Mission to Our Very Own Star
PASADENA CA — Standing on the warm, protected surface of the Earth, it's hard to imagine our Sun as being anything other than benign. Even as stars go, it's relatively small and gentle, pumping out vast quantities of easy energy without complaint. I still remember the profound moment when my grade-school teacher taught us that all of the energy in our bodies, from the force produced by our muscles to propel us around the playground, to the electrical pulses in our brains as we learned how to read, originally came from the Sun.
It made sense even to a third grader: all the energy in our food could be traced back to plants (and animals, who had themselves eaten plants). But where did the plants get their energy? From an ingenious chemical reaction called photosynthesis, which converts sunlight into sugars and starches, very efficient forms of stored chemical energy. All energy on Earth is captured sunlight from our quiet, generous Sun.
But it may surprise you to learn just how much we need to be protected from our star (and not just in terms of sunscreen). Taken in human-sized terms, even our Sun is a mind-bogglingly huge beast. It clocks in at nearly a million miles across, and contains over 100,000,000,000,000,000,000,000,000 tons of hydrogen and helium, as well as a smattering of other chemicals.
The Sun is, in the most basic sense, hydrogen bombs that are too big to actually explode. At the very center of the Sun, conditions are extreme. The super-dense solar core contains about 50% of the entire mass of the Sun, but only takes up about 1% of the Sun's volume. That much material squashed so close together drives the temperature up to around ten million degrees, hot enough to ignite a thermonuclear reaction.
Hydrogen nuclei are stripped of their electrons and rammed together, forcing them to fuse into Helium atoms, which is the exact reaction that powers a Hydrogen bomb. And the explosions at the center of the Sun are no more controlled than any man-made monstrosity. What keeps the Sun from blowing itself apart is the pull of gravity, which keeps the massive outer layers of the Sun pressing down on the central explosion. That's all there really is to stellar structure - a run-away fusion reaction at the very center, squashed down by titanic weight of the rest of the Sun.
Einstein, as you may know, figured out just how much energy you can get out of a fusion reaction, where a tiny amount of matter is converted into energy (just take the speed of light, hardly a small number, and square it!). Even in the case of our most destructive bombs, only a fraction of a gram is converted to energy. Compare that to the Sun, which converts 5 billion kilograms of matter every second, and you begin to see what forces are at work inside our friendly star. Almost all the energy produced at the center of the Sun is in the form of X-rays. Suffice it to say that if for some reason the core of the Sun was exposed to our view, we wouldn't be around very long to comment on it.
Luckily, things are moderated somewhat by the structure of the Sun. Once created, an X-ray has to navigate its way through half a million miles of solar interior before it can get out. Near the center, the matter is so dense that the X-ray can't move far until some atom absorbs it or scatters it, sending it in an entirely different direction. In a fashion, the X-ray is "bounced" around inside the Sun, leaving behind a little of its energy (which is transformed into heat) each bounce it makes. And it makes a lot of bounces. A typical X-ray takes about a million years to escape the Sun's interior, by which time it's been cooled down into a more palatable kind of light, like visible or ultraviolet. It's an amazing thought that the sunlight you see today was actually created in a fusion reaction that took place a million years ago. It's sort of like fossilized light.
Some of the X-rays manage to get out of the Sun faster than others, so besides the warm yellow sunlight that life-forms like us have adapted to, the Sun throws off huge amounts of dangerous radiation and high-energy charged particles. In fact, the Sun would fry all life on the surface of the Earth, were it not for two things: the Earth's atmosphere and magnetic field. As insubstantial as our atmosphere looks, it is, in truth, an incredibly efficient shield against harsh radiation from space. Almost all the X-rays and gamma-rays produced by the Sun are absorbed by our atmosphere, as well as ultraviolet light (I'm sure you've heard of the ozone layer, and how we're busily depleting it).
