"lookback" Options

[MiniTES]

So, now that the shouting and the explosions are over, let's look back and examine:
What exactly have we learned about comets or cometary interiors that we did not know before Deep Impact, other than at least one of them is mantled in thick dust?

And just why did Deep Impact have its event on July 4? It sounds to me like something that Dan the man Goldin would have directed the mission to be built around, just for better PR.

[lyford]

Points for style?
If that was the only reason then I would agree with you, but if it didn't affect the science and the orbital trajectories work out fine, then why not?
(I have no idea if it did or not... anybody else?)

[mike]

The July 4th date does seem a bit PR-y, but regardless nobody had ever slammed something into a comet before Deep Impact. Maybe the comet's debris cloud will yet reveal something or other.. and even if it doesn't I'm sure we learned something or other.

[Guest_BruceMoomaw_*]

Let's not get too cocky in our criticism -- this mission was very far from being a dead loss scientifically. We may yet have ended up getting some good spectral data on the composition both of the comet's natural coma and of the internal ices vaporized by the impact, as well as flash spectra of the dust's elemental composition. And we definitely got unprecedentedly high-resolution photos of a comet's surface -- which would hve been useful in any case to compare to Borrelly and Wild 2.

But this is separate from the question of whether there were better alternate Discovery missions to fly. (By the way, A'Hearn has yet to respond to my questions about this mission -- but then, he has other things to do right now, and probably for quite some time.)

[Comga]

So, now that the shouting and the explosions are over, let's look back and examine:
What exactly have we learned about comets or cometary interiors that we did not know before Deep Impact, other than at least one of them is mantled in thick dust?

And just why did Deep Impact have its event on July 4? It sounds to me like something that Dan the man Goldin would have directed the mission to be built around, just for better PR.

A great deal has been learned about the structure and strength of the nucleus, including that it is bound by gravity, not mechancial strength, and that the dust is as fine as talcum, not granular like sand. The temperature map will show the thermal inertia of the outer layer and indicate the porosity and density of the dust. There are gigabytes of spectral data with information on chemistry. There are structures on the surface, including that large featureless band, that will indicate much about how comets evolve and are modifed. These things were pretty much unknown before Deep Impact.

July 4 is close to, if not at, the date when the comet crossed the ecliptic. Missions out of the ecliptic take much more rocket power, or conversely, carry less mass for a given rocket. The science requirements were specifically written so that the actual impact date was not constrained to July 4. The exact instant of impact was adjusted to coordiate observations by ground and space based telescopes including Hubble. It was a little puzzling, though, why it was not three or six hours earlier when it would have been higher in the sky for the giant telescopes in South America. There must have been a reason.

[edstrick]

The hour of day of the encounter was timed for optimum visibility from two deep space network stations, to prevent non-zero probability simultaneous loss of a DSN station and loss of the flyby spacecraft due to a dust clump impact from causing loss-of-mission.

Regarding why July 4, it's primarily because of the requirement that the intercept be essentially in the ecliptic. I have *NEVER* seen a statement in the general PR and briefing materials of the exact range of arrival date/times that was in fact available to the mission once they had a nominal launch. It may have been July 3 to July 5 but not much more, and maybe only part of those days, especially Jul 3.

[Bob Shaw]

For myself, I'd welcome a US decision to stage space science spectaculars *every* 4th of July. And on Washington's Birthday, Labor Day, Star Wars Day...

...it'd be tough, I know, but I'd *try* to cope!