Effects of Impact and Target Parameters on the Results of a Kinetic Impactor: Predictions for the Double Asteroid Redirection Test (DART) Mission

Abstract

The Double Asteroid Redirection Test (DART) spacecraft will impact into the asteroid Dimorphos on 2022 September 26 as a test of the kinetic impactor technique for planetary defense. The efficiency of the deflection following a kinetic impactor can be represented using the momentum enhancement factor, β, which is dependent on factors such as impact geometry and the specific target material properties. Currently, very little is known about Dimorphos and its material properties, which introduces uncertainty in the results of the deflection efficiency observables, including crater formation, ejecta distribution, and β. The DART Impact Modeling Working Group (IWG) is responsible for using impact simulations to better understand the results of the DART impact. Pre-impact simulation studies also provide considerable insight into how different properties and impact scenarios affect momentum enhancement following a kinetic impact. This insight provides a basis for predicting the effects of the DART impact and the first understanding of how to interpret results following the encounter. Following the DART impact, the knowledge gained from these studies will inform the initial simulations that will recreate the impact conditions, including providing estimates for potential material properties of Dimorphos and β resulting from DART’s impact. This paper summarizes, at a high level, what has been learned from the IWG simulations and experiments in preparation for the DART impact. While unknown, estimates for reasonable potential material properties of Dimorphos provide predictions for β of 1–5, depending on end-member cases in the strength regime.

Momentum Transfer from the DART Mission Kinetic Impact on Asteroid Dimorphos

“…Our result for 𝛽 is consistent with numerical simulations [11][12][13][14][15][16][17][18][19][20][21] and laboratory experiments [22][23][24][25][26][27][28] of kinetic impacts, which have consistently indicated that 𝛽 is expected to fall between about 1 and 6. However, non-unique combinations of asteroid mechanical properties (e.g., cohesive 5 strength, porosity, and friction angle) can produce similar values of 𝛽 in impact simulations 18 . Future studies that combine estimates of 𝛽 with additional constraints from the DART impact site geology 1 and ejecta observations 3,4 will provide greater insight into Dimorphos's material properties.…”

Orbital period change of Dimorphos due to the DART kinetic impact

“…Impact simulations conducted in preparation for DART’s kinetic impact test indicated that, depending on the material strength, impact conditions and other properties, the value of the momentum enhancement factor, β , could be considerable, with predicted values as high as 5 (ref. 2 ) or 6 (ref. 3 ), with a resulting orbital period change of more than 40 min (ref.…”

“… The Double Asteroid Redirection Test (DART) spacecraft successfully performed the first test of a kinetic impactor for asteroid deflection by impacting Dimorphos, the secondary of near-Earth binary asteroid (65803) Didymos, and changing the orbital period of Dimorphos. A change in orbital period of approximately 7 min was expected if the incident momentum from the DART spacecraft was directly transferred to the asteroid target in a perfectly inelastic collision 1 , but studies of the probable impact conditions and asteroid properties indicated that a considerable momentum enhancement ( β ) was possible 2 , 3 . In the years before impact, we used lightcurve observations to accurately determine the pre-impact orbit parameters of Dimorphos with respect to Didymos 4 – 6 .…”

Observational characterization of the ejecta from the DART-Dimorphos impact

“…A key parameter that the DART mission aimed to measure is the momentum gain factor (β ), defined as the momentum change of the asteroid divided by the momentum of the impactor. This resulted in a large effort to predict the outcome of the DART impact 4,5,7,20 . Since only 1/6 of NEOs are binary systems (such as the Didymos system), it is essential to develop methods to estimate the momentum gain of non-binary asteroids in a timely manner, using ground-based observations of the impact ejecta.…”

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The Double Asteroid Redirection Test 1, 2 (DART) was intended to test planetary defense strategies via the reaction of the near-Earth asteroid Dimorphos, a satellite of Didymous, to a spacecraft impact. Pre-impact modeling [3][4][5][6][7] predicted a momentum gain of the impacted asteroid in the range of 1 − 5 times that of the impactor. These models further suggest that the velocity distribution of the ejecta depends, for example, on the material cohesion and porosity.

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