“Say Goodbye to Mr. Burns: The Groundbreaking Space Mission That Will Revolutionize Eclipses!”

Within an unremarkable structure, situated behind a moist parking lot in Antwerp, Belgium, researchers are instructing two spacecraft to become dance partners for an event that will unfold in front of the Sun.

It’s early April, just days prior to the Great American Eclipse, when the Moon will traverse between us and our celestial body, providing eager astronomers a fleeting, yet exceptional, opportunity to observe its corona – the ‘crown’ that constitutes its outer atmosphere.

If the two spacecraft can be trained to execute their movements accurately, the researchers in Belgium will be capable of producing their own, artificial eclipses and studying the corona at will. Why? Because this could assist us in uncovering one of the most significant enigmas in solar physics: what is occurring within the Sun’s dimmer coronal ring.

Numerous aspects regarding the corona remain unknown – for example, why it exceeds one million degrees hotter than the Sun’s surface. Or why solar weather (the radiation, particles, magnetic fields, and matter expelled by the Sun that can interact with Earth’s atmosphere and interfere with our electrical systems) arises from it.

We lack knowledge on this because the Sun’s brilliance overwhelms the corona, rendering it invisible, unless something obscures the Sun’s light. Something akin to the Moon during an eclipse… or a duo of spacecraft executing a meticulously choreographed dance.

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The spacecraft in question are part of the European Space Agency’s (ESA’s) Proba-3 initiative and must learn to dance with each other as they will be too distant from Earth to be controlled with the accuracy necessary to create artificial eclipses.

The mission’s complete designation, PRoject for OnBoard Autonomy, offers a hint (though an awkward one) regarding the level of engagement its operators on Earth anticipate to have.

Genuine total eclipses, as noted earlier, are uncommon and fleeting occurrences. They take place approximately 60 times every century and, weather permitting, provide viewers (wearing eclipse-safe spectacles) merely a few minutes of viewing opportunity. Artificial eclipses would enable us to augment the frequency and duration with which we can observe and analyze the corona.

We currently possess ground- and space-based telescopes, referred to as coronagraphs, that obstruct the star’s light so that nearby objects, which would usually be concealed in the Sun’s brightness, can be examined. They depend on an ‘occulting disc,’ a small circle in the eyepiece that functions like the Moon during an eclipse.

Essentially, Proba-3 operates similarly, but ESA has removed the occulting disc from the telescope’s eyepiece and transformed it into an independent spacecraft.

Proba-3 consists of two spacecraft that need to operate as one, to execute the precisely controlled motions necessary to obscure the Sun’s illumination. They are dance partners on a dance floor that’s 60,000km (37,300 miles) distant from Earth.

The Black Halo

The duo of spacecraft comprising the Proba-3 initiative are known as the Occulter and the Coronagraph. If the choreography succeeds, the Occulter will fly into a position that permits its 1.4m-diameter (4.6ft) disc to cover the Sun’s face.

Instead of casting a massive shadow over Earth, the disc will project an 8cm (3in) shadow on the other Proba-3 spacecraft, the Coronagraph, positioned approximately 150m (nearly 500ft) away.

The Coronagraph will produce new, fresh views of the Sun’s coronal ring that astronomers like Dr. Francisco Diego from University College London will examine. Witnessing the initial image emerging will be, according to Diego, incredibly thrilling. “It’ll feel akin to witnessing a prolonged total solar eclipse, but without our atmosphere acting as a filter.”

This marks the inaugural instance where a telescope has been fashioned from two independent units operating as a single, colossal spacecraft. The Coronagraph and Occulter will be joined upon their launch from India in November, but once they detach, they’ll align to commence their dance.

The eclipses produced by Proba-3’s movements will endure six hours, not the mere minutes we are accustomed to witnessing on Earth. And the images it generates will differ from those of existing coronagraphs, such as the ESA-NASA SOHO (SOlar and Heliospheric Observatory) spacecraft, as they’ll unveil the corona’s ethereal inner ring typically hidden by a black halo.

These black halos arise from light ‘leaking’ around the occulting discs in current coronagraphs, preventing scientists from capturing an image of the Sun devoid of them.

However, because Proba-3’s occulting disc will be situated much farther from its coronagraph, and both components of the spacecraft will be considerably closer to the Sun, reduced light can seep in to influence the images they capture.

In other terms, Proba-3 could ultimately reveal the entirety of the corona and unveil what lies beneath.

“You’ll get to see the complete corona and chromosphere as well – and the transition region, which is where the action is occurring,” explains Diego. “It holds the potential to be a significant breakthrough in solar physics.”

Sun Safety

For astronomers not participating in the initiative, like Diego, Proba-3 serves as a reminder that underneath those black halos lies the potential to answer questions regarding the Sun and other stars, and even the planet we inhabit.

“In the years to come, the advantages will be immense, because the technology utilized to achieve this will be applied to other space missions,” states Diego. “However, the scientific knowledge gained later is crucial. The science relating to solar physics, if accomplished, will be remarkable.”

