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Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua)


‘Oumuamua (1I/2017 U1) is the first known object of interstellar origin to have entered the Solar System on an unbound and hyperbolic trajectory with respect to the Sun1. Various physical observations collected during its visit to the Solar System showed that it has an unusually elongated shape and a tumbling rotation state1,2,3,4 and that the physical properties of its surface resemble those of cometary nuclei5,6, even though it showed no evidence of cometary activity1,5,7. The motion of all celestial bodies is governed mostly by gravity, but the trajectories of comets can also be affected by non-gravitational forces due to cometary outgassing8. Because non-gravitational accelerations are at least three to four orders of magnitude weaker than gravitational acceleration, the detection of any deviation from a purely gravity-driven trajectory requires high-quality astrometry over a long arc. As a result, non-gravitational effects have been measured on only a limited subset of the small-body population9. Here we report the detection, at 30σ significance, of non-gravitational acceleration in the motion of ‘Oumuamua. We analyse imaging data from extensive observations by ground-based and orbiting facilities. This analysis rules out systematic biases and shows that all astrometric data can be described once a non-gravitational component representing a heliocentric radial acceleration proportional to r−2 or r−1 (where r is the heliocentric distance) is included in the model. After ruling out solar-radiation pressure, drag- and friction-like forces, interaction with solar wind for a highly magnetized object, and geometric effects originating from ‘Oumuamua potentially being composed of several spatially separated bodies or having a pronounced offset between its photocentre and centre of mass, we find comet-like outgassing to be a physically viable explanation, provided that ‘Oumuamua has thermal properties similar to comets.

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Fig. 1: Deep stacked images for dust detection.
Fig. 2: Astrometric residuals of ‘Oumuamua observations.

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K.J.M., J.T.K. and J.V.K. acknowledge support through NSF awards AST1413736 and AST1617015, in addition to support for HST programmes GO/DD-15405 and -15447 provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS 5-26555. R.J.W. and R.W. acknowledge support through NASA under grant NNX14AM74G issued to support Pan-STARRS1 through the SSO Near Earth Object Observation Program. D.F., P.W.C. and A.E.P. conducted this research at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. We thank S. Sheppard for obtaining the Magellan observations, and E. J. Christensen, W. H. Ryan and M. Mommert for providing astrometric uncertainty information related to the Catalina Sky Survey, Magdalena Ridge Observatory and Discovery Channel Telescope observations of ‘Oumuamua. This work is based on observations obtained at CFHT, which is operated by the National Research Council of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France and the University of Hawai‘i . It is based in part on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 2100.C-5008(A) and in part on observations obtained under programme GS-2017B-DD-7 obtained at the Gemini Observatory, which is operated by AURA under cooperative agreement with the NSF on behalf of the Gemini partnership: NSF (United States), NRC (Canada), CONICYT (Chile), MINCYT (Argentina) and MCT (Brazil). This is work is also based on observations made with NASA/ESA HST, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS 5-26555. This work has made use of data from the ESA mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium (DPAC; Funding for DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

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Nature thanks A. Fitzsimmons, M. Granvik and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Authors and Affiliations



M.M. discovered the non-gravitational acceleration and extracted the high-precision astrometry from most ground-based observations obtained by the team. D.F. performed the different fits and modelling of the non-gravitational acceleration. K.J.M. secured the HST time and designed the observation programme, computed sublimation dust and gas outgassing limits, and provided the assessment of outgassing. M.W.B. led the design of the HST observations and contributed precision astrometry from HST images. O.R.H. obtained the deep stack of images, searched them for dust and companion, and estimated production rates. D.P. performed the thermal sublimation modelling. N.S. conducted thermal model calculations. H.A.W. managed the HST observations and the initial reduction of images. P.W.C. provided support in analysing possible explanations for the observed non-gravitational acceleration. J.T.K. assembled the deep stack of CFHT data to search for dust and outgassing. R.W. identified and searched pre-discovery images of ‘Oumuamua in Pan-STARRS1 data. R.J.W. obtained the observations using CFHT and searched for pre-discovery observations of ‘Oumuamua. H.E. contributed to the HST proposal and to the design of the HST observations. J.V.K. and K.C.C. contributed to the HST proposal. D.K. provided support in analysing possible explanations for the observed non-gravitational acceleration. A.E.P. investigated the magnetic hypothesis.

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Correspondence to Marco Micheli.

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Extended data figures and tables

Extended Data Fig. 1 Non-gravitational accelerations of Solar System comets and ‘Oumuamua.

Measured non-gravitational radial accelerations A1 for short-period (red) and long-period (blue) comets from the JPL Small Body Database ( The solid vertical black line indicates the A1 value for ‘Oumuamua, which falls within the range observed for Solar System comets; the dashed vertical black lines mark the corresponding 1σ uncertainty.

Source Data

Extended Data Table 1 Ground-based astrometry
Extended Data Table 2 HST astrometry
Extended Data Table 3 Uncertainty assumptions for existing astrometry

Source Data

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Micheli, M., Farnocchia, D., Meech, K.J. et al. Non-gravitational acceleration in the trajectory of 1I/2017 U1 (‘Oumuamua). Nature 559, 223–226 (2018).

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