Why Aren't The Van Allen Belts A Barrier To Spaceflight?

I follow all kinds of information about space and the stars. My brother has only recently started paying attention to these issues, but has been reading some naysayer websites. Because of this, he says he has doubts about the ‘truth’ of the space shuttle, the flight to the moon and other missions, as some claim that they would be impossible because of the heated layers of atmosphere around the earth, which would destroy them—the Van Allen belts. I know that heat shields are used, and am assuming that the rarefied atmosphere might not conduct heat as well. But what is the real reason why these flights are possible and are not eliminated by the heat of the Van Allen belts or other layers?
In a very unique setting over Earth's colorful horizon, the silhouette of the space shuttle Endeavour is featured in this photo by an Expedition 22 crew member on board the International Space Station, as the shuttle approached for its docking on Feb. 9 during the STS-130 mission. Image credit: NASA

In a very unique setting over Earth's colorful horizon, the silhouette of the space shuttle Endeavour is featured in this photo by an Expedition 22 crew member on board the International Space Station, as the shuttle approached for its docking on Feb. 9 during the STS-130 mission. Image credit: NASA

Originally posted on Forbes!

So there are two questions mixed up in here - the first is about traversing the atmosphere without burning up, and the second about traversing the Van Allen belts.

It’s true that re-entering the atmosphere from space is a delicate business, and there are only a few safe paths to do so. The atmosphere, as easily as we move through it on the surface of the Earth, can pose a significant barrier to fast-moving objects. Air resistance is a major factor in designing everything from cars to parachutes to space shuttles. If you’ve ever been out in high winds, you’ve felt the kind of barrier wind can produce to your own motion, and how much force it takes to move in resistance to it.

Objects which encounter our atmosphere from space are generally travelling much faster than any winds we’d encounter during a storm here on Earth (thank goodness), and so the air resistance they hit is significant; the atmosphere, if hit directly, is almost as solid a barrier as encountering rock. Crew-carrying spacecraft will never plunge straight down into the atmosphere, but encounter it at a shallow angle, which allows the craft to encounter the atmosphere’s resistance less abruptly.

This computer-generated art depicts Orion's heat shield protecting the crew module as it enters the Earth's atmosphere. Image credit: NASA

This computer-generated art depicts Orion's heat shield protecting the crew module as it enters the Earth's atmosphere. Image credit: NASA

So what does this atmospheric resistance do? It slows down the spacecraft, by absorbing some of the spacecraft’s energy. That energy heats up the atmosphere immediately around the craft, encasing the craft in a superheated plasma for part of its descent, until much of the forward motion of the craft has been lost. By approaching the atmosphere at an angle, this process takes a longer time, and the craft can be safely slowed. If we tried to drop straight down into the atmosphere, the craft would not be able to slow down as much, and the sudden increase in pressure from the atmosphere would put so much stress on the craft that it might break. If you have humans in the craft, this is not a good idea. If, on the other hand, you’re just trying to get a satellite out of orbit, you can drop them into the atmosphere at a steeper angle, as they don’t need to be functional when they plunge into the Pacific Ocean. (That’s usually where we put them.)

So yes, there’s a heating problem when you re-enter the atmosphere, but the atmosphere itself isn’t heated any more than ambient air temperature. It's only the air surrounding the craft which heats, and only because there's a spacecraft barreling through. The upper atmosphere is actually quite cold, so there’s no intrinsic heated barrier to traverse. We don’t have the same heating problem when launching a spacecraft, after all. This heating is simply atmospheric drag, though this is dangerous enough - the loss of heat tiles protecting the wings of the space shuttle was what led to the loss of the Shuttle Columbia.

NASA's Van Allen Probes orbit through two giant radiation belts that surround Earth. Their observations help improve computer simulations of events in the belts that can affect technology in space. Image credit: JHU/APL, NASA

NASA's Van Allen Probes orbit through two giant radiation belts that surround Earth. Their observations help improve computer simulations of events in the belts that can affect technology in space. Image credit: JHU/APL, NASA

The Van Allen belts, on the other hand, are not actually part of our atmosphere. They’re well beyond it, extending hundreds of miles outwards into space. There are two, both donut-shaped rings surrounding our planet, and are a consequence of our planet’s magnetic field. The Space Shuttle typically orbited at a height of 190 miles to 330 miles above the surface, and the International Space Station orbits at a height of somewhere between 205 and 270 miles above the surface of the Earth.

