Near-Earth Objects (NEO's) are asteroids and comets that have been kicked out of their places of origin as a result of the gravitational bullying of nearby planets--only to eventually wander into dangerous orbits that are uncomfortably close to Earth. Leftover primitive rocky and metallic objects, asteroids are all that is left of what was once a vast population of ancient planetary building blocks, termed planetesimals, that went into the construction of the four rocky, inner terrestrial worlds--Mercury, Venus, Earth, and Mars--when our Solar System was young and still forming. Asteroids mainly inhabit a region of space located between the orbits of two major planets, Mars and Jupiter, that is appropriately dubbed the Main Asteroid Belt. In August 2015, astronomers presented a new NASA study that has traced some members of the near-Earth asteroid population back to their likely source--the Euphrosyne asteroids, which are dark objects on tilted orbits in the outer portion of the Main Asteroid Belt. The study used data derived from NASA's NEOWISE space telescope, which successfully managed to have a second mission after its reactivation in 2013.
The NEOWISE project is the asteroid-hunting component of the Wide-field Infrared Survey Explorer (WISE) mission. NEOWISE takes measurements of asteroids and comets derived from the original WISE images and provides an archive for searching WISE data for Solar System objects. Launched in December 2009, WISE surveyed the entire sky in the infrared bands, until the frozen hydrogen cooling the telescope was used up in September 2010. The survey then continued as NEOWISE.
The astronomers involved in the new NASA study observed a one-of-a-kind family of space rocks. These interplanetary "oddballs", the Euphrosyne (pronounced you-FROH-seh-nee) asteroids, named for the ancient Greek goddess of mirth, have been dark, distant, and delightfully mysterious--until now.
The Euphrosyne family of asteroids do their bewitching, distant dance at the outer edge of the Main Asteroid Belt. They also display a strange and intriguing orbital path, jutting well above the ecliptic--which is the equator of our Solar System. The asteroid after which they all derive their name, Euphrosyne, is approximately 156 miles across and is one of the 10 largest asteroids located in the Main Belt. Many astronomers think that the asteroid Euphrosyne is really the tattle-tale relic of an ancient, catastrophic collision, that occurred about 700 million years ago, producing the "oddball" family of smaller asteroids bearing its name. Scientists think this horrific blast from the past was one of the last major collisions to rock our Solar System.
NEOs are objects whose orbits around our Sun take them dangerously close to the orbit of Earth. This population does not last long on astronomical timescales and is replenished by other reservoirs of similar bodies dwelling in our Solar System. As they travel around our Sun, NEOs can--every so often--travel a bit too close to our planet for comfort. For this reason, the study of such strange objects is important.
NEOs
Composed primarily of water ice containing embedded dust motes, comets originally formed in the frozen outer limits of our Solar System, while most of the rocky, metallic asteroids formed in the balmy inner region closer to the comforting light and warmth of our Star. Because they are relatively unchanged debris from our Solar System's ancient formation, comets and asteroids can serve as time capsules revealing ancient secrets about our Solar System's formation about 4.6 billion years ago. The quartet of giant outer, gaseous planets--Jupiter, Saturn, Uranus, and Neptune--evolved from a treasure trove of billions of icy comets. The comets that we see today are really the lingering, left over frozen chunks that tell the captivating story of this very ancient process. Similarly, the asteroids that we see today are the fragments left over from the initial agglomeration of the quartet of rocky, inner planets, including Earth.
As primitive, relic building blocks from our Solar System's formation, comets and asteroids provide valuable clues in regard to the chemical soup from which the planets formed. If scientists want to know the composition of the primordial mixture from which the planets were born, they must determine the chemical constituents of the leftover debris from this formation process--the comets and asteroids.
Our Solar System emerged from the billowing depths of a giant, cold, dark molecular cloud. It was born as the result of the collapse of a relatively small and superdense pocket embedded within one of these ghostly cold clouds that haunt our Milky Way in huge numbers. Most of the collapsing, superdense pocket collected at the center, and eventually ignited as a result of the process of nuclear fusion--forming our Sun. The remaining mass flattened out and became what is termed a protoplanetary accretion disk from which the planets, moons, comets, asteroids, and other small Solar System objects finally emerged.
