by Lucie Green
Then, as quickly as it started, the eclipse was over – 6 minutes and 42 seconds can really fly by. I had no complaints about my trip to the Pacific, but not all eclipses have been in such great holiday destinations. The early discoveries and the curious observations of the corona meant it wasn’t long before a telescope had been invented that mimicked nature and could create an artificial solar eclipse. No longer did astronomers need to travel across the globe for a fleeting glimpse of the corona; they could study at leisure from home.
A French astronomer, Bernard Lyot, invented this telescope, which is known as a ‘coronagraph’, and in 1930 successfully made the first observations from an observatory in the Pyrenees. In this telescope the image of the Sun was focused onto a circular disc that played the role of the Moon in a real eclipse. The disc was just larger than the image of the photosphere and prevented the photospheric light from going any further into the telescope. The faint light from the corona was not blocked and a further lens was used to focus this light into an image. Much of my job involves looking at images taken with a coronagraph, which is how I am used to seeing the corona – and why it was so great to finally see it for myself.
Now we are left with the problem of why the chromosphere and corona are so hot. It seems the Sun’s atmosphere gets hotter the further it gets from the surface, not cooler, as we expect. And this raises the very natural question: does anything come after the corona? Just how far does the Sun’s atmosphere extend?
9. Bon Voyage
In 1977 NASA launched two identical spacecraft, both on a mission to study the planets Jupiter and Saturn, with the option of one spacecraft then going on to Uranus and Neptune. The 1970s were an exciting time, when the plans for Solar System exploration became much more ambitious, moving beyond exploration of the Moon, Venus and Mars to the outermost planets. And it was a well-timed trip because the planets Jupiter, Saturn, Uranus and Neptune were about to move into a special arrangement that meant a spacecraft could visit all of them in turn, hopping on from planet to planet. This alignment only happens once every 175 years and NASA had plans to make the most of this rare opportunity. So the Voyager 1 and Voyager 2 spacecraft were launched and, despite being built to survive for five years, they are still journeying through space and sending information back to us today.
The Voyager spacecraft were sent off on their mission from Cape Canaveral, Florida, being launched just two weeks apart. Their expeditions had been designed so that as each spacecraft reached its first planet it would take observations and then be flung on to the next in a gravitational slingshot. The slingshot is a clever way to use gravity to our advantage. As Voyager 1 and 2 approached their target planets, the gravitational pull that they felt from the planet increased and the spacecraft speeded up. The path of the spacecraft bent as it passed close by and for Voyager 1 and 2 this propelled them further out into the Solar System with no need to use their own fuel. It meant that the Voyagers could be sent much further out into the Solar System than the rocket that launched them could do alone.
The Voyagers were designed to journey to the far reaches of the Solar System; their journey was one of epic proportions. Jupiter is just over five times more distant from the Sun than we are – 778 million kilometres – and that means it receives less than 4 per cent of the sunlight that we do. To power the spacecraft in the darkness they carry space batteries that use a radioactive source: plutonium-238. All materials that are radioactive naturally emit particles of energy as the nuclei, which are unstable, reconfigure and rearrange themselves. For plutonium-238 an alpha particle is emitted which is made of two neutrons and two protons – it has the same composition as the nucleus of a helium atom. This pared-down particle of plutonium changes into the element uranium, but it’s not the element left behind that is important. The constant emission of the alpha particles generates heat as the moving particles are absorbed into the surrounding material. This heat, also called ‘thermal energy’, is then turned into electricity.
When the spacecraft were launched these space batteries provided them with the same power as roughly four 100-watt light bulbs. It may not sound like much, but all space missions are designed to run on what we would consider a very frugal energy diet. They have to, in fact, as it would be simply too expensive to equip them with anything more. And this small amount of power is all there is to run everything – onboard computers, heaters, instruments and the system that allows the spacecraft to communicate back to Earth. But the radioactive power also meant that each spacecraft had the potential to survive into middle age despite the plans for a five-year working life.
The visits to Jupiter and Saturn were a huge success and spurred the NASA scientists on. The mission was extended so that Voyager 2 would slingshot on to become the first spacecraft to Uranus and Neptune. But after these visits, with no further planets out there – well, except for Pluto at that time – the primary mission ended after an incredible twelve years. It had been a phenomenal success, during which time the Voyagers had discovered that Jupiter’s moon Io is the most volcanically active object in the entire Solar System, made studies of forty-eight different moons around the outer planets, and found that Titan, Saturn’s largest moon, has a thick nitrogen atmosphere and looks like a planet in its own right.
Even though the primary mission was over, the two Voyager spacecraft were very much alive and functioning well. The plutonium power source had been declining though, a natural outcome of the radioactive nature of the material. The more time passes, the more plutonium atoms have reconfigured themselves and the fewer there are left to decay by emitting an alpha particle. Radioactive elements are characterized by what is called their ‘half-life’, which describes how long it takes for the radioactivity to fall to half its original level. Plutonium-238 has a half-life of 87.7 years.
