The true identity of the objects, dubbed centaurs, has been one of the enduring mysteries of astrophysics, the space agency said, with debate of whether they are asteroids or comets. Astronomers have long disagreed about whether centaurs are asteroids flung out from the inner solar system or comets traveling toward the sun from afar.
Their mysterious dual nature saw them dubbed centaurs, after the creature in Greek mythology whose head and torso are human but whose body and legs are those of a horse.
Observations by NASA's Wide-field Infrared Survey Explorer found that most are comets, scientists said.
"Just like the mythical creatures, the centaur objects seem to have a double life," said James Bauer of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Our data point to a cometary origin for most of the objects, suggesting they are coming from deeper out in the solar system."
The WISE instrument completed the largest infrared survey to date of centaurs and their more distant cousins, called scattered disk objects.
The color of many of the centaurs was also a clue to their composition, scientists said.
"Comets have a dark, soot-like coating on their icy surfaces, making them darker than most asteroids," said study co-author Tommy Grav of the Planetary Science Institute in Tucson. "Comet surfaces tend to be more like charcoal, while asteroids are usually shinier like the moon."
The results indicate that roughly two-thirds of the centaur population are comets, the researchers said.