Inside an unassuming building, behind a damp car park in Antwerp, Belgium, scientists are teaching two spacecraft to be dance partners for a performance that will take place in front of the Sun.
It’s early April, just days before the Great American Eclipse, when the Moon will slip in between us and our star, giving excited astronomers a rare, but brief, chance to see its corona – the ‘crown’ that forms its outer atmosphere.
If the two spacecraft can be taught to perform their dance correctly, the scientists in Belgium will be able to create their own, artificial eclipses and observe the corona whenever they want. Why? Because doing so could help us solve one of the biggest mysteries in solar physics: what’s happening inside the Sun’s fainter coronal ring.
There’s a lot we don’t know about the corona – why it’s over a million degrees hotter than the Sun’s surface, for instance. Or why space weather (the radiation, particles, magnetic fields and matter ejected by the Sun that can interact with Earth’s atmosphere and disrupt our electrical systems) originates from it.
We don’t know because the Sun’s light outshines the corona, making it impossible to see, unless something blocks the Sun’s light. Something like the Moon during an eclipse… or a pair of spacecraft performing a carefully choreographed dance.
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The spacecraft in question are of part the European Space Agency’s (ESA’s) Proba-3 mission and have to be taught to dance with each other because they’ll be too far from Earth to control with the precision required to produce artificial eclipses.
The mission’s full name, PRoject for OnBoard Autonomy, gives a clue (albeit an unwieldy one) as to the level of involvement its controllers on Earth expect to have.
Real total eclipses, as previously stated, are rare and brief events. They occur roughly 60 times every 100 years and, weather permitting, give viewers (wearing eclipse-safe glasses) only a few minutes of observing opportunity. Artificial eclipses would allow us to increase the frequency and duration with which we can see and study the corona.
We already have ground- and space-based telescopes, known as coronagraphs, that block out the light of the star so that nearby objects, which would otherwise be lost in the Sun’s glare, can be observed. They rely on an ‘occulting disc’, a small circle in the eyepiece that acts like the Moon during an eclipse.
Essentially, Proba-3 works in the same way, but ESA has taken the occulting disc out of the telescope’s eyepiece and turned it into a separate spacecraft.
Proba-3 is two spacecraft that need to work as one, in order to perform the perfectly controlled movements required to block out the Sun’s light. They’re dance partners, on a dancefloor that’s 60,000km (37,300 miles) away from Earth.
The Black Halo
The two spacecraft that make up the Proba-3 mission are known as the Occulter and the Coronagraph. If the choreography works, the Occulter will fly to a position that allows its 1.4m-diameter (4.6ft) disc to cover the face of the Sun.
Rather than creating a massive shadow over Earth, the disc will cast an 8cm (3in) shadow on the other Proba-3 spacecraft, the Coronagraph, which will be positioned about 150m (almost 500ft) away.
The Coronagraph will generate fresh, new views of the Sun’s coronal ring that astronomers like Dr Francisco Diego of University College London, will study. Seeing the first image come through will be, Diego says, very exciting. “It’ll feel like observing a long total solar eclipse, but without the filtering of our atmosphere.”
This is the first time that a telescope has been made of two independent units acting as one, giant spacecraft. The Coronagraph and Occulter will be joined when they launch from India in November, but once they separate, they’ll fall into formation to begin their dance.
The eclipses Proba-3’s dancing will produce will last six hours, not the few minutes we’re used to seeing on Earth. And the images it produces will differ from those of existing coronagraphs, such as the ESA-NASA SOHO (SOlar and Heliospheric Observatory) spacecraft, as they’ll reveal the corona’s ghostly inner ring that’s typically obscured by a black halo.
These black haloes are caused by light ‘leaking’ around the occulting discs in existing coronagraphs and so scientists have been unable to produce an image of the Sun without them.
But as Proba-3’s occulting disc will be so far away from its coronagraph, and both parts of the spacecraft will be so much closer to the Sun, less light can leak in to affect the images they produce.
In other words, Proba-3 could finally expose the entire corona and show us what’s going on underneath.
“You’ll be able to see the whole of the corona and chromosphere as well – and the transition region, which is where the action is taking place,” says Diego. “It promises to be a major revolution in solar physics.”
Sun Safety
For astronomers not involved in the project, like Diego, Proba-3 is a reminder that underneath those black halos could be answers to questions about the Sun and other stars, and even the planet we call home.
“In the future, the benefit will be enormous, because the technology to achieve this is going to be used for other space missions,” says Diego. “But the science that comes later is important. The science for solar physics, if successful, is going to be fantastic.”
This includes new discoveries about coronal mass ejections (CMEs), and the expulsions of plasma and magnetic fields from the Sun’s surface that send large amounts of energy through space.
