Measuring Earth’s Energy Imbalance with “Space Balls”

The presented research aims at refining the “Space Balls” mission concept, which is in its infancy of formulation and aims to obtain high-accuracy estimates of Earth’s Energy Imbalance (EEI), the globally and annually integrated net radiative flux at the top-of-the-atmosphere (TOA). The measurement of net radiative flux is facilitated through sensing radiation pressure accelerations acting on a (or multiple) near-spherical low-Earth orbiting spacecraft(s). While an accelerometer at the center of the spacecraft senses the non-gravitational orbit perturbations, the absorbing-reflecting spacecraft skin represents the detector itself, translating the impact of radiation pressure force into sensible accelerations. As with any observing system, this idea faces a multitude of technological and scientific challenges, such as related to the spacecraft characteristics themselves. The shape and thermo-optical properties are crucial for establishing a proportional relationship between impinging photons and the induced acceleration. Another difficulty arises from confounding forces and effects that may impede on radiation pressure acceleration, such as drag. To simulate and assess associated uncertainty, we develop a simulation environment based on Monte, a high-fidelity orbit navigation and mission design software. EEI represents the rate of planetary heat uptake in response to anthropogenic and natural radiative forcings and feedbacks and drives climate change as we see and feel it. Existing radiometers measure the incoming and outgoing radiative fluxes at TOA, but their uncertainties are too large to derive the residual net radiative flux with sufficient accuracy. This direct EEI measurement would be unprecedented and a vital piece in quantifying and better understanding global climate change. The overall feasibility of this approach has been demonstrated in the late 1970’s (Cactus accelerometer on Castor satellite), but a dedicated EEI mission does not exist to this day. The capabilities of accelerometers are now at a stage where this measurement might become feasible at the required accuracy and precision.

本研究旨在完善尚处于初步构想阶段的“太空球（Space Balls）”任务概念，该任务旨在高精度估算地球能量失衡（Earth’s Energy Imbalance, EEI）——即大气顶（top-of-the-atmosphere, TOA）处全球年际积分净辐射通量。该任务通过探测作用于单个或多个近球形近地轨道航天器的辐射压力加速度，实现净辐射通量的测量。设置于航天器质心处的加速度计可感知非引力轨道摄动，而兼具吸收与反射特性的航天器蒙皮本身即为探测器，将辐射压力的影响转化为可被检测的加速度。与任何观测系统一样，该构想面临诸多技术与科学挑战，例如与航天器自身特性相关的难题。航天器外形与热光学特性是建立入射光子与感应加速度之间比例关系的关键因素。另一项挑战来自可能干扰辐射压力加速度测量的混杂力与效应，例如大气阻力。为模拟并评估相关不确定性，我们基于高保真轨道导航与任务设计软件Monte开发了一套仿真环境。地球能量失衡（EEI）指的是地球系统响应人为与自然辐射强迫及反馈所产生的全球热量吸收速率，正是该速率驱动了我们当前观测与感知到的气候变化。现有辐射计可测量大气顶（TOA）处的入射与出射辐射通量，但其不确定性过大，无法以足够精度推导剩余净辐射通量。这种直接测量EEI的方法此前尚无先例，将成为量化并深化对全球气候变化理解的关键一环。该方法的整体可行性已于20世纪70年代末得到验证（Castor卫星搭载的Cactus加速度计），但截至今日仍未推出专门针对EEI的任务。如今加速度计的性能已达到可按所需准确度与精密度实现该测量的水平。