Students watch an orbit shorten
Nasa estimated that the impact would shorten the orbit by about 10 minutes. When astronomers started observing the asteroid several hours after the impact, they found that the orbit had indeed shortened—in fact, Nasa reports a reduction of about 32 minutes, to about 11 hours and 23 minutes. As you might expect, that number is not precise—these rocks are 11 million km away, after all.
The “margin of uncertainty", though, is only about two minutes, which itself gives you an idea of how powerful the telescopes used to observe Didymos are. Also, it’s worth noting that this change in the orbital period is due to two things: first, the collision itself; second, the debris that the collision launched into space. As the debris flies off Dimorphos, it pushes the asteroid in the opposite direction.
Admittedly, it’s not as if this debris is streaming off Dimorphos in one direction. More likely, it was ejected all over the place, so its overall effect on Dimorphos is hard to pin down. Still, hold on to that thought about this “ejecta".
Among those watching Dimorphos at the time were some folks at the Thacher School. For 10 nights over several weeks, starting before the collision, Swift and some of his motivated students aimed the Thacher telescope at Dimorphos. Note that this is hardly just a matter of looking through the telescope at distant points of light, as I would do with my telescope.
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