VALSECCHI (INAF): «TROIANO SÌ, MA NON PER SEMPRE»Un asteroide “troiano” per la TerraAvvistato un piccolo asteroide, TK7, che accompagna stabilmente la Terra nel suo percorso intorno al Sole, in un’orbita detta “troiana”. La scoperta, che si è guadagnata la copertina di Nature, aggiunge il nostro pianeta a Giove, Marte e Nettuno, gli unici per i quali si sapesse dell’esistenza di compagni troiani.[img=left]http://www.media.inaf.it/wp-content/uploads/2011/07/Cover_28July-258x340.jpg[/img=left]
TK7 sulla copertina di Nature[/f]“Trojan” è una di quelle paroline che ci mettono sempre un po’ d’ansia, associata com’è a virus e altro malware in grado di mettere in pericolo il nostro computer. E anche “asteroide” non scherza, soprattutto se l’oggetto celeste in questione bazzica nei dintorni del nostro pianeta. Che dire, allora, d’un trojan asteroid, un asteroide troiano? Qualunque cosa sia, più ne stiamo alla larga e meglio è, vien da pensare.
Ebbene, gli asteroidi troiani esistono: sono chiamati così quando condividono l’orbita di un pianeta, e gli astronomi li conoscono da tempo. Fino a ieri, però, i soli pianeti del Sistema solare che potessero vantare tali amabili compagni erano Giove, Marte e Nettuno. Ora invece, come annuncia con enfasi l’ultima copertina di Nature, al trio dotato di troiani s’è ufficialmente aggiunto un quarto pianeta: la Terra.
Già, proprio il nostro pianeta. A condividerne l’orbita attorno al Sole, e per l’esattezza a precederci di circa due mesi (60 gradi), c’è 2010 TK7, questo il nome del nostro nuovo compagno di viaggio: un asteroide dalle dimensioni di tutto rispetto – circa 300 metri – che oscilla attorno al cosiddetto L4, uno dei punti lagrangiani. Un po’ come fanno i satelliti Planck e Herschel dell’ESA, che ci seguono fedelmente orbitando attorno a un altro punto lagrangiano, L2. A scovarlo fra i dati di WISE (Wide-field Infrared Survey Explorer), il satellite infrarosso lanciato dalla NASA nel 2009, un trio di astronomi guidati da Martin Connors, dell’Athabasca University (Canada). Scoperta poi confermata da successive osservazioni ottiche da telescopi terrestri.
Dunque, è confermato? Anche la Terra ha il suo compagno troiano? «Sì, ma è un troiano solo temporaneo», precisa Giovanni Valsecchi, astronomo dell’INAF-IASF di Roma ed esperto di asteroidi, «a differenza, per esempio, dei troiani di Giove. Questi ultimi, infatti, sono in buona parte su orbite sostanzialmente stabili, parliamo di tempi scala nell’ordine della vita del Sistema solare. L’oggetto appena scoperto, invece, è su un’orbita attualmente di tipo troiano, ma instabile».
Insomma, compagno sì, ma non per sempre. Gli autori, comunque, hanno calcolato che la sua orbita è rimasta stabile per almeno 10mila anni, e continuerà a rimanerlo a lungo, anche se le previsioni, al momento, non possono andare oltre i prossimi 7mila anni. In ogni caso, c’è tutto il tempo per conoscersi meglio. Magari, non troppo da vicino: con certi compagni di viaggio, preferiamo mantenere le distanze. E quelle che separano TK7 dalla la Terra non destano alcuna preoccupazione: attualmente, da quanto
riporta la NASA, parliamo di 80 milioni di chilometri, destinati a non scendere mai, almeno per i prossimi 100 anni, al di sotto dei 24 milioni.
Per saperne di più:
Leggi sul sito di Nature l’articolo “
Earth’s Trojan asteroid“
In quest’animazione, TK7 (in verde il suo percorso: ogni loop richiede un anno di tempo) e la Terra in orbita attorno al Sole: (crediti: Connors et al., Nature):
<EMBED WIDTH="640" HEIGHT="500" SRC="http://www.media.inaf.it/video/flv/animazioni/asteroide-troiano.flv" HIDDEN="false" AUTOSTART="true" LOOP="true" volume="100"></EMBED> Cita:
Credit:
Autore e data : di Marco Malaspina27/07/2011 19:01
Fonte: http://www.media.inaf.it/2011/07/27/un- ... -la-terra/Credit:
FONTI UFFICIALI
Cita:
NASA WISE NASA's WISE Mission Finds First Trojan Asteroid Sharing Earth's Orbit[bbfw]
A Glimmer in the Eye of WISE
Asteroid 2010 TK7 is circled in green, in this single frame taken by NASA's Wide-field Infrared Survey Explorer, or WISE. The majority of the other dots are stars or galaxies far beyond our solar system.
Astronomers discovered this object -- the first known Earth Trojan asteroid -- after sifting through asteroid candidates identified by WISE.
This image was taken in infrared light at a wavelength of 4.6 microns in Oct. 2010.
