NOAA's GOES-19 satellite, a key instrument for monitoring space weather, captured a unique view of the partial solar eclipse on September 21. From its position in orbit, the satellite's coronagraph recorded the Moon passing in front of the Sun, an event that produced unusual imagery due to a combination of image processing and a scheduled spacecraft maneuver.
Key Takeaways
- NOAA's GOES-19 satellite captured a natural solar eclipse using an instrument designed to create artificial ones.
- Standard image processing techniques caused the Sun's outer atmosphere, the corona, to disappear from the final images.
- The Moon's path appeared irregular due to a planned calibration maneuver of the satellite, not the Moon's actual movement.
- This may be the first documented instance of a space-based coronagraph in Earth's orbit recording a natural solar eclipse.
A Unique Observation from Space
On September 21, while observers on Earth watched a partial solar eclipse, the National Oceanic and Atmospheric Administration's (NOAA) GOES-19 satellite had a front-row seat. The satellite's Compact Coronagraph-1 (CCOR-1) instrument documented the celestial event from its vantage point high above the planet.
A coronagraph is a specialized telescope designed to study the Sun's faint outer atmosphere, known as the corona. To do this, it uses a small, opaque disk to block the overwhelming brightness of the Sun's main body, effectively creating an artificial eclipse.
In this instance, however, the instrument witnessed a natural event. The Moon passed directly between the CCOR-1's lens and the Sun, allowing the satellite to record a genuine solar eclipse from space. This is a rare occurrence for instruments of this type.
Why This Event is Rare
The ability of GOES-19 to see this eclipse is due to its orbit around Earth. This proximity allows the Moon to occasionally cross its line of sight to the Sun. In contrast, other solar observatories, such as the Solar and Heliospheric Observatory (SOHO), are positioned much farther from Earth at a location known as the L1 Lagrange point. From that distance, the Moon never passes through their field of view, making such an observation impossible.
Understanding the Vanishing Corona
The images returned by GOES-19 during the eclipse were puzzling at first. Instead of just the Sun's disk being obscured by the Moon, the entire corona seemed to vanish as well. This was not a physical phenomenon but a result of the instrument's automated image processing.
To reveal the faint details of the corona, the CCOR-1 system is programmed to subtract the bright, scattered light that bounces around inside the telescope. This process works by creating a model of the scattered light and removing it from each image.
"Normally, each image seen by CCOR-1's detector is a combination of the corona and bright sunlight scattered within the telescope," Bill Thompson of NASA's Goddard Space Flight Center explained to Spaceweather.com. "During the eclipse, the scattered light went away, but the subtraction proceeded as if it hadn't. It ended up subtracting too much."
Essentially, the software continued to remove an amount of light that was no longer present because the Moon was blocking it. This over-subtraction caused the faint light of the corona to be removed from the final image along with the non-existent scattered light.
The Mystery of the Zig-Zagging Moon
Another peculiar aspect of the observation was the Moon's apparent trajectory. In the sequence of images, the Moon did not travel in a straight line as expected. Instead, its path appeared to zig-zag across the coronagraph's field of view.
This unusual motion was not caused by the Moon itself but by the GOES-19 spacecraft. The satellite was undergoing preparations for a scheduled calibration procedure called a "yaw-flip maneuver," which took place on September 22.
What is a Yaw-Flip Maneuver?
A yaw-flip is a routine operation where a satellite is rotated 180 degrees. This maneuver helps calibrate instruments and manage thermal stress on the spacecraft by changing which side faces the Sun. It requires precise adjustments to the satellite's orientation, or "attitude."
Spacecraft Maneuvers Alter the View
The yaw-flip maneuver requires carefully altering the satellite's orientation in space. It is likely that preparatory adjustments made on September 21, the day of the eclipse, caused the spacecraft to shift slightly. These small changes in the satellite's pointing direction were captured by the CCOR-1 instrument.
Because the coronagraph's view was shifting, the Moon's steady path appeared crooked in the sequence of processed images. The zig-zag was an illusion created by the movement of the satellite, not the Moon.
This event, potentially the first of its kind recorded by a coronagraph in Earth orbit, provides scientists with valuable data on instrument performance under unique conditions. It serves as a powerful reminder of the complex interplay between celestial mechanics and the advanced technology used to observe them.
