Open Access
Volume 672, April 2023
Article Number A174
Number of page(s) 4
Section Planets and planetary systems
Published online 17 April 2023

© The Authors 2023

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1 Introduction

The NASA Lucy mission launched in October 2021 will perform the first spacecraft exploration of the Jupiter Trojans. This group of objects resides in the very stable L4 and L5 Lagrangian points of the orbit of Jupiter and is estimated to be nearly as populous as the main belt but far more homogeneous, with low albedos and a colour bimodality in the visible to near-infrared wavelengths (Emery et al. 2015). According to the most recent dynamical models, such as the 'jumping-Jupiter' version of the Nice model (Walsh et al. 2011), Trojans are considered remnants from the primordial trans-Neptunian belt and are therefore believed to contain pristine information about the origins of our Solar System. For more information about the Trojans and the Lucy mission, see Levison et al. (2021) and references therein.

Lucy will accomplish its mission with a series of close targeted flybys of seven Trojans: (3548) Eurybates and its small satellite Queta, (15094) Polymele, (11351) Leucus, and (21900) Orus, all in the L4 (leading) Trojan cloud, and the Patroclus—Menoetius binary system in the L5 Trojan cloud. It will also visit one main-belt asteroid, (52246) Donaldjohanson, in the Erigone collisional family (Souza-Feliciano et al. 2020). When launched, Lucy was scheduled to get the first close-up view of a target in 2025, but recently, a new target in the main belt has been added to this list. After a series of small manoeuvres that will start in May 2023, the spacecraft will make an approach to this new target at a distance of about 450 km on November 1, 2023.

The selected target is the asteroid 152830 Dinkinesh (1997 VD57, Dinkinesh hereafter). It was discovered in 1999 by the LINEAR survey1, and recently identified as a potential fly-by target for the Lucy mission by researchers at the Nice Observatory in France. Dinkinesh is located in the inner part of the belt, having proper orbital elements of ap =2.19137 au, ep = 0.1469, and sin(ip) = 0.0215. According to the AstDyS-2 webpage2, it is considered a background asteroid; that is, it is not associated with any particular collisional family. Apart from its dynamical properties, little is known about this object except its absolute magnitude, which is H = 17.4 according to the Minor Planet Center. Therefore, the Lucy mission team contacted all the mission collaborators with access to telescope facilities and observational time in an attempt to obtain a better characterisation of this new target. In this work, we present the results from the observations performed by researchers of the Solar System Group at the Institute de Astrofísica de Canarias using the telescope facilities of the Observatorios de Canarias (OCAN) in collaboration with researchers from the Lowell Observatory in Arizona (USA), who used their Lowell Discovery Telescope. We describe the observations and the data-reduction process in Sect. 2. Results and a discussion are presented in Sect. 3 and we present our conclusions in Sect. 4.

thumbnail Fig. 1

Visible spectrum of asteroid Dinkinesh obtained with the 10.4m Gran Telescopio Canarias (GTC). Error bars correspond to the 1σ of the average of the individual spectra. Reflectance values from the colour photometry obtained with the 4.3 m Lowell Discovery Telescope (LDT) using Sloan griz filters are shown as blue squares. Coloured lines are the template spectra of the S-type (red), Sq-type (orange), and Sv-type (yellow) taxons from the DeMeo et al. (2009) taxonomy. The hatched region accounts for the ± 1σ dispersion of the spectral classes.

2 Observations and data reduction

2.1 Spectroscopy

The visible spectrum of Dinkinesh was obtained on the night of December 2, 2022, using the OSIRIS camera-spectrograph (Cepa et al. 2000; Cepa 2010) at the 10.4 m Gran Telescopio Canarias (GTC) under the program GTC32-22B. The GTC is located at the El Roque de Los Muchachos Observatory (La Palma, Canary Islands, Spain). The OSIRIS instrument is equipped with two 2k × 4k pixel detectors and a total unvignetted field of view of 7.8 × 7.8 arcmin2. We used the 1.2" slit and the R300R grism (dispersion of 7.74 Å pixel−1, resolution R = 348 for a 0.6" slit) covering the 0.48–0.92 μm wavelength range. The slit was oriented along the parallactic angle to minimise the effects of atmospheric differential refraction and the telescope tracking was set to the proper motion of the asteroid. Details on the observational circumstances are shown in Table 1. Two spectra were obtained, each of 900 s exposure time, with an offset of 10" in the slit direction in between them. To obtain the reflectance spectrum, we observed solar analogues from the Landolt catalogue, namely Landolt (1992) SA93-101, SA98-978, and SA102-1081, at a similar airmass to that of the asteroid.

