Characterisation of the new target of the NASA Lucy mission: Asteroid 152830 Dinkinesh (1999 VD57)

Context. The NASA Lucy mission is designed to collect data that will be used to study the very interesting population of Jupiter Trojans, which are considered to be time capsules from the time of birth of our Solar System. During its journey, the mission will pass near a main belt asteroid, Donaldjohanson. Recently, NASA announced that a new asteroid in the belt will also be visited by Lucy: 152830 Dinkinesh (1999 VD57). Aims. The main goal of this work is to characterise this newly selected target, asteroid Dinkinesh, in order to provide critical information to the mission team. This information includes the most likely surface composition, albedo, and size of the asteroid, which will be used to better plan the data acquisition strategy at the time of the fly-by. Methods. To this end, we obtained visible spectra, colour photometry, and time-series photometry for Dinkinesh using several telescopes located at different observatories. For the spectra, we used the 10.4 m Gran Telescopio Canarias (GTC) on the island of La Palma (Spain); for the colour photometry, we used the 4.3 m Lowell Discovery Telescope (LDT) near Happy Jack, Arizona (USA); and for the time-series photometry, we used the 82 cm IAC80 telescope located on the island of Tenerife (Spain). We used the visible reflectance spectrum to obtain the taxonomical class of the asteroid in order to constrain its albedo value. Colour and time-series photometry were used to compute the absolute magnitude of Dinkinesh, which was used in conjunction with an albedo estimation to constrain its size. Results. Both the visible spectrum and reflectance values computed from colour photometry show that Dinkinesh is an S-type asteroid; that is, it is composed mainly of silicates and some metal. According to observations carried out as part of the NEOWISE survey, S-type asteroids have a typical geometric albedo of p(V) = 0.223 +/- 0.073. From our time-series photometry, we obtain an asteroid mean magnitude of r' = 19.99 +/- 0.05, which provides an absolute magnitude H-r' = 17.53 +/- 0.07 assuming G = 0.19 +/- 0.25 for S-types. Using our colour-photometry, we transformed H-r' to H-V = 17.48 +/- 0.05. This value of absolute magnitude combined with the geometric albedo provides a mean diameter for Dinkinesh of similar to 900 m, ranging between a minimum size of 542 m and a maximum size of 1309 m.