The on-sky evolution of Echidna’s orbit from 2006 to 2025 in our non-Keplerian (left) and Keplerian (right) orbit fits. Colored lines show the on-sky projection of the orbit during different epochs. Open circles show the predicted positions of Echidna against the measured astrometry. The non-Keplerian orbit fit shows good agreement with the observations, while the Keplerian orbit fit has substantial residuals—especially for the 2013–2025 observations.