Libration-induced Orbit Period Variations Following the DART Impact

The Double Asteroid Redirection Test (DART) mission will be the first test of a kinetic impactor as a means of planetary defense. In late 2022, DART will collide with Dimorphos, the secondary in the Didymos binary asteroid system. The impact will cause a momentum transfer from the spacecraft to the binary asteroid, changing the orbit period of Dimorphos and forcing it to librate in its orbit. Owing to the coupled dynamics in binary asteroid systems, the orbit and libration state of Dimorphos are intertwined. Thus, as the secondary librates, it also experiences fluctuations in its orbit period. These variations in the orbit period are dependent on the magnitude of the impact perturbation, as well as the system’s state at impact and the moments of inertia of the secondary. In general, any binary asteroid system whose secondary is librating will have a nonconstant orbit period on account of the secondary’s fluctuating spin rate. The orbit period variations are typically driven by two modes: a long period and a short period, each with significant amplitudes on the order of tens of seconds to several minutes. The fluctuating orbit period offers both a challenge and an opportunity in the context of the DART mission. Orbit period oscillations will make determining the post-impact orbit period more difficult but can also provide information about the system’s libration state and the DART impact.

Forced Resonance Orbit Analysis of Binary Asteroid System

The solar radiation pressure becomes one of the major perturbations to orbits in the study of binary asteroid system, since asteroids have relatively weak gravity fields. In this paper, based on the idea of treating the solar radiation pressure as periodic external excitation, one novel family of orbits due to primary resonance and another novel family of orbits due to both primary resonance and internal resonance have been found by the classical perturbation method. The two types of steady-state orbits due to external resonance with different area-to-mass ratios have been determined and discussed by the frequency-response equations. Five binary asteroid systems, 283 Emma-S/2003 (283) 1, 22 Kalliope-Linus, 31 Euphrosyne-S/2019 (31) 1, 2006 Polonskaya-S/2005 (2006) 1 and 4029 Bridges have been taken as examples to show the validity of the proposed mechanism in the explanation of orbits formation due to resonance.

Juventas Cubesat for the Hera mision

<p>The Juventas CubeSat, will be delivered to the Didymos binary asteroid system by ESA's Hera mission within the context of the Asteroid Impact and Deflection Assessment (AIDA) international collaboration. AIDA is a technology demonstration of the kinetic impactor concept to deflect a small asteroid and to characterize its physical properties. Due to launch in 2024, Hera would travel to the binary asteroid system Didymos. It will explore the binary asteroid and the crater formed by the kinetic impact the NASA’s Double Asteroid Redirection Test (DART). HERA will carry two 6U CubeSats, one of which is the Juventas CubeSat developed by GomSpace Luxembourg with the Royal Observatory of Belgium as principal investigator. The spacecraft will attempt to characterize the internal structure of Didymos’ secondary body, Dimorphos, over a period of roughly 2 months using a low-frequency radar, JuRa. During this period, Juventas will also perform radio science measurements using its Inter-Satellite-Link to characterize the mass and mass distribution of Dimorphos. Afterwards, Juventas will attempt to land on Dimorphos, during which the spacecraft is expected to perform several bounces. Once landed, Juventas will use its gravimeter GRASS to obtain measurements of the surface acceleration on Dimorphos for a nominal duration of two orbits. Through the monitoring of dynamics for landing and bouncing impacts as well as measurements from the GRASS gravimeter payload while on the surface, Juventas will determine surface mechanical properties and provide further information on subsurface structure and dynamical properties of Dimorphos.</p>

On the post-impact spin state of the secondary component of the Didymos-Dimorphos binary asteroid system

