The ships to carry out orbital solar eclipses on demand

Solar eclipses offer a brief glimpse of the ghostly solar corona that surrounds the Sun, which normally cannot be observed due to the Sun's brightness. But the corona can soon be studied more continuously: the European Space Agency (ESA) has publicly presented the pair of spacecraft that make up its new Proba-3 mission, intended to produce orbital solar eclipses on demand.

The Proba-3 Occulter spacecraft will fly about 150 meters away from the second spacecraft, Coronagraph, unveiled to the media at the Redwire Space facility in Kruibeke, Belgium, where they are undergoing to pre-flight tests. The pair will align with the Sun so precisely that the occultator will cast a shadow on the face of the coronagraph, obscuring the Sun so that the corona is visible.

“The two spacecraft will act as if they were one huge 150-metre-long instrument,” explains Dietmar Pilz, ESA's Director of Technology, Engineering and Quality. «However, achieving this will be an extreme technical challenge, because if there is the slightest mismatch, it will not work. “The development process has been equally long, carried out by a consortium of ESA Member States led by Spain and Belgium, so I am very pleased to see Proba-3 here today, preparing for launch.”

Create artificial solar eclipses in orbit

The underlying idea is not new: a cylindrical Apollo capsule attempted to do the same with a Soviet Soyuz spacecraft during the Apollo-Soyuz mission in 1975. But the goal with Proba-3 is to routinely produce these artificial eclipses through a formation in precise flight, up to six hours in a row for each orbit of 19 hours and 36 minutes.

Solar eclipses occur due to a remarkable cosmic coincidence: the Sun is 400 times larger than Earth's Moon, but it is also exactly 400 times farther away. This means that when the two bodies are exactly aligned in space, the Moon covers the fiery face of the Sun, revealing the solar corona, which extends millions of kilometers from our parent star.

The two Proba-3 ships, one in the foreground and the other behind. (Photo: © ESA / P. Sebirot)

A little-observed region of our solar system

This little-observed region of our solar system is of both scientific and practical interest: a million degrees hotter than the surface of the Sun below it, the corona gives rise to the solar wind and space weather, along with violent ejections known as “coronal mass ejections” that cause space weather and solar storms, and can affect both satellites in orbit and terrestrial energy and communications networks.

To observe the corona, specialized ground-based and orbiting telescopes called “coronographs” can incorporate “occultation disks,” carefully designed shields to cover the Sun within their field of view, mimicking a solar eclipse. But their effectiveness is limited by a phenomenon called “diffraction,” in which scattered light leaks around the edges of coronagraphs. The way to minimize this effect is to move the occultation disk far away from the observing coronagraph, but practical limits on the size of spacecraft made that solution impractical for space.

Until now… By flying the two spacecraft in precise formation and with millimeter accuracy, the main ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) instrument images of the solar corona) from Proba-3 will provide data as if aboard a single rigid spacecraft, opening up a hitherto elusive region of study between 3 and 1.1 solar radii around the Sun.

This precision will be obtained by combining a set of increasingly precise positioning technologies: satellite navigation; radio links between satellites, visible light cameras centered on LEDs and, finally, a laser beam reflected between spacecraft. The second instrument on Proba-3 is a radiometer that measures the Sun's total energy output, important for meteorological modeling.

Fully autonomous formation flight

The formation flight will be carried out completely autonomously, towards the top of each 60,000 kilometer altitude orbit, where gravitational, atmospheric and magnetic disturbances are minimized. The rest of the time, the pair will spend the rest of its orbit in safe passive drift. (Source: ESA)