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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Does the Moon cause tides in Earth's atmosphere?

On Feb. 1, 2014, Japan Aerospace Exploration Agency astronaut Koichi Wakata tweeted this view of a crescent moon rising and the cusp of Earth's atmosphere. Distinct colors are visible because the dominant gases and particles in each layer of the atmosphere act as prisms, filtering out certain colors of light. Image credit: NASA.

On Feb. 1, 2014, Japan Aerospace Exploration Agency astronaut Koichi Wakata tweeted this view of a crescent moon rising and the cusp of Earth's atmosphere. Distinct colors are visible because the dominant gases and particles in each layer of the atmosphere act as prisms, filtering out certain colors of light. Image credit: NASA.

Originally posted at Forbes!

Our atmosphere does have tides, and the gravity of the moon is part of why they exist, but it turns out that the moon’s influence on our atmosphere is absolutely swamped by the influence the sun has on our atmosphere. However, it’s not the gravity of the sun that’s making such a large impact.

You can guess that the sun’s gravitational impact on atmospheric tides must be small by looking at the ocean tides, which are largely driven by simple gravity. The ocean tides follow the moon as it circles us - if the sun were a major player in the ocean tides, then high tide would happen at local noon every day, instead of varying based on the moon’s orbit. The sun is still a player; the highest tides occur when the sun and the moon align so both of them are pulling in the same direction, but the difference between a normal high tide and high tide when all forces align isn’t as large as the difference between high tide and low tide; the moon wins this round.

Unlike our liquid oceans, the atmosphere is a gas, and that introduces some other possibilities. Liquids are fairly straightforward under normal conditions; they fill their vessels, and are relatively constant in density - it’s hard to compress or expand a liquid by very much. Gases, on the other hand, are highly variable in density - it’s fairly easy to compress a gas - we do this simply by talking. Gases are also incredibly sensitive to temperature; one of the easiest way to make a gas more dense is to simply cool it down. Conversely, if you want to make a gas puff up and take up more space, heat is an easy way. Each particle of gas gains a little more energy than it had before, and it bounces around a little faster, and so the whole thing winds up slightly more puffed up than it used to be. So the sun, as an enormous source of heat, has a pretty major influence on the gas by simply heating it up.

When the sun is shining on the atmosphere (anytime it’s daytime), it heats up the gas that makes up our atmosphere, which makes the whole atmosphere expand outwards towards space. This expansion can be measured at sea level as a very slight reduction in atmospheric pressure, but is a lot more dramatic at higher altitudes, where the density of gas is really low to start with – as the atmosphere underneath it expands, suddenly there’s an upwelling of denser gas from below.

This isn’t a tide in the way that we’re used to thinking about it – usually if we say “tide” we mean a gravitational tide, but if by a tide we mean a regular, cyclical change, then these are definitely tides, and indeed they are officially termed atmospheric tides.

Our moon does have a gravitational pull on the atmosphere as well, but like the sun’s impact on our ocean tides, it’s a much weaker effect than the heating provided by the sun. If the moon were the main cause behind this atmospheric stretching, it would work the same way as the ocean tides; high tide would mean that you also had the most atmosphere above you, instead of what we see; a 24 hour cycle of our atmosphere heating and cooling under the sun’s rays.


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