Protoplanetary accretion disks have been observed surrounding a large number of young stars inhabiting newly formed stellar clusters. The swirling disks, composed of gas and dust, form at about the same time the fiery, brilliant baby star is born, but at the first stages of their development they cannot be observed because they are shrouded in thick, obscuring blankets of opaque dust and gas. The nurturing accretion disk feeds the ravenous, active baby star, or protostar, this nourishing stellar feast of gaseous, dusty material. At this point, the protoplanetary accretion disk is both searing-hot and very massive. These swirling, whirling disks can linger around their young stars for as long as 10 million years.
By the time the toddler star has evolved to reach the T Tauri stage of its development, the nutritious protoplanetary accretion disk has thinned out and cooled off. A T Tauri is a youthful, very active, variable stellar tot that is less than 10 million years of age--which, in star-years, makes it very young. T Tauris possess diameters that are several times larger than that of our own Sun--which is a middle-aged, relatively small star--but they are still in the process of shrinking. Unlike human toddlers, T Tauris shrink as they grow up. By the time the young star has reached this stage in its early development, less volatile materials have started to condense close to the center of the protoplanetary accretion disk, creating extremely sticky particles of very fine dust. These tiny, tiny dust motes harbor crystalline silicates.
The tiny dust grains bump into one another and merge together to create bigger and bigger objects within the crowded, dense environment of the disk. The little motes of dust form objects up to several centimeters in size, and these go on and on to merge together to form planetesimals. The building-blocks of planets, planetesimals can grow to be 1 kilometer across--or even larger. The planetesimals are very abundant, and they travel throughout the entire accretion disk. Some of these primordial objects can survive long enough to remain as tattle-tale relics billions of years after the formation of the mature planetary system..
Outcast Asteroids
The new study that was conducted by astronomers at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, used the space agency's NEOWISE telescope to peer at the "oddball" family of dark, distant Euphrosyne asteroids in order to learn more about NEOs, and their potential threat to our planet.
As a result of their study, the JPL astronomers came to the realization that the Euphrosynes may be the source of some of the dark NEOs observed to be on highly inclined, long orbits. The astronomers found that, as a result of gravitational interactions with the giant, ringed-planet Saturn, Euphrosyne asteroids can evolve into NEOs over the passage of millions of years.
NEOs can originate from either the Main Asteroid Belt or from the more remote outer regions of our Solar System. Those that wander in from the Main Asteroid Belt are probably herded uncomfortably close to Earth's orbit as a result of collisions, as well as the gravitational influence of planets. Because they originate from well above the ecliptic, and near the distant outer edge of the Asteroid Belt, the forces that determine their trajectories in the direction of Earth are considerably more moderate.
"The Euphrosynes have a gentle resonance with the orbit of Saturn that slowly moves these objects, eventually turning some of them into NEOs. This particular gravitational resonance tends to push some of the larger fragments of the Euphrosyne family into near-Earth space," explained Dr. Joseph Masiero in an August 3, 2015 JPL Press Release. Dr. Masiero is JPL's lead scientist on the Euphrosyne study.
By observing the Euphrosyne asteroids with NEOWISE, JPL astronomers have successfully measured their sizes and the quantity of solar energy that they reflect. Because NEOWISE operates in the infrared portion of the spectrum, it detects heat, and can therefore observe dark objects much better than telescopes that are operating at visible wavelengths, which sense reflected light from our Star. Its heat-detecting ability also enables it to measure the sizes of objects with greater accuracy.
Dr. Masiero and his team studied 1,400 Euphrosyne family asteroids that turned out to be large and dark. They also sported highly inclined and elliptical orbits that made them excellent candidates as the source of some of the dark NEOs detected by NEOWISE. "NEOWISE is a great tool for searching for near-Earth asteroids, particularly high-inclination, dark objects," Dr. Masiero continued to explain in the August 3, 2015 JPL Press Release.
The Main Asteroid Belt is known to be the home of over 700,000 asteroidal bodies, and it is generally thought that many more wait to be discovered. These asteroids range in size from large boulders to about 60 percent of the diameter of Earth's Moon. Therefore, determining the exact place of origin for most of the NEOs is a difficult task.
However, the Euphrosynes are different. "Most near-Earth objects come from a number of sources in the inner region of the Main Belt, and they are quickly mixed around. But with objects coming from this family, in such a unique region, we are able to draw a likely path for some of the unusual, dark NEOs we find back to the collision in which they were born," Dr. Masiero added.
A greater understanding of the behavior and mysterious origin of these dark, distant "oddballs" will provide astronomers with a better picture of asteroids in general, and the NEOs that come screaming into Earth's backyard, in particular.
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