At near-light speeds, single atoms in space strike a spacecraft with the force of solid bullets.

Interstellar space is not a perfect vacuum but contains a thin gas known as the Interstellar Medium (ISM). On average, the ISM consists of approximately one hydrogen atom per cubic centimeter. While this density is extremely low, the kinetic energy of these particles becomes devastating at relativistic speeds. At 90% the speed of light, a single hydrogen nucleus carries an energy of about 1.2 gigaelectronvolts (GeV).A study published by researchers like Arthur G. Suits and others highlights that these impacts are not just mechanical but also ionizing. When a spacecraft travels at these speeds, the hydrogen atoms penetrate the hull and deposit their energy deep within the material. This process creates a constant stream of high-energy radiation, specifically gamma rays and secondary particles, which can degrade electronics and harm biological passengers. The heat generated by these collisions is also significant, potentially melting standard aerospace materials.To combat this, the Breakthrough Starshot initiative, launched in 2016, explores using thin shields or laser-pushed sails to mitigate damage. Other theoretical designs suggest using powerful magnetic fields to deflect charged particles or thick layers of water ice to absorb the radiation. Without these protections, a ship traveling to Proxima Centauri would be eroded away long before reaching its destination. The sheer energy involved means that even the smallest particles in the void of space become lethal projectiles.