None of the approximately 750,000 known asteroids and comets in the Solar System is thought to have originated outside it, despite models of the formation of planetary systems suggesting that orbital migration of giant planets ejects a large fraction of the original planetesimals into interstellar space. The high predicted number density of icy interstellar objects (2.4 × 10 per cubic astronomical unit) suggests that some should have been detected, yet hitherto none has been seen. Many decades of asteroid and comet characterization have yielded formation models that explain the mass distribution, chemical abundances and planetary configuration of the Solar System today, but there has been no way of telling whether the Solar System is typical of planetary systems. Here we report observations and analysis of the object 1I/2017 U1 ('Oumuamua) that demonstrate its extrasolar trajectory, and that thus enable comparisons to be made between material from another planetary system and from our own. Our observations during the brief visit by the object to the inner Solar System reveal it to be asteroidal, with no hint of cometary activity despite an approach within 0.25 astronomical units of the Sun. Spectroscopic measurements show that the surface of the object is spectrally red, consistent with comets or organic-rich asteroids that reside within the Solar System. Light-curve observations indicate that the object has an extremely oblong shape, with a length about ten times its width, and a mean radius of about 102 metres assuming an albedo of 0.04. No known objects in the Solar System have such extreme dimensions. The presence of 'Oumuamua in the Solar System suggests that previous estimates of the number density of interstellar objects, based on the assumption that all such objects were cometary, were pessimistically low. Planned upgrades to contemporary asteroid survey instruments and improved data processing techniques are likely to result in the detection of more interstellar objects in the coming years.

'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 Sun. Various physical observations collected during its visit to the Solar System showed that it has an unusually elongated shape and a tumbling rotation state and that the physical properties of its surface resemble those of cometary nuclei, even though it showed no evidence of cometary activity. 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 outgassing. 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 population. 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 or r (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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Context. Ragozzine & Brown presented a list of candidate members of the first collisional family to be found among the transNeptunian objects (TNOs), the one associated with (136108) Haumea (2003 EL 61 ). Aims. We aim to identify which of the candidate members of the Haumea collisional family are true members, by searching for water ice on their surfaces. We also attempt to test the theory that the family members are made of almost pure water ice by using optical light-curves to constrain their densities. Methods. We use optical and near-infrared photometry to identify water ice, in particular using the (J − H S ) colour as a sensitive measure of the absorption feature at 1.6 μm. We use the CH 4 filter of the new Hawk-I instrument at the VLT as a short H-band (H S ) for this as it is more sensitive to the water ice feature than the usual H filter. Results. We report colours for 22 candidate family members, including NIR colours for 15. We confirm that 2003 SQ 317 and 2005 CB 79 are family members, bringing the total number of confirmed family members to 10. We reject 8 candidates as having no water ice absorption based on our Hawk-I measurements, and 5 more based on their optical colours. The combination of the large proportion of rejected candidates and time lost to weather prevent us from putting strong constraints on the density of the family members based on the light-curves obtained so far; we can still say that none of the family members (except Haumea) require a large density to explain their light-curve.

We show that 'Oumuamua's excited spin could be in a high energy LAM state, which implies that its shape could be far from the highly elongated shape found in previous studies. CLEAN and ANOVA algorithms are used to analyze 'Oumuamua's lightcurve using 818 observations over 29.3 days. Two fundamental periodicities are found at frequencies (2.77±0.11) and (6.42±0.18) cycles/day, corresponding to (8.67±0.34) h and (3.74±0.11) h, respectively. The phased data show that the lightcurve does not repeat in a simple manner, but approximately shows a double minimum at 2.77 cycles/day and a single minimum at 6.42 cycles/day. This is characteristic of an excited spin state. 'Oumuamua could be spinning in either the long (LAM) or short (SAM) axis mode. For both, the long axis precesses around the total angular momentum vector with an average period of (8.67±0.34) h. For the three LAMs we have found, the possible rotation periods around the long axis are 6.58, 13.15, or 54.48 h, with 54.48 h being the most likely. 'Oumuamua may also be nutating with respective periods of half of these values. We have also found two possible SAM states where 'Oumuamua oscillates around the long axis with possible periods at 13.15 and 54.48 h, the latter as the most likely. In this case any nutation will occur with the same periods. Determination of the spin state, the amplitude of the nutation, the direction of the TAMV, and the average total spin period may be possible with a direct model fit to the lightcurve. We find that 'Oumuamua is "cigar-shaped", if close to its lowest rotational energy, and an extremely oblate spheroid if close to its highest energy state for its total angular momentum.