The Hera Radio Science Experiment at Didymos

Hera represents the European Space Agency's inaugural planetary defense space mission and plays a pivotal role in the Asteroid Impact and Deflection Assessment international collaboration with NASA DART mission that performed the first asteroid deflection experiment using the kinetic impactor techniques. With the primary objective of conducting a detailed post-impact survey of the Didymos binary asteroid following the DART impact on its small moon called Dimorphos, Hera aims to comprehensively assess and characterize the feasibility of the kinetic impactor technique in asteroid deflection while conducting an in-depth investigation of the asteroid binary, including its physical and compositional properties as well as the effect of the impact on the surface and shape of Dimorphos. In this work, we describe the Hera radio science experiment, which will allow us to precisely estimate critical parameters, including the mass, which is required to determine the momentum enhancement resulting from the DART impact, mass distribution, rotational states, relative orbits, and dynamics of the asteroids Didymos and Dimorphos. Through a multi-arc covariance analysis, we present the achievable accuracy for these parameters, which consider the full expected asteroid phase and are based on ground radiometric, Hera optical images, and Hera to CubeSats InterSatellite Link radiometric measurements. The expected formal uncertainties for Didymos and Dimorphos GM are better than 0.01% and 0.1%, respectively, while their J2 formal uncertainties are better than 0.1% and 10%, respectively. Regarding their rotational state, the absolute spin pole orientations of the bodies can be recovered to better than 1 degree, and Dimorphos’ spin rate to better than 10-3%. Dimorphos reconstructed relative orbit can be estimated at the sub-m level. Preliminary results, using a higher-fidelity dynamical model of the coupled motion between rotational and orbital dynamics, show uncertainties in the main parameters of interest that are comparable to those in standard radio science models. A first-order estimate of the expected uncertainty in the momentum transfer efficiency from DART's impact, obtainable with Hera, yields a value of about 0.25. This represents a significant improvement compared to current estimates. Overall, the retrieved values meet the Hera radio science requirements and goals, and remain valid under the condition that the system is determined to be in an excited but non-chaotic (or tumbling) state. The Hera radio science experiment will play an integral role in the exploration of the Didymos binary asteroid system and will provide unique scientific measurements, which, when combined with other observables such as optical images, altimetry measurements, and satellite-to-satellite tracking of the CubeSats, will support the mission's overarching goals in planetary defense and the deep understanding of binary asteroids.

赫拉（Hera）是欧洲空间局（European Space Agency, ESA）的首个行星防御太空任务，在小行星撞击与偏转评估（Asteroid Impact and Deflection Assessment, AIDA）国际协作框架中扮演关键角色，该协作与美国国家航空航天局（National Aeronautics and Space Administration, NASA）的双小行星重定向测试（Double Asteroid Redirection Test, DART）任务配合——后者是首个利用动能撞击技术开展小行星偏转实验的任务。其核心目标是在DART撞击双小行星系统迪迪莫斯（Didymos）的小卫星迪莫弗斯（Dimorphos）后，对该系统开展详细的撞击后勘测。赫拉旨在全面评估并表征动能撞击技术用于小行星偏转的可行性，同时深入研究该双小行星系统，包括其物理与组成特性，以及撞击对迪莫弗斯表面与形态的影响。本研究详述了赫拉的射电科学实验（radio science experiment）方案，该实验可精准估算多项关键参数：包括用于确定DART撞击产生的动量增强所需的质量、质量分布、自旋状态、双小行星迪迪莫斯与迪莫弗斯的相对轨道及动力学特性。通过多弧段协方差分析，我们给出了上述参数可达到的估算精度——该分析考量了小行星全阶段预期演化情况，且基于地面辐射测量数据、赫拉获取的光学影像以及赫拉与立方星（CubeSats）间星间链路的辐射测量数据。迪迪莫斯与迪莫弗斯的引力参数（GM）的预期形式化不确定度分别优于0.01%与0.1%，其二阶带谐项（J2）的形式化不确定度分别优于0.1%与10%。关于自旋状态，两颗天体的绝对自旋极取向可被恢复至优于1度的精度，迪莫弗斯的自旋速率不确定度优于10^-3%。迪莫弗斯的重构相对轨道可被估算至亚米级精度。采用耦合转动与轨道动力学的高保真动力学模型得到的初步结果显示，核心关注参数的不确定度与标准射电科学模型中的结果相当。对赫拉可获取的DART撞击动量传递效率的预期不确定度进行一阶估算，结果约为0.25，这相较当前估算值有显著提升。总体而言，获取的参数值满足赫拉射电科学实验的任务要求与目标，且在系统处于受激但非混沌（或翻滚）状态的前提下依然成立。赫拉射电科学实验将在迪迪莫斯双小行星系统的探测中发挥不可或缺的作用，并将提供独特的科学测量数据；这些数据结合光学影像、测高测量以及立方星的星间跟踪等其他观测手段，将助力该任务达成行星防御与深入理解双小行星系统的总体目标。