Enormous exploding comet is visible along the Wasatch Front

The sun is no longer the largest object in the solar system. That honor, though probably fleeting, now belongs to Comet 17P/Holmes, an exploding comet drifting in the blackness between Mars and Jupiter.

The comet is not the most massive thing in our system; that's the sun, no contest. 17P's coma is little more than thin gas, dust and ice particles. But in terms of overall dimension, calculating by angular size and known distance, scientists say the expanding coma is a bit bigger than the sun's diameter of 864,000 miles.

The diameter doesn't include the comet's odd short tail, wagging in the solar wind, which was recently discovered.

For nearly all of the 115 years that 17P/Holmes has been known, it has been so dim in its far-off orbit that it was difficult or impossible to see, even with sophisticated telescopes. But in late October, it brightened in the course of a day, making it easily visible to the naked eye throughout the Wasatch Front, including lightly-polluted Salt Lake City.

"It's got to be one of the weirdest comets I've ever heard of ... ," said Patrick Wiggins, NASA solar system ambassador to Utah. "In 24 hours, literally a day, it went from very, very faint to very bright, nearly a million-fold."

Why?

The two prevailing theories:

• It erupts sometimes because material inside the comet effervesces when it nears the sun.

• It ran into an asteroid.

A piece of evidence in favor of the first is that the comet was discovered in 1892 when it suddenly brightened. After the eruption died down, 17P/Holmes was lost to sight until the 1960s. Favoring the second idea is that it perpetually circulates in the asteroid belt.

Meanwhile, many Utahns are keeping their eyes, binoculars, telescopes and cameras on it while the show lasts. Nobody is sure when it will fade.

Star-watcher Kurt Fisher has been taking a look with binoculars about 11 p.m. nightly. Holmes is in the constellation Perseus, easily seen in the northeast after dusk. It's around 15 degrees, the distance of one and a half fists held at arm's length, below the familiar W of the constellation Cassiopeia.

Comet 17P/Holmes is a gray-looking gas ball with stars around it. It is such thin gas that stars can be seen through it, also. Near the center is a bright elongated spot, the icy nucleus from which the gas and other material blew off.