On The Lunar Origin of Near-Earth Asteroid 2024 PT5

The Near-Earth Asteroid (NEA) 2024 PT5 is on an Earth-like orbit which remained in Earth’s immediate vicinity for several months at the end of 2024. PT5’s orbit is challenging to populate with asteroids originating from the Main Belt and is more commonly associated with rocket bodies mistakenly identified as natural objects or with debris ejected from impacts on the Moon. We obtained visible and near-infrared reflectance spectra of PT5 with the Lowell Discovery Telescope and NASA Infrared Telescope Facility on 2024 August 16. The combined reflectance spectrum matches lunar samples but does not match any known asteroid types – it is pyroxene-rich while asteroids of comparable spectral redness are olivine-rich. Moreover, the amount of solar radiation pressure observed on the PT5 trajectory is orders of magnitude lower than what would expected for an artificial object. We therefore conclude that 2024 PT5 is ejecta from an impact on the Moon, thus making PT5 the second NEA suggested to be sourced from the surface of the Moon. While one object might be an outlier, two suggest that there is an underlying population to be characterized. Long-term predictions of the position of 2024 PT5 are challenging due to the slow Earth encounters characteristic of objects in these orbits. A population of near-Earth objects which are sourced by the Moon would be important to characterize for understanding how impacts work on our nearest neighbor and for identifying the source regions of asteroids and meteorites from this under-studied population of objects on very Earth-like orbits.

近地小行星（Near-Earth Asteroid，NEA）2024 PT5处于类地轨道，于2024年末在地球近邻区域停留了数月之久。PT5的轨道难以用主小行星带（Main Belt）起源的小行星来拟合，其更常被归类为被误认作自然天体的火箭箭体，或是月球撞击溅射的碎屑。我们于2024年8月16日利用洛厄尔发现望远镜（Lowell Discovery Telescope）与美国国家航空航天局红外望远镜设施（NASA Infrared Telescope Facility）获取了PT5的可见光与近红外反射光谱。该联合反射光谱与月球样品高度匹配，但与所有已知小行星类型均不吻合：其富含辉石（pyroxene），而光谱红度相当的小行星则以橄榄石（olivine）为主要成分。此外，在PT5轨道上观测到的太阳辐射压数值，较人造天体的理论预期值低数个数量级。因此我们推断，2024 PT5是月球撞击事件产生的溅射物，这使其成为第二个被认为源自月球表面的近地小行星。尽管单个此类天体可能属于异常值，但两个案例表明存在待表征的潜在天体种群。由于这类轨道天体的近地交会周期缓慢，对2024 PT5进行长期位置预测颇具挑战。若存在一批源自月球的近地天体，对其进行表征将有助于理解我们最近邻天体上的撞击过程，并识别这类研究不足的类地轨道小行星与陨石的源区。