Photometric Observations of the Binary Near-Earth Asteroid (65803) Didymos in 2015–2021 Prior to DART Impact

Abstract

We performed photometric observations of the binary near-Earth asteroid (65803) Didymos in support of the Double Asteroid Redirection Test (DART) mission that will test the Kinetic Impactor technology for diverting dangerous asteroids. It will hit the Didymos secondary, called Dimorphos, on 2022 September 26. We observed Didymos with 11 telescopes with diameters from 3.5 to 10.4 m during four apparitions in 2015–2021, obtaining data with rms residuals from 0.006 to 0.030 mag. We analyzed the light-curve data and decomposed them into the primary rotational and secondary orbital light curves. We detected 37 mutual eclipse/occultation events between the binary system components. The data presented here, in combination with 18 mutual events detected in 2003, provide the basis for modeling the Dimorphos orbit around the Didymos primary. The orbit modeling is discussed in detail by Scheirich & Pravec and Naidu et al. The primary light curves were complex, showing multiple extrema on some epochs. They suggest a presence of complex topography on the primary’s surface that is apparent in specific viewing/illumination geometries; the primary shape model by Naidu et al. (Icarus 348, 113777, 2020) needs to be refined. The secondary rotational light-curve data were limited and did not provide a clear solution for the rotation period and equatorial elongation of Dimorphos. We define the requirements for observations of the secondary light curve to provide the needed information on Dimorphos’s rotation and elongation when Didymos is bright in 2022 July–September before the DART impact.

Spectral Rotational Characterization of the Didymos System prior to the DART Impact*

“…After the 2021 photometric observational campaign (Pravec et al 2022) we were left with only one solution for each mutual event. Using a helpful lookup table available on the DART Observation WG repository, 21 we identified mutual events that occurred during our observational timeframe.…”

“…The Didymos rotation period is extremely well known after the 2021 observational campaign (T = 2.2593 ± 0.0002, Pravec et al 2022). Therefore, it is possible to assign to each spectrum a rotational phase, corresponding to spectra obtained at different phases of its lightcurve.…”

“…Hints of its binarity first arose in Goldstone radar observations, and were later confirmed with optical lightcurve analysis, along with Arecibo radar imaging in 2003 (Pravec et al 2003). Radar and lightcurve data have established a diameter of 780 ± 80 m, with a well-known rotational period of 2.259 3 ± 0.0002 hr, for the primary (Didymos), while the secondary, later renamed Dimorphos, has a projected diameter of 163 ± 18 m and an orbital period of 11.920 ± 0.005 hr (Pravec et al 2022). The study of the rotational properties of the system are even more crucial due to the fact that Didymos belongs to a rare class of binaries whose primary is at risk of rotational disruption (Walsh et al 2008).…”

Anticipating the DART Impact: Orbit Estimation of Dimorphos Using a Simplified Model

“…We used light curves from Pravec et al (2006) and Pravec et al (2022) and measured the times of mutual events detected in the orbital component of the light curves of Didymos. The decomposition of the total light curve into the primary and the primary-subtracted components was performed by Pravec et al (2006) for the 2003 data, by Pravec et al (2022) for the 2015, 2017, and 2019 data, and by one of us (NM) for the 2020 and 2021 data. We used the Scheirich & Pravec (2022) model to determine the types of events.…”

Shape Modeling of Dimorphos for the Double Asteroid Redirection Test (DART)

“…Hence, shape modeling for DART using DRACO images alone is an exercise in how well one can estimate the shape of an object from images with essentially no stereo in which less than half of the body is both seen by the camera and illuminated. To constrain the shape model in areas that are neither visible nor illuminated, we will use constraints on axial ratios (a/b and b/c) obtained from light curves from the ground (Pravec et al 2006(Pravec et al , 2012(Pravec et al , 2016(Pravec et al , 2022Scheirich & Pravec 2022), from earlier, unresolved DRACO images of the Didymos system that will be taken in the days leading up to impact, and from the a/b and b/c axial ratios of secondaries in other binary asteroid systems.…”