Aiming at Comet Tempel 1
The Deep Impact flyby and impactor spacecraft arrived at comet Tempel 1 in July to make a crater in the surface of its nucleus and study the results. During the last minutes before impact, the team had instructed the two spacecraft to look at the same location on the nucleus - the targeted impact point. Both spacecraft were programmed to accomplish this. First, both were to look for the brightest spot on the nucleus and then offset a prescribed amount to identify a location for impact. After the collision, the team wanted the flyby spacecraft to continue observing the impact site until the comet flew overhead 13 minutes later.
During the actual encounter, the flyby and impactor spacecraft were affected by different circumstances and each had their own reactions. The returning images showed that both spacecraft veered from pointing at the targeted site, which resulted in images of different locations.
After the encounter, the team looked very carefully at all the images from both the impactor and flyby spacecraft and found several surprises. The pictures from the impactor showed that just before the collision, something had caused that spacecraft to repeatedly tilt to the side resulting in a view away from the predicted impact site. The pictures taken by the flyby spacecraft showed that its camera stayed locked on the predicted impact site up to the impact, but after the impact, that spacecraft tilted off a little to a new view and stayed there until the nucleus flew overhead.
Steve Collins, an "Attitude Control" engineer for the Deep Impact mission can tell us what happened to both spacecraft but can you figure it out on your own? Look at the questions and clues.
Question #1: Why did the impactor's camera view keep tilting away from the predicted point of impact?
Clues:
- The closer you get to a comet nucleus, the more particles of ice and dust you encounter being cast off its surface.
- The software on the impactor is continually giving that spacecraft information on the predicted point of impact.
- Certain pieces of hardware on the impactor have the job of assisting with the aiming of the impactor so its cameras can see the point of impact. Which parts would be important in this situation?
Question #2: Why did the camera on the observing flyby spacecraft point at the predicted place of impact up until the collision and then suddenly off to a new view?
Clues:
- Remember what the team had programmed both spacecraft to do to find the point of impact by looking for center of brightness and think about what difference this could have made at the point of impact?
- There was much more material thrown from the nucleus (ejecta) during the collision than the team expected and the reflection from the Sun made it very bright.
Do you think you know the answer to one or both of the questions? Find out if you are right.
Answers:
Question #1: Why did the impactor's camera view keep tilting away from the predicted point of impact?
In the last few moments before hitting Tempel 1, the Impactor spacecraft was pelted by particles of ice and dust drifting away from the comet nucleus. These are the same particles that are left behind to make up the comet's tail sometimes seen from Earth. Most of these particles were very small, the size of baby powder or smoke but a few were bigger, the size of sand grains. Moving at over 10 km/sec, the largest of the particles are big enough to bump the spacecraft and tilt the camera away from the impact site when they hit the spacecraft. Take a look at the movie of the final impactor images. See if you can pick out some of the places where the impactor is hit by dust particles. The impactor has a built-in software "control system" that signals hardware on the spacecraft called thrusters. These thrusters operate to "re-aim" the spacecraft so that the camera is pointed at the predicted impact site. Impactor telemetry showed 4 "bumps". At each bump, the camera suddenly began to tilt until the control system signaled the thrusters, which brought the camera back where it was supposed to point.
From the size of the bumps it's possible to estimate out how massive the particles must have been. The science team expected to see some dust impacts. It was even possible that an unlucky dust particle hit might destroy the impactor camera before the impact, but it survived to the end, returning its final picture just a few seconds from impact. Amazing!
Question #2: Why did the camera on the observing flyby spacecraft point at the predicted place of impact up until the collision and then suddenly veer off to a new view?
The navigation software on the flyby spacecraft (AUTONAV) takes pictures of the approaching nucleus and locks onto the brightest point it can see. It uses this "center of brightness" to predict where the impactor will hit the comet and then orders the spacecraft control system to point the science cameras at the predicted impact site. This programming was also like that of the impactor which it was hoped would keep them both looking at the same location on the surface of the nucleus - the planned point of impact.
When the impact occurred, the scientists were surprised (and excited because scientists love surprises!) to see that the impact threw up a large plume of very fine particles. These particles formed a bright, long-lasting plume that hung over the impact site for several hours.
Since this plume was brighter than the surface of the nucleus, the navigation software locked onto it instead of the impact point. This caused a small change in the direction the camera was pointed for pictures taken after impact. The change didn't affect the science very much, but was big enough for the engineers who wrote the software to notice and want to question. The change in the camera pointing caused by the bright plume is small and difficult to see, but if you look at the impact movie made from the flyby pictures, you can see how bright the ejecta plume becomes compared to the surface of the comet nucleus.
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