Why did NASA need to observationally confirm whether DART successfully redirected Dimorphos? NASA's DART impactor made a head-on collision with the asteroid Dimorphos on September 26, 2022. A real-time video feed gave immediate confirmation of the direct hit. But according to this press release, NASA had to observe Dimorphos for two more weeks before being able to confirm that Dimorphos's trajectory was indeed noticeably altered (as planned).
Why? It seems to me that determining the collision's effect on Dimorphos's orbit would be a very simple exercise in Newtonian mechanics. I assume that Dimorphos's total mass was well-known from its orbital dynamics with Didymos. I know that its internal composition wasn't well understood, but is that really so important for understanding its post-collision dynamics? Conservation of momentum means that the subsequent overall motion of Dimorphos's center of mass should not be affected by the details of its internal composition.
I know that the collision ejected some material off of Dimorphos's surface, so there's a bit of a semantic question as to whether after the collision, the term "Dimorphos" should refer to "all of that material that made up Dimorphos before the collision" or "what's left on the largest connected component of that material after the collision". But it doesn't seem to me that this would make a big difference regarding Dimorphos's overall dynamics. It seems to me that approximating the collision as a perfectly inelastic collision between two point particles would probably give a pretty good model. Even if the impactor did knock off a significant fraction of Dimorphos's mass (which seems unlikely), then it seems to me that this outcome would count as "significantly changing its trajectory" almost by definition.
Was there ever really any genuine uncertainty whether DART would redirect Dimorphos given that DART directly impacted Dimorphos? What kind of plausible internal composition of Dimorphos could have led to a failure to be redirected?
Edit to clarify question scope: As is often the case, many people are interpreting the title of my question too literally. (My understanding is that Stack Exchange's convention is that the "official" version of an SE question is found in the question body, and the purpose of the question's title is to draw attention rather to precisely state the question.) I'm not trying to have a general philosophical debate about how much you should trust theory vs. experiment. Nor am I trying to understand why NASA actually did observationally confirm the redirection, as a lot of complicated non-physics factors enter into that decision. (So any speculation about NASA's political incentives, etc. are out of scope for this question.) I'm just asking, very concretely, what were the main sources of scientific uncertainty in the extent to which Dimorphos would be redirected given a successful collision, and how those uncertainties would affect the extent of redirection. "The composition of Dimorphos" would not be a concrete enough uncertainty; I'd like to know how the composition of Dimorphos would change the redirection. Of the many comments and answer to this question so far, only John Doty's answer addresses my question within the scope that I intended it.
 A: The spacecraft had a large amount of energy, but not a lot of momentum. Most of the impulse delivered to the target was due to the momentum of the ejecta. Energy scales as $mv^2$, but momentum scales as $mv$. For a given energy, cut the ejecta velocity in half, eject four times as much, and deliver twice the impulse. But whether the energy produces a small quantity of fast ejecta or a large quantity of slow ejecta depends the the material properties of the target. These were poorly known.
A: Biliards or snooker are games where you can perfectly calculate the mechanics, nonetheless there is still a great amount of variability.
In the case of DART the impact point might have been solid rock or rubble slipping sideways and dispersing part of the momentum or something in between. Another factor of uncertainty is the angle, it is very difficult to hit with great precision a rotating body.
A: Because sometimes things don't quite turn out as the theory expects, and the only way to know is to experiment.
Aa an example, take the main result of this experiment: the orbital period was changed by ~32min (https://www.nasa.gov/press-release/nasa-confirms-dart-mission-impact-changed-asteroid-s-motion-in-space).
If we take for correct the calculations in DART crash on Dimorphos: computation of orbital period change and we add to that an estimate of the effect due to reshaping outlined in https://iopscience.iop.org/article/10.3847/PSJ/ac7566, taking the larger estimates, we arrive at a change of 18 minutes, well short of the real world 32.
I'm no physicist and I haven't read all the details of either calculation, so there may well be some assumption made both times that could be integrated (there probably is), but I think it well shows how much value there is in actually preforming the experiment and not trusting theory blindly.
