Proba-3: A mission to see the Sun like we've never seen it before

The PROBA-3 mission by the European Space Agency (ESA) is designed to explore and study the sun in unique ways.

It consists of two small satellites – the Occulter and the Coronagraph – that will work together to create a giant, space-based solar coronagraph.

Viewing the Sun in unprecedented detail

The spacecraft aims to capture the sun’s faint ambiance while enduring the intense heat of the solar disk.

By positioning one satellite to block the Sun’s direct light, the other can capture images of the solar corona, allowing scientists to study the Sun’s outer atmosphere in unprecedented detail.

The sun-facing side of the Occulter is equipped with a unique instrument to continuously measure the sun’s total energy output, also known as total solar irradiance. This is a crucial variable for climate studies.

Proba-3’s mighty radiometer

The key instrument is the Swiss-made, shoebox-sized Davos Absolute Radiometer or DARA, procured from the Physical Meteorological Observatory Davos (PMOD).

The question arises, why is it so important to keep track of the total solar irradiance? Wolfgang Finsterle, DARA Principal Investigator at PMOD, elaborated on this question.

“Researchers used to talk about the ‘solar constant’ but in fact it is always changing slightly. And it’s essential to keep track of the total solar irradiance, because it is the dominant energy input to the surface of the Earth,” noted Finsterle.

“It amounts to something like 99.978% of the energy available on Earth, including the conserved solar energy stored in coal and oil. It drives all the dynamic processes of Earth’s climate, so even the tiniest variations are hugely significant.”

Measuring the Sun’s energy output

Having studied total solar irradiance for over a century, PMOD began with ground-based instruments and later escalated to deploying space-based radiometers in the 1970s, to maintain a continuous dataset.

The noble endeavor earned PMOD the title of World Radiation Centre from the World Meteorological Organization to calibrate radiation measurements across global UN monitoring programs.

How Proba-3’s DARA works

DARA’s basic operating principle can be easily understood, but its execution showcases sophisticated engineering tailored for precision.

It has a 5-mm diameter cavity made from black-painted silver, which is known for its low temperature emissivity and high absorption efficiency.

Sunlight warms the cavity’s interior for 15 seconds, then a shutter automatically closes at its entrance.

For the next 15 seconds, electric heat maintains the cavity’s previous temperature, and this energy is extrapolated to the unit of total solar irradiance.

New possibilities with DARA

The new and improved DARA is ready to embark on a lifetime journey with Proba-3, ensuring millions of opening and closings, as proven in PMOD’s vacuum chamber tests.

“DARA is an improvement on previous radiometer designs with an optimized cavity design to minimize unwanted straylight and a multi-channel measuring system for self-calibration,” noted Finsterle.

“This generation of instrument also possesses a fully digital control loop, allowing the possibility of experimenting with higher frequency observations.”

Proba-3’s unique orbit

The main difference that sets Proba-3’s DARA apart is its elongated orbit that will propel it 60,000 km above the Earth’s surface.

It can adjust to slight changes in the solar disk’s size based on its distance from the sun, which changes during Earth’s annual elliptical orbit.

All the radiometer needs to know is its position in space, and its data gathering compensates for the shift.

In the end, Proba-3’s journey will offer us new insights into the sun’s energy outputs and the influences they have on Earth’s climate.

Proba-3’s optimal view of the Sun

One of the standout aspects of Proba-3’s mission is its unique orbit. Unlike typical low-Earth orbits, Proba-3’s extended trajectory is essential for achieving the mission’s objectives. This specific path enables precise positioning for optimal observation of the Sun.

By positioning itself farther from Earth, Proba-3 can better align its two spacecraft, the Occulter and the Coronagraph, to create an artificial solar eclipse. This alignment allows scientists to observe the Sun’s faint corona without interference from its blinding surface light.

Such observations are vital for studying solar winds and their impact on space weather, which can disrupt satellite communications and power grids.

Image Credit: ESA-P. Carril

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