"Which is hotter -- lava or re-entry from space?"

Photo from WikiImages

Leave it to a seven-year-old to pose such a dramatic question as this!  Lava and re-entry both make things red-hot, right?  So are they comparable?

 
Lava -- which, to review, is what we call magma when it’s on Earth’s surface -- has varying temperatures depending on what kind of rock has melted to create it.  The hottest lava is made of basalt and flows in the range of 1850-2200°F.

The heat created by an object falling through our atmosphere is a bit more than the glowing warmth of lava.  The Orion spacecraft comes down facing temperatures up to 4000°F, and meteors, with their varying shapes and trajectories and lack of heat shields, often get to burn up at even hotter temperatures.  But, to give lava a fair shake at impressiveness-of-hotness, let’s take the space shuttle as our comparative re-entry representative.  The space shuttle’s surfaces reached temperatures in the 2700-2900°F range.

So, re-entry is hotter than lava, but a really hot volcano at 2200°F is not so far off from a slow space shuttle blazing in at 2700°F that it couldn’t at least compete in a metal-melting contest.

Both lava and re-entry are hot enough to melt all of the “softer” metals, meeting a division of ability right around iron.  Cast iron melts in the 2060-2200°F range, so whether you cast your frying pan into the fires or Etna or out the shuttle window, you can kiss it goodbye. 

Your wrought iron furniture, however, melts at 2700-2900°F, so while it would be OK in a volcano, it would most likely melt away in the atmosphere with so many other shooting stars.  So in the metal melt-off, we do have to hand it to re-entry as the heat master, because if you just dropped a wrought-iron gate from on high, it would be toast. 

However, if that iron gate was balled up into a wrought iron wad, it just might actually stand a chance of surviving a tumble through the atmosphere.  In regular gate form, it would need some propulsion assistance and/or heat shields and/or a nice capsule covering to keep it intact and it would probably still be red-hot and perhaps a bit wobbly, but if it was wadded into a blunt shape and could create a good shockwave, at space shuttle speeds, it could survive. 

 

You know what else is made of wrought iron?  The Eiffel Tower.  That just begs for some visualization, doesn’t it?

So, we have determined that a lava flow would not liquefy the Eiffel Tower, but a straight fall from space would.  Now let’s picture making some special modifications ourselves to help the Tower conquer re-entry.  The problem for the Eiffel Tower in reality is that, while the heat levels of re-entry built up in the space shuttle’s descent would not be enough to melt it, its shape would not create the kind of heat-protective shockwave that blunt-shaped re-entry vehicles use to survive falling through the atmosphere.  You would end up with a bunch of giant molten strips of slag falling at you.  But say you equipped the Tower with shuttle-like descent capabilities – add a big domed heat-shielded side to hide above and some thrusters to maintain alignment for proper descent speed – then, were the Eiffel Tower, for some good and plausible reason, to come careening down from space, it might survive the heat of the fall.  The sudden stop at the end would be a problem, though.  Now picture a glowing-hot Eiffel Tower deploying its gigantic parachute…where do you think it should land?

 
Now on to the next thing to ponder on the topics of lava and re-entry… what if you needed to re-enter from space onto a planet that was already hotter than lava?  Hmmm…