Comet Flare

Comets are usually fairly predictable. But a normally small, faint comet recently did something really odd.

In less than 24 hours late last month, the ball of ice, rock, and dust, named Comet 17P/Holmes suddenly grew 400,000 times brighter than normal.

The image above shows what Comet 17P/Holmes looked like from the ground on Nov. 1. Below is a view of the suddenly bright comet from the Hubble Space Telescope on Nov. 4.

A. Dyer

Weaver, et al., NASA, ESA

Three weeks after the flare-up, people could still see the object without telescopes. (In the United States, it was directly overhead around 2 a.m. in the constellation Perseus.)

Scientists continue to puzzle over the event.

Many comets often brighten. They travel around the sun in oval-shaped orbits. As they get closer to the sun, the star’s heat vaporizes ice on their surfaces. The process releases fine dust, which reflects light. As a result, the heated comets look extrabright.

No one expected Holmes to get especially bright, however. That’s because, even at its closest, the comet is still twice as far from the sun as Earth is. So, it never gets much heat.

Even more puzzling, the comet’s recent brightening happened 5 months after the comet was at its closest location to the sun. It takes Holmes 6.88 years to complete one orbit.

To explain the rapid brightening, scientists speculate that a layer of material lifted off the comet and disintegrated. The resulting halo of dust around the object would have reflected lots of light.

It’s possible that the entire comet may be made up of similarly fragile layers stacked one atop another and glued together by ice, says Zdenek Sekanina of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

Why couldn’t people see the outburst until 5 months after Holmes passed its point closest to the sun? That might be how long it took for the comet’s outer layer to soak up enough of the sun’s heat to peel off, Sekanina says.

This wasn’t the first time that Holmes has experienced a burst in brightness. In fact, an English astronomer discovered the comet in the fall of 1892, when it grew suddenly brighter—again, 5 months after passing its position closest to the sun.

About 2.5 months after its discovery, in January 1893, the comet experienced another brightening. But after that, the comet grew very dim. For decades, Holmes was so faint that astronomers actually lost track of it between 1906 and 1964.

On the basis of what happened 114 years ago, astronomers are now watching closely for a follow-up outburst from the comet.

Some scientists think that earlier event and the modern one are related. It’s possible, for example, that dust from the first event fell back onto Holmes. That could have dimmed the comet for more than a century. Last month, internal pressure may have finally become strong enough to eject the debris.—Emily Sohn

Going Deeper:

Cowen, Ron. 2007. Flare-up: Comet Holmes’ surprise bloom. Science News 172(Nov. 17):309. Available at http://www.sciencenews.org/articles/20071117/fob5.asp .