One measurement provides lots of data
Today, the albedo is primarily monitored using satellites that overfly the Earth and measure how much light is reflected back. It takes many overflights to cover larger areas and subsequently extensive post-processing of data to get the full picture. So it’s an advantage to measure the albedo using earthshine if you want large-scale measurements, says Katcha Koch Winther.
“Instead of doing a lot of measurements and then factoring in whether they are taken over the sea, forest, or savannah, the earthshine method enables us to basically use a single measurement. When we take a picture of the New Moon, where we see almost the entire night side of the Moon, we get a signal from almost half of the Earth in one measurement,” says Katcha Koch Winther.
Measuring the right light
But it is important to ensure that the instrument measures the right light.
Quality control has therefore played a major role in Katcha Koch Winther’s development of the instrument. Among other things, she has had a strong focus on the mirror surfaces of the camera optics, where she wants to avoid the surfaces scattering the light inside the instrument, which can also lead to faulty measurements. And it is also important to know the Moon’s own albedo.
“When we measure earthshine, we use the Moon as a kind of mirror. So we should also take into account the Moon’s own reflectivity. If it’s included incorrectly in our measurements, we risk that our calculations of the Earth’s albedo are wrong,” says Katcha Koch Winther.
The fact that earthshine is much weaker than moonshine is also a challenge in the development of a useful instrument. This is due to the large difference in brightness, which the usual camera solutions cannot take into account. Since it’s crucial to be able to separate the two light sources (earthshine and moonshine), DTU Space had to develop its own solution, and this was done by MSc Eng René Fléron.
“There are two types of information that we need to get from the instrument. We need to know exactly where on the Moon the instrument takes photos, and how much light there is. It was the realization of this duality in the task that provided the solution. The first part is solved by the camera, which takes photos and gathers data about where on the Moon photos are taken. The second part is solved by adding a photometer to the instrument. A photometer is a sensitive light meter that can handle the large differences in brightness from moonshine and earthshine, respectively,” explains René Fléron, who together with Peter Thejll at DMI was the man behind the juLIET project.
DTU helping NASA
The Earth’s albedo is also of interest to NASA. In 2025, the US space agency will mount an instrument on the outside of the International Space Station (ISS) that will take measurements of both sunlight reflected from Earth and of earthshine on the Moon. DTU has supplied navigation and positioning equipment for the mission named CLARREO Pathfinder. CLARREO is short for Climate Absolute Radiance and Refractivity Observatory.
DTU’s equipment will control the CLARREO instruments so that they with high precision point to the selected areas where measurements will be made, says Professor John Leif Jørgensen.
“A more precise uncovering of the Earth’s albedo is important because it says something about how much solar radiation remains and contributes to global warming. Solar radiation is an important climate driver, and by measuring it, we can get much more accurate climate models in the future,” says John Leif Jørgensen.
For many years, the professor and his colleagues have designed, built, and delivered a steady stream of camera and navigation equipment for the NASA and ESA space missions. Many of them are climate missions whose purpose is to provide better data and measurements.
“When we build and deliver instruments for climate missions, DTU contributes to improving input to the climate models that enable politicians to decide how the world should act in relation to climate change,” says John Leif Jørgensen.
