: NASA is planning a series of improvements to the Space Shuttles that it considers to be "crucial" for the safety of astronauts. Money to solve these problems, however, is in short supply and the crucial improvements will take years to fund. These include:

1. Replace the hydrazine-fueled auxiliary power unit that runs the hydraulics with an electric unit, eliminating the dangerous and toxic hydrazine. This will cost $224 million, of which $39 million has been approved.

2. New sensors to monitor the main engine will reduce the chances of a catastrophic failure by 25%. This will cost $108 million of which $18 million has been approved.

3. Improved controls will reduce workload during ascent. These would cost $380 million, of which $40 million has been approved.

4. Advanced vector controls for the solid boosters would eliminate the hydrazine-fueled hydraulic pumps. This would cost $208 million of which $11 million has been approved.

5. Changes to the main engine would improve reliability. These would cost $400 million, but no money for this (or any later projects on this list) has been approved to date.

6. Improvements to the solid booster hold-down hardware would remove the chance of human error for a mere $5 million.

7. Changes to the solid rocket booster fuel would improve performance and reduce the risk to the ground crew for only $10 million.

8. Changes to the welding methods for the external tank would improve reliability and quality control for $20 million.

The shuttles have used 25% of their service lives in ten years, and could in theory keep flying until 2030 if the upgrade program continues at a steady pace.--Stephen V Cole