Astronomy and Astrophysics

Distant Asteroid Revealed to be a Complex Mini Geological World

After 8 years of observations, scientists from the SETI Institute have found an exotic orbit for the largest Trojan asteroid, (624) Hektor — the only one known to possess a moon. The formation of this system made of a dual primary and a small moon is still a mystery, but they found the asteroid could be a captured Kuiper body product of the reshuffling of giant planets in our solar system. The results are being published today in Astrophysical Letters.

This study, based on W. M. Keck Observatory data and photometric observations from telescopes throughout the world, suggests that the asteroid and its moon are products of the collision of two icy asteroids. This work sheds light on the complex youth of our solar system, when the building blocks that formed the core of Giant planets and their satellites were tossed around or captured during the giant planet migrations.

Keck ObservatoryKeck Observatory Credit: NASA

In 2006, a small team of astronomers led by Franck Marchis, astronomer at the Carl Sagan center of the SETI Institute, detected the presence of a small 12 km diameter moon around the large Trojan asteroid (624) Hektor using the 10 m Keck II telescope atop Mauna Kea, fitted with the NIRC-2 (Near-Infrared Camera 2) instrument behind the adaptive optics and laser guide star system (LGS-AO).

Since then, they collaborated with several researchers from University of California at Berkeley in order to determine the orbit of this moon and understand the origin of the system. Trojan asteroids are those that are temporarily trapped in regions 60 degrees in front or 60 degrees behind the planet Jupiter in its orbit around the Sun. They are difficult to study since they are small and faint.

While the asteroid has been studied for 8 years, there were a couple of significant challenges before a paper could be published, according to Marchis. “The major one was technical: the satellite can be seen only with a telescope like Keck Observatory’s fitted with LSG-AO, but time on the mighty Keck’s is highly prized and in limited availability,” he said. “Secondly, the orbit of the satellite is so bizarre that we had to develop a complex new algorithm to be able to pin it down and understand its stability over time.”

artist render
Artistic representation of the Trojan system showing the large 250 km dual shape Hektor and its 12 km moon. Download full size Credit: H. Marchis & F. Marchis

The research, conducted with expert assistance from colleagues at the Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE) of the Observatoire de Paris, revealed that the 12 km moon orbits the large 250 km asteroid every 3 days at a distance of 600 km in an ellipse inclined almost 45 degrees with respect to the asteroid’s equator.

“The orbit of the moon is elliptical and tilted relative to the spin of Hektor, which is very different from other asteroids with satellites seen in the main-belt,” said Matija Cuk, coauthor and scientist at the Carl Sagan Center of the SETI Institute. “However, we did computer simulations, which include Hektor being a spinning football shape asteroid and orbiting the Sun, and we found that the moon’s orbit is stable over billions of years.”

Hektor has been known since the 1970s to be spinning rapidly (less than 7 hours) and extremely elongated. Using the high-angular resolution of the Keck II telescope, combined with a large number of photometric observations taken since 1957, the team built a refined shape hoping to get a clue to the origin of the system.

“We built several models of equal quality from the photometric data, but we favored a model made of two lobes since some of the best adaptive optics observations suggest that the Trojan asteroid has a dual structure,” said Josef Durech, co-author and researcher at the Charles University in Prague.

A complex shape for the asteroid and a bizarre orbit for the moon will be matters of discussion for the scientific community. The team speculated that the moon could be ejecta produced by a slow encounter that formed the bi-lobed asteroid, but emphasized the need for robust and accurate simulations.

“We also show that Hektor could be made of a mixture of rock and ices, similar to the composition of Kuiper belt objects, Triton and Pluto. How Hektor became a Trojan asteroid, located at only 5 times the Earth–Sun distance, is probably related to the large scale reshuffling that occurred when the giant planets were still migrating,” said Julie Castillo-Rogez, researcher at the Jet Propulsion Laboratory, California Institute of Technology.

Hektor was discovered in 1907 by August Kopff. The satellite of Hektor, discovered in 2006 by Franck Marchis and his team has not been named yet. The team welcomes any idea for naming the satellite, keeping in mind that the satellite should receive a name closely related to the name of the primary and reflecting the relative sizes between these objects.

