Rendezvous with `Oumuamua

Astronomical Science DOI: 10.18727/0722-6691/5092 Rendezvous with Òumuamua Olivier R. Hainaut 1 through interstellar space, some of them Canada-France-Hawaii Telescope (CFHT), Karen J. Meech 2 eventually crossing the Solar System. respectively, pinned it down to a hyper- Marco Micheli 3, 4 bolic orbit with an eccentricity of 1.188 ± Michael S. J. Belton 5 These interstellar objects (ISOs) are icy 0.016 (Figure 2). The IAU’s Minor Planet planetesimals that are expected to Center (MPC) registered the discovery and the 1I/2017 U1 team a behave like the long-period comets of under the cometary designation C/2017 the Solar System; volatile ices sublimate U1 (Williams, 2017). when the ISO approaches the Sun, 1 ESO developing a coma and a dust tail — We immediately started a follow-up 2 Institute for Astronomy, University of features that should make them bright campaign via a series of Director’s Discre- Hawai‘i, Honolulu, USA and therefore easy to spot. The rocky tionary Time proposals — using the Very 3 ESA SSA-NEO Coordination Centre, ISOs, on the other hand, only reflect Large Telescope (VLT), Gemini South, Frascati, Italy sunlight. As their albedo is expected to the United Kingdom Infra-Red Telescope 4 INAF–Osservatorio Astronomico di be extremely low they become dark (after (UKIRT), the NASA/ESA Hubble Space Roma, Monte Porzio Catone, Italy eons of bombardment by high-energy Telescope (HST) — and additional time 5 Belton Space Exploration Initiatives, cosmic rays), they would be extremely on the CFHT. Whilst we were reminded of Tucson, USA faint and hard to detect. Overall, because the Rama spacecraft from the novel by of the overwhelming majority of icy over Arthur C. Clarke (1973), our first surprise rocky objects, and thanks to the brighter came from a deep stacked image, con- On 19 October 2017 the Panoramic aspect of the icy ones, the community firming what the discovery images indi- Survey Telescope And Rapid Response expected that the first ISO signature cated; the object does not display any System (Pan-STARRS) discovered a would be a comet discovered on a hyper- cometary activity (Figure 1b). The deep rapidly moving object near the Earth. bolic orbit. The hyperbolic orbit would limiting magnitude reached corresponds In itself this was nothing unusual but indicate an object not bound to the Sun. to an extremely low dust production limit, over the course of a few days astrono- With various ongoing all-sky surveys amounting to less than 1 kg of micron- mers realised that this was the first hunting for transient and moving objects sized dust within 750 km of the ISO. The detection of an unbound object travel- (for example, the Catalina Sky Survey, object is therefore similar to asteroids, ling through the Solar System. At the Pan-STARRS, the ESO Public Survey prompting the MPC to swiftly change its time of its discovery, the interstellar ATLAS, the All-Sky Automated Survey designation to A/2017 U1. However, as visitor, 1I/2017 U1 (Òumuamua), was for Super Novae [ASAS-SN]), the time its origin from outside the Solar System quite faint and already speeding away. was ripe for such a discovery. had now been established without a In the ensuing days, thanks to the doubt, the object received its final desig- efforts of about 10 separate teams, over nation 1I/2017 U1, and a Hawaiian name 100 hours on 2.5- to 10-metre tele- The discovery in honour of the place of its first discov- scopes were devoted to observing the ery, Òumuamua. The number 1I reflects object during the short, exhilarating We were nevertheless caught by surprise that this is the first interstellar object to be and frantic period over which it was when object P10Ee5V was discovered identified, and the Hawaiian name means visible. This is an account of our obser- by Pan-STARRS1 (PS1) at Mauna Kea on scout or a messenger from our distant vations and how they have contributed 19 October 2017 (Figure 1a). Immediate past reaching out to us from far away. We to the current view that 1I/2017 U1 is follow-up observations from the ESA refer to the object as Òumuamua for the an elongated object in an excited rota- Optical Ground Station discarded the rest of this article. tion state with surface colours similar data because of the “unrealistically large to those of Solar System comets and eccentricity of the orbit”. Fortunately, The measured colours from the object’s asteroids. pre-discovery images from PS1 and follow- surface indicate a linear reflection spec- up observations on 20 and 22 October trum with a fairly red slope (Figure 3), from the Catalina Sky Survey and the which is typical