Data reduction was performed using standard procedures (see de Leon et al. 2016). The images were initially bias- and flat-field corrected. Sky background was subtracted and a one-dimensional spectrum was extracted using a variable aperture, corresponding to the pixel where the intensity was 10% of the peak value. A wavelength calibration was applied using Xe+Ne+HgAr lamps. This procedure was applied to the spectra of the asteroid and the stars. We then divided the individual spectra of the asteroid by the spectra of the solar analogues, and the resulting ratios were averaged to obtain the final reflectance spectrum of Dinkinesh, which is shown in Fig. 1.

Table 1

Observational circumstances of Dinkinesh.

2.2 Time-series photometry

Time-series photometry of Dinkinesh was obtained on November 17, 2022, using the IAC80 telescope at Teide Observatory (Tenerife, Canary Islands, Spain). Table 1 provides information on the observational circumstances. The IAC80 is a 82 cm telescope with f/D = 11.3 in the Cassegrain focus configuration and is equipped with the CAMELOT-2 camera, with a back-illuminated e2v 4k × 4k pixel CCD of 15 μm2 pixels, a plate scale of 0.32 arcsec pixel−1, and a field of view of 21.98 × 22.06 arcmin2. We used a Sloan r filter and 70 s exposure time with the telescope in sidereal tracking.

Images were bias- and flat-field corrected using standard procedures. Aperture photometry was carried out using the Photometry Pipeline3 (PP) software (Mommert 2017). PP uses the Source Extractor software for source identification and aperture photometry and the SCAMP4 software for image registration. Image registration and photometric calibration are based on matching field stars with the USNO-B1 (Monet et al. 2003) and PanSTARRS catalogues, respectively. The circular aperture photometry is performed using Source Extractor; an optimum aperture radius is identified using a curve-of-growth analysis by the PP. The final calibrated photometry for each field source is written into a queryable database, and target photometric results are extracted from this database. Moving targets are identified using JPL Horizons ephemerides service (Giorgini et al. 1996). The resulting light curve is shown in Fig. 2. The observed gap between Julian Dates JD 2459900.67 and JD 2459900.70 is due to the fact that the measured asteroid magnitude was affected by a close and bright star.

thumbnail Fig. 2

Light curve of Dinkinesh obtained on November 17, 2022, with the 82 cm IAC80 telescope.

2.3 Colour photometry

Broad-band colours were obtained with the Large Monolithic Imager (LMI) at the 4.3 m Lowell Discovery Telescope (LDT) on UT 19 November 2022. The LMI is a 6k × 6k e2v CCD that images a 12.3 arcmin2 field of view. The images were obtained using the Sloan Digital Sky Survey (SDSS) griz filter set and were sampled at 0.36 arcseconds per pixel in 3 × 3 binning mode. Exposures were 60 seconds each, with the filters sequenced in an interleaved pattern: rgrirzr. Given the unknown rotational light curve of Dinkinesh, this strategy provided a means to correct for rotational variability from one exposure to the next by monitoring the magnitude of the target in r-band. This can be important for deriving colours (independent of rotation effects) when the individual filters are collected non-simultaneously. However, in this case, no significant light-curve variability was detected over a ~33 minute observing window, which is consistent with the time-series photometry presented in Sect. 2.2. A total of 4, 5, 4, and 8 exposures were taken in g, r, i, and z bands, respectively. The extra z-band exposures were taken to offset the lower signal-to-noise ratio (S/N) per exposure in that band. Observing conditions were suboptimal with poor seeing (~3") and significant cloud cover, which caused variability at the level of 0.5–1 magnitude in the zero points for each filter.

