In preparation for launch later this month, ground teams have successfully completed the delicate operation of loading the James Webb Space Telescope with the propellant it will use to steer itself while in space.
In order to make critical course corrections shortly after launch, to maintain its prescribed orbit nearly 1 million miles from Earth, and to repoint the observatory and manage its momentum during operations, Webb was built with a total of 12 rocket thrusters. These rocket thrusters use either hydrazine fuel or a special mixture of hydrazine fuel and dinitrogen tetroxide oxidizer.
To safely handle these extremely toxic propellants, Webb was moved to the fueling section of the Ariane payload preparation facility at Europe’s Spaceport in French Guiana. Specialists wore Self-Contained Atmospheric Protective Ensemble, or “SCAPE,” suits while loading the observatory. The nearly 10-day procedure began November 25.
Webb’s spacecraft bus, built by Northrop Grumman, was filled with 369 pounds (168 kilograms) of hydrazine fuel and 292 pounds (133 kilograms) of dinitrogen tetroxide oxidizer. Both fuel and oxidizer will be used together to maximize power for the biggest “burns” by Webb’s Secondary Combustion Augmented Thrusters, which are for mid-course corrections and inserting into L2 orbit, as well as for orbit maintenance around L2 during the mission. However, hydrazine alone will be used for the small “burns” by Webb’s tiny precision Monopropellant Reaction Engine thrusters, which are used for large-angle repointing and for managing spacecraft momentum. The fuel loading system was formally disconnected on December 3, followed by inspections and closeouts that concluded over the weekend.
Combined operations between the Arianespace and NASA teams preparing Webb and its Ariane 5 rocket are now set to begin. The next large milestones for the joint teams will be to move Webb to the Bâtiment d’Assemblage Final (BAF), or Final Assembly Building; place it atop its rocket; and encapsulate it inside its protective fairing. With final closeouts complete, the full stack of rocket and payload atop its mobile launch platform will be rolled out of the BAF to the launch pad, two days before its scheduled December 22 launch.