Lunar Imaging Earthshine Telescope, juLIET, for Earth Albedo Measurements

The average global temperature of Earth is governed by the energy balance equation, comparing energy entering and leaving the Earth system. A key parameter in this balance is the Earth’s albedo, determining the ratio of the Sun’s energy being reflected from or absorbed by Earth. The global albedo varies on several different timescales – daily due to changes in cloud cover, seasonally due to changes in foliage and snowfall, and on greater timescales a change in albedo is a reflection of our changing climate. To measure these changes, multi-decadal data is needed.

Data of top-of-the-atmosphere shortwave radiation used in albedo estimation, are primarily gathered by LEO satellites using absolute measurement techniques. These are however affected by the harsh space environment, especially radiation, which causes drift errors in the data, requiring in-flight calibration. The purpose of NASA’s and ESA’s upcoming missions CLARREO and TRUHTS respectively, is to provide state of the art calibration data to account for these errors. However, they do not remove the issue all together.

As an alternative to these absolute measurements, the space based earthshine telescope juLIET (ju Lunar Imaging Earthshine Telescope) aims to estimate the albedo through relative measurements. The Earthshine albedo technique is based on comparing the intensity of Moonlight coming from the visible dayside of the Moon and the Earthshine reflected off the visible nightside of the Moon. As a relative measurement, it is more resilient to calibration drift.

Albedo measurements using the Earthshine technique have been successfully carried out from Earth, but due to Moonlight being several magnitudes brighter than Earthshine, atmospheric scattering of Moonlight reduces the possible precision on the Earthshine intensity. While the issue of atmospheric scattering is removed by going into orbit, measuring the dim Earthshine with a sufficiently high precision to be used for albedo estimation, using the same sensor that measures the Moonlight, still poses a significant challenge, due to scattering and diffraction of Moonlight within the telescope.

To determine the feasibility of the juLIET instrument, an analysis of the optical noise of the telescope is conducted. This analysis is carried out using Zemax OpticStudio and MATLAB, where main contributors to the uncertainty of the measurement are isolated and quantified.

The results of this noise analysis will be extended to determine which lunar phases juLIET can provide measurements of the Earth albedo, during its mission time as primary payload on the small-sat ROMEO developed by IRS, University of Stuttgart.