Reduction of the images followed standard bias and flat-field correction techniques. The photometry was also measured with the Photometry Pipeline, and the photometric calibration of each frame based on PanSTARRS field stars served to counteract the effects of the variable extinction. For each filter, a weighted mean magnitude was computed and the differences of these magnitudes were used to derive colours. The weighted mean magnitudes and average errors are , , , and . This yields the following colours: (gr) = 0.67 + 0.04, (ri) = 0.19 + 0.04, and (rz) = −0.05 + 0.10. These colours are transformed into flux or reflectance values using the SDSS estimate for solar magnitudes (Holmberg et al. 2006): g = 5.12, r = 4.68, i = 4.57, and z = 4.54. The resulting reflectance values are plotted in Fig. 1 as blue squares. For comparison, the reflectance values are represented with a normalisation at 0.55 μm, which is calculated based on a linear interpolation of the data between the g and r bands.

3 Results and discussion

Visible spectroscopy enabled us to perform a taxonomical classification of the asteroid. We did so using the M4AST online tool (Popescu et al. 2012), which fits a curve to the data and compares it to the taxons defined by DeMeo et al. (2009) using a χ2 fitting procedure. A list with the best three results in order of decreasing goodness of fit is provided. For the case of Dinkinesh, the best three fits are S, Sq, and Sv (see Fig. 1). The reflectance values obtained from the visible colours are in very good agreement with this classification. Therefore, we can confidently conclude that the asteroid belongs to the S-complex.

The taxonomical classification of Dinkinesh provides a statistical estimation of its albedo and consequently a more solid estimation of its size. Using measurements of the NEOWISE survey, Mainzer et al. (2011) obtained a median albedo value of pv = 0.223, with a maximum value of 0.557 and a minimum value of 0.114 for the DeMeo et al. (2009) S-complex.

The computed asteroid mean magnitude in the 6 h period of observations with the IAC80 on November 17 was . Due to the apparent visual magnitude of the asteroid at the time of observations (see Table 1) and the small aperture of the IAC80 telescope, the measured S/N was only ~10. Nevertheless, it allowed us to conclude that the asteroid brightness does not vary more than 0.1 mag in a 6 h period. On the other hand, during the observations made with the LDT on November 19, the mean magnitude of Dinkinesh was .

Assuming a G-value for the S-complex taxonomy, we can derive the asteroid's absolute magnitude using the HG phase function described in Muinonen et al. (2010). A slope of G = 0.15 is commonly assumed for asteroids, but several works show that this value can be different for different taxonomical classes. Recently, Colazo et al. (2021) used photometry of asteroids from the Gaia DR2 and the Asteroid Photometric Catalogue (Lagerkvist et al. 1995) to obtain mean phase-slopes for S-, C-, D-, and X-type objects. In their Table 3, these authors compare their results with those obtained from PanSTARRS PS1 (Vereš et al. 2015) and those from Pravec et al. (2012) using their own observations and data from Warner et al. (2009). In the case of S-type asteroids, Colazo et al. (2021) obtained a value of G = 0.19 ± 0.44, while (Vereš et al. 2015) obtained G = 0.16 + 0.26 and Pravec et al. (2012) G = 0.23 + 0.05. We computed a simple average of these three values, propagating their associated errors, obtaining G = 0.19 + 0.25. Using this value, we obtain an absolute magnitude of Hr = Π.13 + 0.07 on November 17 and Hr = 17.15 + 0.04 on November 19 (we note that both values agree well within the uncertainties). To compute Hv, we used our colour photometry and the transformation equations from Jester et al. (2005). Our final absolute magnitude for Dinkinesh is Hv = 17.48 + 0.05.

