Planned Radar Observations of Binary near-Earth Asteroid Didymos in 2022

Binary near-Earth asteroid 65803 Didymos is the target of NASA’s Double Asteroid Redirection Test (DART) mission, a test of the kinetic impactor approach to planetary defense (Cheng et al., 2016). The spacecraft was launched in 2021 November, and will impact Dimorphos, the satellite of Didymos, on 2022 September 26 at 23:14 UT. The primary objectives of the DART mission are to shorten the orbital period of Dimorphos and measure the change using ground-based optical observations.

We are planning Goldstone radar (8560 MHz, 3.5 cm) observations of Didymos starting on September 27 and bistatic Goldstone to Green Bank Telescope (GBT) observations from October 2-22, starting about a week after the DART impact. These observations could yield delay-Doppler images with range resolutions as fine as 75 m as well as echo power spectra. We will measure the secondary-to-primary range and Doppler separations, and use them to estimate the mutual orbit parameters, including the change in mean motion, of Dimorphos. The echo power spectra of the system will allow us to place limits on the size and velocity distribution of the impact ejecta at the time of the observations.

The primary, Didymos, is roughly spherical with a volume equivalent diameter of 780 ± 30 m and has a ridge along the equator (Naidu et al., 2020). The secondary, Dimorphos, has a diameter of 150 ± 30 m and a spin period of 12.4 ± 3.0 h that is consistent with rotation synchronized with its orbital period of 11.92 h (Naidu et al., 2020). The mutual orbit has a semimajor axis of 1190 ± 30 m (Naidu et al, 2020), a nearly circular eccentricity of < 0.03, and an orbit pole in ecliptic coordinates of (λ, β) = (320.6 ± 13.7, -78.6 ± 1.8)° (Scheirich and Pravec, 2022).

We will present our plans for the upcoming radar observations, expected results, and how the data will contribute to the measurement of the change in mutual orbit due to the DART impact.