A payload fairing is the aerodynamic shell at the top of a launch vehicle that protects the payload from aerodynamic pressure, heating, and acoustic vibration during ascent through the atmosphere. Once the vehicle reaches the near-vacuum of space (typically around 100-120 km altitude), the fairing halves separate and fall away, exposing the payload for deployment.
Fairings are expensive components, often costing several million dollars each. SpaceX pioneered fairing recovery by catching them with ships equipped with large nets and later by fishing them from the ocean, enabling fairing reuse that further reduces per-launch costs. Fairing diameter is a critical constraint for payload designers, with most medium-lift vehicles offering 4-5 meter diameter fairings and heavy-lift vehicles reaching 5-7 meters.