<p>NASA’s Double Asteroid Redirection Test (DART) is designed to be the first demonstration of a kinetic impactor for planetary defense against a small body impact hazard. The target is the smaller component of the Didymos-Dimorphos binary asteroid system. The DART impact will abruptly change the relative velocity of the secondary (Dimorphos), increasing the binary eccentricity and exciting librations in the secondary. The observed change in the binary orbit period will be used to infer the “beta factor”, or the momentum transfer efficiency, an important parameter used in planetary defense. The post-impact spin and librational dynamics are expected to be highly dependent on the momentum transferred to the target (i.e., beta) and the shape of the secondary, which is still unconstrained.</p> <p>In this work, we explore the possible post-impact spin state of Dimorphos, as a function of its shape and beta, assuming it has an ellipsoidal shape and that both bodies have a uniform density. We have conducted attitude dynamics simulations with a modified 3-D spin-orbit model, accounting for the secondary’s shape and the primary’s oblateness, to understand the underlying dynamical structure of the system. In addition, we have used the radar-derived polyhedral shape model of Didymos in high-fidelity Full Rigid Two-Body Problem (FR2BP) simulations to capture the fully three-dimensional nature of the problem. We consider the outcomes from a simplified planar impact, where the DART momentum is transferred within the binary orbit plane, opposite the motion of Dimorphos, in addition to a more realistic case that accounts for the full DART velocity vector (which contains out-of-plane components).</p> <p>With both simulation tools, we produce the expected signatures of the 1:1 and 2:1 secondary resonances between the free and forced libration periods, corresponding to axial ratios of a/b = 1.414 and a/b = 1.087, respectively. For moderate values of beta (~3), we find that the libration amplitude can exceed ~40 degrees in most cases. For some possible axial ratios, it is even possible to achieve a libration amplitude exceeding 40 degrees with beta values as low as 1. In addition, both codes reveal that the secondary may be attitude unstable in many cases, and can enter a chaotic tumbling state for larger values of beta (~5). In some cases, Dimorphos is able to break from its assumed 1:1 spin-orbit resonance.</p> <p>In the case with a more realistic impact geometry (where some momentum is transferred out-of-plane), the results are relatively similar. The most noticeable difference is in the cases that result in a chaotic tumbling state. In those cases, the characteristic timescale for entering the chaotic tumbling state is much shorter – typically only several orbit periods are required. We also discuss the feasibility of detecting the post-impact spin state of Dimorphos with ground-based observations.</p> <p>This study was supported in part by the DART mission, NASA Contract # NNN06AA01C to JHU/APL. The work of K.T. and I.G. is supported by the EC Horizon 2020 research and innovation programme, under grant agreement No. 870377 (project "NEO-MAPP"). Some of the simulations herein were carried out on The University of Maryland Astronomy Department’s YORP cluster, administered by the Center for Theory and Computation.</p>

LICIACube: Italian deep space small satellite technology for the Asteroid Redirection Test (DART) mission

<p>The Double Asteroid Redirection Test (DART) mission is part of the plan developed by NASA for the Planetary Defence program, since space mission towards asteroid have become crucial to study their composition. Moreover, these missions are the future of space exploration, providing opportunities for testing novel technologies for extreme conditions. These are some of the many reasons why NASA developed the Double Asteroid Redirection Test (DART) mission and the Italian Space Agency joined the effort. DART is a spacecraft acting as a kinetic impactor that will deflect the orbit of a binary asteroid by crashing itself into the moonlet of the Didymos binary system. In order to increase the accuracy of the deflection measurement, the ASI 6U Light Italian CubeSat for Imaging of Asteroid (LICIACube) will be carried on DART and released by the main probe in proximity of the target. The effects of the impact will be observed also from ground-based telescopes. The small satellite that will be the only witness of this event, LICIACube, is an Italian Space Agency project, and has been designed, integrated and tested by the assigned aerospace company Argotec. The primary objective of LICIACube is to capture photographs of DART impact ejecta plume over a span of times and phase angles in order to confirm the DART impact on the secondary body of the Didymos binary asteroid system and to observe the ejecta plume dynamics. After the deployment from the DART spacecraft, LICIACube will perform braking manoeuvers, to increase the relative velocity with respect to DART spacecraft, allowing LICIACube to perform the scientific phase and fulfil the mission objectives. Following this phase, the LICIACube satellite will continue on its path for few months, transferring scientific data and performing radio-science experiments. Many of the scientific objectives will be accomplished by using the autonomous navigation algorithm and the imaging capabilities provided by the baseline platform, based on the heritage of the Argotec company. The images acquired by LICIACube will help the Italian involved scientific community to obtain relevant discoveries about the binary asteroid system.</p> <p>The mission is articulated in a series of single critical moments: LICIACube will be deployed by DART 120 hours before the impact on Didymos B; the satellite will fly-by the asteroid with a relative velocity of 6.5 km/s, and it will document the effects of the impact, the crater and the evolution of the plume generated by the collision. To acquire images with the best spatial resolution, LICIACube will aim at fly-bying the asteroid close to the Didymos-B surface: considering the high relative velocity at the close approach, LICIACube will have to maintain the asteroid's pointing at an angular speed of approximately 10 deg/s. Scientific objectives will be accomplished by using the autonomous navigation algorithm and the imaging capabilities provided by the platform, based on the heritage of the Argotec company. The two optical payloads embarked on LICIACube have the duty of acquiring the images that are then processed on board through the navigation algorithm, thus allowing to identify the asteroid system, distinguish the main and secondary bodies and control the satellite attitude in order to keep the asteroid pointing during fly-by. The navigation algorithm is mainly based on neural network trained on ground using photorealistic images of the binary asteroid system and the plume generated by the impact.</p> <p>The images acquired and downlinked by the LICIACube satellite will help the scientific community to obtain more detailed results about the binary asteroid, and provide feedback to the Planetary Defense program, pioneered by the Space Agencies. The scientific team is enriched by University of Bologna team, supporting the orbit determination and the satellite navigation, Polytechnic of Milan, for mission analysis support and optimization and INAF (National Institute of Astrophysics), providing support in the scientific operations of the satellite. The LICIACube mission will be a challenging opportunity for the entire Italian technical and scientific community leading to the implementation of a deep space mission based on a small scale but highly technological platform.</p>