The paper entitled “The puzzling mutual orbit of the binary Trojan asteroid (624) Hektor” published today by ApJL is co-authored by F. Marchis (SETI Institute), J. Durech (Charles University), J. Castillo-Rogez (Jet Propulsion Laboratory), F. Vachier (IMCCE-Obs. De Paris), M. Cuk (SETI Institute), J. Berthier (IMCCE-Obs. De Paris), M.H. Wong (UC Berkeley), P. Kalas (UC Berkeley), G. Duchene (UC Berkeley), M. A. van Dam (Flat Wavefronts), H. Hamanowa (Hamanowa observatory)and M. Viikinkoski (Tampere University)

The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and world-leading laser guide star adaptive optics systems.

Meteor Astronomer Peter Jenniskens Participates in Chelyabinsk Meteor Impact Investigation

Peter Inspecting Meteors
Jennikens and Popova examine meteor fragments, credit: P. Jenniskens

Peter Jenniskens and international scientists for the first time have gathered a detailed understanding of the effects on Earth from a small asteroid impact. 

The unprecedented data obtained as the result of the airburst of a meteoroid over the Russian city of Chelyabinsk on Feb. 15, 2013, has revolutionized scientists' understanding of this natural phenomenon.

The Chelyabinsk incident was well observed by citizen cameras and other assets. This provided a unique opportunity for researchers to calibrate the event, with implications for the study of near-Earth objects (NEOs) and developing hazard mitigation strategies for planetary defense. Scientists from nine countries have now established a new benchmark for future asteroid impact modeling.

"Our goal was to understand all circumstances that resulted in the shock wave," said meteor expert Peter Jenniskens, co-lead author of a report published in the journal Science. Jenniskens, a meteor astronomer at NASA’s Ames Research Center and the SETI Institute, participated in a field study led by Olga Popova of the Institute for Dynamics of Geospheres of the Russian Academy of Sciences in Moscow in the weeks following the event.

“It was important that we followed up with the many citizens who had firsthand accounts of the event and recorded incredible video while the experience was still fresh in their minds," said Popova.

By calibrating the video images by the position of the stars in the night sky, Jenniskens and Popova calculated the impact speed of the meteor at 42,500 mph (19 kilometers per second). As the meteor penetrated through the atmosphere, it efficiently fragmented into pieces, peaking at 19 miles (30 kilometers) above the surface. At that point the light of the meteor appeared brighter than the sun, even for people 62 miles (100 kilometers) away.

Due to the extreme heat, many of the pieces of the debris vaporized before falling out of the orange glowing debris cloud. Scientists believe that between 9,000 to 13,000 pound (4,000 to 6,000 kilograms) of meteorites fell to the ground. This included one fragment approximately 1,400 pound (650 kilogram) recovered from Lake Chebarkul on October 16, 2013, by professional divers guided by Ural Federal University researchers.

NASA researchers participating in the 59 member consortium study suspect that the abundance of shock fractures in the rock contributed its break up in the upper atmosphere.   Meteorites made available by Chelyabinsk State University researchers were analyzed to learn about the origin of the shock veins and their physical properties.

"One of these meteorites broke along one of these shock veins when we pressed on it during our analysis," said Derek Sears, a meteoriticist at Ames.

Mike Zolensky, a cosmochemist at NASA’s Johnson Space Center in Houston, may have found why these shock veins (or shock fractures), were so frail. They contained layers of small iron grains just inside the vein, which had precipitated out of the glassy material when it cooled.

"There are cases where impact melt increases a meteorite's mechanical strength, but Chelyabinsk was weakened by it," said Zolensky.

The impact that created the shock veins may have occurred as long ago as 4.4 billion years. This would have been 115 million years after the formation of the solar system, according to the research team, who found that the meteorites had experienced a significant impact event at that time.

“Events that long ago affected how the Chelyabinsk meteoroid broke up in the atmosphere, influencing the damaging shockwave,” said Jenniskens.

Research is being conducted to better understand the origin and nature of NEOs. These essential studies are needed to inform our approach to preparing for the potential discovery and deflection of an object on a collision course with the Earth. 

