asteroids are a large group of objects that swarm around in the same orbit as the gas-giant planet Jupiter--the largest planet in our Solar System. The total number of Trojan asteroids larger than 1 kilometer is thought to be about 1 million. Like wild horses, the asteroids gallop through space in herds, with one tumbling, zipping swarm leading the way in front of Jupiter, and a second swarm whizzing in from behind. But where did the Trojans come from?
There are now thousands of Trojans known--amounting to about the same number thought to inhabit the Main Asteroid Belt between Mars and Jupiter. The first Trojan was spotted on February 22, 1906, by German astronomer Max Wolf, who found the galloping object in the herd leading ahead of Jupiter. It was named Achilles by Wolf, and it is approximately 135 kilometers in diameter. Each of the Trojans is named for a hero of the Trojan War, in honor of the legend in which Greek soldiers hid inside a giant wooden horse statue in order to launch a surprise invasion of Troy, and attack that ancient city's people. The largest of the Trojans is 624 Hektor, which has a diameter of about 203 kilometers. It is thought that the smallest of the swarming Trojans are merely the leftovers from collisions between crashing larger Trojans.
Asteroids and comets are lingering relics of our Solar System's ancient past. Our Solar System was born about 4.6 billion years ago due to the gravitational collapse of a small dense knot within a giant, cold, dark molecular cloud. Most of the collapsing mass congealed at the center, giving birth to our Star, the Sun. The rest flattened into what is termed a protoplanetary disk from which the planets, moons, asteroids, comets, and other small Solar System bodies emerged. This generally accepted model, termed the nebular hypothesis, was first suggested back in the 18th century by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace.
Protoplanetary disks have been spotted circling a number of stars inhabiting young star clusters. They form when a baby star is born, but at the earliest stages cannot be seen because they are swathed by an impenetrable, opaque, veiling envelope. The accretion disk, which nourishes the central baby protostar is thought to be both very massive and searing-hot. The temperature can easily skyrocket above 1,000 Kelvin within 1 Astronomical Unit (AU) from the baby star, and 400 Kelvin inside 5 AU.One AU is the average distance between Earth and the Sun, which is 93,000,000 miles.
Accretion disks can hang around for about 10 million years. By the time the young star reaches what is termed the T Tauri stage, the disk has grown both cooler and thinner. A T Tauri star is an extremely youthful and energetic variable star that is less than 10 million years old, and possesses a mass that is similar to, or perhaps a bit less, than that of our Sun. T Tauri stars have diameters that are several times greater than that of our Star, and they are still in the process of shrinking. By the time a bouncing baby star has reached this stage, less volatile materials have already started to condense near the center of the accretion disk, forming very tiny, smoke-like dust grains that contain crystalline silicates.
These dust particles are bestowed with a natural stickiness and therefore tend to glue themselves together in the dense disk environment, leading to the formation of ever larger particles up to several centimeters in size. Further aggregation results in the formation of planetesimals, which are the building blocks of planets. The planetesimals can be 1 kilometer across or even larger. Planetesimals are extremely abundant, and they tend to spread throughout the protoplanetary disk--and some remain as relics long after the formation of a planetary system. Asteroids, such as those found in our own Solar System, are believed to be left-over rocky planetesimals. Comets, on the other hand, are thought to be the relic icy planetesimals from the outer limits of a Solar System such as our own. The asteroids are the leftover building blocks of the rocky inner planets--Mercury, Venus, Earth, and Mars--while the comets are the leftover building blocks of the outer giant planets, which are Jupiter, Saturn, Uranus and Neptune.
Astronomers using data obtained from NASA's Wide-field Infrared Survey Explorer (WISE), have succeeded in obtaining some new and important clues in respect to the mysterious origins of Jupiter's herds of Trojans. These observations are the first to obtain a detailed analysis of the Trojans' colors. The new observations reveal that the leading and trailing Jovian attendant herds are composed of mainly dark, reddish rocks with non-reflecting, somewhat dull surfaces. Also, the leading herd is more numerous than the trailing one.
WISE has succeeded in revealing that the two separate herds of galloping Trojans are similar, and it therefore sheds some light on the baffling origin of these asteroids. Apparently, both herds do not contain invaders from other regions of our Solar System! Furthermore, the Trojans do not resemble asteroids dwelling in the Main Asteroid Belt--neither do they bear a family resemblance to the comet-like objects spinning around in the icier, darker, and more remote region near the dwarf planet Pluto, known as the Kuiper Belt.
"Jupiter and Saturn are in calm, stable orbits today, but in their past, they rumbled around and disrupted any asteroids that were in orbit with these planets. Later, Jupiter re-captured the Trojan asteroids, but we don't know where they came from. Our results suggest that they may have been captured locally. 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," explained Dr. Tommy Grave in an October 15 2012 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Grav is a WISE scientist from the Planetary Science Institute in Tucson, Arizona. He is also a member of the NEOWISE team, which is the asteroid-hunting component of the WISE mission. JPL manages, and operates, WISE for NASA's Science Mission Directorate. The spacecraft soared into orbit on December 14, 2009, and was put into hibernation mode in 2011, after it had succeeded in scanning the entire sky twice, completing its primary mission.
Before WISE, the primary mystery in regard to this population of objects was determining how many of them pranced around in the two herds of space rock and ice, both leading and following Jupiter.
Grav continued to note that "The two asteroid camps even have their own 'spy'. After having discovered a handful of Trojans, astronomers decided to name the asteroid in the leading camp after the Greek heroes and the ones in the trailing after the heroes of Troy. But each of the camps already had an 'enemy' in their midst, with asteroid 'Hector' in the Greek camp and 'Patroclus' in the Trojan camp."
The NEOWISE team has successfully determined the colors of hundreds of Jupiter's Trojans. This has enabled many of these objects to be sorted according to asteroid classification schemes for the very first time.
"We didn't see any ultra-red asteroids, typical of the Main Belt and Kuiper Belt populations. 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- types, which are more grey-bluish in color. More research is needed, but it's possible we are looking at some of the oldest material known in the Solar System," Grav continued to explain in the October 15, 2012 JPL Press Release.
Other planets have also been found to have their own retinues of Trojans--they are Mars, Neptune, and our own Earth! WISE was responsible for discovering the very first Earth Trojan!
I am a writer and astronomer whose articles have been published since 1981 in various newspapers, journals, and magazines. Although I have written on a variety of topics, I particularly love writing about astronomy because it gives me the chance to communicate to others the many wonders of my field. My first book, "Wisps, Ashes, and Smoke," will be published soon.