Astrophysicists studying cosmic rays have spotted “the most abundant source of antiprotons near the Earth” — i.e., antimatter — according to a new paper in Astrophysical Journal Letters by scientists on NASA’s PAMELA project.
PAMELA stands for Payload for Antimatter Exploration and Light-nuclei Astrophysics, and the satellite-based experiment was launched in 2006 for the purpose of studying cosmic rays, high-energy subatomic particles that are slamming into the Earth’s atmosphere constantly, creating showers of “daughter” particles in the process. That’s right, Nature is running her very own particle accelerator experiment at the edge of Earth’s atmosphere.
That means that the showers of daughter particles should include small amounts of antiprotons, just like in manmade particle accelerators. Most of those would annihilate the moment they came into contact with their ordinary matter counterparts, but astrophysicists have long hypothesized that a few remaining antiprotons could become trapped within the Earth’s magnetic field, resulting in “an antiproton radiation belt” similar to the Van Allen radiation belts that already exist.
That’s what PAMELA scientists think they’ve found, nestled within the Van Allen belts — specifically, a region called the South Atlantic Anomaly. There’s so many high energy particles trapped in this region, the Hubble Space Telescope needs to switch off whenever it passes through several times a day. So it seemed like the best place to look for elusive antiprotons.
“Trapped antiprotons can be lost in the interactions with atmospheric constituents, especially at low altitudes where the annihilation becomes the main loss mechanism,” study co-author Alessandro Bruno of the University of Bari told BBC News. “Above altitudes of several hundred kilometers, the loss rate is significantly lower, allowing a large supply of antiprotons to be produced.”
PAMELA scientists analyzed 850 days worth of data, focusing on those periods when the satellite was in the anomalous region. And lo and behold, they detected a small amount of antiprotons trapped in the Van Allen belts. We do mean “small”: 28 antiprotons. It’s still roughly three times more than one would expect to find from the solar wind, so it seems as if the hypothesis is correct.
And that, in turn, might shed light on one of the foremost mysteries in modern cosmology: what gave rise to the matter/antimatter asymmetry in the earliest moments of our universe that gave us the matter-based cosmos we know and love?
Interesting and noteworthy for physicists, sure, but is it especially exciting otherwise? I wondered, until I read the fourth sentence in the BBC News story about the paper: “The researchers say there may be enough to implement a scheme using antimatter to fuel future spacecraft.”
Talk about burying your lede! But I’ll go ahead and be the wet blanket here and point out that this is a pretty optimistic interpretation of “the most abundant source of antiprotons near the Earth.” We’re talking about exactly 28 antiprotons, here.
I get it: everyone loves Star Trek, and geeks around the world still dream of one day being able to harness the power of antimatter to fuel their very own version of the USS Enterprise, which relies on large quantities of the stuff to supply sufficient propulsion to boost the starship into its famous “warp drive.”
As he did with many technical aspects of the series, for the Enterprise propulsion system, creator Gene Roddenberry drew on established scientific fact. The concept of matter/antimatter propulsion is not just the stuff of science fiction.
The “Star Wars” strategic defense initiative in the 1980s included several projects that proposed using antimatter as rocket fuel, and as recently as October 2000, NASA scientists announced early designs for an antimatter engine for future missions to Mars.
When antimatter meets matter, the result is an explosion. Both particles are annihilated in the process, and their combined masses are converted into pure energy. The effect is ably illustrated in Star Trek III: The Search for Spock: after surrendering his ship to the Klingons, Kirk himself sabotages it, programming the computer to mix matter and antimatter indiscriminately, causing a huge explosion and destroying the Enterprise:
Antimatter is an ideal rocket fuel because all of the mass in matter/antimatter collisions is converted into energy. Matter/antimatter reactions produce 10 million times the energy produced by conventional chemical reactions such as the hydrogen and oxygen combustion used to fuel the space shuttle. They are 1,000 times more powerful than the nuclear fission produced at a nuclear power plant, or by the atomic bombs dropped on Hiroshima and Nagasaki. And they are 300 times more powerful than the energy released by nuclear fusion
If it weren’t so difficult and expensive to produce in bulk, antimatter would be the most efficient fuel known to man. Alas, the only way to produce antimatter is in so-called “atom smashers”: enormous circular tunnels lined with powerful magnets that propel atoms around and around until they approach the speed of light. Then they slam into a target, creating a shower of particles. Some of those are antiparticles, which are separated out by the magnetic field.
Here’s the sticking point: Even the most powerful atom smashers only produce minute amounts of antiprotons each year — as little as a trillionth of a gram, which would barely light a 100-watt bulb for three seconds. It would take tons of antimatter to fuel a trip to distant stars, in case anyone was excitedly planning their first interstellar vacation.
Even if we had an ample supply of antimatter, a secure means of storage must then be devised; the antimatter must be kept separate from matter until the spacecraft needs more power. Mixing can’t occur all willy-nilly, because then the two would annihilate each other uncontrollably, with no means of harnessing the energy –- plus, the resulting explosion would most likely destroy the spacecraft.
So what prompted BBC News to raise the specter of antimatter propulsion? Turns out it was an April 2006 report for NASA’s Institute for Advanced Concepts, “Extraction of Antiparticles Concentrated in Planetary Magnetic Fields,” by principal investigator James Bickford of the Draper Laboratory.
Bickford’s scheme involves concentrating and harvesting antiprotons trapped in the magnetic fields around Earth (and other planets). He proposes using superconducting rings to create an electric current loop capable of “scooping” up trapped antiparticles:
Of course, you’ll likely still have similar challenges to overcome, namely, harvesting sufficiently large quantities of antimatter, and figuring out how to safely store it when you do. That said, it’s got to be gratifying for physicists like Bickford to see some observational data emerge in support for their ideas. Ultimately that’s the real significance of what PAMELA found. As Technology Review said, “It’s always interesting to have theoretical predictions confirmed. That’s good science at work.”
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