Comet definitions are getting blurry NASA/JPL-Caltech/UCLA

When is a comet not a comet? It’s a question astronomers are asking themselves more and more often. Now it seems one of these supposed ice balls might actually be an asteroid that gets within 8 million kilometres of the sun – a cosmic hair’s breadth.

Traditionally, the two kinds of space rocks are thought to be very different. Comets are loose piles of rock and ice on long, elliptical orbits that heat up and develop a tail of gases as they near the sun. Asteroids, on the other hand, are lumpy bodies of hard rock and metal that mostly orbit the sun at a distance that falls between Mars and Jupiter.

But an increasing number of objects are being discovered that blur the line between comet and asteroid. The latest is comet 322P/SOHO 1, which was discovered in 1999 by the Solar and Heliospheric Observatory, a space telescope that examines the area around the sun. But SOHO’s view is shaded to protect it from intense sunlight and its resolution is comparatively low, meaning it can’t get a good look at 322P during the comet’s closest approach to the sun.


Now, Matthew Knight of the University of Maryland in College Park and his colleagues have used ground-based telescopes and the Spitzer Space Telescope to take another look. They found that there was no sign of a tail from 322P as it got close to the sun. They also found that its density is at least 1000 kilograms per cubic metre, double that of the famous comet 67P/Churyumov-Gerasimenko.

The readings are a big clue that 322P may actually be an asteroid, says Knight: “We’ve never seen a comet with density like this before.”

If so, that would make it the closest-approaching asteroid to the sun, coming to within about 8 million kilometres of it – or roughly 5 per cent of the distance between Earth and the sun.

Cometsteroid or asteromet?

Exactly where the line falls between asteroid and comet is more than idle stamp collecting, because tracking the evolution of these rocks allows us to trace the history of the solar system.

“It tells us about how the solar system formed and where different objects may have formed,” says Knight. The latest models of solar system formation suggest that Saturn and Jupiter could have moved about, mixing up the orbits of smaller space rocks.

So should we drop the distinction between asteroids and comets? Knight says the situation is a bit like the realisation that there are many small planets on the outskirts of the solar system – which ultimately resulted in astronomers classifying them, along with Pluto, as dwarf planets.

“It’s another case where having artificial names for things gets difficult,” says Knight, but it’s unlikely the terms will fall out of favour. “I think those terminologies will stick around, but scientists may end up with more distinctions.”

Journal reference: The Astrophysical Journal Letters, DOI: 10.3847/2041-8205/823/1/L6