The innermost Van Allen belt sits somewhere between 400 to 6,000 miles above the surface of our planet. Even if the innermost belt is at its closest, the ISS (and the space shuttle in its day) are more than 100 miles away from the Van Allen Belts. For near-Earth missions, the Van Allen belts are not a hazard to spacefarers.

It was, however, a hazard for the Apollo missions. The Van Allen belts are not a physical barrier to spacecraft, and so, in principle, we could have sent the Apollo spacecraft through the belts. It would not have been a good idea. The Van Allen belts are a kind of trap for charged particles like protons and electrons. They’re held in place by the magnetic field of the Earth, and so they trace the shape of the magnetic field itself. The problem with the Van Allen belts lies not in them being impassable, but in the charged particles they contain.

In this 1966 photo, a plasma thruster at NASA's Lewis Research Center simulates Van Allen Belts, rings of radiation around the Earth. The Cleveland, Ohio, center is now John H. Glenn Research Center. Image credit: NASA

In this 1966 photo, a plasma thruster at NASA's Lewis Research Center simulates Van Allen Belts, rings of radiation around the Earth. The Cleveland, Ohio, center is now John H. Glenn Research Center. Image credit: NASA

Charged particles are damaging to human bodies, but the amount of damage done can range from none to lethal, depending on the energy those particles deposit, the density of those particles, and the length of time you spend being exposed to them.

In the case of the Apollo missions, the solution was to minimize the second two factors. We can’t control the energy of those particles, though they can be large. The density of the Van Allen belts is well known (from sending uncrewed probes through them), and there are hotspots you can definitely avoid. In particular, the innermost belt is a rather tightly defined region, and it was possible to stay out of it for the trip to the Moon. The second belt is much larger, and harder to avoid, but there are still denser regions to avoid. For the Apollo trips, we wanted to send the astronauts through a sparse region of the belts, and to try and get through them quickly. This was necessary in any case; the crafts had to make it to the Moon in a reasonable amount of time, and the shorter the trip, the less exposure to all sorts of radiation the astronauts would get.

An artist's depiction with cutaway section of the two giant donuts of radiation, called the Van Allen Belts, that surround Earth. Image credit: NASA/Goddard Space Flight Center/Scientific Visualization Studio

An artist's depiction with cutaway section of the two giant donuts of radiation, called the Van Allen Belts, that surround Earth. Image credit: NASA/Goddard Space Flight Center/Scientific Visualization Studio

In the end, it seemed that these tactics worked; the on-board dose counters for the Apollo missions registered average radiation doses to the skin of the astronauts of 0.38 rad. This is about the same radiation dose as getting two CT scans of your head, or half the dose of a single chest CT scan; not too bad, though not something you should do every week.

Your brother is right that both the atmosphere and the Van Allen belts can be dangers to space exploration, but with careful observations, orbital maneuvering, and inventiveness, we’ve navigated our way beyond them many times. Hopefully, we'll continue to do so in the future many times more.

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How Come The Oort Cloud Isn't Torn Away From Our Sun By Nearby Stars?

If the Oort Cloud is three light years away from our Sun, then it’s closer to Alpha Centauri than our Sun, right? So how can it stay around our Sun if the mass of Alpha Centauri is 1.1 times the mass of our Sun - wouldn’t the gravity of Alpha Centauri rip it away?
An illustration of the Kuiper Belt and Oort Cloud in relation to our solar system. Image credit: NASA

An illustration of the Kuiper Belt and Oort Cloud in relation to our solar system. Image credit: NASA

Originally posted on Forbes!