Using this value and the geometric albedo associated to S-types, pv = 0.223, we estimate a mean diameter for Dinkinesh using the standard diameter-albedo relationship . Considering the range in G values of 0.16 < G < 0.23, and the corresponding range in computed Hv, namely 17.44 + 0.05 < HV < 17.53 + 0.05, as well as the minimum and maximum values for the geometric albedo (see Fig. 3), the estimated mean diameter can vary from 542 m to 1309 m. The upper limit on the amplitude of the light curve (0.1 mag in a 6 h period) suggests that either the shape of Dinkinesh is not very elongated, that the polar axis was oriented close to the line-of-sight during the observations, or that the asteroid is a very slow rotator.

thumbnail Fig. 3

Mean estimated diameter of Dinkinesh (898 m) and values computed for the range of albedo values associated with its taxonomical class, the S-complex (black crosses), i.e., 0.114 < pv < 0.557 (Mainzer et al. 2011), the range of G values for S-types, 0.16 < G < 0.23, and the corresponding range in Hv, using those G values, Hv =17.39 to Hv = 17.58.

4 Conclusions

Here, we present visible spectra, colours, and visible time-series photometry for the main-belt asteroid 152830 Dinkinesh (1999 VD57), which was recently selected by the NASA Lucy mission as an additional target in its journey to the Jupiter Trojans. The visible spectrum of Dinkinesh and its time-series photometry were obtained with the 10.4 m Gran Telescopio Canarias and the 82cm IAC80 telescopes, respectively (Spain), while visible colour photometry was obtained with the 4.3 m Lowell Discovery Telescope (USA). Our conclusions can be summarised as follows:

  1. From its visible spectrum and colours, Dinkinesh can be classified as an S-type asteroid, that is, it is mainly composed of silicates and metal;

  2. From colour- and time-series photometry we compute an absolute magnitude of Hv = 17.48 + 0.05, which is in good agreement with the value reported by the Minor Planet Center (Hv = 17.4). Observations of Dinkinesh at different phase angles will enable a more accurate determination of its absolute magnitude, and therefore a better estimation of its mean diameter;

  3. Using the geometric albedo value associated with S-type asteroids (pv= 0.223), we obtain that Dinkinesh has a mean diameter of , with a range of values that goes from D = 542 m to D = 1309 m;

  4. The obtained upper limit on the light curve amplitude (0.1 mag in a 6 h period) is not conclusive, and only allows us to suggest several possibilities, including that the asteroid is a slow rotator.

After this paper was accepted for publication in A&A, a preprint appeared on the Earth and Planetary Astrophysics section of the arxiv by Bolin et al. (2023) showing spectra that confirm our results for Dinkinesh.


J.dL. and J.L. acknowledge support from the ACIISI, Consejeria de Economía, Conocimiento y Empleo del Gobierno de Canarias and the European Regional Development Fund (ERDF) under grant with reference ProID2021010134. NPA acknowledges support from the Center for Lunar and Asteroid Surface Science (CLASS) a NASA's SSERVI team funded in CAN3. The work of MP is financed by a grant of the Romanian National Authority for Scientific Research and Innovation CNCS - UEFISCDI, project number PN-III-P2-2.1-PED-2021-3625. Based on observations made with the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma.


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All Tables

Table 1

Observational circumstances of Dinkinesh.

All Figures

thumbnail Fig. 1

Visible spectrum of asteroid Dinkinesh obtained with the 10.4m Gran Telescopio Canarias (GTC). Error bars correspond to the 1σ of the average of the individual spectra. Reflectance values from the colour photometry obtained with the 4.3 m Lowell Discovery Telescope (LDT) using Sloan griz filters are shown as blue squares. Coloured lines are the template spectra of the S-type (red), Sq-type (orange), and Sv-type (yellow) taxons from the DeMeo et al. (2009) taxonomy. The hatched region accounts for the ± 1σ dispersion of the spectral classes.

In the text
thumbnail Fig. 2

Light curve of Dinkinesh obtained on November 17, 2022, with the 82 cm IAC80 telescope.

In the text
thumbnail Fig. 3

Mean estimated diameter of Dinkinesh (898 m) and values computed for the range of albedo values associated with its taxonomical class, the S-complex (black crosses), i.e., 0.114 < pv < 0.557 (Mainzer et al. 2011), the range of G values for S-types, 0.16 < G < 0.23, and the corresponding range in Hv, using those G values, Hv =17.39 to Hv = 17.58.

In the text

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