NASA's recently announced asteroid initiative will be the first mission to capture and relocate an asteroid. It represents an unprecedented technological feat that will lead to new scientific discoveries and technological capabilities that will help protect our home planet.

Aside from representing a potential threat, the study of asteroids and comets represent a valuable opportunity to learn more about the origins of our solar system, the source of water on the Earth, and even the origin of organic molecules that lead to the development of life. 

For more information about the Chelyabinsk field study visit: http://cams.seti.org/index-chelyabinsk.html

For more information on asteroids and comets, visit: http://www.nasa.gov/mission_pages/asteroids/main/

Telescopes Large and Small Team Up to Study Triple Asteroid

Combining observations from the world’s largest telescopes with small telescopes used by amateur astronomers, a team of astronomers discovered that the large main-belt asteroid (87) Sylvia has a complex interior, probably linked to the way the multiple system was formed. The findings are being revealed today at the 45th annual Division of Planetary Sciences meeting in Denver, Colorado.

This work illustrates a new trend in astronomy in which backyard amateur astronomers team up with professional astronomers to expand our knowledge of our solar system. The study of multiple asteroids such as (87) Sylvia gives astronomers an opportunity to peek through the past history of our solar system and constrain the internal composition of asteroids. In 2005, the triple asteroid was discovered to possess two moons.

The team, led by Franck Marchis, senior research scientist at the Carl Sagan Center of the SETI Institute, has continued to observe this triple asteroid system by gathering 66 adaptive optics observations from 8-10m class telescopes including those at the W. M. Keck Observatory, the European Southern Observatory, and Gemini North.

“Because (87) Sylvia is a large, bright asteroid located in the main belt, it is a great target for the first generation of adaptive optics systems available on these large telescopes. We have combined data from our team with archival data to get a good understanding of the orbits of these moons,” Marchis said.

With expert assistance from colleagues at the Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE) of the Observatoire de Paris, the team developed an accurate dynamical model of the system, allowing them to predict the position of the moons around the asteroid at any time.

The “drop test” of this work was the prediction of the relative positions of the moons during an occultation on Jan. 6, 2013. Observers equipped with small telescopes located on a narrow path across the south of France, Italy and Greece could see the triple system (87) Sylvia passing in front of a bright 11-mag star. Such occultations allow exquisitely precise measurements of the relative positions and sizes of the occulting objects.

In collaboration with EURASTER, a group of amateur and professional astronomers, the team successfully motivated ~50 observers to watch the event. Twelve of them detected the occultation by the primary of the system which lasted between 4 and 10 seconds depending on the observer’s position on Earth.

“Additionally, four observers detected a two-second eclipse of the star caused by Romulus, the outermost satellite, at a relative position close to our prediction. This result confirmed the accuracy of our model and provided a rare opportunity to directly measure the size and shape of the satellite”, Jérôme Berthier, astronomer at IMCCE said.

The chords of this occultation observations revealed that Romulus is 24 km in diameter with an extremely elongated shape, possibly made of two lobes joined together like a dumbbell. This is not surprising if the satellite formed from the accretion of fragments created by the disruption of a proto-Sylvia by an impact, several billion years ago.

The team derived the shape of the 270-km primary asteroid Sylvia by combining data from the occultation of the asteroid with other sources of information. These included archived recordings of the variation of light caused by the spin of the asteroid, and direct imaging by adaptive optics systems. Because the satellites’ orbits do not seem to be affected by the irregular shape of the asteroid, the team concluded that the large asteroid is most likely differentiated. The asteroid likely has a spherical core of dense material, surrounded by a fluffy or fractured outer surface layer.

“Combined observations from small and large telescopes provide a unique opportunity to understand the nature of this complex and enigmatic triple asteroid system,” Marchis said. “Thanks to the presence of these moons, we can constrain the density and interior of an asteroid, without the need for a spacecraft’s visit. Knowledge of the internal structure of asteroids is key to understanding how the planets of our solar system formed”.

Support for this work was provided by NASA through grant number NNX11AD62G.

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