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The secrets of asteroid Minerva and its two moons

A multiple-telescope campaign has revealed that Minerva is unusually round for an asteroid and has a possibly unique structure.
Minerva with two moons
An artist’s impression depicting the 97-mile (156km) asteroid Minerva and its two moons. Credit: D Futselaar
Since the discovery of its two moons, the triple asteroid Minerva has been the focus of space- and ground-based telescope studies that have attempted to unravel the secrets of this intriguing system. A multiple-telescope campaign has now revealed that Minerva is unusually round for an asteroid, and has a possibly unique structure.

The campaign to “weigh” the asteroid and derive its density and other characteristics was undertaken by an international team of planetary astronomers led by Franck Marchis from the Carl Sagan Center of the SETI Institute in Mountain View, California.

Minerva is the fourth asteroid located in the main belt known to possess two moons. With a diameter of 97 miles (156 kilometers) and two tiny 3-mile (5km) size moons, this triple system orbits around the Sun between Mars and Jupiter. Members of Marchis’ team discovered the two moons in 2009 using the Keck II Telescope. Other triple asteroids in the main belt are 87 Sylvia (triplicity discovered in 2005), 45 Eugenia (2006), and 216 Kleopatra (2008). Marchis and his team were involved in the discovery of the triplicity of these asteroids and the follow-up studies.

“Very little was known about Minerva apart from the asteroid’s orbit around the Sun and a rough estimate of its size and shape,” said Marchis. “Shortly after the discovery of its two moons, our group focused on re-analyzing previous data from ground-based and space telescopes, and organizing a campaign of new observations to better understand the nature of this intriguing asteroid.”

The team studied the asteroid in detail using the large W.M. Keck telescope in Hawaii and a small robotic telescope at Kitt Peak. Together, these observations enabled the astronomers to make precise determinations of the orbits of the moons by directly imaging the system and by detecting an eclipse event.

“Eighteen months after the discovery of the moons with the 10-meter Keck II Telescope, we requested telescope time with the super-LOTIS telescope, a small 60-centimeter robotic telescope at Kitt Peak, to refine the spin period and shape of the large 156-kilometer (97 miles) asteroid,” said Descamps from the Institut de Mecanique Celeste et de Calcul des Ephemerides at Paris, France.

This new result on the shape of Minerva was derived by combining optical data recorded over the past 30 years, the high-resolution images from the sophisticated optics system available at the Keck Observatory, and the result of a stellar occultation observed by U.S.-based amateur astronomers December 24, 2010.

“The determination of the shape was complicated by Minerva’s unusually round form,” said Josef Durech from Charles University in Prague, Czech Republic. “The shape model could not have been derived without combining these three types of data.”

Interestingly, the three other main belt asteroids known to have moons have very elongated shapes, suggesting that Minerva may have a different interior composition or structure.

The existence of moons around an asteroid provides a direct means of measuring the mass of the system, and if the size of the central asteroid is well-known, astronomers can derive its density, as well. In the case of Minerva, it was possible to determine the size of the asteroid in two different ways: by analyzing the stellar occultation event observed December 24, 2010, and by re-analyzing archived data of the IRAS infrared space telescope obtained in 1983. Both methods indicate that Minerva has a diameter of about 97 miles (156km). Assuming a similar composition for the moons and the asteroid, and using the adaptive-optics observations from the Keck Telescope, the team of astronomers have now concluded that the moons are tiny — around 3 miles (5km) in diameter.

From the shape, size, and mass of the asteroid, the astronomers calculated its density to be 1.9 grams per cubic centimeter. Minerva appears to be a primitive type of asteroid known as a carbonaceous chondrite. Assuming it has the same composition as the denser carbonaceous chondrite meteorites collected on Earth, its macro-porosity, or percentage of empty space, is around 30 percent.

“All large main belt asteroids known to possess one or several moons have large porosities, possibly due to a rubble-pile interior,” Marchis said. “However, Minerva has a significantly higher density than other carbonaceous-type asteroids in multiple systems. We may finally be detecting subtle differences in the compositions of these types of asteroids, something we suspected from studying the composition of carbonaceous chondrite meteorites. These results may provide insight not only into the history and formation of multiple asteroid systems, but also the structure and origin of asteroids in general.”

