Proba 3: Shadowing the Sun
On Proba 3, Europe's mission to eclipse the Sun on demand
This week I’m looking at the Proba-3 mission, launched on December 5. In the next week I plan another article for paying subscribers on the Parker Solar Probe, scheduled to make its closest approach to the Sun on December 24. I’m also hoping to get one more long article out on dark matter before the end of the year. As always, please think about subscribing if you haven’t done so already - paid subscriptions are low-cost and they really help me spend more time researching and writing!
It is possible, if you can move fast enough, to outpace the shadow of the Moon. In 1973 someone actually did this: they took a prototype of Concorde, serial number 001, cut holes in the roof for cameras and telescopes, and then flew at supersonic speeds across the deserts of northern Africa.
Their goal was a total eclipse of the Sun, or, more precisely, the darkest moment of that eclipse: the point of totality. This is a special moment. The Earth, Moon and Sun fall into perfect alignment; the disc of the Moon, thanks to an odd coincidence, exactly covers the disc of the Sun, and for a few brief minutes the world falls dark.
Strange things happen in those minutes. Birds stop singing. Animals prepare, as though it were night, to sleep. Temperatures fall, winds slow. The Moon itself appears inky black and around it, streaming out in all directions, radiates a cloud of shimmering white filaments. Even the stars themselves can seem to shift: it was during an eclipse, in 1919, that Arthur Eddington first proved the truth of Einstein’s relativity.
And then it is over. The Earth rotates, the Moon drifts on, and so, after a moment of awesome darkness, the Sun returns. Totality cannot last long: at most it may reach seven minutes, though often it lasts less than five.
The eclipse itself, however, does not end. It simply moves on, tracing the shadow of the Moon as it races over the surface of the Earth. If you can follow that shadow, you can prolong the moment of totality - not indefinitely, because the shadow eventually moves off the surface of the Earth entirely, but for much longer than seven minutes. Aboard the Concorde, flying at supersonic speeds, darkness was maintained for seventy-four minutes. It remains the longest period of totality ever experienced.
But not for much longer. Sometime in the next few months a pair of European satellites together known as Proba-3 will begin creating eclipses of their own far above the Earth. If things go to plan their totality will last not for seven minutes, nor even seventy-four, but for six hours at a stretch. Even more astonishingly the satellites will repeat this day after day, recording a sequence of artificial eclipses lasting over a thousand hours in total.
All of this is done not just for fun - although, the idea of creating and experiencing eclipses is very cool - but because there are things we can only study during totality. One, as Eddington showed in 1919, is the subtle shift of starlight thanks to the warping of space and time by the Sun’s mass. The other is the corona, that pearl white halo that appears only when the brightness of the Sun is obscured.
I. The Ultimate Coronagraph
It is this corona, often described as the Sun’s atmosphere, that is the target of Proba-3. In essence the two spacecraft will form an instrument known as a coronagraph, a device that allows the corona to become visible. Most other coronagraphs are simple: a disc is placed in front of a telescope and then positioned so as to block the light of the Sun without obscuring the surrounding corona, much as the Moon does in an eclipse.
Proba-3 will take a similar, though more sophisticated, approach. One of the two spacecraft, known as the Occulter, has a large disc fixed to one side of it. The other, the Coronagrapher, carries the cameras and instruments needed to observe the corona.
In order to create an eclipse, the two spacecraft align themselves with the Sun. From the viewpoint of the Coronagrapher, the Occulter moves directly in front of our star, positioning itself so that its disc blots out the Sun and so reveals the surrounding corona. Of course, both spacecraft are also moving around the Earth, and so they must continuously adjust their positions to keep the alignment - an act known as formation flying.
This is tricky to do at the best of times. Proba-3 will need to keep the satellites and the Sun aligned almost perfectly in order to create its artificial eclipses. The Occulter must, for example, fly 144 metres in front of its partner, and then align itself to within five millimetres of its target position. Any error and the eclipse it produces will be flawed: sunlight will leak past its disk and the measurements of the corona will be lost.
To aid them in this task, the satellites are placed into an unusual highly elliptical orbit. This takes them to a point over sixty thousand kilometers above the Earth, and then has them fall back until they reach a second point just six hundred kilometers high. At this time they are moving fast, too fast to maintain alignment, and so they separate as they fall and then reunite on the way back up.
They do this once every twenty hours, and form the eclipses only during the highest portions of their journey. The laws of orbital mechanics dictate that the spacecraft are moving more slowly at that time, and so fine adjustments in their positions are easier to make. Even so, the period of eclipse only lasts for the six hours in which Proba-3 is nearest the apex of its orbit.
Both spacecraft are also equipped with a set of tools to work out where they are, both relative to the Sun and to each other. This includes cameras, which are able to detect and lock on to markings placed on the spacecraft. Laser reflectors help the pair measure the distance between them, and a radio link allows them to share information and coordinate.