This encompasses new insights about coronal mass ejections (CMEs), and the expulsion of plasma and magnetic fields from the Sun’s exterior that propel substantial amounts of energy through space.

When they encounter Earth’s magnetic field, roughly one to two days later, the highly charged particles they carry disrupt Earth’s atmosphere, resulting in auroras like the Northern Lights.

These phenomena do not merely yield a beautiful display, however. CMEs also pose risks to our electrical infrastructure: a severe event could result in radio blackouts and failures in the power grid. Diego aims to uncover the origins of these CMEs. “That will be exceedingly beneficial for Proba-3 to determine.”

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CMEs also influence space weather beyond our planet, the streams of highly charged particles impacting other celestial bodies in the Solar System, the Moon, and satellites outside Earth’s protective magnetic field. (In 2022, space weather led to the destruction of approximately 40 Starlink satellites.)

The International Space Station resides within Earth’s protective field but,for astronauts venturing beyond it, CMEs could be fatal. As per NASA, an astronaut struck by a CME on the lunar or Martian terrain would face 30 times more radiation than a standard chest X-ray.

For forthcoming crewed expeditions to the Moon or Mars, Proba-3’s insight into the genesis of CMEs could be lifesaving.

“The more we comprehend the Sun’s behavior, the safer human endeavors in space will be,” Diego indicates. Solar radiation, he cautions, “is one of the principal challenges of returning to the Moon” for missions such as Artemis II. “The [astronauts during the] Apollo missions were ‘fortunate’ because we weren’t aware of these issues,” he remarks.

Understanding more about solar storms might assist scientists in enhancing their predictive capabilities – and could even pave the way for an advanced early warning system for individuals who find themselves residing and working on the Moon or Mars.

Space Ballet

It’s not just Proba-3’s visuals that could create a stir; its maneuvering is expected to establish new standards as well. To position the occulting disc precisely to obscure the Sun and capture its corona, the two spacecraft – each comparable to a small automobile – must adhere to an exact formation.

To achieve this, ESA has innovated cutting-edge, highly precise laser technology that can keep the spacecraft within their spatial relationship with a precision of a single millimeter – despite being 144m (472ft) apart.

Imagine it as two vehicles racing around a massive circuit but remaining entirely parallel, while the drivers, instead of watching the road, maintain eye contact. However, rather than race car velocities, the two spacecraft are soaring through space at approximately 10km/s (up to 6.2 miles per second).

“I believe many people do not fully grasp the magnitude of this challenge,” says Dr. Jorg Versluys, ESA’s build manager for Proba-3. “One millimeter at 144m is astonishing.”

To visualize the scale of the challenge, think of ten London buses lined up end-to-end. The two spacecraft would need to be positioned at opposite ends of this line and aligned with millimeter precision. Yet, the spacecraft will be in motion, traveling at speed.

Versluys recalls walking the distance the instruments must maintain one day during testing. “At that moment, I realized the challenge we’re attempting to overcome.”

To confront it, ESA has modified standard star trackers to assist the Proba-3 spacecraft in locating one another after they separate post-launch.

Typically, star trackers search for specific constellations to enable a satellite to orient itself, but ESA has equipped one of the Proba-3 spacecraft with a distinct pattern of LEDs and trained the other to locate this artificial constellation.

But that’s not all. The two spacecraft will also transmit a laser signal between them, allowing the Occulter to compute how much it needs to adjust to cast its shadow in the appropriate location.

According to Versluys, these criteria have compelled the team to be “imaginative and innovative, sometimes taking a few risks, occasionally bending the rules slightly, and at times breaking them.”

It’s an ambitious endeavor and, for many involved, the primary objective is simply to showcase that it can indeed be done.

Diego believes this groundbreaking formation flying, “will be crucial for planning and organizing future space missions that necessitate two or more satellites.”

This encompasses missions with gravitational wave detectors, aiming to measure the expansion of space triggered by low-frequency disturbances in space-time. Astronomers are eager to locate the sources of these waves, and Diego is optimistic that spacecraft flying in formation could assist in that pursuit.

“You need a three-dimensional instrument, which can be [constructed] in space – but you must fly spacecraft in formation with a fraction of a millimeter precision,” explains Diego. “Proba-3 will be very beneficial for this.”

This is to say there’s a great deal at stake regarding Proba-3’s functionality. With the launch approaching in November, Versluys is already experiencing pre-launch jitters: “It’s essential to ensure every detail is rehearsed: you must be fully prepared to establish contact and issue commands once the spacecraft are launched.”

Investigating the Sun may be the driving force behind all the effort dedicated to the choreography, and if executed flawlessly, it would represent a significant triumph. However, the real excitement will commence when Proba-3 transmits its first image.

“That will be a moment filled with emotion,” Versluys smiles. “It will be enchanting.”

