Spacecraft instruments are highly specialized and can take years to design, build, and test. However, sometimes a simple innovation can have a significant impact on their capabilities. In the case of the ESA’s Solar Orbiter, one astronomer’s last-minute hack has allowed the spacecraft to take groundbreaking observations it would not have been able to capture otherwise.
The Solar Orbiter, launched by the ESA in February 2020, is a mission dedicated to studying the Sun’s inner heliosphere and polar regions in high resolution. Its strategic approach involves making close approaches to the Sun every six months, allowing it to monitor the build-up of magnetic activity that precedes solar flares and eruptions. One of the crucial instruments onboard is the Extreme Ultraviolet Imager (EUI), contributed by Belgium, which offers detailed imaging of the Sun’s atmosphere from the chromosphere to the corona.
During the final construction phase of the EUI, an ingenious idea was conceived by a member of the Solar Orbiter team. The EUI already had a safety door to protect it during spaceflight and when not in use, but what if this door could be modified to enhance the imager’s functionality? Frédéric Auchère, a member of the EUI team from Institut d’Astrophysique Spatiale, Université Paris-Sud, proposed a straightforward yet effective solution – adding a small protruding “thumb” to the door.
The addition of the thumb on the instrument door proved to be a game-changer. With the thumb placed in a halfway-opened position, it effectively covered the Sun’s bright disc during operations. As a result, the EUI could detect the much fainter ultraviolet (UV) light emanating from the surrounding corona. This innovative approach, termed the occulter mode of operation, essentially combines the functionalities of a coronagraph and camera into a single instrument, enabling deeper penetration into the Sun’s atmosphere.
Extensive testing of the EUI in the occulter mode began in 2021, and the results have been published in Astronomy and Astrophysics. Frédéric Auchère, the astronomer behind the idea, is the lead author of the paper. The team’s findings demonstrate the success of this new instrument configuration, expanding possibilities for both imaging the Sun and its corona. Daniel Müller, ESA’s Project Scientist for Solar Orbiter, commends the breakthrough, highlighting the potential for future instruments with similar capabilities.
The modification to the EUI’s instrument door has unlocked a previously inaccessible region of the Sun’s atmosphere for extreme UV imagers. Traditional separate coronagraphs typically block this region due to technical restrictions. However, the EUI’s innovative occulter now enables easier imaging of this challenging area. The Sun’s lower atmosphere holds valuable insights into physics and magnetic structures that were previously obscured. David Berghmans, the EUI Principal Investigator from the Royal Observatory of Belgium, expresses excitement about the opportunity to unveil the secrets hidden within.
While Auchère’s addition to the EUI’s instrument door proved effective, there are opportunities for improvement. Due to its sub-optimal position, the thumb requires longer exposures to capture the desired images. Some test images presented in the paper necessitated 1000-second exposures. Nevertheless, the proven effectiveness of this design opens the door for future improvements in upcoming missions.
The implications of the EUI’s modified occulter extend beyond its immediate capabilities. The authors of the paper envision the efficiency of FSI-based coronagraphs being increased significantly, potentially enabling the acquisition of similar images in just 10 seconds with minor modifications. This advancement paves the way for EUV instruments to supersede visible light instruments and separate coronagraphs. The absence of background emission from dust scattering makes EUV coronagraphs more manageable and less demanding in terms of platform pointing accuracy and stability.
Thanks to Frédéric Auchère’s innovative hack, our understanding of the Sun is poised for a significant leap forward. The modified occulter on the EUI instrument has proven its ability to capture intricate details of the Sun’s lower atmosphere, revealing new physics and magnetic structures. As future missions embrace similar modifications and improvements, the field of solar exploration is set to expand, unraveling the mysteries of our nearest star like never before.