The Oort cloud is an interesting feature of our solar system; a nebulous, spherical cloud of comets which marks the very outer limit of our solar system. The Oort cloud is also the source of our long period comets - those icy fragments of the early solar system which orbit our Sun very infrequently. To be classified as a long period comet, more than 200 years must pass between trips near the Sun. Hale-Bopp is probably the most well known of these, as it was visible to the naked eye for a long time in 1998. A more recent visitor was the Lovejoy comet, which swung near the Sun in 2011.

The Oort cloud is very far from the Sun. It is outside the bubble produced by our Sun’s solar wind and magnetic field by a considerable distance. While Voyager 1 has left this magnetic bubble, and entered what is called “interstellar space”, it has several hundred more years of traveling before it even reaches the inner edge of the Oort cloud. How is part of the solar system in interstellar space? Well, this means that the solar system at such a large distance from the Sun is not entirely ruled by our own star - the presence of other stars is mixing with the influence of our Sun.

This artist's concept puts solar system distances in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. The inner edge of the main part of the Oort Cloud could be as close as 1,000 AU from our sun. The outer edge is estimated to be around 100,000 AU. Image credit: NASA/JPL-Caltech

This artist's concept puts solar system distances in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. The inner edge of the main part of the Oort Cloud could be as close as 1,000 AU from our sun. The outer edge is estimated to be around 100,000 AU. Image credit: NASA/JPL-Caltech

The inner edge of the Oort cloud is typically quoted as beginning at somewhere between 1,000 and 5,000 au from the Sun. 5,000 au is about 0.08 light years away from the Sun, which is a little over four weeks of travel time for a beam of light, and considerably closer to our Sun than to Proxima Centauri, the closest star. These Oort cloud objects at the inner edge of their cloud are fairly reasonably more attached to our Sun than they are to anything else, and there are a lot of them here.

As we travel from the inner Oort cloud to the outer region, we should note that the Oort cloud is not an even assembly of objects, from some inner bound to a fixed outer bound. Instead, while there is something of an inner boundary, the outer boundary is more of a fizzling out, with objects getting fewer and farther between as you go farther and farther from the Sun. This means that the “outer boundary” is a very tricky thing to attach a number to. How many objects need to be out there to still count as part of the Oort cloud? Just one? Or do we need a higher density of objects before we’re dropping our delineation down? As a result of this fuzziness, plus the fact that it’s very hard to spot Oort cloud objects in the first place, estimates of the outer bound of the Oort cloud range from 50,000 to 200,000 au. It’s that 200,000 au that works out to 3.1 light years away from our Sun. NASA often quotes this outer edge as sitting at 100,000 au, which is about 1.6 light years, which means that this fuzzy “edge” is extending less than half the way out to Alpha Centauri.

Comet Lovejoy is visible near Earth's horizon in this nighttime image photographed by NASA astronaut Dan Burbank, Expedition 30 commander, onboard the International Space Station on Dec. 22, 2011. Image credit: NASA

Comet Lovejoy is visible near Earth's horizon in this nighttime image photographed by NASA astronaut Dan Burbank, Expedition 30 commander, onboard the International Space Station on Dec. 22, 2011. Image credit: NASA

All these numbers are for a sense of scale. In actual fact, the Oort cloud is incredibly sensitive to gravitational forces from objects other than our Sun. One of these is a very large-scale gravitational inequality; our solar system is not at the center of the Milky Way galaxy. The gravitational pull from our Galaxy is therefore stronger on one side of the solar system than it is on the other, and this galactic tide is enough to gradually jostle the Oort cloud. This kind of perturbation is part of how we think we get the long period comets, which can come blazing into the inner solar system, and, if they are unlucky, sometimes completely evaporated by the Sun.

The Oort cloud is also sensitive to the motions of other stars nearby in the Galaxy, and other extrasolar objects, like clouds of gas. As stars pass nearby (or through) the outer reaches of the Oort cloud, they will disturb the delicate gravitational balance that keeps these objects in their long, distant orbits. Stars aren’t likely to smash directly into a comet out there, but they might jostle it out of its orbit, and send it down into the inner solar system - another way of getting comets into the rest of the solar system.

