On December 4, 2024, ESA is set to launch an audacious two-part telescope into space to study the sun’s atmosphere.

If you’ve ever seen a total solar eclipse, you’ll have caught a glimpse of this wispy outer atmosphere, called the corona. With the moon eclipsing the dazzling body of the sun, this faint light streaming from our star suddenly becomes visible. And coronagraphs – telescopes that study the sun’s atmosphere – work the same way. They block out the sun’s glare with a disk called an occulter attached to the front of the telescope.

But ESA’s new spacecraft, Proba-3, isn’t like any other coronagraph. The occulter isn’t attached to the telescope … but will instead be on a separate satellite, 492 feet (150 meters) away. Soaring through space in perfect coordination, the two spacecraft will form the largest coronagraph ever made.

This new instrument is set to launch from Satish Dhawan Space Centre in Sriharikota, India, on December 4, at 10:38 UTC.

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EarthSky’s Will Triggs explains the ground-breaking Proba-3 mission in this short video.

Proba-3, reinventing the coronagraph

Scientists have been using coronagraphs to study the sun for almost a century, with French astronomer Bernard Lyot building the first in 1931. Since then we’ve sent several coronagraphs into space, in order to image the sun without the distortion of Earth’s atmosphere. So what’s the need for this unique two-part coronagraph?

Damien Galano, Proba-3’s mission manager, explained:

[Designing a coronagraph] might sound simple, but it’s rendered much harder by the peculiar fact that light acts as both particles and waves. This means some light spills around the edge of whatever’s blocking it, like waves around a seawall. This phenomenon is known as diffraction; it needs to be designed against to minimize unwanted sunlight reaching your instrument.

And the best way to avoid diffraction? Move the occulting disk farther from the telescope. As Andrei Zhukov, Principal Investigator of Proba-3’s main instrument, explained:

This is why total solar eclipses give us such an excellent view of the corona, because the moon is around 238,000 miles (384,000 kilometers) away from Earth, so diffraction effects are minimal.

But a large distance between telescope and the occulter is hard to achieve on a single spacecraft. The LASCO C2 coronagraph on NASA’S SOHO spacecraft – which we often rely on for our daily sun news updates – has its occulter just 70 centimeters (28 inches) away.

It was to overcome this limitation that, two decades ago, scientists at the Laboratoire d’Astrophysique de Marseille came up with Proba-3’s system, in which an entirely separate spacecraft occults the sun from afar.

The fantastic feat of formation flying

To function as a coronagraph, the pair of satellites that make up Proba-3 will have to successfully perform what ESA is calling the ‘world’s first precision formation flying mission’.

The satellites will launch together and then separate into tandem orbits around Earth. These 19.7-hour orbits will be highly elliptical, bringing them just 373 miles (600 kilometers) away at perigee – their closest to Earth – and 37,612 miles (60,530 kilometers) away at apogee, their farthest from Earth.

And during apogee, when Earth’s gravitational pull is weaker and requires less fuel, the satellites will maneuver into formation. They’ll line up 492 feet (150 meters) apart, so that the outer spacecraft’s 4.6 feet (1.4 meter) occulting disk creates an artificial solar eclipse for the inner satellite.

To keep the eclipse stable, they’ll maintain their separation to a precise single millimeter. The spacecraft will do this autonomously, communicating with each other via LEDs and lasers. A shadow detector on the telescope satellite will make corrections if the occulting satellite’s 3 inch (8 centimeters) shadow is in any way misaligned. And they’ll maintain this precise positioning for six hours at a time.

Proba-3 will provide a unique view of the sun

The increased distance between Proba-3’s telescope and its occulter will allow it to see much closer to the edge of our star without bright sunlight bleeding into the image. Proba-3 will be able to see the corona from just 1.1 times the radius of the solar disk. For contrast, LASCO C2’s field of view starts at 1.5 times the solar radius.

So scientists using Proba-3 will be able to see the sun’s inner corona, which is normally visible only during a total solar eclipse. Total solar eclipses only occur an average of once every 18 months, and typically last under seven minutes. But Proba-3 will be able to study the inner corona for six hours at a time around 50 times a year.

And it will study this region with a better frame rate (number of frames per second) than other space-based coronagraphs. While LASCO C2 takes an image roughly every 12 minutes, Proba-3 can image the corona up to every 30 seconds.

Scientists hope this detailed view of the inner corona will provide insight into the development of coronal mass ejections (CMEs): blobs of solar material and magnetic fields blasted out during events on the sun. CMEs can cause auroras if they reach Earth, as they disturb our magnetic field. And particularly strong CMEs can pose a threat to satellites and even power grids on Earth.

A new look at the corona, coming soon

Observations with Proba-3 are set to begin after a roughly four-month commissioning stage. And the team currently plans to have two six-hour observing periods every week, depending on how much fuel needs to be saved across the planned two years of operation.

Proba-3 is an ambitious innovation, which ESA describes as a “technology demonstration mission”. And if all goes to plan, it should be a useful tool in scientists’ growing sun-observing arsenal. Mission scientist Joe Zender said:

Success will rely on the formation flying technology working as planned, of course, but the closer we get to launch, the more I realize the excitement of what we are doing, including co-observations with many other solar observing missions.

Bottom line: ESA is launching an ambitious spacecraft to study the sun’s atmosphere. Proba-3 is comprised of 2 satellites that will align to form an artificial solar eclipse.

Via ESA

Read more: Why is the sun’s atmosphere hotter than its surface?