Comets in Asteroids’ Clothes
I enjoy writing about astronomy exactly for the reason I once feared it: there’s so much we don’t know. There’s rarely any shame in asking about the nature of a black hole, for example, because scientists are grappling with it too. Usually, if I get confused while writing an astronomy story, it’s because I’ve stumbled on an area of research that is still new, uncertain — and confusing! And those are the frontiers where the theories compete and the stories get fun.
This uncertainty has pervaded many lines of inquiry in recent headlines: discoveries on Saturn’s moon Titan, the nature of black holes and the contents of our solar system, with all its new members. And now, scientists are going back to the drawing board when it comes to the distinction between asteroids and comets.
Take this news flash out of the Planetary Science Institute (PSI) in Tucson, Arizona: “Research aims to unmask comets posing as asteroids.”
The difference between asteroids and comets was once clear. The story went that a giant cloud of gas and dust collapsed to create the solar system about 4.6 billion years ago. Asteroids formed from rock in the warmer, inner areas of the solar system; the so-called “asteroid belt” orbits between Mars and Jupiter. Comets were made of rock and ice and inhabited the colder, more distant regions.
Comets, historically, were defined by their tails. Because their wide orbits keep them for most of their lives in the frozen zone beyond Neptune, their surfaces are covered with dirty ice that vaporizes in fiery displays once they get closer to the sun. Comets are often much smaller than asteroids. The famed Hale-Bopp was thirty to fifty miles across. Halley boasted a ten-mile diameter. While many asteroids are big enough that gravity works to make them round, comets tend to have elongated shapes that get even more odd as the sun’s heat vaporizes their interior patches of ice.
But sometimes, a comet looks like an asteroid. Comet Halley could one day be an example. When it buzzed Earth in 1910, Halley made a dazzling appearance that inspired all kinds of poems and songs. But Halley didn’t sing its old song with quite as much gusto in 1986. Eventually, the Sun will burn off Halley’s volatile components and it’ll cease to be a spectacular show not because of its position but because there’s nothing left to vaporize. And therein lies one of the rubs in distinguishing asteroids and comets: “I expect Halley,” Flagstaff astronomer David Schleicher once said, “to finally get to the point when it looks like an asteroid.”
The research team at Tucson’s Planetary Science Institute, led by PSI astronomer Paul Abell, estimates that between five and 10 percent of Near Earth Objects could be comets impersonating asteroids. Some could be dying comets, and others could be dormant – with the potential to display comet-like traits after colliding with other objects.
Abell and his colleagues are using NASA’s Infrared Telescope Facility at the Mauna Kea Observatories in Hawaii and the MMT telescope on Mount Hopkins, south of Tucson, to revisit objects that have been labeled asteroids.
One imposter was already exposed: 2001 OG108, Lowell Observatory astronomers (in Flagstaff) first raised a flag on the object, formerly known as an asteroid. Other telescopes were aimed at it, and sure enough, detected a comet’s nucleus.
It’s useful to tell the difference partly because nearby comets could provide water and other materials become supply depots to support future space missions. And comets or asteroids could be threats to Earth if they collide with us — but comets could perhaps be more dangerous. Comets are structurally weak and likely to break up as they enter the atmosphere, leading to a heat and shockwave blast that would be much more devastating than the impact from an asteroid of the same size, the study authors say.
Finally, the PSI methods could reveal clues as to the origins of the comets it glimpses — most are thought to come from Pluto’s neighborhood, called the Kuiper Belt, or the theoretical Oort Cloud beyond it. Both regions are believed to contain clues in deep freeze that will help us understand the solar system’s origins.