Comets disguised

as asteroids
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 04, 2008

Planetary scientists have discovered that between five and ten percent of Near Earth Objects could be comets impersonating asteroids, and plan to find ways to reveal their true identity.

Most Near Earth Objects (NEOs) – objects whose orbits bring them in proximity to Earth – are rocky asteroids or dying comets that have lost most of their volatile components which form their characteristic tails. Other NEOs could be dormant comets that might again display comet-like features after colliding with another object, but which skulk around the Solar System outwardly appearing as asteroids.

A dormant comet may look a lot like an asteroid at first glance. Although still active, the image on the left shows the nucleus of comet Wild-2 as imaged by the Stardust mission, and the image on the right shows asteroid Gaspra. Images: NASA.

NEO 2001 OG108, discovered by the Lowell Observatory Near Earth Asteroid Search telescope, is one such example. Although this NEO exhibited an orbit similar to that of cometary objects coming in from the Oort cloud, it was thought to belong to a group of asteroids called the Damocloids, which have Comet Halley-type orbits, but lack cometary tails.

Several different groups of astronomers turned their telescopes to 2001 OG108 because it looked suspiciously like a comet, and it turned out that they were actually looking at a bare comet nucleus. Such an observation is rare, since the nuclei of comets are very dark and difficult to observe when they're far from Earth. But when they eventually approach Earth, the Sun's heat vaporises their icy ingredients, creating the clouds of dust and gas that make up the coma and tail, and shrouding the nucleus from view. Surely enough, in early 2002 and less than a year after its discovery, NEO 2001 OG108 flared up with a coma, revealing its true identity and forcing a re-classification to C/2001 OG108 (LONEOS).

2001 OG108 set the scene for the new observing project led by Paul Abell of the Planetary Science Institute and colleagues, including Faith Vilas, director of the MMT Observatory and an affiliate senior scientist at the Planetary Science Institute. Using NASA's Infrared Telescope Facility at the Mauna Kea Observatories in Hawaii and the MMT telescope on Mount Hopkins in Arizona, Abell and colleagues will determine observational signatures that separate extinct or dormant comets from near-Earth asteroids. And by working in both the visible (0.35-1.0 microns) and near-infrared (0.7-2.5 microns) spectral ranges, they hope to map different spectral signatures for objects that originate in the Oort Cloud, the Kuiper Belt and the Asteroid Belt.

Views of our Solar System showing the relative locations of the main Asteroid Belt between the orbits of Mars and Jupiter, the Kuiper Belt surrounding the orbit of Pluto, and the Oort Cloud, from which many minor bodies such as Sedna and 2006 SQ372 are thought to originate. Image: SST.

"Hopefully, by combining data from these two wavelength regions, we may be able to find some signal or some observational discriminator that will help us identify whether something is an extinct comet or just an asteroid," says Abell.

"Are all these comets made of the same type of material or are they different?" he asks. "If they're composed of different materials, they may have different spectral signatures, and our preliminary work on Jupiter-family comets and Halley-type comets shows that this may be true.” Abell wants to know why that may be the case, and wonders if it may be something to do with the initial conditions of their formation regions or due to the different environments in which they spend most of their time – that is, nature or nurture?

The results could have important implications for the early evolution of the Solar System and future destiny of the Earth, since comets are thought to contain unmodified remnants of the primordial materials that formed the Solar System. And just like other NEOs, comets can pose a definite threat to civilisation should they find themselves forced onto a collision course with our planet. Unlike rocky asteroids that blast out craters when they slam into solid ground, comets are structurally weak and more prone to disintegrating as they plough through the atmosphere, leading to a shockwave blast focused towards the ground which has the potential to be much more devastating than the impact from an asteroid of the same size. Furthermore, dormant comets in near-Earth space could become supply depots to support future exploration activities with water and other materials.