Two Asteroids Found to be Comets

With the help of NASA’s Spitzer Space Telescope, astronomers have discovered that what was thought to be a large asteroid called Don Quixote is in fact a comet. The left image shows Don Quixote’s coma and tail — features of comets — as seen in infrared light by Spitzer. The coma appears as a faint glow around the center of the body, caused by dust and gas. The tail, which appears more clearly in the right image, points towards the right-hand side of Don Quixote, into the direction opposite of the sun. The right image represents a more elaborate image processing step, in which the glow of the coma has been removed based on a model comet coma. Bright speckles around Don Quixote are background stars; the horizontal bar covers image artifacts caused by the image processing. Image Credit: NASA/JPL-Caltech/DLR/NAU

Two different teams are reporting that what scientists long thought were asteroids are actually comets. Michael Mommert said that (3552) Don Quixote, whose journey stretches to the orbit of Jupiter, now appears to be a comet.

David Jewitt said that the Sun-grazing asteroid, Phaethon, has betrayed its true nature by showing a comet-like tail of dust particles blown backwards by radiation pressure from the Sun. Unlike a comet, however, Phaethon’s tail doesn’t arise through the vaporization of an icy nucleus. During its closest approach to the Sun, researchers believe that Phaethon becomes so hot that rocks on the surface crack and crumble to dust under the extreme heat. Studying the nature of asteroids and comets is important in determining their composition and whether or not such objects could have delivered materials to the early Earth that were important for the origin of life.

The announcements were made at the European Planetary Science Congress (EPSC) 2013 in London on Tuesday 10 September.

Don Quixote

The discovery of (3552) Don Quixote resulted from an ongoing project coordinated by researchers at Northern Arizona University using the Spitzer Space Telescope. Through a lot of focused attention and a little bit of luck, they found evidence of cometary activity that had evaded detection for three decades.

“Don Quixote’s orbit resembles that of a comet, so people assumed it was a comet that had gotten rid of all its ice deposits thousands of years ago,” said Mommert, a PhD student of team member Prof. Alan Harris at the German Aerospace Center (DLR) in Berlin at the time this work was carried out. Near-Earth asteroids that are former comets make up roughly 5 percent of the whole near-Earth asteroid population, as found by Mommert and colleagues in a related study. These objects are mostly “dead comets” – comets that had shed the carbon dioxide and water that give them their spectacular comae and tails long time ago.

What Mommert, now a post-doctoral researcher at NAU, and an international team of researchers discovered, though, was that Don Quixote was not actually a dead comet. In fact, the third-biggest near-Earth asteroid out there, skirting Earth with an erratic, extended orbit, is “sopping wet,” said NAU associate professor David Trilling, with large deposits of carbon dioxide and presumably water ice.

Geminids over Pendleton, Oregon. Credit: Thomas W. Earle

Finding evidence of carbon dioxide wasn’t easy. During an observation of the object using Spitzer in August 2009, Mommert and colleagues found that it was far brighter than they expected. “The images were not as clean as we would like, so we set them aside,” Trilling said.

Much later, though, Mommert prompted a closer look, and partners at the Harvard-Smithsonian Center for Astrophysics found something unusual when comparing the infrared images of the object: something, that is, where an asteroid would have shown nothing. The processed images indicated that Don Quixote had a coma and a faint tail.

“This discovery of carbon dioxide emission from Don Quixote required the sensitivity and infrared wavelengths of the Spitzer telescope and would not have been possible using optical telescopes on the ground,” Mommert said. This discovery implies that carbon dioxide and water ice might be present on other near-Earth asteroids, as well.

The implications have less to do with a potential impact, which is extremely unlikely in this case, and more with “the origins of water on Earth,” Trilling said. Comets may be the source of at least some of it, and the amount on Don Quixote represents about 100 billion tons of water—roughly the same amount that can be found in Lake Tahoe, California.

This study has confirmed Don Quixote’s size and the low, comet-like reflectivity of its surface. The results of this study have been submitted to the Astrophysical Journal for publication. Michael Mommert’s work at the DLR’s Institute of Planetary Research in Berlin was funded by Grant HA 2914/2-2 from the German Research Foundation (DFG).


Most meteor showers arise when the Earth ploughs through streams of debris released from comets in the inner solar system. The Geminids, which grace the night sky annually in December, are one of the best known and most spectacular of the dozens of meteor showers. However, astronomers have known for 30 years that the Geminids are not caused by a comet but by a 5 km diameter asteroid called (3200) Phaethon.

Zoomed, contoured STEREO image showing the south-eastward extension of the image of (3200) Phaethon. Credit: Jewitt, Li, Agarwal /NASA/STEREO

Until recently, though, and much to their puzzlement, astronomer’s attempts to catch Phaethon in the act of throwing out particles all ended in failure. The tide began to turn in 2010 when Jewitt and colleague, Jing Li, found Phaethon to be anomalously bright when closest to the Sun. The key to success was their use of NASA’s STEREO Sun-observing spacecraft. Phaethon at perihelion appears only 8 degrees (16 solar diameters) from the sun, making observations with normal telescopes impossible. Now, in further STEREO observations from 2009 and 2012, Jewitt, Li and Jessica Agarwal have spotted a comet-like tail extending from Phaethon.

“The tail gives incontrovertible evidence that Phaethon ejects dust,” said Jewitt. ‘That still leaves the question: why? Comets do it because they contain ice that vaporizes in the heat of the Sun, creating a wind that blows embedded dust particles from the nucleus. Phaethon’s closest approach to the Sun is just 14 per cent of the average Earth-Sun distance (1AU). That means that Phaethon will reach temperatures over 700 degrees Celsius – far too hot for ice to survive.”

The team believes that thermal fracture and desiccation fracture (formed like mud cracks in a dry lake bed) may be launching small dust particles that are then picked up by sunlight and pushed into the tail. While this is the first time that thermal disintegration has been found to play an important role in the Solar System, astronomers have already detected unexpected amounts of hot dust around some nearby stars that might have been similarly-produced.

So, is Phaethon an asteroid or a comet? Asteroids and comets derive from entirely different regions of the solar system; asteroids from between Mars and Jupiter (roughly 2 to 3.5 AU) and comets from the frigid trans-Neptunian realms (30 AU and beyond).

“By the shape of its orbit, Phaethon is definitely an asteroid. But by ejecting dust it behaves like a ‘rock comet’,” said Jewitt.

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