An overview of NASA Lucy misson’s encounter with the main belt asteroid Dinkinesh

On November 1, 2023, <i>Lucy</i> passed within 430 km of the smallest Main Belt asteroid yet to be encountered by a spacecraft, (152830) Dinkinesh. This target was a late addition to the <i>Lucy</i> mission [1] and was intended primarly as an in-flight test of <i>Lucy</i>’s autonomous range-finding and tracking system [2]. Dinkinesh orbits the Sun near the inner edge of the Main Asteroid Belt with a heliocentric semimajor axis of 2.19 AU. Its ground-based reflectance spectrum shows a silicate band at 0.95 µm best fit as an Sq-type [3,4].

<i>Lucy</i> approached Dinkinesh at a solar phase angle of 120°; at close approach the phase dropped rapidly, going through near-zero and then increased to an out-bound phase of 60°. The relative velocity of <i>Lucy</i> and Dinkinesh was 4.5 km/s.

All of the instruments aboard <i>Lucy</i> [2] successfully observed Dinkinesh during the encounter.  Here we present the results from L'LORRI and L'Ralph.  L'LORRI, which stands for <i>Lucy</i>’s LOng Range Reconnaissance Imager, is a panchromatic (350 — 850 nm) imager with a 20.8 cm, f/13 telescope feeding a 1024 × 1024 pixel CCD focal plane [5]. L'LORRI revealed Dinkinesh, which has an effective diameter of only ~720 m, to be an unexpectedly complex system. Of particular note is the discovery of the first confirmed contact binary asteroid satellite, now named (152830) Dinkinesh I Selam.  Figure 1 shows a L’LORRI image of the Dininesh system taken about 6 minutes after close approach when the spacecraft was ~1600 km from the primary.  It clearly shows that Selam consists of two near-equal sized lobes of ∼200 m each. It orbits Dinkinesh at a distance of roughly 3km.  

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<b> Figure 1</b>
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Figure 2 shows three cross-eyed stereo images of Dinkinesh taken on approach, near close approach, and on departure, respectively. Dinkinesh has two major geological features: a longitudinal trough and an equatorial ridge (the yellow and rose colored dots, respectively). The ridge overlays the trough implying that it is the younger of the two structures. However, there is as yet no information to better constrain their relative ages, and thus they could potentially have formed in the same event.  Indeed, Dinkinesh’s ridge and trough are likely the result of mass failure and the reaccretion of material, and may both be linked to the formation of Selam.

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<b> Figure 2</b>
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The L’Ralph instrument on the <i>Lucy</i> mission consists of two focal planes sharing a common optical path: MVIC, a multi-spectral visible/NIR camera and LEISA, a shortwave infrared hyperspectral imager [6]. MVIC obtains images in 5 color bands with band-centers ranging from 430 nm to 850 nm,  while LEISA covers the spectral range from 0.9 to 3.95 mm.  Observations of the Dinkinesh system were taken near closest approach.  As a result, the phase angle was changing rapidly, and the reflectance had to be corrected accordingly.  It was also found that both MVIC and LEISA data were consistent with that expected for an S/Sq-type asteroid such as Dinkinesh (e.g. see Figure 3 for a comparison to 433 Eros[7]).  The spectra of Dinkinesh and Selam were identical to within measurement uncertainties.

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<b> Figure 3</b>
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<b>Acknowledgements:</b> The <i>Lucy</i> mission is funded through the NASA Discovery program on contract No. NNM16AA08C. The authors thank the entire <i>Lucy</i> mission team for their hard work and dedication.

<b>References:</b> [1] Levison H. et al. (2021), PSJ 2(5), 171. [2] Olkin C. et al. (2021) PSJ 2(5), 172. [3] Bolin B. (2023) Icarus 400, 115562. [4] de León J. et al. (2023) A&A, 672, A174. [5] Weaver H. et al. (2023) SSRv, 219, 82. [6] Reuter, D. et al. (2023), Space Science Rev. 219, 69. [7] Binzel, R. et al. (2004), Icarus 170, 259-294