WASHINGTON -- Between now and 2005, NASA plans to spend nearly $1.6 billion equipping the space shuttle fleet with safety upgrades that affect every major aspect of the shuttle system. Here, according to the General Accounting Office, are the costs and purpose of the various improvements:

• Electric auxiliary power unit: Eliminates the use of hazardous fuel and high-speed equipment to generate power for the orbiter's hydraulic system. Cost: $224 million.
• Main engine advanced health monitoring: Provides real-time data to the astronauts on engine performance. Cost: $108 million.
• Orbiter avionics: Cuts astronauts' workload in space and improves their situational awareness. Cost: $380 million.
• Main engine: Improves engine operating environment, boosting engine reliability. Cost: $400 million.
• Solid-fuel rocket booster auxiliary power unit: Eliminates the use of hazardous fuel in the booster unit that controls thrust direction. Cost: $208 million.
• Solid-fuel rocket booster hold-down hardware: Cuts potential for human error and improves booster reliability. Cost: $5 million.
• Solid-fuel rocket motor propellant redesign: Improves performance of solid fuel and boosts safety of ground crews. Cost: $10 million.
• External tank friction stir welding: Improves reliability of the manufacturing quality control process. Cost: $20 million.
• Implementation and additional studies: Allows upgrades to be installed, tested and maintained on the shuttle fleet. Cost: $233 million.

NASA also wants to spend an additional $630 million on supportability upgrades to improve the checkout and launch control systems at the Kennedy Space Center. That would bring the total five-year bill for upgrades to about $2.2 billion.