at maximum power. Post-test examination indicated the engine was not approaching failure. NSTAR operated for years on Dawn. The NASA Evolutionary Xenon Thruster (NEXT) project operated continuously for more than 48,000 hours. The test was conducted in a high-vacuum test chamber. Over the course of the test, which lasted more than five and a half years, the engine consumed approximately 870 kilograms of xenon propellant. The total impulse generated would require over 10,000 kilograms of conventional rocket propellant for a similar application. Hall-effect thruster life Hall-effect thrusters suffer from strong erosion of the ceramic discharge chamber by impact of energetic ions: a test reported in 2010 showed erosion of around 1 mm per hundred hours of operation, though this is inconsistent with observed on-orbit lifetimes of a few thousand hours. The Advanced Electric Propulsion System (AEPS) is expected to accumulate about 5,000 hours and the design aims to achieve a flight model that offers a half-life of at least 23,000 hours and a full life of about 50,000 hours. Propellants Ionization energy represents a large percentage of the energy needed to run ion drives. The ideal propellant is thus easy to ionize and has a high mass/ionization energy ratio. In addition, the propellant should not erode the thruster to any great degree, so as to permit long life, and should not contaminate the vehicle. Many current designs use xenon gas, as it is easy to ionize, has a reasonably high atomic number, is inert and causes low erosion. However, xenon is globally in short supply and expensive (approximately $3,000 per kg in 2021). Some older ion thruster designs used mercury propellant. However, mercury is toxic, tended to contaminate spacecraft, and was difficult to feed accurately. A modern commercial prototype may be using mercury successfully. Mercury was formally banned as a propellant in 2022 by the Minamata Convention on Mercury. From 2018-2023, krypton was used to fuel the Hall-effect thrusters aboard Starlink internet satellites, in part due to its lower cost than conventional xenon propellant. Starlink satellites have since switched to argon Hall-effect thrusters, providing higher specific impulse. Other propellants, such as bismuth and iodine, show promise both for gridless designs such as Hall-effect thrusters, and gridded ion thrusters. Iodine was used as a propellant for the first time in space, in the NPT30-I2 gridded ion thruster by ThrustMe, on board the Beihangkongshi-1 mission launched in November 2020, with an extensive