Comet Hale-Bopp. Alex Krainov shot this image at Zabriskie Point in Death Valley in April 1997. Image credit: Alex Krainov, CC BY-SA 3.0

Comet Hale-Bopp. Alex Krainov shot this image at Zabriskie Point in Death Valley in April 1997. Image credit: Alex Krainov, CC BY-SA 3.0

But these perturbing stars are in motion too, and they will pass through relatively quickly, on an astronomical timescale. Alpha Centauri is still arriving into the solar neighborhood, and isn't yet close enough to do much influencing. With the combination of the fading density of objects, the short time frame with which a star will be close enough to really dramatically pull on the objects sitting out there, and the length of time between stellar close passes being quite long, we don't expect the Oort cloud to have been stripped away from our star. But it is absolutely influenced by the presence of those stars, and by the Galaxy at large, and our long, once-a-millennia comets like Hale-Bopp are the result.

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Is Space Curved? Can We See The Milky Way In The Past?

Is it possible that space-time is curved in such a way that one (or many) of the galaxies we see in telescopes is actually our own Milky Way a few billion years earlier?
This infrared view reveals galaxies far, far away that existed long, long ago. Taken by the Near Infrared Camera and Multi-Object Spectrometer aboard the NASA/ESA Hubble Space Telescope, the image is part of the Hubble Ultra Deep Field survey, the deepest portrait ever taken of the universe. Image credit: NASA, ESA and R. Thompson (Univ. Arizona)

This infrared view reveals galaxies far, far away that existed long, long ago. Taken by the Near Infrared Camera and Multi-Object Spectrometer aboard the NASA/ESA Hubble Space Telescope, the image is part of the Hubble Ultra Deep Field survey, the deepest portrait ever taken of the universe. Image credit: NASAESA and R. Thompson (Univ. Arizona)

Originally posted on Forbes!

It is mathematically possible for a universe to be shaped this way, but not our Universe. Our Universe is as close to flat as we can measure right now, though it’s only possible for it to be very slightly curved, considering the wiggle room we have remaining on our measurements.

The universe that you describe could be round, donut-shaped or cylindrical; some shape where at least in one direction, it connects back to itself. These aren’t your only options for a universe - you could also invent a saddle shaped or other, more exotic shape to place your universe in.

For now, let’s roll with a cylindrical universe. And let’s put a star somewhere on the surface. If the light from this star is going along the length of the cylinder, all it can ever do is go out, because the surface is flat in that direction; there’s no curve or loop. This flat, uncurved behavior is how we believe our Universe behaves in every direction. Light in our Universe departs its star, and travels in a straight line forever (as far as we can tell) unless it is intercepted by another astrophysical object, another star, planet, or telescope detector.

However, the light that leaves our star in the cylindrical universe has one other option. The light that goes in the other direction - around the curve of the cylinder - will also travel in a straight path. But this path loops back on itself, and if the light doesn’t hit anything else, after it has completed its tour of the cylinder’s circumference, it will arrive back where it began, on the other side of the star, delayed by the length of time it took to do its loop.

The three possible geometries of space. At the top is a sphere, followed by a saddle-shaped universe, and then flat. Each geometry will affect the path of light traveling through it. Image credit: NASA/WMAP Science Team

The three possible geometries of space. At the top is a sphere, followed by a saddle-shaped universe, and then flat. Each geometry will affect the path of light traveling through it. Image credit: NASA/WMAP Science Team

 

What happens if you make your universe spherical? It’s a very similar thing, except now every path that light can take will loop back onto itself, given enough time. There’s another curious thing about the light this time, though, which is that the beams of light, even though they’re all travelling “out”, will all cross each other at some other point on the sphere. If the star was on a flat surface, these beams of light would only ever get further apart; there’s nothing that would ever curve the light back towards each other.

In our Universe, we know that there’s no bending of the light as it comes through space (this is from an analysis of the map of the oldest light in the Universe) beyond what you would expect from gravitational forces. This lack of a large scale bending rules out the spherical and saddle-shaped options, and all that’s left are the ones which can be considered flat. While we can’t observe the entire universe to objectively figure out what the global shape of the entire thing is, we know that on the scales of the observable universe, our Universe is pretty darn flat.

