Although some approaches to such shields can be heavy and/or complex, a University of Manchester PhD student has developed one that's simple, cheap and lightweight.

As a spacecraft plummets through a planet's atmosphere, the friction of the air against the rapidly-passing underside of the craft causes heat to build up. Heat shields serve to dissipate that heat, keeping it from damaging the spacecraft itself, while also helping to slow the spacecraft's descent by creating aerodynamic drag. 

Presently-used shields include ones that inflate when needed, or that are mechanically deployed. Rui Wu, however, created a prototype that's a little different. 

Made of a flexible, strong and heat-resistant material that folds down when not in use, his shield automatically starts spinning like a samara-type tree seed when exposed to the onrush of air that a spacecraft would experience when dropping through a planet's atmosphere.