If you’re in the Northern Hemisphere, March 2024 could be an exciting month for comet-watching. If you’re reading this later or elsewhere, there’ll be other opportunities later in the year or down the track. On the other hand, only the seriously hard-core tend to get excited about asteroid spotting. So, what’s the difference, and how do you tell them apart?

Comets vs Asteroids

Comets have probably been part of human awareness as long as we have been able to communicate to each other that something looked odd in the skies. City lights make the sight of a comet a rare thing for most people these days, but once there would be several “hairy stars” bright enough to be noticed in a typical lifetime, even when those were shorter. 

On the other hand, the first asteroid was discovered on New Year’s Day 1801. Almost certainly at some point previously one had come close enough that people could see it pass during a night or two, but no records are known. It was only after that we started to realize a new category existed, and had to think how to distinguish it from the old.

Comets are usually similar in size to small-medium asteroids, and don’t necessarily come any closer. The reason they can be so much more visible is that comets are primarily made of ice, with some dust and possibly rock mixed in. As they approach the Sun, the outer layers of ice get above their freezing points (which differ depending on the type of ice). With no atmosphere to keep the products liquid, the former ices escape as gases, a process known as subliming, taking some of the dust with them. 

A fairly small amount of ice can cover a vast area in gaseous form, and reflect enough sunlight that they can often be quite bright. In the best cases, a long tail pushed away from the Sun by the solar wind creates a streamer across the sky. Even when the tail requires instruments to see, there is often a glowing head or coma to the comet that looks distinctively different from stellar points of light.

Asteroids, being lumps of rock, with maybe some dust, do none of that, so the difference may seem straightforward. Ice creating fuzziness means a comet, nothing but a point of light moving over hours or days and it’s an asteroid, yes? Simple really.

Except that nature seldom favors such clear divisions.

Phaethon makes trouble

Another feature of comets is that they often cause meteor showers in their wake. The dust particles that stream off comets as they sublime form patches around the inner Solar System. When a patch lies in the path of the Earth’s orbit, our planet plows into it on the same day each year. The dust particles produce meteors as they hit the atmosphere, in the best cases lighting the sky with a breathtaking display. 

We now know of dozens of meteor showers, and new ones are being added all the time, although few are bright enough to attract much attention. Most of these showers come from comets. The exception is the Geminids, which happens to be one of the year’s best and comes from material dropped by the asteroid 3200 Phaethon. 

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

The process has raised questions about how this asteroid alone comes to produce a meteor shower. The reasons are still debated, but the initial assumption was that Phaethon is a “dead comet”, one that originally contained a mix of rock and ice, but now only has rock remaining.

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

Some people might argue that meteor showers are not a key identifier of comets, and therefore there is no question that Phaethon is an asteroid. However, if the defining feature of a comet is a tail, then sometimes Phaethon seems to get suspiciously close there as well, something that has been attributed to sodium fizzing out of cracks. 

Other exceptions

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

Several circumstances are thought to produce this sort of activity. A collision between two asteroids can throw off dust that can look comet-like, like what occurred after the DART mission slammed into Dimorphos. Some asteroids rotate so quickly that dust is thrown off that can become a tail. In 2009 DQ118’s case, we don’t really know what the cause is.

Desperately searching for a simple division, some might say that comets have ice, no matter how little remaining, while asteroids are only rock, even if it sometimes produces tails. However, Ceres, the largest asteroid and first to be discovered, has ice volcanoes. No one is proposing it be reclassified as a comet. Why, therefore, should 24 Themis, a smaller object found to have ice, have its status changed?

If all this isn’t complex enough, consider the case of 288P, a pair of objects in the main asteroid belt that show occasional bursts of comet-like activity. A proposed explanation is that a fast-spinning object long since lost ice from its outer layers. To anyone perceiving it, 288P would have appeared an ordinary asteroid at the time. Eventually, however, its rapid rate of spin caused it to split into two pieces, exposing ice previously trapped in the interior to sunlight, and causing these occasional bursts.

There’s clearly no correct way to classify a sort of zombie comet that only revives after tearing itself apart. Instead, we have what some astronomers call the comet-asteroid continuum, with which we are still wrestling. 

Eventually, for convenience’s sake, some of these border cases will get cleared up, perhaps by the International Astronomical Union voting on a definition, as it did for planets. Probably, however, new examples will appear to baffle us. Nature does not so much abhor a vacuum as puncture simplicity.