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Acceleration of 1I/‘Oumuamua from radiolytically produced H2 in H2O ice


In 2017, 1I/‘Oumuamua was identified as the first known interstellar object in the Solar System1. Although typical cometary activity tracers were not detected2,3,4,5,6, ‘Oumuamua showed a notable non-gravitational acceleration7. So far, there has been no explanation that can reconcile these constraints8. Owing to energetic considerations, outgassing of hyper-volatile molecules is favoured over heavier volatiles such as H2O and CO2 (ref. 9). However, there are theoretical and/or observational inconsistencies10 with existing models invoking the sublimation of pure H2 (ref. 9), N2 (ref. 11) and CO (ref. 12). Non-outgassing explanations require fine-tuned formation mechanisms and/or unrealistic progenitor production rates7,13,14,15. Here we report that the acceleration of ‘Oumuamua is due to the release of entrapped molecular hydrogen that formed through energetic processing of an H2O-rich icy body. In this model, ‘Oumuamua began as an icy planetesimal that was irradiated at low temperatures by cosmic rays during its interstellar journey, and experienced warming during its passage through the Solar System. This explanation is supported by a large body of experimental work showing that H2 is efficiently and generically produced from H2O ice processing, and that the entrapped H2 is released over a broad range of temperatures during annealing of the amorphous water matrix16,17,18,19,20,21,22. We show that this mechanism can explain many of ‘Oumuamua’s peculiar properties without fine-tuning. This provides further support3 that ‘Oumuamua originated as a planetesimal relic broadly similar to Solar System comets.

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Fig. 1: The model feasibility for a range of possible parameter space.
Fig. 2: Thermal model of ‘Oumuamua during the observational arc in which non-gravitational acceleration is required.
Fig. 3: Thermal model of ‘Oumuamua over the course of the entire trajectory interior to the orbit of Neptune.

Data availability

The datasets generated and analysed during the current study are available on GitHub at

Code availability

All code used for this manuscript is available on GitHub at


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We thank D. Jewitt, M. Brown, D. Farnocchia, K. Meech, D. Lai, A. Morbidelli and L. Kelley for useful conversations and suggestions. We thank the scientific editor, L. Sage, for insightful comments and constructive suggestions that greatly strengthened the scientific content of this paper. D.Z.S. acknowledges financial support from the National Science Foundation grant no. AST-17152, NASA grant no. 80NSSC19K0444 and NASA contract no. NNX17AL71A from the NASA Goddard Spaceflight Center. J.B.B. acknowledges support from NASA through the NASA Hubble Fellowship grant no. HST-HF2-51429.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract no. NAS5-26555.

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J.B.B. performed the H2 budget modelling and led the writing of the manuscript. D.Z.S. performed the thermal modelling and contributed to the manuscript text.

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Correspondence to Jennifer B. Bergner or Darryl Z. Seligman.

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Nature thanks Alan Fitzsimmons and Marco Micheli for their contribution to the peer review of this work. Peer reviewer reports are available.

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Bergner, J.B., Seligman, D.Z. Acceleration of 1I/‘Oumuamua from radiolytically produced H2 in H2O ice. Nature 615, 610–613 (2023).

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