But even though the power had been dropping both Voyagers still kept going. And with some clever adjustments that included turning off some of the instruments, the spacecraft continued to do their science. The work of Voyager 1 and 2 was far from over and a unique opportunity lay ahead. Attention turned away from the planets and in 1989 the Voyagers were given a new mission: the Voyager Interstellar Mission. Their space batteries were expected to remain useful until around 2020, which might give them enough time to leave the Solar System and become the first human-made objects to make measurements of interstellar space. Even though they had left all the planets behind, they hadn’t yet left the Solar System. NASA had a very clear idea of what the edge is and it actually begins with the Sun.
TAILS
Up until now we have been looking at the Sun as a sphere of plasma, which can be broadly split into different shells. Then, during the time of a total solar eclipse, we get a glimpse that the plasma stretches up from the photosphere to form an atmosphere. The impression that we get from the total-eclipse observations is that the atmosphere gets thinner and thinner with height until it eventually fades and runs out of material.
But over the centuries there have been some icy visitors to the inner Solar System which hinted that the tenuous corona we saw in the previous chapter doesn’t end where we see it end with our eyes during an eclipse. They gave the first clue that the atmosphere of the Sun extends beyond what our eyes perceive. These icy visitors are the comets.
Comets are frozen lumpy bodies of ice and dust several kilometres across that mostly reside in a region called the ‘Oort Cloud’, named after the Dutch astronomer who proposed that a vast shell of comets surrounds the Solar System at one fifth of the distance to the nearest star. That’s over 10,000 times further out than Jupiter. At that distance, there is no way that we c
an see any comets directly, not even with the most powerful telescope. But, still, the Oort Cloud is thought to exist. At that vast distance, comets are only loosely gravitationally bound to the Sun. So they can be easily knocked out of their icy ghetto by the gravitational influence of a star passing nearby. This interaction can change the orbit of a comet so that it comes in closer to the Sun. Again, this may seem far-fetched, but during the lifetime of the Solar System, as we fly through the Galaxy, such incredible events can happen. This gravitational disruption is thought to explain why we have comets that repeatedly come and visit us in the inner Solar System, like Halley’s comet (named after Edmond Halley, whom we met before), which comes back to us every seventy-six years.
At their distant outpost comets can take millions of years to orbit the Sun, but when one gets dislodged from the Oort Cloud and comes into the inner Solar System, its journey close to the Sun causes it to heat up. Basking in the increasing amount of sunlight falling on it, ice that has been frozen for millennia turns into gas and particles of dust trapped amongst the ice grains are released. As the comet reaches roughly the orbit of Jupiter, a diffuse cloud of gas and dust called a ‘coma’ starts to form around the giant iceberg. And then something spectacular happens. A beautiful tail forms behind the comet that stretches out for millions of kilometres. And it is this tail that shows something intriguing.
Comet tails all have something in common: they always point away from the Sun – within a few degrees – no matter whether the comet is approaching the Sun or whether it has passed around the Sun and is heading back out into the depths of space. So on its way out the comet’s tail is actually ahead of it! This tells us that the motion of the comet itself is not the major factor in setting the direction of the tail. The material of the tail isn’t simply being laid down behind the comet as if it were leaving a trail behind it. Something is always pushing the tail out from the Sun.
One possibility is sunlight itself. The pressure of light falling on the dust particles in the coma around the comet can provide a slight force that pushes the particles out and results in a tail. There is no mystery there. But comets have another tail, which is slightly different in colour, and this one cannot be blown back by sunlight – it is a tail of ionized gas. (See plate 6.) Gas particles in the comet become ionized by the ultraviolet light coming from the Sun: electrons are knocked off their atoms, which creates positively charged ions. These are then blown back into what is really an ion tail. But sunlight doesn’t have the same effect on electrically charged particles as it does on dust particles. Something else must be washing over the comet that is not sunlight but which is still originating at the Sun.
The only observational clues to work with first of all came from the detailed viewings of the comet tails. They showed that there is a very small difference in angle between a comet’s ion tail and the direction directly away from the Sun. This was a key piece of information because this difference could be explained if the comet was cutting through an outflow of some sort, perhaps a wind from the Sun. If the comet were stationary it would form a tail that pointed directly away from the Sun. But if it were moving across something flowing out from the Sun, its trail would go off at a slight angle. Which is exactly what was observed. The Sun must be producing some kind of invisible wind.
LIVING IN THE SUN’S ATMOSPHERE
Work on where the Sun’s atmosphere might actually end really accelerated in the 1950s. In this decade, a British physicist, Sydney Chapman, used the fact that the Sun’s atmosphere is hotter than its surface to calculate that this would cause the corona to be ‘puffed up’. The high-temperature plasma would exert a pressure and so would expand outwards – just like bread rising in the oven because the air trapped in the dough expands as it warms.
