms and fusing them into Helium atoms thus releasing energy and light. In fact, the famous equation E=mc2 is what governs energy creation in the Sun. Every second in the Sun, 700 million tons of Hydrogen atoms are converted to 695 million tons of Helium atoms. The missing mass is converted into energy through the proportionality constant the speed of light squared, c2. If the sun were big enough, it could also fuse Helium atoms together to make Lithium and so on down the periodic table, creating heavier and heavier elements. However, due to the structure of heavier atoms, Iron is the most massive element that can be produced in the center of a star. However, we know that heavier elements than Iron can be found in our solar system, Uranium for example. There is only one way that elements heavier than Iron can be created naturally; in a supernova. That’s right, our solar system was created from the remains of a supernova.
Our Sun and the rest of our solar system was created from the stuff left over from a dead star that exploded as it died. The gas and dust that was left over from the supernova slowly gathered together through gravity. As most of the material gathered in one central point, friction heated the material up. Once it got hot enough, Hydrogen was fused into Helium and our Sun was born. If this is still confusing, I will defer to Stephen Hawking and Benedict Cumberbatch to explain the origins of our solar system:
Why should I care about it?:
Well, I think we all know that we owe the existence of life to the Sun, but here are some other interesting things about the Sun that impact our lives.
Reason 1: The Sun is Dynamic (and dangerous)
The Sun looks pretty boring from the surface of the Earth; it is round and bright. However, it is so much more than a glowing ball. The surface is churning with tremendous energy, equivalent to many many nuclear warheads and all of this energy manifests itself in several ways.
On Earth, we have earthquakes, where two tectonic plates slide against one another; but on the Sun there are sunquakes, caused by the massive turbulence of superheated gas churning within the sun. The rapid movement of gas causes shockwaves that propagate through the Sun. They are so powerful that we can actually measure them using satellites because the surface of the sun is constantly shaking and moving due to them. In fact, this is exactly what NASA’s SDO project does.
In addition to sunquakes, there are events called flares, prominences, and coronal mass ejections. These three events are related to one another and are all caused by fluctuations in energy and the Sun’s magnetic field. Basically these events shoot plasma out of the sun. If the material loops back and returns to the Sun, it is called a prominence. If it extends straight out of the sun, it is called a flare. Finally, the material that is flung out of the sun through a flare is called a coronal mass ejection (CME). These CMS’s can be especially important for Earthling’s because if they are powerful enough, they could knock out any satellites in its way. For this reason, there are many satellites orbiting the sun giving a space weather forecast to try to avoid such complications.
Reason 2: The Sun is our Protector
Remember how those CME’s are caused by the Sun’s magnetic field? Well it turns out that we rely upon the magnetic field of the Sun to protect us from the harsh radiation of space. The magnetic field of the Sun is incredibly powerful and incredibly complicated. It is so powerful in fact that its influence can be felt far outside of Pluto. It creates a shell around our solar system called the heliosphere. If we were to travel outside of the heliosphere, we would be bombarded by radiation from deep space. These particles don’t normally reach Earth because they are deflected away by the Sun’s large magnetic field. On a side note, Voyager I is close to becoming the first man-made object to ever travel outside of the heliosphere.
Summing it all up:
The Sun can giveth and the Sun can taketh. On one hand, the Sun is necessary for life on Earth though the light and energy heating our planet and making photosynthesis possible. On the other hand, CME’s pose a risk to space travel and satellites. Also, in about 5 billion years or so, the Sun will begin to run low on Hydrogen fuel. When that happens, it will expand in size, engulfing and incinerating our planet. So, next time you wake up to a bright sunny day, remember that there is a lot more than meets the eye when it comes to the Sun.
This is a series of posts about our celestial neighbors, highlighting facts you may not have known about them. In this first installment we will learn about how we owe our very existence to the closest neighbor on the block: The Moon.
Just the Facts:
The moon is a bit over a quarter million miles away from the Earth, or about 60 times the radius of the Earth. The moon has a radius roughly one quarter of the radius of the Earth and the mass is about 1/80 that of Earth. Putting those facts together means that the gravity you would feel on the moon is about 1/6 of that on Earth. The moon has little to no atmosphere which means no wind to erode craters or astronaut footprints.
Where did it come from?:
There have been many different origin stories for the moon throughout human history ranging from the supernatural to the scientific. However, the currently accepted theory, first seriously proposed in 1976, is called the giant impact hypothesis. To fully understand the power of this theory, we need to travel back in time to the early days of the solar system.