But one thing you may not have thought about before is just how much we owe to our magnetic field. The Earth's core contains huge quantities of molten metal, the motions and currents of which generate a magnetic field around our planet. Okay, but aside from being able to line-up compass needles with the magnetic field, what good is it to us? Magnetic fields have the very useful property of being able to deflect charged particles, like protons and electrons. The Sun hurls heavy charged particles like protons and helium nuclei into space in a continuous stream which astronomers call the "solar wind." If it weren't for the Earth's magnetic field, these high-energy projectiles would penetrate to the Earth's surface, destroying any living cells in their path.
You might not feel quite so warm and fuzzy when you look up into the sunlit sky, but at least you can take comfort in the fact that we are well protected by our natural environment. The Earth acts like a hermetically sealed bottle, keeping the more threatening aspects of the Sun well at bay. The problem for scientists, however, is that we're a bit too well sealed-off from our immediate neighborhood in space.
Despite the face that we can readily see the Sun in the sky, and study it from afar, we remain isolated from it. Everything we know about the Sun has come from remote sensing. We can study sunspots and solar cycles with our telescopes, as well as determine the chemical composition of the Sun with our spectrographs. But so far, we've never been able to directly sample what the Sun is really like. We've never been able to touch, sniff, or taste material taken directly from our star. We have many questions that can't be answered by studying the Sun at a distance.
On August 8th, 2001, NASA took a first step to address this discrepancy. The Genesis spacecraft was launched on a mission to return a real sample of the Sun. Now, Genesis isn't designed to get very close to the Sun. It will only get about a million miles closer to the Sun than the Earth is (our average distance from the Sun is 93 million miles), orbiting around a point in space called L1, where the pull of gravity from Earth exactly balances that of the Sun. The important thing is that Genesis will travel completely out of the Earth's magnetic field, allowing it direct exposure to the solar wind.
Plates of crystalline glass will be held up towards the Sun, catching particles of the solar wind as they fly by. The glass is incredibly pure, so that scientists will know any chemicals or nuclei they find embedded in the glass will have doubtlessly come from the Sun. Then, in the fall of 2004, Genesis will stow its glass plates and return to Earth. After its million-mile return flight, Genesis will plunge through the atmosphere into the waiting grasp of special helicopters, which will snatch the spacecraft up before it can fall to Earth and damage the precious cargo.
The solar wind, despite the huge energies of its particles, is extremely sparse and ephemeral. Even after three years of exposure, the glass plates will collect only a tiny fraction of a gram of material. But what clues that material may have in store for us!
What scientists are really after is an accurate measurement of the chemistry of the Sun. They want to know exactly which elements are present in the Sun, and in what abundances. The answers to those questions are far from trivial. Knowing the exact chemical nature of the Sun will help us understand how energy is transmitted through its structure, which may help us solve fundamental mysteries like what generates the Sun's weird magnetic field (which is responsible for sunspots and solar flares), and why the magnetic fields "flips" every eleven years, bringing on a season of violent solar activity.
Measuring the exact chemical abundance of the Sun will also help us get a grip on how the Sun formed in the first place (and hence the name of the Genesis mission). Not much has altered the surface of the Sun since it formed about five billion years ago. What does the chemistry of a collapsing dust cloud (which the Sun formed out of) have to be like to give birth to a star? Now that we're able to observe other dust clouds forming new stars (like the Orion Nebula), knowing the chemistry of the Sun will allow us to search for similar environments in other parts of our galaxy, and look for the onset of stellar birth.
Speaking as an astronomer who specializes in the structure and evolution of stars, I am constantly humbled by just how little we know about even the closest, easiest to observe star, let alone the billions of other stars that make up our galaxy. And how can we even begin to understand how a galaxy works, if we know so little about the stars that make it up? A good first step is to find out as much about the Sun as we possibly can, then extend that knowledge to the different colors, sizes and flavors of stars we see all around us. But we'll start with our very own star.