When a spacecraft enters a thick atmosphere at a high velocity, it rapidly compresses the gas in front of it. This creates temperatures high enough to ionize the gas molecules into a hot, dense plasma. To protect against damage, spacecraft are typically covered by a heat shield material that burns in a controlled manner. This process is called ablation. Though current materials are effective for present-day missions, future missions require better heat shields.
Click here for original story, Tokamak experiments provide unique data for validating spacecraft heat shield ablation models
Source: Phys.org