As most people are aware, planets form slowly over time as particles and atoms are gravitationally attracted to one another and coalesce into a bigger body. As the object becomes bigger, it gets hotter. Eventually it gets hot enough that all of the material in it melts into a giant ball of magma. However, just like how oil floats on top of water, the more dense elements such as uranium and iron (the metals in general) slowly sank down to the center of this ball of magma while the lighter elements (the ones that make up rock and the atmosphere) stayed closer to the surface. This separation process is called differentiation and as the Earth was well underway with this process is when something incredible happened.
Roughly around the time that the outermost layer of the Earth would have been cool enough to solidify, a planet roughly the size of Mars collided with Earth at an oblique angle. Such a collision would have been terrifically violent. Think about a meteor hitting earth today. There is a crater that is left behind from the rock and dirt that is ejected out from the impact. Now, scale that up to the size of another planet hitting Earth. There was so much material flung out into the far reaches of space that much of it is probably still flying through space somewhere, never hitting anything since. However, there was also a significant amount of material that was ejected with just the right amount of energy to fall into an orbit around the Earth. This stuff eventually coalesced into what we now call our Moon. There are many good reasons to think that the giant impact hypothesis is true (and a few to not think it is true or not entirely), but for the sake of brevity I will let you look at them yourself or do a separate post about it.
Why should I care about it?:
I will try to convince you that if it were not for the moon, human life, or even life itself, would not have evolved on Earth or had much of a hope of becoming a technologically advanced society.
Reason 1: The Tides and the Beginnings of Life
As everybody knows, the Moon is what has the biggest influence over the ocean tides. Also, many know that the Moon is slowly (38mm per year) moving farther away from Earth. However, this slow motion away from earth, over billions of years, is quite significant. Billions of years ago, when the first oceans were forming on Earth, the Moon was much closer to the Earth. That means that its gravitational influence on the tides would have been much greater. In fact, it would have been so great that as the Earth rotated there would have been two massive tidal waves that traveled around the Earth pointing to and opposite the Moon the whole time. It sounds too incredible to be true, but in this early Earth there was a massive wave that washed over the entire Earth twice a day!
You might be wondering how massive waves could help life develop, but you have to keep in mind the state of the Earth at that time. It was still quite volcanically active and hot. All of this thermal energy would have been able to initiate countless chemical reactions creating new, organic compounds left and right. However, this in itself would be unlikely to be sufficient for self-sustaining life. What needed to happen is the organic compounds created at one part of Earth to get to some other organic compounds created elsewhere. This is where the massive tidal waves play an integral part. These waves would have made sure that the oceans stayed well mixed so that every organic compound could interact with every other compound, eventually creating the right mix to form life!
Reason 2: Our Axis and the Seasons
All of us are familiar with the seasons that happen every year. We have summer, fall, winter, and then spring. These four seasons come one after another year after year. Most of you are also probably familiar with the fact that seasons happen because the Earth is tilted in relation to the sun. As the Earth revolves around the sun different sides of the Earth face the sun more directly at different times, creating the seasons. The regularity of the changing of the seasons have become so important for life on earth that all different kinds of life have adapted to them: trees, birds, fish, insects, mammals. Many different species have some behavioral trait, whether it be migration, hibernation, or shedding leaves that depends on the regular changing of the seasons.
What you may not realize is that we owe this steady progression to the moon. If we did not have such a large celestial satellite orbiting our planet, the axis of Earth’s tilt would be very different today than it is now. In fact, there is no way to really predict what it would be. The moon pulls on the Earth strongly enough to stabilize the tilt axis so that it virtually does not change over time. However, without the moon there, the axis would have been very unstable and would change quite unpredictably. This means that what is the north pole one year could lie on the equator only 1000 years later. This means that there would have been massive climate swings all over the planet; making the steady seasonal cycles we are accustomed to quite impossible.
Having stable seasons was a big requirement for the development of advanced life because it allowed plants, that cannot move to avoid arctic conditions, to take root in one location and flourish and evolve. More importantly for humans, regular seasons make things like farming and agriculture possible. It was our ability to grow our own food that allowed humanity to grow out of the hunter/gatherer phase of existence and create civilization.
Summing it all up:
The moon is a pretty cool neighbor to have. It is at once familiar because we see it every day and mysterious. For example, it was just recently proposed that the Earth had two moons at one time that merged into the one we have today. It may just be a big lump of rock for the most part, lacking the sex appeal of some other moons in the solar system; but it has inspired and helped the human race reach out and touch it. And now, as we strive to go further, let us not fall into the trap of selling the moon short in this wondrous thing called the universe, because without it, we would never have made it this far.