COMET CONUNDRUM

The Monster of the Night Skies

The comet 17P/Holmes has astronomers around the world scratching their heads in confusion. The heavenly body just keeps getting bigger -- and is now twice the diameter of our sun.

Comets, of course, are no rarity. And it seems like every couple of years or so, one becomes big and bright enough that it can easily be seen from Earth. But the behavior of 17P/Holmes has mystified both hobby astronomers and professionals around the globe.

PHOTO GALLERY: GIANT IN THE NIGHT SKIES

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Rather than shrinking as it gets further from the sun as most comets do, this one just keeps getting bigger and brighter. At the beginning of the week, the cloud of dust and gas surrounding the comet's core -- called the coma -- had already grown larger than the sun. Now, just a few days later, the coma's diameter is twice that of the sun -- the dust cloud measures some 2.7 million kilometers across whereas the sun is just 1.39 million kilometers across. And there is no sign that it is finished.

"The comet is now a long ways away, but the dust cloud is still growing," Dr. Maciej Mikolajewski from the Torun Center for Astronomy at Nicolaus Copernicus University told SPIEGEL ONLINE. "It's the first time I've ever seen such a thing. I've never seen such a bright comet in my life."

Even more interesting than the celestial body's sheer size, however, is that scientists aren't totally sure why it suddenly exploded in brightness throughout October and November. The comet is no longer close enough to the Earth to be readily visible without a telescope, but the head scratching in the scientific community continues.

Mikolajewski, whose institute has been following the path of the comet for more than a month, says that a chemical reaction within the core of 17P/Holmes could be to blame for the explosion in size and brightness. Or, he says, it could have run into an asteroid as it sped through the inner part of the solar system. "Both of those hypotheses are possible," he says.

Frank Brenker, a comet researcher at the University of Frankfurt, likewise thinks that the comet may have run into something. "The impact theory is most plausible," he told SPIEGEL ONLINE. Most comets do gain in size and brightness as they get closer to the sun. But this comet, oddly, doubled in size as it was traveling away from the sun, making it unlikely that the sun was to blame.

The comet 17P/Holmes was discovered in 1892 by the British astronomer Edwin Holmes. That year, the comet was briefly bright enough to be visible with the naked eye before disappearing once again into the nether reaches of the solar system. Every seven years, the four-kilometer wide chunk of ice swings through the center part of the solar system before once again being catapulted back out into space.

Comets often achieve impressive sizes when they are warmed by the sun, and tails can sometimes stretch for thousands of kilometers. "A gigantic comet is nothing all that special," confirms Ulrich Christensen, head of comet and planet research at the Max Planck Institute for Solar System Research. "But a diameter larger than the sun is certainly unusual."

It could be that researchers never find out exactly what happened to 17P/Holmes. It has been speeding away from the Earth ever since Nov. 7 according to the US space administration NASA. It will be back in 2014.

With reporting by Holger Dambeck and Charles Hawley