STEX's (Space Technology EXperiments) main equipment was provided by the Naval Research Laboratory. These included:

• Hall Effect electric thrusters derived from Russian technology
• Experimental solar arrays and batteries
• ATEX advanced tether experiment, a follow-on to the earlier TIPS satellite. This comprised two end masses connected by a 6 kilometer polyethylene tether. The upper end mass was to be deployed first, while the lower end mass remained attached to STEX. After a series of dynamics experiments in this configuration, the lower end mass was to be separated from STEX and the ATEX became a free-flying pair subsatellite. In the event, the ATEX was jettisoned from STEX after the tether had only deployed 21 m.

Typical orbit: 665 km x 665 km at 85 degrees inclination. Associated Launch Vehicle: Taurus.

• 1998 October 3 - STEX - Launch Site: Vandenberg. Launch Vehicle: Taurus. Perigee: 744 km (462 mi). Apogee: 759 km (472 mi). Inclination: 84.99 deg. Period: 99.77 min.

  Tested TAL-D55 plasma engine and was to be part of the NRO's ATex tether experiment. The ATeX lower end mass was meant to remain attached to the STEX parent spacecraft, but with only 21 m of tether deployed, it appeared the tether was so far off vertical that automatic safety systems jettisoned the base to protect the remainder of the STEX satellite. Thus, the upper and lower ATeX end masses were in orbit as one object connected by a 21 m tether, and designated USA 141 (1998-055C). The main STEX satellite was in orbit as a separate object, 1998-055A.