The orbit of Didymoon around around its primary is about 4in/s or 10cm/s. Wolfram gives its comparison to the speed of a garden snail. They are hoping to see

The orbit of Didymoon around around its primary is about 4in/s or 10cm/s. Wolfram gives its comparison to the speed of a garden snail. They are hoping to see a change in its velocity of a fraction of a mm/s. https://youtu.be/U0nZvOm3bNs?t=415 https://youtu.be/U0nZvOm3bNs?t=415 It is a small value, but its a measurable fraction of the the velocity of its orbit around the primary (about 12h). They know the momentum that the impacter had. They had an estimate of the mass of Didymoon and its velocity. They are interested in how much of that momentum was transfered to Didymoon and how much was carried away from ejecta.