When they hit Earth’s magnetic field, around one to two days later, the highly charged particles they contain disturb Earth’s atmosphere and cause aurorae like the Northern Lights.
They don’t only result in a pretty spectacle, however. CMEs also threaten our electrical infrastructure: a severe one could cause radio blackouts and outages in the power grid. Diego wants to know the origin of these CMEs. “That will be extremely useful for Proba-3 to find out.”
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CMEs also affect space weather beyond Earth, the streams of highly charged particles striking other planets in the Solar System, the Moon and satellites outside Earth’s protective magnetic field. (In 2022, space weather destroyed around 40 Starlink satellites.)
The International Space Station is within Earth’s protective field but, for astronauts travelling beyond it, CMEs could be lethal. According to NASA, an astronaut hit by a CME on the lunar or Martian surface would be exposed to 30 times the radiation of a normal chest X-ray.
For future crewed missions to the Moon or Mars, Proba-3’s glimpse into the origin of CMEs could prove to be life-saving.
“The better we know the behaviour of the Sun, the safer human missions in space will be,” Diego says. Solar radiation, he warns, “is one of the main problems with going back to the Moon” for missions like Artemis II. “The [astronauts on the] Apollo missions were ‘lucky’ because we didn’t know about these things,” he adds.
Knowing more about solar storms could help scientists become better at predicting them – and may even lead to the development of an early warning system for anyone who ends up living and working on the Moon or Mars.
Space Ballet
It isn’t just Proba-3’s images that could make waves, though; its dance is likely to set new benchmarks too. To get the occulting disc in exactly the right place to block out the Sun and photograph its corona, the two spacecraft – each the size of a small car – need to fly in a precise formation.
In order to do so, ESA has developed new, ultra-accurate laser technology that can keep the spacecraft in their relative positions with an accuracy of a single millimetre – even though they’ll be 144m (472ft) apart.
Think of it like two cars racing around a giant track but staying absolutely parallel to each other, while the drivers, instead of looking where they’re going, maintain eye contact. But instead of race car speeds, the two spacecraft are hurtling through space at around 10km/s (up to 6.2 miles per second).
“I think a lot of people don’t fully realise what kind of challenge this really is,” says Dr Jorg Versluys, ESA’s build manager for Proba-3. “I mean, one millimetre at 144m is crazy.”
To get an idea of the challenge they face, imagine 10 London buses parked end-to-end. The two spacecraft would sit at either end of the line of buses and need to be aligned to within a millimetre. But the spacecraft will be moving, at speed.
Versluys remembers walking the gap the instruments need to maintain one day while running tests. “At that moment, it dawned on me what kind of challenge we’re trying to tackle.”
In an attempt to tackle it, ESA has adapted conventional star trackers to help the Proba-3 spacecraft locate each other once they separate after launch.
Usually, star trackers look for specific constellations to help a satellite orient itself, but ESA has fitted one of the Proba-3 spacecraft with a unique pattern of LEDs and trained the other to search for this artificial constellation.
It doesn’t end there. The two spacecraft will also beam a laser between them so the Occulter can calculate how much it needs to move to cast its shadow in the correct place.
According to Versluys, these requirements have forced the team to be “creative and inventive, sometimes taking a bit of a risk, sometimes bending the rules a bit, sometimes breaking the rules.”
It’s an ambitious project and, for many of those involved in it, the goal is simply demonstrating that it’s possible.
According to Diego, this boundary-pushing formation flying, “will be valuable for planning and organising future space missions that require two or more satellites.”
That includes missions involving gravitational wave detectors, with the hope of measuring the dilation of space caused by low-frequency disturbances in space-time. Astronomers are racing to detect where these waves come from and Diego thinks spacecraft flying in formation could help us do that.
“You need a three-dimensional instrument, and that can be [created] in space – but you have to fly spacecraft in formation with a fraction of a millimetre precision,” says Diego. “Proba-3 is going to be very useful for that.”
All of which is to say that there’s a lot riding on Proba-3’s performance. With the launch looming in November, Versluys is already feeling stage fright: “You need to make sure that you’ve rehearsed everything: you need to make sure you’re ready, when the spacecraft have launched, to contact them and give them commands.”
Studying the Sun may be the purpose behind all the effort being put into the choreography and, if the dance is performed correctly, that would be an enormous victory. But the real show will start when Proba-3 sends back its first image.
“That will be an emotional moment,” Versluys smiles. “It will be magic.”
