The Alpha Magnetic Spectrometer (AMS-02) is visible at center left, as installed on the International Space Station. Credit: NASA
Millions of light-years away, millions of years ago, a star exploded. In this violent process, it ejected incredible amounts of mass, including carbon, nitrogen and oxygen—the building blocks of life. In fact, the star may have produced elements on the periodic table all the way up to iron. As it exploded, it spewed these elements into deep space. Only a burnt-out core remained.
No one on Earth noticed this exploding star at the time. Besides the fact that humans had not yet evolved, those cosmic sprays of particles—called cosmic rays—are just now reaching us. Even though millions of years separate us from their source, these particles can provide us with insight into the workings of our universe.
Recent results from the Alpha Magnetic Spectrometer (AMS-02)—an experiment on the International Space Station—are expanding our understanding of these cosmic rays. The findings are published in the journal Physical Review Letters .
Particles from the stars
While X-rays or UV rays are forms of energy, cosmic rays are different. They're groups of particles raining down from space. These cosmic rays can include a variety of elements, including phosphorus, chlorine, potassium, argon and calcium. Exploding stars (supernovae) produce some of these elements. Others come about when nuclei from heavier elements (originally from supernovae) collide with gases in space. Or, as astronomer Carl Sagan famously said, "We are made of star-stuff."
Even though cosmic rays are constantly hitting Earth's atmosphere, we don't know much about them. One of the biggest questions is why these particles are moving so fast. While a star's explosion provides an initial burst of speed, these particles should slow down over time. The fact that they're still moving near the speed of light suggests that something has accelerated them.
What these rays may reveal
Decades ago, the United States studied cosmic rays to find out if the government of the former USSR was conducting nuclear testing. However, researchers realized that cosmic rays were coming down from the sky rather than up from the Earth, making it a moot point.
While they weren't helpful for spying, NASA and the Department of Defense recognized other reasons to study cosmic rays. As space exploration expands, future astronauts will be exposed to more cosmic rays, whether on a space station or on the moon.
Cosmic rays are also bombarding satellites used for communications, GPS and other purposes. These agencies want to understand these rays' effects on both humans and machinery.
Cosmic rays may also help us solve one of science's biggest questions—dark matter.
Unlike ordinary matter, dark matter only interacts via gravity. Astronomers only know it exists because of its effects on stars, galaxies and the astrophysical records of the early universe. Despite estimates that it makes up about a quarter of the universe's mass-energy, scientists have yet to detect it directly or indirectly. The DOE's Office of Science is interested in detecting dark matter as part of understanding the building blocks of the universe.
Some models of dark matter suggest that dark matter particles could collide and annihilate each other. If dark matter does act like this, the process would create positrons, the antimatter counterpart to electrons. This excess of positrons should show up in cosmic ray data.
On Earth, our atmosphere protects us from the potential harm of these cosmic rays. The amount of water vapor in the atmosphere is equivalent to 10 meters (33 feet) of water. However, this barrier means that it's impossible to accurately measure cosmic rays on the ground. To study them, we must go into space itself.
Introducing the Alpha Magnetic Spectrometer
The AMS-02 isn't just an instrument to measure cosmic rays in space—it's the perfect instrument to measure cosmic rays in space. The AMS-02 is the only tool to have collected these data and has done so for a long time. It's been on the ISS and exposed to the harsh conditions of space for more than 13 years.
At its most basic, the AMS-02 is a particle detector. Particle detectors on Earth are often attached to accelerators that smash particles into each other, like the Large Hadron Collider at CERN. Others are in remote locations to capture particles away from human interference. But AMS-02's location in space means that it doesn't need a source of particles or a location away from humans. At the size of a large coffee table, it's also much smaller than most particle detectors.
Since its installation in 2011, the AMS has collected 230 trillion cosmic ray events. Most of these events are from common elements in the universe, such as hydrogen and helium. These interactions are in the billions. Some are rarer, with around 100,000 interactions with lithium. And some of the interactions with heavy elements like iron, n…
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