Pluto's mass was poorly known before 1978. what happened in that year to change this

The answer is ‘The return echo of a radar pulse, sent many years before, returned to Earth allowing a Doppler shift measurement of its rotation rate’.  This allowed the astronomers to  calculate its centripetal force hence able to derive force by using the law of universal gravitation.

Answeraquifer;

Lol why dont you actually do the work instead of have people do it for you. jk i do it too lol