This explained the size of the corona seen during a total solar eclipse. But there was more to Chapman’s findings. Plasma conducts heat exceptionally well because of its freely moving particles and this means that it can conduct heat, and remain hot, out to very large distances. Chapman realized that its high temperature must be maintained far beyond the corona that is seen during a total solar eclipse. The Sun’s atmosphere was bigger than anyone had ever expected.
In fact, Chapman’s theory had the Sun’s atmosphere extending out to a distance far beyond the Earth. Chapman realized that the Earth orbits inside the atmosphere of the Sun. An astonishing thought that shows we are more intimately connected to the Sun than we ever imagined. Thankfully the Earth’s magnetic field protects us from the Sun’s massive atmosphere as we fly through it; something we’ll visit again later.
Chapman’s theory took us one step closer to understanding how the Sun can reach far out into the Solar System but it didn’t explain what was actually causing the ion tails of comets. A large tenuous atmosphere was not enough to cause ionized tails to blow back off comets. There must be more to the Sun’s atmosphere than Chapman’s model was able to capture. To explain why comets have ion tails, we had to answer the question: is the Sun’s atmosphere static or dynamic? Does it simply extend into space, or is it actively expanding?
In 1958 another revelation came when a controversial theory was developed that was able to unite the ideas about the formation and orientation of comets’ tails and the model of a very extended solar corona. And this new model provided an important clue about the Sun’s atmosphere. It showed that it isn’t possible to have a corona that is static and merely conducting heat out to very large distances. Instead, since the hot plasma is exerting a pressure it means it would be constantly expanding the plasma outwards, against the Sun’s gravitational pull.
Close to the Sun the static model appeared to be correct because the Sun’s gravitational pull is stronger there. But further out the Sun’s gravity weakens and it gets to a point where the Sun cannot hold on to the hot plasma of its atmosphere and it actively expands out into the Solar System, forming a solar wind – just as comets’ ion tails indicated. The constant expansion of the corona provides a flow of plasma that would blow as a wind of charged particles into the Solar System. This solar wind would blow over the comets and produce their ion tails.
A few words should be said about the man behind this model: the American astronomer Eugene Parker. His theory really was a game changer and it wasn’t easy for the research community to accept this new idea. Parker followed the normal route for disseminating new ideas and wrote his theory up for publication in one of the community journals, the Astrophysical Journal (the journal co-founded by Hale). The paper was sent to two referees, who act as a quality control and provide independent advice to the journal that the paper is worthy of publication. But the referees thought the paper had no scientific value and rejected it. Parker’s theory was simply too much of a radical departure from the accepted view at that time.
Luckily, the editor of the journal at that point was Subrahmanyan Chandrasekhar, who worked down the corridor from Parker at the University of Chicago. He was an outstanding astrophysicist himself and in his work he showed that dying stars known as white dwarfs which have more than one and a half times the mass of the Sun collapse under gravity and become unimaginably dense. These objects were later given the name ‘black holes’. Chandrasekhar walked down the hall, poked his head into Parker’s office and asked if he was really sure about his work. Parker was. Luckily for solar physics, Chandrasekhar ignored the referees.* Written in late 1957 and submitted to the journal at the start of 1958, the paper was finally published in November 1958.
Even though that exchange took place almost twenty years before the Voyagers left the launch pad, it is centrally important to their Interstellar Miss
ion. Parker’s model described a wind that could extend the atmosphere of the Sun to 100 times further out than the Earth. The wind would only stop when its own pressure (which would get smaller as it went out from the Sun) became the same as the pressure of the interstellar gas beyond. The solar wind would create a vast plasma bubble within which the Solar System is immersed.
According to this model, the edge of the Sun’s atmosphere would be just over twice as far as the distance to the (now) dwarf planet Pluto. This is the boundary that the Voyagers were looking for. Once they moved beyond the bubble, they would be in interstellar space. But to understand exactly where the edge might lie, and how the edge might be detected, needs a good description and understanding of exactly what the solar wind is like. And this needs measurements to be taken from much closer to the Sun.
It didn’t take long to test Parker’s theory as his work coincided with the start of the space age. Within just a few years spacecraft ventured outside the Earth’s magnetic field, meaning that if the solar wind really did exist, they would be able to detect it outside the Earth’s own protective magnetic bubble. And that’s exactly what happened. The measurements revealed the existence of a solar wind that is gusty and that sometimes blows fast and sometimes slow – relatively speaking, that is. The fast solar wind streams outwards at an incredible speed of around 800 kilometres per second and makes the 150-million-kilometre journey from the Sun to the Earth in just over two days. The slow wind still moves at a cracking pace and has a speed of around 400 kilometres per second, and takes around four days to reach the Earth. Even at its slowest, the solar wind is faster than any wind on Earth. In fact, no wind in the Solar System moves faster.
