A reader has pointed out that JPL has changed their online press kit for Deep Impact, which was my primary source material for the encounter timeline. I don't know if these changes reflect actual changes to the mission timeline, or just errors in the previously posted version of the press kit. There's a new timeline posted now. The main changes between the June 9 and June 28 versions of JPL's press kit:
- The times of the release of the impactor and the last spacecraft Trajectory Correction Maneuver have been shifted later, by 15 minutes.
- Post-release telemetry from the impactor, and also the flyby spacecraft deflection maneuver, have been shifted later, by 8 minutes.
- They no longer note the times at which the first images will be acquired by the flyby craft or the impactor after the impactor is released.
- The length of time that the spacecraft stays in Shield Mode (during its closest approach to Tempel 1) has been increased by 15 minutes.
The last change is really significant, because it means that after flying by Tempel 1, Deep Impact will have traveled much farther past the comet before it is allowed to turn back and capture departing shots. When you are a spacecraft, more distance translates directly to worse resolving power, and lower detail in your images.
For that reason, scientists on a mission are always arguing with engineers. The scientists want to get really close so they can get the best pictures. Engineers, on the other hand, have as their primary goal the safety of the spacecraft, so they are more cautious. Of course, scientists want to protect the spacecraft too, and engineers want to see good science results, so it's not that black-and-white. But that basic tension between scientists and engineers exists on every mission. If this is a real change and not just an error correction, it looks like there may have been a recent argument between safety and science, with increased concerns about safety carrying the day.