Imagine there was a way to make a giant satellite, 100 metres across, without having to launch it as a single, huge payload.
This could be possible using a technology called ‘formation flying’ – two or more satellites flying in formation, acting together as one big satellite to form, for example, an enormous telescope.
The European Space Agency’s Proba-3 mission, due to launch by the end of 2024, will accomplish precisely this by using two satellites flying in formation at a distance of 144 metres apart.
PROBA-3 and the solar corona
PROBA-3 will capture images of the Sun’s outer layer, the corona, from which coronal mass ejections originate.
These large outbursts produce tiny, high-speed particles that can disrupt anything electrical in their path, potentially causing satellite and aircraft failures here on Earth.
Capturing images of the corona requires blocking the bright sunlight from the Sun using a mask, creating an artificial solar eclipse.
Though satellites have been built in the past to image the Sun’s corona, these held the mask very close to the camera.
The resulting diffraction effects make capturing images very close to the Sun’s surface challenging.
Formation flying
The solution is formation flying. PROBA-3 consists of two satellites.
One functions as the coronagraph that captures images.
The other satellite is the occulter, equipped with a round disc to serve as the occulting mask, blocking sunlight.
The pair maintain a distance of 144 metres from each other, using their own guidance, navigation and propulsion systems, all controlled by an onboard computer called the Advanced Data and Power Management System, developed by Redwire, where I work.
Additionally, the satellites are equipped with specialised formation flying units.
The first is the Visual Based Sensor (VBS) on board the occulter spacecraft.
This uses blinking lights on the coronagraph to locate the other spacecraft.
Once the VBS has aligned the two spacecraft down to a few centimetres, the occulter satellite activates its Fine Longitudinal and Lateral Sensor (FLLS).
The FLLS sends a laser to the coronagraph satellite, which reflects it back, allowing the spacecraft to line up with millimetre precision.
Finally, the Shadow Position Sensor (SPS) on the occulter satellite measures the shadow cast onto the door covering the instrument in charge of imaging the corona.
Once the shadow alignment is done, the door opens, allowing the spacecraft to image the Sun’s corona.
The satellites fly in a 20-hour, highly elliptical orbit around Earth.
Every time they’re at apogee – the part of the orbit furthest away from Earth – PROBA-3 can take pictures of the corona.
Testing the spacecraft
Around 2020, the flight units started arriving at Redwire in Kruibeke, Belgium, where me and my team made sure each unit could communicate with the onboard computer.
This was my favourite part of the job.
In some cases it took many fun hours trying to find out why the unit wasn’t connecting or ‘speaking the same language’ as the onboard computer.
Once all the units were working, we could start more complex testing to ensure they could interact with each other.
It’s important to test both spacecraft could work together, as they will need to when flying in formation.
PROBA-3 launch and mission
The next step will be preparation for launch and, just as importantly, what comes afterwards.
At first, the satellites will be stacked upon each other, and only the coronagraph will be made operational.
After a month of testing, when we’re sure all units have made it through launch intact, we’ll separate the two satellites.
The occulter will then start living its own life. When that spacecraft is also fully commissioned, both satellites will be made operational.
After three months of flying around Earth, PROBA-3 will finally take its first picture of the Sun’s corona.
You can bet that I will print that picture out and hang it in my bedroom!
This article appeared in the June 2024 issue of BBC Sky at Night Magazine.
