Characterization of the Ejecta from the NASA/DART Impact on Dimorphos: Observations and Monte Carlo Models

Abstract

The NASA Double Asteroid Redirection Test (DART) spacecraft successfully crashed on Dimorphos, the secondary component of the binary (65803) Didymos system. Following the impact, a large dust cloud was released, and a long-lasting dust tail developed. We have extensively monitored the dust tail from the ground and the Hubble Space Telescope. We provide a characterization of the ejecta dust properties, i.e., particle size distribution and ejection speeds, ejection geometric parameters, and mass, by combining both observational data sets and using Monte Carlo models of the observed dust tail. The size distribution function that best fits the imaging data is a broken power law having a power index of –2.5 for particles of r ≤ 3 mm and –3.7 for larger particles. The particles range in size from 1 μm up to 5 cm. The ejecta is characterized by two components, depending on velocity and ejection direction. The northern component of the double tail, observed since 2022 October 8, might be associated with a secondary ejection event from impacting debris on Didymos, although is also possible that this feature results from the binary system dynamics alone. The lower limit to the total dust mass ejected is estimated at ∼6 × 106 kg, half of this mass being ejected to interplanetary space.

Optical Monitoring of the Didymos–Dimorphos Asteroid System with the Danish Telescope around the DART Mission Impact

“…They find a model that contains two components (low and high velocity, ejected hemispherically and into an ejecta cone, respectively), with a third component needed to explain the appearance of the second tail. Both Lin et al (2023) and Moreno et al (2023) agree that this secondary dust release event is necessary to produce this tail and that the timing of this is consistent with the "bump" in the photometry we observe around 8 days after impact. Interestingly, an alternative model demonstrating the secondary tail to be an effect of changing viewing geometry of the ejecta cone created in the initial impact event was recently presented by , but it is inconsistent with the ≈8 day brightening we note (Section 3 and Figure 2).…”

“…These authors find an expansion velocity in the anti-Sun (tail) direction of 31 m s −1 on September 27, consistent with acceleration of millimeter-centimeter particles by radiation pressure, but our deeper images (acquired approximately 14 hr earlier from Chile, versus Lin et al 2023ʼs observations from Lulin observatory in Taiwan) already show a longer tail. More detailed Monte Carlo modeling of the ejecta motion by Moreno et al (2023) fits the ejecta using a broken power-law size distribution between micron and 5 cm radius particles. They find a model that contains two components (low and high velocity, ejected hemispherically and into an ejecta cone, respectively), with a third component needed to explain the appearance of the second tail.…”

“…In practice there are too many unknowns within ζ (primarily the 3D distribution of initial velocities, and any size dependence on those) for this to be of much use in fitting the fading relationship seen in Figure 2. Modeling the full motion of ejecta is beyond the scope of this work, but has been tackled by others (e.g., Moreno et al 2023).…”

Ejecta Evolution Following a Planned Impact into an Asteroid: The First Five Weeks

“…This is a critical constraint not addressed by the first two scenarios. The short duration of the dimming pause could suggest that only a narrow size range of particles was ejected due to expected low re-impact speeds (Rossi et al 2022), as opposed to the wide range of particle sizes implied by the persistence of the main ejecta tail on a several-month timescale and its lack of an observed disconnection event (Moreno et al 2023). The short duration of the second tail is consistent with this scenario.…”

VLT/MUSE Characterization of Dimorphos Ejecta from the DART Impact

“…Photometric evidence supports this theory, since HST and other optical facilities detected a noticeable pause in dimming between October 1 and 3 in multiple apertures (Kareta et al 2023;Li et al 2023;Rożek et al 2023), which allows us to infer that new ejecta is likely being excavated rather than old ejecta turning around (that should be within the apertures). Additional Monte Carlo (MC) models of tail formation by Moreno et al (2023) indicate that transfer of linear momentum, via ejecta impacting, to the surface of both Didymos and Dimorphos exhibits a secondary peak around T + 5D post-impact, which may suggest that the secondary tail is the product of numerous secondary impacts across both binary bodies. Lastly, our width measurements hint that a secondary impactor might be more likely, since the blown-back ejecta would have a longer time to radially disperse and create a much thicker tail, while the secondary impactor would produce lower-velocity ejecta that would create a much thinner tail due to less time to disperse and a much lower ejection energy as compared to the primary impact.…”