This artist’s impression shows how photons in the Cosmic Microwave Background (CMB, as detected by ESA’s Planck space telescope) are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the Universe. Image credit: ESA and the Planck Collaboration

This artist’s impression shows how photons in the Cosmic Microwave Background (CMB, as detected by ESA’s Planck space telescope) are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the Universe. Image credit: ESA and the Planck Collaboration

How do we know that the Universe isn’t a tightly rolled cylinder? Well, we can’t rule out a gigantic cylinder, but it would have to be so large that we couldn’t ever detect a difference between light going “out” along the length of the cylinder and the light going “around”, because as far as we can observe, the Universe is the same in every direction. If there were a preferred direction, where the Universe appeared considerably younger than in the other direction, then we’d get suspicious of a cylindrical shape. But since there’s no evidence for that, we usually describe our Universe as an unwarped, three dimensional, grid. And with that kind of shape, we don’t expect any of the light from the distant universe to be taking a looping path to show us our own Milky Way.

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When Was Dark Matter Formed?

Was dark matter created during the Big Bang?
This full-sky map from the Planck mission shows matter between Earth and the edge of the observable universe. Regions with more mass show up as lighter areas while regions with less mass are darker. The grayed-out areas are where light from our own galaxy was too bright, blocking Planck's ability to map the more distant matter. Image credit: ESA/NASA/JPL-Caltech

This full-sky map from the Planck mission shows matter between Earth and the edge of the observable universe. Regions with more mass show up as lighter areas while regions with less mass are darker. The grayed-out areas are where light from our own galaxy was too bright, blocking Planck's ability to map the more distant matter. Image credit: ESA/NASA/JPL-Caltech

Originally posted on Forbes!

Our current understanding of the event dubbed the Big Bang implies that all matter, and all energy, appeared in our Universe at that moment. Ever since then, the Universe has been changing, and those changes are reflected in the behavior of matter and energy.

Unfortunately, we can’t observe this event directly in order to get a better understanding of how the earliest stages of the Universe’s evolution unfolded. This isn’t something that can be fixed with a more powerful telescope, either; at an early point in time, the Universe was quite simply opaque to light. And much in the same way that I can’t see through furniture, even the best observatories can’t penetrate into the depths of time beyond this point.

To explain how we know that dark matter has been around since the very early Universe, I have to talk a bit about how we’ve managed to learn about the early Universe in general.

This map shows the oldest light in our universe, as detected with the greatest precision yet by the Planck mission. The ancient light, called the cosmic microwave background, was imprinted on the sky when the universe was 370,000 years old. Image credit: ESA and the Planck Collaboration

This map shows the oldest light in our universe, as detected with the greatest precision yet by the Planck mission. The ancient light, called the cosmic microwave background, was imprinted on the sky when the universe was 370,000 years old. Image credit: ESA and the Planck Collaboration

You may have seen this image before. The cosmic microwave background, which has the alternate name of the “oldest light in the Universe” is the surface of the cloudy Universe, just as it became transparent to light. There’s a lot of information about the early Universe embedded in this image, which tells us about the temperature of the Universe, and how the matter which later became clusters of galaxies was distributed around.

It’s this latter point which points to the existence of dark matter in the very early Universe.

One of the ways this image is analysed is by looking at the characteristic spacing between bright and cool patches. If this were 100% randomly distributed, you wouldn’t expect to find things at any particular distance more frequently than any other distance. And that could have been the way our Universe was arranged, but it isn’t. Our Universe has a few preferred spacings.

One of these sets of spacings is formally termed Baryon Acoustic Oscillations. Baryons are simply any matter, the protons and neutrons and electrons that make up humans and stars and dogs. Acoustic oscillations should sound like a musical term more than a physics one, and it’s not without reason. The early Universe was much like a fluid in many ways, and information travelled through it like sound through water. The Baryon Acoustic Oscillations, therefore, refers to the way that the stuff of atoms rippled through the early Universe.

