Friday, July 9, 2021

Why are Van Allen belts not a barrier to spaceflight?

Of course, there are two mixed issues here.

The first is about how to get through the atmosphere without getting burned and the second about how to get through the Van Allen belts.

 It's true that re-entering the atmosphere from outer space is a delicate business and there are only a few safe ways to do it.  The atmosphere, as easily as we move across the Earth's surface, can pose a significant barrier to fast-moving objects.  

Air resistance is an important factor in all designs, from cars to parachutes and space shuttles.  If you've ever been outdoors in high winds, you've felt the kind of barrier the wind can produce in its own movement and the amount of force it takes to move to resist it.

 Objects that encounter our atmosphere from space are generally traveling much faster than any winds we would encounter during a storm here on Earth (thank goodness) and therefore the resistance of the air they hit is significant;  the atmosphere, if hit directly, is a barrier almost as solid as finding a rock.  The spacecraft carrying the crew will never dive straight into the atmosphere, but will encounter it at a shallow angle, which allows the spacecraft to encounter atmospheric resistance less abruptly.

 So what does this atmospheric resistance do?  It slows down the spacecraft, absorbing some of its energy.  This energy heats the atmosphere immediately around the ship, enveloping the ship in superheated plasma for part of its descent, until much of the ship's forward motion has been lost.  When approaching the atmosphere at an angle, this process takes longer and the ship can be safely decelerated.  If we tried to descend straight into the atmosphere, the ship would not be able to decelerate as much, and the sudden increase in atmospheric pressure would put so much stress on the ship that it could be seriously damaged.  If you have humans on the ship, that's not a good idea.  If, on the other hand, you're just trying to get a satellite out of orbit, you can drop it into the atmosphere at a steeper angle, as they don't need to be functional when diving into the Pacific Ocean.  (Usually this is where we put them.).

 So yes, there is a heating issue when you re-enter the atmosphere, but the atmosphere itself is not heated any more than the ambient air temperature.  It's just the air around the ship that heats up, and only because there's a spaceship passing through it.  The upper atmosphere is actually quite cold, so there is no intrinsic heat barrier to cross.  After all, we don't have the same heating problem when launching a spaceship .  This heating is simply atmospheric drag, although it is dangerous enough.  The loss of heat plates protecting the shuttle's wings is what led to the loss of the shuttle Columbia.

 Van Allen belts, on the other hand, are not really part of our atmosphere.  They are far beyond it, stretching hundreds of kilometers into space.  There are two donut-shaped rings around our planet, and they are a consequence of our planet's magnetic field.  The space shuttle normally orbits 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 Earth's surface.

 The innermost Van Allen belt is between 400 and 6,000 miles above the surface of our planet.  Even if the innermost belt is as close as possible, the ISS (and the space shuttle of its day) will be more than 160 kilometers away from the Van Allen belts.  For missions close to Earth, Van Allen's belts are not a danger to space travelers.

 It was, however, a danger to the Apollo missions.  Van Allen's belts are not a physical barrier for spacecraft, and so, in principle, we could have sent the Apollo spacecraft through the belts.  It wouldn't have been a good idea.  Van Allen belts are a kind of trap for charged particles like protons and electrons.  They are held in place by the Earth's magnetic field and therefore trace the shape of the magnetic field itself.  The problem with Van Allen belts is not that they are insurmountable, but the charged particles they contain.

 Charged particles are harmful to human bodies, but the amount of damage they do can range from none to lethal, depending on the energy these particles deposit, the density of those particles, and the amount 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 cannot control the energy of these particles, although they may be large.  The density of Van Allen belts is well known (by sending probes through them), and there are hotspots you can definitely avoid.  In particular, the innermost belt is a well-defined region, and it was possible to be left out during the trip to the Moon. The second belt is much larger and more difficult to avoid, but there are still denser regions to be avoided.  For Apollo trips, we wanted to send astronauts through a sparse region of the belts and try to get past them quickly.  In any case, this was necessary;  ships had to reach the moon in a reasonable amount of time, and the shorter the trip, the less exposure to all types of radiation the astronauts would have.

 In the end, it seemed that these tactics worked;  Onboard dose counters for the Apollo missions recorded average radiation doses to the astronauts' skin of 0.38 rad.  It's about the same radiation dose when taking two CT scans of your head, or half the dose of a single chest CT scan;  it's not that bad, although it's not something you should do every week.

 ♦ You are right that both the atmosphere and the Van Allen belts can be dangerous for space exploration, but with careful observations, orbital maneuvers, and ingenuity, we sail past them many times over.  Hopefully, we will continue to do this many more times in the future.

 Images:

 1- In a unique setting on Earth's colorful horizon, the silhouette of the space shuttle Endeavor is featured in this photo by an Expedition 22 crew member aboard the International Space Station as the space shuttle approached to dock on February 9 during the mission. STS-130.  Credit: NASA

 2- NASA's Van Allen probes orbit through two giant radiation belts that encircle the Earth.  His observations help improve computer simulations of belt events that can affect technology in space.  Credit: NASA

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