Evidence against non-gravitational acceleration of 1I/2017 U1 ‘Oumuamua

Micheli et al. (Nature 559:223, 2018) reported that a seven-parameter fit to the orbit of 1I/2017 U1 ‘Oumuamua indicated a non-gravitational acceleration in the anti-Solar direction, and attributed it to recoil from comet-like outgassing. The implied gas to dust ratio is at least 100 times greater than that of known Solar System comets. The reported collapse of the scatter of nearly contemporaneous coordinate residuals upon inclusion of the non-gravitational term in the orbital fits is difficult to understand. There are grounds for skepticism.