The Solar Orbiter spacecraft, a joint Nasa and European Space Agency (ESA) mission, is set to be launched from Cape Canaveral just after 4am UK time on Monday morning, and will reach its vantage point above the planetary plane by the end of 2021.
From Earth, the sun looks like a perfectly uniform fiery ball, but scientists say that its extremities could look unfamiliar, possibly featuring gaping dark holes or angular geometric structures.
“There’s no rational reasons why the poles shouldn’t be different”, said Mark McCaughrean, senior advisor for science & exploration at ESA. “Be prepared for surprises.”
The only previous mission to have flown above the sun’s poles was Nasa’s Ulysses probe, launched in 1990, but while it took measurements of the sun’s magnetic field and solar wind, it did not have a camera.
Over the next year, the Solar Orbiter will conduct several flybys of Venus and one of the Earth, using the planets’ gravity to slingshot into a tilted elliptical orbit. By the end of 2021, its orbit will have an inclination of 17° – high enough to take images of the sun’s extremities – and if the mission is extended it could reach an inclination of 33°.
Computational models and magnetic field measurements suggest that the sun’s poles could be dominated by huge coronal holes – areas of the surface that are less dense than their surroundings and will show up as ominous dark pools.
“These coronal holes are enormous at the poles. We’ll be able to peer over the edges … and see how it grows and shrinks with solar activity”, said Nicola Fox, director of Nasa’s heliophysics division. “It’s something we’ve never been able to do before.”
Striking geometric features are also a possibility, according to McCaughrean. The first images of Jupiter’s north pole surprised astronomers with an octagonal arrangement of storms twirling like cogs around a central vortex, while Saturn’s north pole features a vast hexagonal cloud structure.
The $1.3bn mission will run parallel to Nasa’s Parker Solar Probe, launched 18 months ago, which has flown closer to the sun than any spacecraft, and the Inouye solar telescope, which has acquired the highest resolution images of the solar surface since it began observations in December.
At just 3.83 million miles from the solar surface – technically inside the sun’s atmosphere – Parker will be seven times closer to the Sun than Solar Orbiter. But, McCaughrean points out, Parker will not be taking any direct images because if it looked at the sun, it would melt. From its slightly less scorching vantage point, Solar Orbiter will be able to make long-range observations of surface features such as sun spots as well as detecting the solar wind streaming past.
“We always wanted to make this seeing and feeling connection. The stuff that you can see at the surface you can feel going past a few days later”, said McCaughrean. “Parker is out there going ‘Ooh, I’m feeling stuff but I’ve got no idea where it came from’.”
Solar Orbiter’s camera and other instruments are housed behind a heat shield, peering out through shuttered peepholes that close whenever the probe’s interior gets too hot. The titanium shield is coated in a substance called SolarBlack, made from charred animal bones, something Esa concluded was uniquely well-suited to keeping the craft at a stable temperature throughout the mission.
The probe’s orbit, which at times will move almost in synch with the sun’s own rotation, will allow it to track sun spots on the solar surface as they appear and vanish, and will investigate the origins of the solar wind as it streams off the sun.
The mission will also use helioseismology – measurements of small and large scale oscillations at the sun’s surface – to try and unravel the sun’s internal structure. The sun’s acoustic “ringing” can also reveal sun spots bubbling up from the interior days before they appear as darkened patches on the surface.
Solar Orbiter is scheduled to begin making science measurements in May, with full operations starting in November 2021, when the probe’s telescopes switch on.
“It will capture the imagination like science fiction does and inspire the next generation of scientists and space explorers,” said Yannis Zouganelis, Esa’s deputy project scientist for Solar Orbiter.