When will we see another bright comet?

Recently, a friend of mine asked when we might be able to see a comet. He was surprised when I said there are several visible right now.

Indeed, you can find a list of upcoming visible comets on this useful site compiled by Japanese comet enthusiast Seiichi Yoshida. The site provides information on comets that are currently visible as well as comets that will be visible each month for the next five years. For this month, Yoshida lists four observable comets.

But none gets brighter than magnitude +12 (lower magnitudes are brighter), and the comets are visible only under dark, clear skies, using moderately large telescopes. Indeed, the vast majority of periodic comets — comets whose orbits are well known and have been observed more than once — fall into this category. These comets quietly come and go and are known only to enthusiastic amateur astronomers who make a concerted effort to hunt them down with good binoculars or telescopes. Generally, they are unimpressive to the eye, usually appearing as nothing more than faint fuzzballs, even in large telescopes.

Of course, when my friend asked his question, I knew exactly what he was alluding to: He wanted to know when we might see a

and/or

comet, perhaps one that would develop a tail stretching a quarter of the way or more across the sky. Unfortunately, such objects give no advance notice as to when they will appear. But with some confidence, I can state that at least one of these comets is heading toward the inner solar system even as I type these words.

But whether it will appear three weeks, three months or three years from now is unknown.

So what are our chances of seeing such a celestial showpiece? More on that in a moment, but first, let's talk about what needs to happen to produce a bright comet.

Recipe for a bright comet

The unpredictability of a passing comet's appearance and brightness is no surprise to those who study these enigmatic objects. What we see depends on many variables — the comet's orbit; the relative locations of the comet, Earth and the sun; and the size and composition of that icy clumping of solar system rubble that forms the comet's nucleus.

The nucleus's dusty, rocky material and frozen gases are similar to the composition of Saturn's rings. This part of a comet, usually only a few miles across, is gradually warmed by the sun's heat, and expels gas and dust into space, often in distinct jets. But such emissions from the nucleus are often nonuniform. To predict comets' activity, astronomers have developed general formulas and models for comet brightness based on the observed behavior of many comets going back to the late 19th century. Usually, a comet's activity increases rapidly as it draws closer to the sun; the brightness typically varies (roughly) as the inverse fourth power of its solar distance.

Put another way, as a comet's distance from the sun is halved, its brightness increases by a factor of 16, or three magnitudes.

But comets can be capricious and, like people, have their individual quirks. The physical appearances and behaviors of comets are as varied as the appearances and behaviors of people; no two are alike.

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