633 mph – nothing to mach
Technicians from the 846th Test Squadron at Holloman Air Force Base, N.M., pump liquid helium into a test sled here Aug. 18, 2015. This sled system runs on four super-conducting magnets that need to be cooled down to a few degrees above absolute zero to ensure the smoothest ride possible. (U.S. Air Force photo by Airman 1st Class Randahl J. Jenson)