Proba-3's Priorities
Marie Beeckman can’t wait to see the first image Proba-3 produces and plans to hang it on her wall. Beeckman is one of the choreographers for the Proba-3 mission (officially, she’s the satellite operations manager for Redwire Space, the mission partner responsible for assembling and testing the spacecraft).
“Seeing the data [from Proba-3] come in for the first time will be the actual interesting part,” she says. Depending on who you ask, the primary goal for the Proba-3 mission changes: it’s either furthering solar physics or executing the intricate dance.
Even though she’s very excited about seeing the photos, Beeckman, who dreams of sending the command to trigger the spacecrafts’ separation after launch, is very much in the second camp.
“It’s what we work towards – and it’s also the most challenging part for us,” she says. As we speak, Proba-3 is sitting in the harsh, artificial light of a lab in that unassuming building behind the Belgian car park.
In this ‘dressing room’, the two spacecraft face away from each other. There’s a lot that stands between now and the moment they come face-to-face in front of the Sun. “Everybody is really excited to have it succeed. But obviously there are things that fail on the way,” Versluys adds. “A lot can go wrong.”
For example, the two spacecraft might not separate after launch – or separate, but one might not ‘wake up’. Something could get damaged or communication could be lost.
Worst of all, the spacecraft could collide. “If they collide, we can’t predict what’s really going to happen, but there will be damage,” Versluys says. “We assume that that would be a loss of mission.”
Beeckman, shaking her head, adds: “The chances of them hitting each other are way too high in my opinion. Having two objects so close to each other in space…”
All they can do to ensure the mission runs smoothly is to test everything over and over again. That testing is down to Beeckman and her team, who are racing to iron out any issues. “Everybody’s always excited about working on this… we love seeing the progress on it.”
In the lab, there’s a palpable buzz among the researchers and engineers, but also a nervousness. For Beeckman, the biggest challenge is creating space-like conditions within that large, bright room.
For the most part, testing has gone smoothly, but there’s no way to rehearse the formation flying, the actual dance until the curtain goes up – until the Occulter and Coronagraph are actually in space and the show begins.
She also worries about transporting the two spacecraft across the world to India, from where they’ll launch, as humidity, temperature, dust and road conditions could cause damage.
In fact, the lab where Beeckman’s team works is a clean room that operates on a traffic-light system: red when there’s a lot of disturbance and potential contamination from human activity; green when the room has recovered.
Despite everyone entering the room through an airlock, the engineers anxiously wait for the particle levels to drop after anyone goes in to check the spacecraft. These are sensitive performers.
In India, the spacecraft will undergo a final ‘health check’. Then, from the control centre in Redu, Belgium, Beeckman’s team will “do one last rehearsal: a fake launch to see how we’re going to deal with everything.”
Versluys will be watching the launch in person. “We stack them together, put them on the launcher, then do the countdown – and keep our fingers crossed,” he laughs, anxiously.
As for Diego, he plans to travel to Spain to watch the next total eclipse in 2025, but, thanks to Proba-3, could see one sooner. He points to a picture of the 1994 Bolivian eclipse hanging on his office wall – a picture that, in just a few months, could be joined by the same print that Beeckman will have hanging on her wall – if everything goes according to plan.
Proba-3 could usher in a new era of eclipses: ones that last hours instead of minutes and are visible even when it’s cloudy. Its dances may finally lift the curtain on the mysteries of the corona, the faint ring that has been obscured by the Sun for so long.
“Certainly, this mission will be a milestone in new space observations,” Diego says. “We’re coming into the new generation: we already had the SOHO space telescope and then James Webb. Now, we’re jumping another step forward. [Proba-3] will be extremely important in proving that this can be done, and the best way of doing it.”
About our experts
Dr Francisco Diego is an astronomer at University College London. After earning his BSc in Mechanical Engineering in Mexico City, he went on to work at the Sociedad Astronomica de Mexico (Astronomical Society of Mexico), Planetario Luis E. Erro, and Instituto de Astronomia (Institute of Astronomy, UNAM) before going on to earn his PhD in Astronomy at UCL.
He has since gone on to be a lecturer at UCL's Department of Physics and Astronomy, the UK Association for Astronomy Education's vice president while being part of the Royal Astronomical Society and International Astronomical Union.
As a presenter, author, producer, and broadcaster as well as a lecturer, Dr Francisco Diego has also been a part of various BBC programmes like The Planets and Wonders of the Universe.
Marie Beeckman is the Satellite Operations System Engineer at Redwire Space. After completing her Bachelor's degree in Electromechanical Engineering at Ghent University, she stayed on to a Master's in Automation Engineering.
Then, she went to KU Leuven for her Master's in Space Studies before combining all of her studies to start her flourishing career in the Space sector, including the Proba-3 project.
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