Image credit: NASA/JPL-Caltech/UCLA
PASADENA, Calif. – Astronomers studying observations taken by NASA's Wide-field Infrared Survey Explorer (WISE) mission have discovered the first known "Trojan" asteroid orbiting the sun along with Earth.
Trojans are asteroids that share an orbit with a planet near stable points in front of or behind the planet. Because they constantly lead or follow in the same orbit as the planet, they never can collide with it. In our solar system, Trojans also share orbits with Neptune, Mars and Jupiter. Two of Saturn's moons share orbits with Trojans.
Scientists had predicted Earth should have Trojans, but they have been difficult to find because they are relatively small and appear near the sun from Earth's point of view.
"These asteroids dwell mostly in the daylight, making them very hard to see," said Martin Connors of Athabasca University in Canada, lead author of a new paper on the discovery in the July 28 issue of the journal Nature. "But we finally found one, because the object has an unusual orbit that takes it farther away from the sun than what is typical for Trojans. WISE was a game-changer, giving us a point of view difficult to have at Earth's surface."
The WISE telescope scanned the entire sky in infrared light from January 2010 to February 2011. Connors and his team began their search for an Earth Trojan using data from NEOWISE, an addition to the WISE mission that focused in part on near-Earth objects, or NEOs, such as asteroids and comets. NEOs are bodies that pass within 28 million miles (45 million kilometers) of Earth's path around the sun. The NEOWISE project observed more than 155,000 asteroids in the main belt between Mars and Jupiter, and more than 500 NEOs, discovering 132 that were previously unknown.
The team's hunt resulted in two Trojan candidates. One called 2010 TK7 was confirmed as an Earth Trojan after follow-up observations with the Canada-France-Hawaii Telescope on Mauna Kea in Hawaii.
The asteroid is roughly 1,000 feet (300 meters) in diameter. It has an unusual orbit that traces a complex motion near a stable point in the plane of Earth's orbit, although the asteroid also moves above and below the plane. The object is about 50 million miles (80 million kilometers) from Earth. The asteroid's orbit is well-defined and for at least the next 100 years, it will not come closer to Earth than 15 million miles (24 million kilometers). An animation showing the orbit is available at:
http://www.nasa.gov/multimedia/videogal ... =103550791 .
Earth's New Trojan Friend
This artist's concept illustrates the first known Earth Trojan asteroid, discovered by NEOWISE, the asteroid-hunting portion of NASA's WISE mission. The asteroid is shown in gray and its extreme orbit is shown in green. Earth's orbit around the sun is indicated by blue dots. The objects are not drawn to scale.
Trojans are asteroids that share an orbit with a planet, circling around the sun in front of or behind the planet. Because they ride in the same orbit as a planet, they never cross its path, and never collide with the planet. They circle around stable gravity wells, called Lagrange points.
In the case of 2010 TK7, it has an extreme orbit that takes it far above and below the plane of Earth's orbit. The asteroid's orbit is well defined and for at least the next 100 years, it will not come closer to Earth than 15 million miles (24 million kilometers).
Typically, Trojan asteroids, for example those that orbit with Jupiter, don't travel so far from the Lagrange points. They stay mostly near these points, located where the angle between the sun and Earth is 60 degrees. Asteroids near a comparable position with respect to Earth would be very difficult to see, because they would appear near the sun from our point of view.
WISE was able to spot 2010 TK7 because of its eccentric orbit, which takes it as far as 90 degrees away from the sun. WISE surveyed the whole sky from a polar orbit, so it had the perfect seat to find 2010 TK7. Follow-up observations with the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii, helped confirm the object's Trojan nature.
Image credit: Paul Wiegert, University of Western Ontario, Canada
"It's as though Earth is playing follow the leader," said Amy Mainzer, the principal investigator of NEOWISE at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Earth always is chasing this asteroid around."
A handful of other asteroids also have orbits similar to Earth. Such objects could make excellent candidates for future robotic or human exploration. Asteroid 2010 TK7 is not a good target because it travels too far above and below the plane of Earth's orbit, which would require large amounts of fuel to reach it.
"This observation illustrates why NASA's NEO Observation program funded the mission enhancement to process data collected by WISE," said Lindley Johnson, NEOWISE program executive at NASA Headquarters in Washington. "We believed there was great potential to find objects in near-Earth space that had not been seen before."
NEOWISE data on orbits from the hundreds of thousands of asteroids and comets it observed are available through the NASA-funded International Astronomical Union's Minor Planet Center at the Smithsonian Astrophysical Observatory in Cambridge, Mass.
JPL manages and operates WISE for NASA's Science Mission Directorate in Washington. The principal investigator, Edward Wright, is a professor at the University of California, Los Angeles. The mission was selected under NASA's Explorers Program, which is managed by the agency's Goddard Space Flight Center in Greenbelt, Md. The science instrument was built by the Space Dynamics Laboratory in Logan, Utah.
The spacecraft was built by Ball Aerospace & Technologies Corp., Boulder, Colo. Science operations and data processing take place at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA.
Cita:
CREDIT : http://www.nasa.gov/mission_pages/WISE/news/wise20110727.html
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