This Saturday, July 20, marks the 50th anniversary of NASA’s Apollo 11 Moon landing, when Neil Armstrong took “one small step for man” and “one giant leap for mankind” on the lunar surface. Edwin “Buzz” Aldrin joined Armstrong to plant the U.S. flag, collect rocks and dust, and observe the Moon with appreciation and awe: “Beautiful, beautiful,” he said. “Magnificent desolation.”
As an aerospace company, the Moon is especially magnificent to us, and we’re excited to celebrate such a momentous day in history. At the time of the Moon landing, governments were responsible for driving progress in space exploration. Now, commercial companies like Planet are leaders in innovation—engaging not just with Earth observation, but Moon observation as well.
Even though Planet’s focus is imaging the Earth’s surface every day, we also have one of the largest archives of Moon images taken from satellites, having collected over three million Moon images since October 2016.
Why does an Earth-observation company need images of the Moon? Because photographing the Moon provides our engineers with important information that allows our satellites to take better photos of the Earth.
To make sure all of our satellites are in proper working order, we radiometrically calibrate them, meaning we compare the readings of each satellite and check their accuracy when collecting data and imagery.
During the calibration process, individual satellites take photos of the same subject (like the Moon). Then, our engineers examine the results and make adjustments to satellites so they all meet our high image quality standards. That way, when it’s time to photograph the Earth, all of the satellites are positioned to deliver top quality imagery.
The best subjects to photograph for calibration are unchanging or near-static, so that when each satellite in our large constellation makes a full rotation and passes a specific subject, not much has changed. For example, if you were trying out the functionality of multiple digital cameras over time, and you wanted to compare image quality, you’d want to select a subject to photograph that would stay the same—like a bowl of wax fruit rather than real fruit. Real fruit will rot, but wax fruit will not. If the subject is unchanging, then when you use different cameras to snap a picture, you don’t have to worry about the subject’s visual changes impacting the results.
Because the Earth’s atmosphere is always shifting, and the Earth’s appearance changes with the seasons and clouds, our planet is quite a fidgety subject to photograph. However, the Moon’s weather and atmosphere are practically nonexistent, and there’s no vegetation and no seasons to change the color of its surface, making it an ideal target for calibration.
“The exceptional stability of the Moon makes it a very useful tool for calibration, particularly for satellite sensors on orbit,” says Tom Stone, principal investigator for lunar calibration at the United States Geological Survey.