Asteroids Following Jupiter’s Solar Orbit Show Their True Colors
October 16, 2012

Asteroids Following Jupiter’s Solar Orbit Show Their True Colors

Lawrence LeBlond for - Your Universe Online

Jovian asteroids that orbit the Sun on the same trajectory as the gas giant Jupiter have been studied more closely by scientists using NASA´s Wide-field Infrared Survey Explorer (WISE).

These asteroids travel in packs, like hungry wolves stalking their prey, some racing ahead of the gas giant, while others trail behind. The observations of these Jovian predators are the first detailed look at their true colors.

Scientists have found that both groups of cosmic hunters are made up of predominantly dark, reddish rocks with a matte, non-reflecting surface. And based on the data researchers have gleaned, the leading pack of these natural satellites, commonly referred to as trojans, outnumber the trailing group.

The research offers new insight into the mystery of the composition and origins of these trojan hunters.

Using WISE, astronomers have found both packs of asteroids do not harbor any interlopers (stray intruders) from other parts of the solar system. Also, these trojans do not resemble the asteroids from the main belt between Mars and Jupiter, nor from the Kuiper belt family from the icier, outer regions near Pluto.

Tommy Grav, a WISE scientist from the Planetary Science Institute in Tucson, Arizona, said in Jupiter´s heyday, back when it “rumbled around” instead of keeping in a stable trajectory, it “disrupted any asteroids that were in orbit [with Jupiter, and Saturn as well].”

“Later, Jupiter re-captured the Trojan asteroids, but we don't know where they came from. Our results suggest they may have been captured locally,” explained Grav in a news release. “If so, that's exciting because it means these asteroids could be made of primordial material from this particular part of the solar system, something we don't know much about.”

Grav is a member of the NEOWISE team, the asteroid-hunting portion of the WISE mission.

The first of these trojans was discovered on February 22, 1906, by German astronomer Max Wolf, who found it racing ahead of Jupiter. Named “Achilles,” this was the first of many Jovian leaders to be detected ahead of the gas giant. Later, asteroids were also found to be trailing behind.

Since then, other planets have been found with trojan asteroids in their midst, including Neptune and Earth--as recently found by WISE.

Before WISE, there was really no accurate measurement as to how many individual trojans were traveling in these clouds of rock and ice debris in either group around the Jovian behemoth. It is believed that there are as many objects in the two swarms around Jupiter as there are in the entirety of the main asteroid belt between Mars and Jupiter.

In order to determine whether this is true, and to find exactly just how many there could be, astronomers began a well-coordinated, well-executed observational campaign. However, the team knew there were many things that would get in the way of making accurate observations--the main being Jupiter itself. Using ground-based telescopes proved difficult in garnering real estimates due to the orientation of Jovian asteroid clouds over the past few decades. While astronomers were able to generate some results, they deemed them unreliable.

But with the launch of WISE in December 2009, things were looking up. The observatory´s 16-inch telescope and infrared cameras scoured the entire sky looking at the glow of celestial heat sources. From January 2010 to February 2011, WISE generated about 7,500 images every day. The NEOWISE project used data from the observatory to catalogue more than 158,000 asteroids and comets throughout the solar system.

“By obtaining accurate diameter and surface reflectivity measurements on 1,750 Jupiter Trojans, we increased by an order of magnitude what we knew about these two gatherings of asteroids,” said Grav. “With this information, we were able to more accurately than ever confirm there are indeed almost 40 percent more objects in the leading cloud.”

Using WISE, Grav and his colleagues were also able to get a better look at the surfaces and interiors of some of the Jovian trojans. WISE´s infrared detectors were sensitive to the thermal glow of objects, unlike visible-light telescopes, meaning WISE could provide better estimates of surface reflectivity (albedo) and more details about their visible and infrared colors (types of light beyond the visible spectrum).

“Seeing asteroids with WISE's many wavelengths is like the scene in 'The Wizard of Oz,' where Dorothy goes from her black-and-white world into the Technicolor land of Oz,” Amy Mainzer, the principal investigator of the NEOWISE project at NASA's Jet Propulsion Laboratory in Pasadena, California, said in the news release. “Because we can see farther into the infrared portion of the light spectrum, we can see more details of the asteroids' colors, or, in essence, more shades or hues.”

So far, the NEOWISE team have analyzed the colors of some 400 trojans, allowing astronomers to properly sort many of these hunters according to asteroid classification schemes for the first time.

“We didn't see any ultra-red asteroids, typical of the main belt and Kuiper belt populations,” said Grav. “Instead, we find a largely uniform population of what we call D-type asteroids, which are dark burgundy in color, with the rest being C- and P-type, which are more grey-bluish in color.”

Grav noted that more research was needed, but he believes what he and his colleagues are seeing is “some of the oldest material known in the solar system.”

Scientists have proposed future space missions to the Jovian trojans to gather more detailed data to determine their age and origins.

Two studies of the latest findings have been accepted for publication in the Astrophysical Journal. Results of the studies were presented at the 44th annual meeting of the Division for Planetary Sciences of the American Astronomical Society in Reno, Nevada.

JPL manages, and operated, WISE for NASA's Science Mission Directorate. The spacecraft was put into hibernation mode in 2011, after it scanned the entire sky twice, completing its main objectives.