Since the discovery of its two moons, the triple asteroid Minerva has been the focus of space- and ground-based telescope studies that have attempted to unravel the secrets of this intriguing system. A multiple-telescope campaign has now revealed that Minerva is unusually round for an asteroid, and has a possibly unique structure.

The campaign to “weigh” the asteroid and derive its density and other characteristics was undertaken by an international team of planetary astronomers led by Franck Marchis from the Carl Sagan Center of the SETI Institute in Mountain View, California.

Minerva is the fourth asteroid located in the main belt known to possess two moons. With a diameter of 97 miles (156 kilometers) and two tiny 3-mile (5km) size moons, this triple system orbits around the Sun between Mars and Jupiter. Members of Marchis’ team discovered the two moons in 2009 using the Keck II Telescope. Other triple asteroids in the main belt are 87 Sylvia (triplicity discovered in 2005), 45 Eugenia (2006), and 216 Kleopatra (2008). Marchis and his team were involved in the discovery of the triplicity of these asteroids and the follow-up studies.

“Very little was known about Minerva apart from the asteroid’s orbit around the Sun and a rough estimate of its size and shape,” said Marchis. “Shortly after the discovery of its two moons, our group focused on re-analyzing previous data from ground-based and space telescopes, and organizing a campaign of new observations to better understand the nature of this intriguing asteroid.”

The team studied the asteroid in detail using the large W.M. Keck telescope in Hawaii and a small robotic telescope at Kitt Peak. Together, these observations enabled the astronomers to make precise determinations of the orbits of the moons by directly imaging the system and by detecting an eclipse event.

“Eighteen months after the discovery of the moons with the 10-meter Keck II Telescope, we requested telescope time with the super-LOTIS telescope, a small 60-centimeter robotic telescope at Kitt Peak, to refine the spin period and shape of the large 156-kilometer (97 miles) asteroid,” said Descamps from the Institut de Mecanique Celeste et de Calcul des Ephemerides at Paris, France.

This new result on the shape of Minerva was derived by combining optical data recorded over the past 30 years, the high-resolution images from the sophisticated optics system available at the Keck Observatory, and the result of a stellar occultation observed by U.S.-based amateur astronomers December 24, 2010.

“The determination of the shape was complicated by Minerva’s unusually round form,” said Josef Durech from Charles University in Prague, Czech Republic. “The shape model could not have been derived without combining these three types of data.”

Interestingly, the three other main belt asteroids known to have moons have very elongated shapes, suggesting that Minerva may have a different interior composition or structure.

The existence of moons around an asteroid provides a direct means of measuring the mass of the system, and if the size of the central asteroid is well-known, astronomers can derive its density, as well. In the case of Minerva, it was possible to determine the size of the asteroid in two different ways: by analyzing the stellar occultation event observed December 24, 2010, and by re-analyzing archived data of the IRAS infrared space telescope obtained in 1983. Both methods indicate that Minerva has a diameter of about 97 miles (156km). Assuming a similar composition for the moons and the asteroid, and using the adaptive-optics observations from the Keck Telescope, the team of astronomers have now concluded that the moons are tiny — around 3 miles (5km) in diameter.

From the shape, size, and mass of the asteroid, the astronomers calculated its density to be 1.9 grams per cubic centimeter. Minerva appears to be a primitive type of asteroid known as a carbonaceous chondrite. Assuming it has the same composition as the denser carbonaceous chondrite meteorites collected on Earth, its macro-porosity, or percentage of empty space, is around 30 percent.

“All large main belt asteroids known to possess one or several moons have large porosities, possibly due to a rubble-pile interior,” Marchis said. “However, Minerva has a significantly higher density than other carbonaceous-type asteroids in multiple systems. We may finally be detecting subtle differences in the compositions of these types of asteroids, something we suspected from studying the composition of carbonaceous chondrite meteorites. These results may provide insight not only into the history and formation of multiple asteroid systems, but also the structure and origin of asteroids in general.”

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