(Intro)

Last week we talked about one of the trickiest aspects of space flight. One that doesn't get nearly as much attention as the launch of a mission . And that's the end of a mission, when a spacecraft re-enters the earth's atmosphere.

Re-entry begins where our atmosphere begins at the so called Karman line 100 km above the surface where air is dense enough to support traditional air crafts.

Sounds inviting enough but careening into this dense atmosphere from the relative emptiness of space makes for a bumpy ride.

Not only does a speeding spacecraft compress all the air in front of it generating a huge amount of heat, it also has to decelerate rapidly creating g-forces that can feel to astronauts like an almost crashing weight.

Now we've already talked about how the power of math can help determine the ideal angle and speed of re-entry to minimize these effects.

But of course the cleverest math in the world can't help you unless you have a spacecraft that is designed and built to endure the trip.

One challenge that engineers have struggled with the longest and experimented with the most is finding a material that can withstand the intense heat of re-entry up to 1600 degrees Celsius.

Some of the most effective solutions involve using heat sinks, materials that absorb and then re-radiate the heat to keep the craft cool.

The earliest heat sinks designs were essentially layers of thick dense materials usually alloys of metals like titanium and beryllium that use their excessive mass to capture and emit lots of heat.

But those materials are very heavy. So while handy for unmanned vehicles like missiles they've proven unsuitable for manned missions.

During the space shuttle era engineers tried a more diverse approach, shielding each orbiter with more than half a dozen different materials each tailored to a particular heat load.

The most famous technology here was the shuttle's insulating tiles which were of almost pure silica the same stuff in glass and sand and were so porous that by volume they were more than 90% empty.

White tiles helped reflect solar radiation, while the black ones on the bottom where designed to emit as much absorbed heat as possible.

And they fared well but it was the failure of another part of the shuttle's heat shield, a cracked carbon panel in the Columbia, that made it clear that the materials we were using were just too fragile.

So you know what technology we're looking at now? The same technology we were using 50 years ago. This involves using ablative materials. Substances that redirect heat energy into chemical reactions.

More specifically they use the heat to trigger a process that causes the heat shield to ablate or erode away. This helps dispel the heat but you're not left with much protection by the time you're on the ground.

This might not sound entirely safe but it was what was used on all of the Apollo missions. Each re-entry vehicle was covered in a ceramic coating that was designed to melt away when it encountered extremely high heat.

On the downside using this method means you have to refurbish or totally rebuild a spacecraft after each mission. But given its proven record of success, NASA has decided to go back to using these materials on its next generation of crew vehicles, the Orion spacecraft.

Finally the last key in engineering a safe re-entry is simply the shape of the vehicle. Have you ever wondered why astronauts never actually land in rockets like they do in cartoons and comic books?

For decades before we ever made it into space, scientists thought that return vehicles had to be shaped like missiles it just stood to reason based on what we knew about aerodynamics.

But in 1951 an engineer named Harvey Allen at what would later become NASA had a revolutionary idea. What if spaceships were actually rounded and blunt instead of pointy? Now it sound counter-intuitive since a blunt shape would interact more with the air than an aerodynamic one.

But Allan's calculations showed that blunt shapes were better at keeping crafts cool because they create a shock wave that insulates the craft providing more surface area for the heat to spread across.

That's why today most spacecrafts used for re-entry aren't shaped like buck rogers rockets. They're all variations on spheres and cones like the Soyuz lander, the Apollo lander and the Orion crew vehicles.

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