Proba-3 to continuously monitor Sun's energy output

Proba-3 to continuously monitor Sun's energy output
by Erica Marchand
Paris, France (SPX) Nov 09, 2024
Proba-3's ambitious mission requires more than one spacecraft to achieve its objectives. To enable the Coronagraph spacecraft to study the Sun's outer atmosphere, the Occulter spacecraft must block the solar disk. This setup places the Occulter in continuous solar alignment, making it an invaluable scientific platform.

The Occulter is equipped with the Davos Absolute Radiometer (DARA), a compact instrument developed by the Physical Meteorological Observatory Davos (PMOD) in Switzerland, designed to consistently measure the Sun's total energy output, or total solar irradiance. This variable is crucial for climate research.

"Researchers used to refer to the 'solar constant,' but it actually changes slightly," said Wolfgang Finsterle, DARA Principal Investigator at PMOD. "Tracking total solar irradiance is essential because it accounts for approximately 99.978% of Earth's energy input, influencing climate processes. Even minor fluctuations are significant."

PMOD has a long history of studying solar irradiance, beginning with ground-based tools over a century ago and transitioning to space-based radiometers in the 1970s for continuous data. Designated as the World Radiation Centre by the World Meteorological Organization, PMOD calibrates radiation measurements for global monitoring programs.

Wolfgang highlighted, "Solar irradiance varies with the 11-year solar cycle. To detect long-term trends, we compare irradiance during successive solar minima. This requires data over long periods from various instruments, as individual radiometers degrade slowly due to continuous ultraviolet exposure. Notably, the SOHO solar observatory's radiometer from 1995 still functions effectively."

DARA operates on a straightforward principle: a 5-mm black-painted silver cavity absorbs sunlight for 15 seconds, after which a shutter closes. Electric heating maintains the cavity's temperature for the next 15 seconds, and the required energy is measured to determine solar irradiance in watts per square meter. This cycle repeats throughout the instrument's life. DARA's shutter mechanism, tested for millions of cycles in PMOD's vacuum chamber, ensures reliability.

"DARA represents an evolution from earlier radiometers, featuring an optimized cavity design to reduce stray light and a multi-channel system for self-calibration," Wolfgang explained. "Its digital control system also allows higher frequency observations."

Two similar radiometers have already proven successful. CLARA, a compact version, launched on Norway's NorSat-1 CubeSat in 2017 and remains functional. Another DARA has been operational aboard China's FY-3E weather satellite since 2021. Werner Schmutz of PMOD noted, "These successful missions bolster confidence in DARA's performance on Proba-3, which can maintain solar alignment within half a degree."

Proba-3's DARA will have the added challenge of an elongated orbit, reaching up to 60,000 km from Earth. It can automatically adapt to variations in the solar disk's size due to the Earth's elliptical orbit. By knowing its position in space, DARA compensates for these changes in real-time.