NEOVST: A mini 4-SDSS-colors Survey of newly-discovered Near-Earth Objects through the VLT Survey Telescope

Introduction: Near-Earth Objects (NEOs) provide a window into the smallest population of asteroids and comets that originated from more distant regions of the Solar System. Most of them are product of collisional processes at various stages of the Solar System's evolution, thus important for probing compositions that may have contributed to the delivery of water and organics to Earth and the inner planets.

As NEO discoveries increase—currently at about 3,000 objects per year—the majority are not followed up with physical characterization. These properties would offer insights into their internal structure and material strength in the event of atmospheric entry [1,2], not to mention their potential as targets for future mining, exploration, and planetary defense missions [3]. Therefore, anticipating a rise in new discoveries following the start of LSST operations, we initiated a year-long, monthly photometric observation campaign of NEOs discovered in the past two years (2022–2023) and visible with the VLT Survey Telescope (VST) at Cerro Paranal. The main instrument on the VST is OmegaCAM, a multi-filter, 32-CCD camera with a Field of View (FoV) of 1 square degree [4]. This capability enables the tracking of very fast-moving NEOs and also brings important collateral science: many other small bodies can be observed simultaneously. These secondary targets are identified and analyzed to enhance their characterization and astrometric positions.

Observations: The observations were carried between February to September 2023, two monthly nights. The targets were observed cycling through SDSS r’i’z’g’r’ filters in exposure times of 10 to 30s depending on the observational conditions. Every target was observed for 1 hour. In total, we acquired 5519 image cubes covering 72 NEAs, and possibly 5 to 10 times this number in SSO in the background depending on the pointing region. The magnitude limit is estimated to r’=21.5 for SNR=3 and exposure time of 20s. The cubes are radiometrically calibrated in the Astro-Wise environment through the VST pipeline available by INAF-OAC [5]. The final products are single Mosaics in absolute Jansky flux and astrometrics. We are also provided with a list of sources measured with SExtractor. See Fig. 1 for an example of final product, where the main target and secondary targets are visible.

Methodology: The analysis is carried through procedures developed in Python3 language and the use of Scikit-image, Astropy, Photutils, and Astroquery libraries, and ordered in three steps:

1. Querying SSOs and Fixed Sources in the Field. We catalog all fixed and moving sources over mag=23 for identification of the main and secondary targets, and obtaining the photometry of field stars for flux quality check. We also operate a check on the celestial coordinate and correct any miss-offsets present. The moving object ephemeris is obtained from OBSPM/IMCEE  service and the fixed sources from several catalogs bundled under the Astropy/VO service.

2. Target Tracking, Cube alignment, stacking. We crop all loci of 0.04°x0.04° from the field that might show potential SSOs, and then align it according to Sky-Fixed and SSO-fixed coordinates. Then fixed sources are removed, we applied Hough Transform [7] to detect linear tracks related to SSO apparent motion. Then, the cube is centered onto the target, and then a stacked cube product is provided as well.

3. Aperture, Lorenz and PSF Photometry. Photometry for the target and field stars are provided through aperture, Lorenz fitting, and 2D PSF photometry analysis. We also check for signs of fast rotation throughout the image sequences. We then compare the field star photometry with SDSS and VST/Atlas sources to cross-check the flux quality [8,9].

Preliminary Results and Future Steps: To date, data reduction has been completed for the first 4 observing months and for all 30 primary targets and secondary objects under exposure time ≥ 20s (the example of a section of a light-curve in Fig. 2). The color analysis is concluded for the March 2023 run, the results (Fig. 3) show that awhile the asteroids cluster around the feature and featureless loci, we have samples of redder and bluer asteroids, generally associated to organic or carbon-rich bulk compositions.

In September 2024, new NEOVST observations have been scheduled between February 2025 and September 2025. We are now focusing on scattered monthly observations of newly discovered NEOs, with exposure times of 30s (r’) to 50s (z’), and two sequences of 1 hrs each target. We now aim to improve the signal of the background stars for better astrometric positioning, and better sampling for the phase and rotational curves by compiling data from two separated nights.

        

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Fig.1: (a) Full mosaic of 32 CCDs for the acquisition of 2023-02-04 UT 05:15:09 in SDSS r’ for the main target NEA 2022 YM5. (b) The main target (magenta) and secondary targets (yellow) in the FOV.

 

Fig.2: A sequence of AB magnitudes obtained for the asteroid 2001 EV2 for the run of March 2023. The error bars are within the symbol size.

Fig.3: SDSS color mosaic of the asteroids observed in the March 2023 run (7NEAs+15MBAs). The data is overlay by the approx. loci of “S-like” and “C-like” asteroids. Many objects lack the SDSS z’ magnitude due to the weaker band efficiency.

References: [1]Perna,D.,Barucci,M.A.,&Fulchignoni,M. 2013, A&ARv,21,65. [2]Perna,D.,Dotto,E.,Ieva,S., et al. 2016, AJ 151,11. [3]Sanchez,J.-P., & McInnes,C.R. 2013, ed. V.Badescu, 439 [4]K.Konrad,R.Bender,E.Cappellaro, et al. 2004, GIA 5492, 484–494. [5]McFarland,J.P.,Verdoes-Kleijn,G., Sikkema,G. et al. 2013, ExpAstron 35, 45–78. [6]Bertin,E.&Arnouts,S. 1996, A&ASupp 317, 393. [7]P.V.C.Hough. 1959, Proc.Int.Conf. High Energy Accelerators and Instrumentation. [8] Gunn,J. E.; Siegmund,W.A.; Mannery,E.J., et al. 2006, ApJ 131,2332–2359. [9]T.Shanks,N.Metcalfe,B.Chehade, et al. 2015. MNRAS 451 (4),21:4238–4252.