Astronomers using NASA's Hubble Space Telescope took advantage of a giant cosmic magnifying glass to create a detailed map of dark matter in the universe. Image credit: NASA, ESA, and D. Coe (NASA JPL/Caltech and STScI)

Astronomers using NASA's Hubble Space Telescope took advantage of a giant cosmic magnifying glass to create a detailed map of dark matter in the universe. Image credit: NASA, ESA, and D. Coe (NASA JPL/Caltech and STScI)

The cause of those ripples? A pressure from dark matter. When the Universe became less fluid, the protons, electrons and neutrons were frozen into place, wherever their ripple had left them. There’s a huge number of these ripples in the Universe, so rather than a single stone thrown into a lake, our image is more like a water surface pummeled by a tropical rainstorm. The spacing between where the dark matter sat, and where some of the baryons in our Universe had rippled outwards to, is reflected in the statistics of the cosmic microwave image. Without the presence of dark matter, we wouldn’t expect to see this exact spacing. And if dark matter was present so early in the Universe, it probably was formed in the big bang. At the very least, it couldn’t have been formed later, the way the heavy elements in our Universe were formed by stars, many years after this snapshot of our early Universe.

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Is Dark Energy Pushing Our Galaxy Somewhere?

If dark energy is pushing our Galaxy in a known and straight line, then where is it pushing us to? What kind of force is driving the dark energy and where is it taking us to? Or is there a force that is drawing dark energy to it, and we are just trapped within it?
Still from an animation illustrating the accelerating expansion of the universe due to dark energy. Image credit: NASA's Goddard Space Flight Center Conceptual Image Lab

Still from an animation illustrating the accelerating expansion of the universe due to dark energy. Image credit: NASA's Goddard Space Flight Center Conceptual Image Lab

Originally posted on Forbes!

Dark energy isn't pushing our Galaxy in a specific direction; it's responsible for the expansion of the space between all objects in space which are not tied to each other by gravity.

So, for instance, dark energy doesn't influence the distance between the Sun and the Earth, because our gravitational ties are much stronger than the gentle stretching that space is doing. Similarly, our Sun isn't being moved relative to the center of our Galaxy, because the force of gravity binding us to the Galaxy is much stronger than what dark energy can exert.

Dark energy can't even shear apart the gravitational ties which attach our Milky Way to the Andromeda Galaxy and the numerous, tiny dwarf galaxies which hover around our Galaxy in their own orbits. The distances here are enormous; Andromeda is 2.5 million light years away. Light arriving to us from Andromeda now will have left that galaxy when our planet Earth had only just seen its first humans.

Dark energy is a force to reckon with only for galaxies much more distant from us, where the gravitational force between our Milky Way Galaxy and that faraway galaxy plays no role. It's often phrased as a gravitational counter-force, but that's only partially correct. It is true that dark energy seems to have a repellent influence on the space surrounding it, but unlike gravity, which is strongest around concentrations of mass in the Universe, dark energy seems to be evenly spread throughout the universe, with not a care for the presence of a galaxy, planet, or supercluster . It's this evenhandedness of dark energy that means that gravity can overpower it in densely populated regions of the Universe.

In this artist's conception, dark energy is represented by the purple grid above, and gravity by the green grid below. Gravity emanates from all matter in the universe, but its effects are localized and drop off quickly over large distances. Image credit: NASA/JPL-Caltech

In this artist's conception, dark energy is represented by the purple grid above, and gravity by the green grid below. Gravity emanates from all matter in the universe, but its effects are localized and drop off quickly over large distances. Image credit: NASA/JPL-Caltech

Dark energy is not a directional force - there's no bulk motion to the left, right, or an arbitrarily defined up that this expansion leans towards. So there's no point to which the universe is being drawn, and equally there's no origin point from which the expansion is unspooling. Any given point in space is simply, and very gradually, becoming more distant from most other points in the universe. It's not that our Galaxy is being pushed around- dark energy is instead ballooning out the space within which our Galaxy sits.

Where does dark energy come from? So far that's a mystery; we can measure its influence to a great degree of confidence, but if we knew the exact nature of why it behaves the way we observe it to, we'd probably rename it something less vague than 'dark energy'.

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