A contact binary satellite of the asteroid (152830) Dinkinesh

Abstract

Asteroids with diameters less than about 5 km have complex histories because they are small enough for radiative torques (that is, YORP, short for the Yarkovsky-O'Keefe-Radzievskii-Paddack effect)¹ to be a notable factor in their evolution². (152830) Dinkinesh is a small asteroid orbiting the Sun near the inner edge of the main asteroid belt with a heliocentric semimajor axis of 2.19 AU; its S-type spectrum^3,4 is typical of bodies in this part of the main belt⁵. Here we report observations by the Lucy spacecraft^6,7 as it passed within 431 km of Dinkinesh. Lucy revealed Dinkinesh, which has an effective diameter of only 720 m, to be unexpectedly complex. Of particular note is the presence of a prominent longitudinal trough overlain by a substantial equatorial ridge and the discovery of the first confirmed contact binary satellite, now named (152830) Dinkinesh I Selam. Selam consists of two near-equal-sized lobes with diameters of 210 m and 230 m. It orbits Dinkinesh at a distance of 3.1 km with an orbital period of about 52.7 h and is tidally locked. The dynamical state, angular momentum and geomorphologic observations of the system lead us to infer that the ridge and trough of Dinkinesh are probably the result of mass failure resulting from spin-up by YORP followed by the partial reaccretion of the shed material. Selam probably accreted from material shed by this event.

Fig. 1. Images of Dinkinesh and Selam obtained by Lucy’s close-approach imaging campaign.

a–f, Cross-eyed stereo versions of the images taken on approach, near-close approach and on departure, respectively (see the ‘Observations’ section in Methods for a description of the imaging campaign). Dinkinesh has two main geological features: a longitudinal trough and an equatorial ridge (the yellow and rose coloured dots, respectively). The two coloured arrows (green and blue) in a point to the northern boundary of the ridge, as determined by visual inspection, at the two fiducial longitudes discussed in Fig. 3. Scale bar, 200 m. g, A simulated image of Dinkinesh with the trough removed. This is a modified version of a, in which the cyan region was moved 79 m to the lower left in the image (26° from horizontal) and rotated 7° clockwise. We take the fact that the limb profile of Dinkinesh is smooth near the colour transitions of this reconstruction to suggest that the trough is a result of a structural failure that moved the cyan region away from the remainder of the body. h–k, Stereo pairs of images of Selam taken on approach and near-close approach, respectively. l, A single image of Selam taken on departure. Selam was outside the L’LORRI field of view from 10 s to 5.5 min after close approach and so stereo imaging is not possible. The images of Selam allow us to visually estimate the dimensions of its lobes by crudely approximating their complex shapes as triaxial ellipsoids. We find that the inner and outer lobe major axes lengths in the directions parallel to the Dinkinesh vector, the orbital direction and the spin pole are roughly 240 × 200 × 200 m and 280 × 220 × 210 m, respectively. Scale bar, 200 m. m, A departure image of the entire system. Scale bar, 1 km. Also, all images are deconvolved except for m. l and m are the same image, so comparison illustrates the effects of deconvolution. Ecliptic north is approximately up in all frames, whereas the body north of Dinkinesh is down because it is a retrograde rotator. Image details are as follows. Times relative to close approach in minutes: a, −1.04; b, −1.29; c, +0.21; d, −0.04; e, +2.21; f, +1.71; h, −2.29; i, −3.29; j, −0.29; k, −0.54; l and m, +5.46. Original pixel scale, m per pixel: a, 2.53; b, 2.72; c, 2.14; d, 2.12; e, 3.63; f, 3.11; h, 3.70; i, 4.85; j, 2.16; k, 2.24; l and m, 7.56. Solar phase angle, °: a, 62.1; b, 68.0; c, 21.5; d, 30.5; e, 25.0; f, 17.8; h, 84.2; i, 93.3; j, 39.3; k, 47.7; l and m, 44.5.

