Nature certainly has a way of one-upping the fruits of human ingenuity. Extreme astrophysical objects have long been known to accelerate the particles that make up cosmic rays to whopping energies that make the Large Hadron Collider look like a child's slingshot. The mammoth collider near Geneva, Switzerland, which resumed service in 2009 after an aborted start-up the year before, will ultimately boost protons to energies of seven trillion electron volts. Cosmic-ray protons, in comparison, have been clocked striking Earth with tens of million times as much energy; a single proton can pack as much punch as a baseball hurled at 60 miles per hour. (For the technically inclined, some cosmic rays have energies exceeding 1020 electron volts.)

These particles do not make it all the way to the planet's surface. Instead, after striking the atmosphere, they send forth a shower of less energetic secondary particles. From these secondary showers, astrophysicists try to determine the specific source of these cosmic speed freaks. In 2007 a collaboration of scientists representing the Pierre Auger Cosmic Ray Observatory South, a massive cosmic-ray detector in Argentina, reported that the arrival path of the highest-energy particles correlated with the location of active galactic nuclei, bright regions in the centers of other galaxies powered by black holes. The implication was that the highest-energy particles stream into the Milky Way from accelerators elsewhere in the universe. But some observational details remained incomplete, and further observation of the rare ultrahigh-energy events diminished the correlation with active galactic nuclei.

Now a study based on data from the same experiment proposes that violent events relatively closer to home, but hundreds of thousands of years in the past, might also fit the bill. Stellar explosions within the Milky Way could spit energized particles into interstellar space, where they would race around, confined by the galaxy's magnetic field, until striking Earth or some other celestial body, according to a paper published in the August 27 Physical Review Letters.

The hypothesis stems from hints of cosmic-ray complexity provided by the observatory (named for a 20th-century French physicist), which comprises 1,600 detectors covering 3,000 square kilometers. The added complexity?: The energetic cosmic rays may not just be protons, but atomic nuclei. "People have always thought that the highest-energy cosmic rays are protons, not nuclei," says study co-author Alexander Kusenko, a physicist at the University of California, Los Angeles. "But Pierre Auger finds that as you go to higher energy, you get more nuclei."

Unlike more fundamental protons, few atomic nuclei are expected to survive the long journey across intergalactic space without dissociating into their component neutrons and protons. "They shouldn't get here from very far away," Kusenko says. But those same cosmic rays do not appear to arrive from the heart of the Milky Way, as might be expected if the particles originated within the galaxy.

The study's authors have proposed that high-energy nuclei may have been spewed into the Milky Way long ago by dramatic stellar explosions known as gamma-ray bursts (GRBs). "They happen in other galaxies, so they should happen in our galaxy every so often—once or 10 times in a million years," Kusenko says. In the intervening years between a GRB and an accelerated nucleus reaching Earth, the cosmic ray's path through the galaxy would be so long and chaotic that its source location would be almost impossible to determine.

In effect, local GRBs could explain both the seeming randomness of the cosmic rays' inbound trajectory as well as the increasing contribution of massive nuclei at the highest energies. "One effect kills two birds with one stone," Kusenko says.

But results from another leading cosmic-ray observatory indicate that there may not be two birds to kill; unlike Pierre Auger, the High Resolution Fly's Eye (HiRes) detector, which ran until 2006, did not speak to a significant nuclear component in the highest-energy cosmic rays. "That's the controversy—we see something that is consistent with protons," says HiRes's Pierre Sokolsky, a physicist at the University of Utah where HiRes was located.

Neither Pierre Auger nor HiRes has the luxury of a large sample of high-energy cosmic rays; a physicist with a square kilometer of detectors might have to wait 100 years to observe a single such event. With more data from Pierre Auger and from HiRes's successor, the Telescope Array outside Hinckley, Utah, in which Sokolsky is involved, the composition of high-energy cosmic rays—whether purely protons or mixed with nuclei—should become clearer. That would in turn help to clarify the source—or sources—of these cosmic bullets. And if the discrepancy cannot be resolved among the observations of Pierre Auger in Argentina and of HiRes and the Telescope Array in Utah, then astrophysicists will be charged with explaining a puzzling hemispheric dichotomy. "That would be very interesting," Sokolsky says.

"If I were a theorist, I would wait for the dust to settle a little bit," he says. "I have an outstanding bet of two very good bottles of wine that these heavy nuclei are going to go away."