#specific impulse

Specific impulse (Isp) is the single most important performance metric in rocket propulsion. What it means physically, how it derives from exhaust velocity, why it governs mission architecture, and how every propulsion type compares — from cold gas to ion drives.

Ion drives and Hall thrusters accelerate xenon to 30–80 km/s and run for years. How electric propulsion works, why it cannot launch from Earth, and how Dawn, Hayabusa2, and BepiColombo redefined what small spacecraft can do.