Proba-3’s Priorities

Marie Beeckman is eagerly anticipating the first image produced by Proba-3, intending to display it on her wall. Beeckman is one of the choreographers for the Proba-3 mission (officially, she serves as the satellite operations manager for Redwire Space, the mission collaborator tasked with assembling and testing the spacecraft).

“Witnessing the data [from Proba-3] come in for the first time will be the truly fascinating aspect,” she states. Depending on whom you ask, the primary aim of the Proba-3 mission fluctuates: it either involves advancing solar physics or executing the intricate maneuvering.

Although she’s extremely enthusiastic about viewing the images, Beeckman, who dreams of sending the command to initiate the spacecraft’s separation post-launch, firmly aligns with the latter focus.

“It’s our ultimate goal – and additionally, it presents the most challenging aspect for us,” she mentions. As we converse, Proba-3 rests in the harsh, artificial light of a laboratory in that unremarkable structure behind the Belgian parking lot.

Within this ‘dressing room’, the two spacecraft face away from one another. There are numerous obstacles to navigate before they can meet directly in front of the Sun. “Everyone is thrilled for it to succeed. However, naturally, there are factors that can go awry,” Versluys adds. “A great deal can potentially go wrong.”

For instance, the two spacecraft might not separate after launch – or they may do so, but one could fail to ‘wake up’. Damage could occur or communication might be lost.

Worst of all, there’s the possibility of a collision. “If they collide, we can’t forecast the exact outcome, but damage will certainly ensue,” Versluys says. “We anticipate that such an event would result in a mission loss.”

Beeckman, shaking her head, adds: “The likelihood of them making contact is far too high, in my view. Having two objects so closely situated in space…”

All they can do to guarantee the smooth execution of the mission is to conduct continual rigorous testing. That responsibility falls to Beeckman and her team, who are working swiftly to resolve any issues. “Everyone is consistently thrilled about being part of this… we love observing the progresson it.”

Within the laboratory, there’s an evident energy among the researchers and engineers, yet there’s also a sense of apprehension. For Beeckman, the primary obstacle is simulating space-like conditions in that expansive, illuminated area.

Overall, the testing has proceeded without major issues, but rehearsing the formation flying, the actual choreography can only be practiced until the moment arrives – until the Occulter and Coronagraph are truly in space and the performance commences.

She also expresses concern regarding the transport of the two spacecraft to India, from where they are scheduled to launch, as factors like humidity, temperature, dust, and road conditions could potentially inflict harm.

Indeed, the laboratory where Beeckman’s team operates is a clean room governed by a traffic-light system: red indicating significant disturbance and potential contamination from human interaction; green denoting recovery of the room.

Even with everyone entering through an airlock, the engineers anxiously monitor for particle levels to decrease after anyone checks the spacecraft. These are delicate instruments.

In India, the spacecraft will undergo a final ‘health assessment’. Following that, from the control facility in Redu, Belgium, Beeckman’s team will “perform one last rehearsal: a simulated launch to prepare for the upcoming challenges.”

Versluys will be attending the launch in person. “We stack them together, place them on the launcher, then initiate the countdown – and hope for the best,” he chuckles nervously.

As for Diego, he intends to visit Spain to witness the next total eclipse in 2025, but, thanks to Proba-3, he might experience one even sooner. He gestures to a photograph of the 1994 Bolivian eclipse displayed on his office wall – a picture that, in just a few months, could be accompanied by the same image that Beeckman will have hanging in her space – if everything unfolds according to plan.

Proba-3 may herald a new chapter of eclipses: ones that endure for hours instead of mere minutes and can be seen even amid cloudy skies. Its movements might finally unveil the secrets of the corona, the faint halo that has remained hidden by the Sun for an extended period.

“Certainly, this mission will mark a pivotal moment in new space observations,” Diego asserts. “We’re entering a new generation: we’ve already had the SOHO space telescope and then the James Webb. Now, we’re making another significant leap forward. [Proba-3] will be crucial in demonstrating that this can be achieved, and in the most effective manner.”

About our specialists

Dr Francisco Diego is an astronomer at University College London. After obtaining his BSc in Mechanical Engineering in Mexico City, he continued his career at the Sociedad Astronomica de Mexico (Astronomical Society of Mexico), Planetario Luis E. Erro, and Instituto de Astronomia (Institute of Astronomy, UNAM) before securing his PhD in Astronomy at UCL.

He has since taken on the role of a lecturer at UCL’s Department of Physics and Astronomy, has served as vice president of the UK Association for Astronomy Education, and has been involved with the Royal Astronomical Society and International Astronomical Union.

As a presenter, author, producer, and broadcaster, as well as a lecturer, Dr Francisco Diego has participated in numerous BBC programmes, such as The Planets and Wonders of the Universe.

Marie Beeckman works as the Satellite Operations System Engineer at Redwire Space. After earning her Bachelor’s degree in Electromechanical Engineering from Ghent University, she pursued a Master’s in Automation Engineering.

Subsequently, she attended KU Leuven for her Master’s in Space Studies before merging all her experiences to launch her thriving career in the space industry, including the Proba-3 initiative.

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