How to tell the difference between comets and asteroids

If you’re in the Northern Hemisphere, March 2024 could be an exciting month for comet watching. If you read this later or elsewhere, there will be other opportunities later this year or later in the future. On the other hand, only the really hardcore people tend to get excited about asteroid spotting. So, what’s the difference, and how can you tell them apart?

Comets vs. asteroids

Comets have probably been part of human consciousness for as long as we have been able to communicate to each other that something seemed strange in the sky. City lights make the sight of a comet rare for most people today, but once upon a time several “hairy stars” would have been bright enough to be noticed in a normal lifetime, even if they were shorter.

On the other hand, the first asteroid was discovered on New Year’s Day, 1801. Almost certainly at some point an asteroid had come close enough for people to see it pass by for a night or two, but no data are known. Only then did we begin to realize that a new category existed, and we had to figure out how to distinguish it from the old one.

Comets are generally similar in size to small to medium-sized asteroids, and do not necessarily come closer. The reason they can be so much more visible is that comets are mostly ice, with some dust and possibly rock mixed in. As they approach the sun, the outer layers of ice rise above their freezing point (which varies depending on the type of ice). of ice). With no atmosphere to keep the products liquid, the former ice escapes as gases, a process known as sublimation, which takes some of the dust with it.

A fairly small amount of ice can cover a large area in gaseous form and reflect enough sunlight that they can often be quite bright. At best, a long tail pushed away from the sun by the solar wind creates a pendulum across the sky. Even if the tail requires instruments to see, there is often a glowing head or coma on the comet, which looks distinctly different from the bright points of stars.

Artist's impression of Phaethon, which is thought to open cracks as it approaches the sun, causing sodium to seep out.

Artist’s impression of Phaethon, which is thought to open cracks as it approaches the sun, causing sodium to seep out.

Image credits: NASA/JPL-Caltech/IPAC

Asteroids, which are boulders, with perhaps some dust, don’t do any of that, so the difference may seem simple. Ice creating blur means a comet, nothing more than a point of light moving for hours or days and it’s an asteroid, yes? Simple really.

Only nature rarely favors such clear divisions.

Phaethon causes problems

Another characteristic of comets is that they often cause meteor showers in their wake. The dust grains that flow from comets as they sublimate form patches around the inner solar system. Whenever there is a patch in Earth’s orbit, our planet plows into it on the same day every year. The dust particles produce meteors when they hit the atmosphere, at best lighting up the sky with a breathtaking display.

We now have dozens of meteor showers, and new ones are happening all the time, even though few are bright enough to attract much attention. Most of these showers come from comets. The exception is the Geminids, which happen to be one of the best of the year and come from material dropped by the asteroid 3200 Phaethon.

So here we have an asteroid that behaves in one of the ways associated with comets.

The process has raised questions about how this asteroid alone can produce a meteor shower. The reasons are still debated, but the initial assumption was that Phaethon is a ‘dead comet’, a comet that originally contained a mix of rock and ice, but now contains only rock.

Phaethon’s story is probably different, but there are quite a few asteroids in long thin orbits that are more typical of comets and are more likely dead comet candidates. It’s certainly plausible that some comets start out with a lot more rocky material than most. If enough of this rock stays together as the ice boils away, what was once a comet could lose all trace of cometary behavior. Is it an asteroid now? Or should a new class be created?

Some people might argue that meteor showers are not an important identifier of comets, and therefore there is no doubt that Phaethon is an asteroid. However, if a comet’s defining feature is a tail, Phaethon sometimes appears suspiciously close to that too, something attributed to sodium seeping out of cracks.

Other exceptions

Phaethon may be unique, but other objects blur the boundaries in different ways. Late last year, citizen scientists discovered DQ118, previously classified as an asteroid, around 2009, which is called “comet-like activity.” Indeed, that discovery was made as part of the Active Asteroids program, which is hunting for exactly this kind of refusal to stay neatly in one box or the other.

Various conditions are believed to trigger this type of activity. A collision between two asteroids can release comet-like dust, like what happened after the DART mission collided with Dimorphos. Some asteroids rotate so quickly that they throw out dust that can become a tail. In the case of DQ118 from 2009, we don’t really know what causes it.

Desperate for a simple division, some might say that comets contain ice, however little is left, while asteroids are just rock, even if they sometimes produce tails. However, Ceres, the largest asteroid and the first to be discovered, has ice volcanoes. No one is proposing to reclassify the planet as a comet. Why would the status of 24 Themis, a smaller object found to contain ice, need to be changed?

If all this isn’t complex enough, consider the case of 288P, a pair of objects in the main asteroid belt that occasionally exhibit bursts of comet-like activity. One proposed explanation is that a rapidly spinning object long ago lost ice from its outer layers. To anyone who observed it, 288P would have been an ordinary asteroid at the time. Eventually, however, its rapid rotation speed caused it to split into two pieces, exposing ice previously stuck inland to sunlight, causing occasional eruptions.

There’s clearly no right way to classify a type of zombie comet that only comes back to life after tearing itself apart. Instead, we have what some astronomers call the comet-asteroid continuum, which we are still struggling with.

Eventually, conveniently, some of these borderline cases will be clarified, perhaps by the International Astronomical Union voting on a definition, as it has done for planets. But new examples are likely to baffle us. Nature abhors not so much a vacuum as a leaky simplicity.

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