All of this is done autonomously. The two spacecraft are designed to function automatically, guided by their onboard software, and so can position themselves, create eclipses, and produce scientific data without human intervention. This capability - that’s to say, the algorithms and techniques needed for autonomous formation flying - is one of the key results of Proba-3.
II. The Crown of the Moon
On August 7th, 1869, an eclipse took place over North America, visible along a narrow path stretching from Alaska to South Carolina. Remarkably, we have a picture of it: the eclipse was photographed by a man named Henry Morton, and then featured in the very first issue of Nature, today the world’s most prestigious scientific journal.
Two other observers spotted something odd in that eclipse. In the light of the corona they found spectral lines that did not correspond to any known substance. An unknown element must be there, they concluded, which they called Coronium.
For more than six decades this element stuck around, and was even placed before hydrogen on the periodic table by Mendeleev. Of course, it didn’t exist: in the 1930s researchers discovered the spectral lines were actually coming from iron atoms heated to extreme temperatures. But there came another mystery. The solar corona, it turned out, was incredibly hot, soaring to millions of degrees above the temperature of the Sun itself.
That is weird. Normally things cool down the further away they are from something hot, but in the case of the corona this usual arrangement is wildly inverted. The further out you look, the hotter the corona becomes. In places, and especially during strong flares, the corona can hit ten million degrees Kelvin, making it almost as hot as the Sun’s inner core.
The question of how this can be possible has long vexed physicists. Probably, most think, it has something to do with the way the Sun’s magnetic field behaves. One theory suggests this field unleashes “nanoflares”, which explode upwards and release heat into the corona. Another thinks waves of magnetic energy send particles flying outwards, pushing heat with them.
The truth may be more complex, with both processes playing a part. Indeed data from the Parker Solar Probe has suggested this is the case. It has spent the last few years flying through the corona itself, an immensely difficult task, and has returned data that should help solve the mystery. But Proba-3 will help as well. It is being launched during the peak of the Sun’s activity, and so should get a view of the corona at its most energetic time.
The key thing, scientists have said, is that Proba-3 will allow for long measurements of how the corona changes in time. So far these have not been possible: the inner corona, where the processes driving the heat probably play out, is not visible to simpler coronagraphs. They are limited by diffraction, in which light bends around the edges of the blocking disc.
This can be solved by placing the disc far away - as, effectively, happens in an eclipse. But until Proba-3 we have not had an instrument capable of doing this, and so the inner corona has only been visible during the rare moments of totality. Once the two spacecraft begin scientific work that will finally change - and so too might our understanding of how the corona behaves.
III. When Spacecraft Work Together
Proba-3, as the name suggests, is not the first Proba satellite. It is not even the third - delays postponed the launch by several years and another, Proba V, got there first. The others in the series were not coronagraphs nor focused on the Sun, but they were all technology demonstrators. The idea of Proba is to test things out, to try new ideas and technologies, and then prove them for use in later missions.
In the case of Proba-3, the new thing is autonomous formation flying. If it works as hoped, the technology could open the way for future spacecraft to work together in space. And that would allow engineers to design new kinds of telescopes, capable of seeing deeper and more clearly than ever before.
When work started on Proba-3, back in 2005, engineers had a follow-up mission in mind. That was XEUS, an X-ray telescope made, like Proba-3, of two separate spacecraft. Instead of blocking the Sun, the pair would have searched for distant black holes, including some formed soon after the Big Bang.
One of the spacecraft, then, would have carried a mirror and the other the instruments and cameras needed to study such objects. Depending on what scientists wanted to look at, the mirror could have repositioned itself, moving precisely as required to zoom in or out and offering a view hundreds of times sharper than anything else achieved so far.
Unfortunately, this mirror would have been heavy, and after the space shuttle stopped flying, getting it into orbit would have been difficult. Progress on the mission thus stalled, and attention has since moved on to other projects.
There are, though, other concepts for formation flying observatories. ESA has studied an idea called Darwin, an observatory that could study the atmospheres of nearby exoplanets. It would need four spacecraft, each flying together in perfect formation, and each focusing their attention on the light coming from these worlds.
By acting in formation such spacecraft could create an interferometer, a device that can combine waves to amplify the light from a target, while removing the light from everything else. Such an observatory could so hone in on a single faraway planet that would, to a more traditional telescope, be drowned out by the brightness of its star.
Before all that, though, scientists and engineers must get Proba-3 working. Right now, and for the next few months, the spacecraft will pass through a period of commissioning. Operators will first test out its platform and instruments, and then move on to begin formation flying. The first eclipses could come by March next year.
This is extremely ingenious implementation to achieve a difficult objective! I am seriously impressed with the careful planning that went into Proba 3- It makes Juno look like a simple job in cramming instrument packaging into limited space (although there is much more to it than that of course). Did the alignment challenge have to take in any distortions caused by the moon's gravity?
I have heart arrhythmia and recently had an operation to correct it. In the slow sequence of solar maximum to solar minimum is there a definite constant? What would it require for our star to develop arrhythmia in its energy output? Are there any observed stars that are highly arrhythmic?