of cometary nuclei, D-type Models of the early Solar System suggest that the migration of forming giant planets a b ejected a large fraction of planetesimals 10 arcseconds Figure 1. (a) The discovery image of into interstellar space. Most of these Òumuamua on 19 Octo- planetesimals are expected to be icy — ber 2017 with Pan- i.e., comet-like — with only a small fraction STARRS. The object is of them being rocky objects. Depending the faint trail in the centre of the circle. (b) A on the model, the ice to rock planetesimal deep image, combining ratio varies between 400:1 and 10 000:1 VLT and Gemini South (Meech et al., 2016). Assuming similar g- and r-band data, processes have been taking place else- illustrating the object’s asteroidal appearance. where in the Galaxy, a large number of Reproduced from planetesimals should be wandering Meech et al. (2017). The Messenger 173 – September 2018 13 Astronomical Science Hainaut O. R. et al., Rendezvous with Òumuamua C-type asteroid Orbit of Jupiter S-type asteroid 1.6 D-type asteroid Comet 67P Mars ty 1.4 Comet 9P ivi Comet 103P Venus flect Orbit A/2017 U1 Mercury Sun 1.2 Earth d re of 1I 1I 1.0 /` Oumuamua Normalise 0.8 Orbit of a comet within Solar System 0.6 Neptune 0.40.5 0.6 0.70.8 0.91.0 1.1 Brooks Bays/SOEST Publication Services/UH Institute for Astronomy for Services/UH Institute Publication Bays/SOEST Brooks Wavelength (μm) Saturn Uranus Sun Jupiter 1I Figure 3. The reflectivity of the surface of Òumuamua is consistent with D-type asteroids and comets (Meech et al., 2017). Figure 2. The orbit of Òumuamua, showing that it but the unknown geometry of the direc- tem (1000 to 3000 kg m – 3), its rotational entered the Solar System from above the plane of tion of the rotation axis makes the implied periods indicate that it must have some the planets, passing inside Mercury’s orbit at peri- helion on 9 September 2017 and making its closest elongation a lower limit. Overall, the large- modest, but non-zero, cohesive strength approach to Earth on 14 October 2017. Òumuamua to-small axis ratio of the body is ~ 10:1. — otherwise the centrifugal forces would passed beyond the distance of Jupiter at 5.2 au in tear it apart. A pancake-shaped object early May; while moving fast, it will still be inside the The photometric light curve does not con- could be held together by gravity only for Solar System for the duration of our lifetimes. It will – 3 reach the outer edge of the Kuiper belt by the end strain the third dimension of Òumuamua. densities above 1500 kg m . of 2025, cross over the heliopause sometime in For the rotation to be stable, the third axis the late 2030s and won’t even reach the innermost should be small — similar to the short edge of the Oort cloud at 1000 au until nearly 2200. axis — which has given rise to the highly Òumuamua: final glimpses and a last popular cigar-shaped artist’s impression. surprise asteroids from the outer asteroid belt, and However, collecting more data and com- some trans-Neptunian objects. In other bining ours with those of other teams, it We continued to monitor Òumuamua as words, Òumuamua’s surface seems simi- became apparent that the light curve is it receded in order to improve the deter- lar to that of objects in the outer Solar not periodic (Figure 4), and that the object mination of its orbit, with the aim of System. This may suggest an organic-rich is in an excited state of complex rotation. extrapolating it back to its origin. The last surface (as seen on comets) or a surface Studying the rotation in detail, we found datapoint was acquired with the HST on with iron-rich minerals. Another team a spin state with two fundamental peri- 2 January 2018, when the object was very observed the very faint Òumuamua with ods at 8.67 ± 0.34 hours and 3.74 ± 0.11 close to its detection limit. The position the wide-band ultraviolet-infrared spectro- hours. The object could be spinning in of Òumuamua was carefully measured graph, X-shooter, on the VLT (Fitzsimmons a short-axis mode — where the short on over 200 ground- and space-based et al., 2018). This attempt resulted in a principal axis of the object circulates images, including the recently published noisy spectrum that was in agreement around the total angular momentum vec- Gaia Data Release 2 (DR2) catalogue. with the reddish photometric colours, and tor (TAMV) — or in a long-axis mode — This brought another surprise; the object the lack of emission lines set independent where the long axis circulates around the was not following a purely gravitational limits on the cometary activity.

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