Penetration into low-density media: In situ observation of penetration process of various projectiles

► We carry out hypervelocity impact experiments using low-density targets. ► We observed the track formation process using a high-speed camera. ► At early stages, the evolution of tracks is similar in any type of projectiles. ► The evolution is different depending on the projectile conditions at later stages. ► We briefly discuss the track formation process based on the experimental results and some previous models.