Fig. 2. Phased light curves for Dinkinesh and Selam.

a, Phased light curve for Dinkinesh folded using a period of 3.7387 h. b, Phased light curve for Selam folded using a period of 52.67 h. These periods were determined from outbound photometry, as developed in the ‘Light-curve analysis’ section in Methods. The raw photometry is shown in Extended Data Fig. 1. The solid black points were used to derive the periods. The light curve of Dinkinesh is more complicated than that of Selam. Indeed, the light curve of Selam is reminiscent of what is expected for a contact binary consisting of two rotating spheroids seen edge-on and at this phase angle (60°). The hollow red points were excluded and correspond to mutual events. The arrows indicate when different types of event would be predicted. Events marked with the Lucy spacecraft symbol, , show occultations (when one object passes in front of the other from the point of view of the spacecraft) if they occur. Events marked with the sun symbol, ⊙, indicate the potential times of eclipses (at which the shadow of one object falls on the other). The observed mutual events are associated with eclipses. Occultations are not seen by Lucy during departure, which is consistent with the fact that its trajectory is slightly inclined with respect to the orbital plane of Dinkinesh. Green arrows show events that occur if Selam were in a prograde orbit about Dinkinesh, whereas orange arrows occur for a retrograde orbit. From this, we can conclude that the orbit of Selam is retrograde.

Fig. 3. The shape model of Dinkinesh.

Three orientations (insets) and topographic cuts designed to emphasize the structure of the equatorial ridge. The model, which is described in the ‘Shape’ section in Methods, consists of two regions. The side of the model that was facing Lucy during the encounter was based on the images taken during the close-approach imaging campaign (see the ‘Observations’ section in Methods). The unilluminated portion of Dinkinesh is estimated with a super-ellipsoid. The rotational (z) axis points up in these figures. The orange curve shows a latitudinal cut that lies along the ridge. The blue and green curves are longitudinal cuts corresponding to the minimum and maximum elevations of the equatorial ridge, respectively. The points labelled X1 and X2 indicate where the green and blue curves cross the orange curve, respectively. The dots show the location of the ridge’s northern boundary, as determined visually in Fig. 1a (the corresponding blue and green arrows) and the horizontal dashed lines show the extent of the ridge. The ridge at its lowest point measures 150 m wide and 40 m high, whereas it is 230 m wide and 100 m high at its highest point. The curves are solid in the locations in which the shape model is reliable, whereas they are dotted elsewhere. The reference locations labelled A1, A2, A3, B1, B2, B3, X1 and X2 are included to allow the reader to associate the shape model to the curves.

Fig. 4. A simplified graphical depiction of a plausible sequence of events leading to the current configuration of the Dinkinesh–Selam system.

a,b, Asteroids with diameters less than approximately 10 km are subject to spin-up by the YORP effect. Rapid spin of the primary and the associated centrifugal force eventually trigger a structural failure that leads to sudden mass shedding. This event might also have created the trough seen on Dinkinesh (Fig. 1a–f) through the mass movement of a portion of the body (Fig. 1g). The shed material forms a ring, with some material coalescing into a satellite(s) and closer material eventually falling back to the surface at the equator to form the ridge. The formation of the contact binary may be the result of a merger of two satellites formed either in a single mass-shedding event (a) or in two separate events (b). c, An alternative scenario is that Selam formed as a single object that subsequently underwent fission owing to spin–orbit coupling^,. It is also possible that some or all of Selam formed from a collision on the primary, but the trough and ridge would not have survived such an event. Thus this mechanism would still require later Dinkinesh spin-up by YORP and mass shedding to form the trough and superposed equatorial ridge.

Extended Data Fig. 1. Post-encounter photometry of the Dinkinesh–Selam system.

a, The observed flux (with arbitrary scale) of the system as a function of time. b, The residuals to the fit described in the ‘Light-curve analysis’ section in Methods. The solid black points are the data used in the combined light-curve fit. The hollow red points are those that were excluded from the fit owing to being affected by mutual events between the components.