Have you ever seen a centaur? NASA believes in them, so they must exist, though even NASA wasn’t exactly sure what their true identity was — until recently, that is, when the truth about them came out.
Centaurs are — to NASA, anyway — not the creatures of mythology who are half human and half horse. But, the true identity of what they actually are, the small celestial bodies orbiting the sun between Jupiter and Neptune, has been one of the enduring mysteries of astrophysics.
Like the centaurs of mythology, NASA’s centaurs exhibit at times the properties of two objects — asteroids and comets. So, what are these mysterious objects? Are they asteroids flung out from the inner solar system or comets traveling in toward the sun from the far reaches of space? According to a new study of observations from NASA’s Wide-field Infrared Survey Explorer (WISE), most centaurs are comets.
James Bauer of NASA’s Jet Propulsion Laboratory in Pasadena, Califonia, the lead author of a paper published online July 22 in the Astrophysical Journal, states that:
Just like the mythical creatures, the centaur objects seem to have a double life. Our data point to a cometary origin for most of the objects, suggesting they are coming from deeper out in the solar system.”
What does “cometary origin” mean”
An object in space can be said to have a “cometary origin” if it is made from the same material as a comet, or has been an active comet in the past, and might possibly be an active comet in the future.
How did NASA come to the conclusion they did about the “centaurs”?
The largest infrared survey to date of centaurs and a distant cousin of their, called “scattered disk objects,” led NASA to their findings about what sorts of objects centaurs really are.
The asteroid-hunting part of the WISE mission, known as NEOWISE, gathered infrared images of 52 centaurs and scattered disk objects. Of the 52 objects, fifteen are new discoveries. They, like all centaurs and scattered disk objects, start off by orbiting in an unstable belt. Eventually, gravity from one of the giant nearby planets will either fling them closer to the sun or further away from their current locations.
Previously, astronomers had occasionally observed centaurs with dusty halos, a common feature of outgassing comets. NASA’s Spitzer Space Telescope seemed to have discovered some evidence for comets in the group, which was one of the difficulties researchers had in trying to estimate the numbers of comets and asteroids there.
That was, they had difficulties in sorting out the population until the astronomers received data from NEOWISE that provided information on the objects’ albedos, or reflectivity. That data anables NEOWISE to tell whether a centaur’s surface is dark and matte, or a shiny one that reflects more light.
When astronomers combined the albedo information with what was already known about the colors of the objects, the puzzle pieces fell into place.
Visible-light sorts of observations have shown centaurs generally to be one of two colors or hues, either blue-gray or reddish. Blue-gray objects could be an asteroid or comet. Because NEOWISE showed that most of the blue-gray objects are dark, a telltale sign of comets, while reddish objects are more likely to be asteroids, it was a relatively safe conclusion to make that most of the centaurs are comets waiting to be born.
According to the study’s co-author, Tommy Gray, of the Planetary Science Institute in Tucson, Arizona:
Comets have a dark, soot-like coating on their icy surfaces, making them darker than most asteroids. Comet surfaces tend to be more like charcoal, while asteroids are usually shinier like the moon.”
Roughly two-thirds of the centaur population are comets, which come from the frigid outer reaches of our solar system, according to the results. The rest of them might be asteroids, though the results of the research so far are inconclusive about exactly what they are. It is possible that future research from NEOWISE may reveal more of the secrets of the centaurs.
WISE is managed by JPL, who are, in turn, managed and operated by the California Institute of Technology in Pasadena for NASA’s Science Mission Directorate.
NASA’s Near Earth Object Observation Program funded the NEOWISE portion of the project. In 2011, WISE completed its key mission objective, two scans of the entire sky. It has been hibernating in space ever since then.
But, with the data that WISE and NEOWISE gathered, they have proven, along with other scientist and researchers at NASA, that centaurs do exist, and that the majority of them are more like comets than asteroids.
Written by: Douglas Cobb