Ejecta from the DART-produced active asteroid Dimorphos

NASA’s Double Asteroid Redirection Test (DART) mission performed the first ever kinetic impact to deflect an asteroid1. The DART kinetic impact test artificially activated an asteroid with a hypervelocity impact, providing a unique opportunity for an extensive observing campaign to monitor the evolutionary process from the formation of the ejecta to its dispersion via a sustained tail. Here we report observations of the impact ejecta with the Hubble Space Telescope (HST) from impact time (T)+15 minutes to T+18.5 days at a resolution of 2.1 km per pixel. Our observations showed that the gravitational interaction between the binary system and dust under the influence of solar radiation pressure produced a complex morphology during the evolution of the ejecta. The dust ejected at speeds much higher than the escape speed of the binary system (0.25 m/s) is directly ejected out of the system. The dust moving at speeds just above the escape speed displayed signatures of gravitational interaction with the binary asteroid system, forming spirals and extended features. Slow ejecta is ultimately pushed in the antisolar direction (nearly opposite the impact direction) by solar radiation pressure to form a tail. These dynamical processes are highly dependent on particle size and ejection direction. The ejecta evolution following DART’s kinetic impact offers a framework for understanding the fundamental mechanisms acting on asteroids disrupted by natural impact2,3 for the first time.

NASA的双小行星重定向测试（Double Asteroid Redirection Test，DART）任务完成了人类历史上首次通过动能撞击（kinetic impact）偏转小行星的壮举¹。DART动能撞击试验通过超高速撞击（hypervelocity impact）人为激活了一颗小行星，为开展大规模观测活动提供了独特契机——该活动旨在监测从喷射物（ejecta）形成到其通过持续尾迹扩散的完整演化过程。本文报告了利用哈勃空间望远镜（Hubble Space Telescope，HST）对撞击喷射物的观测结果，观测时段为撞击时刻（T）+15分钟至T+18.5天，分辨率为每像素2.1千米。观测结果表明，在太阳辐射压（solar radiation pressure）的作用下，双星系统（binary system）与尘埃之间的引力相互作用，使得喷射物在演化过程中呈现出复杂的形态。以远高于双星系统逃逸速度（escape speed，0.25米/秒）的速率被抛出的尘埃，会直接脱离该系统。速率略高于逃逸速度的尘埃，则会表现出与双小行星系统引力相互作用的特征，形成螺旋及延伸结构。低速喷射物最终会在太阳辐射压的作用下被推向背阳方向（antisolar direction，与撞击方向几乎相反），从而形成尾迹。这些动力学过程（dynamical processes）高度依赖于颗粒大小和喷射方向。DART动能撞击后喷射物的演化，首次为理解自然撞击导致小行星破碎的基本机制提供了框架²,³。