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Post Info TOPIC: Patroclus and Menoetius


Posts: 131433
617 Patroclus

A low density of 0.8 g/cc for the Trojan binary asteroid 617 Patroclus
Authors: Franck Marchis, Daniel Hestroffer, Pascal Descamps, Jerome Berthier, Antonin H. Bouchez, Randall D. Campbell, Jason C. Y. Chin, Marcos A. van Dam, Scott K. Hartman, Erik M. Johansson, Robert E. Lafon, David Le Mignant, Imke de Pater, Paul J. Stomski, Doug M. Summers, Frederic Vachier, Peter L. Wizinovich, Michael H. Wong

The Trojan population consists of two swarms of asteroids following the same orbit as Jupiter and located at the L4 and L5 Lagrange points of the Jupiter-Sun system (leading and following Jupiter by 60 degrees). The asteroid 617 Patroclus is the only known binary Trojan (Merline et al. 2001). The orbit of this double system was hitherto unknown. Here we report that the components, separated by 680 km, move around the system centre of mass, describing roughly a circular orbit. Using the orbital parameters, combined with thermal measurements to estimate the size of the components, we derive a very low density of 0.8 g/cc. The components of Patroclus are therefore very porous or composed mostly of water ice, suggesting that they could have been formed in the outer part of the solar system.

Patroclus observed with the Keck 10-m telescope on the Mauna Kea
summit in Hawaii and its adaptive optics system.
The observations of this faint object (integrated magnitude in the visible, magv15.8) were made possible using the LGS system. An artificial star (with mv12) is created at proximity of the target, exciting the thin sodium layer at an altitude of ~100 km using a dye laser with a power output of ~13 W. The asteroid itself was used for the tip-tilt analysis, which is acquired by an avalanche photodiode tip-tilt sensor, and then corrected by balancing a flat mirror.
The lowest-order aberrations are corrected by controlling a 384-actuator deformable mirror at a rate of 400 Hz. These observations were taken on 28 May 2005 with the NIRC-2 infrared camera through the H, centred at 1.6 µm (a), and Kp, centred at 2.2 µm (b), broadband filters. The angular resolution of the data is estimated to 58 mas, close to the diffraction limit of the telescope (indicated by the scale bar, 50 mas). In these two images, the angular separation of both components is 150 mas, corresponding to 640 km, close to the maximum separation of the system. The magnitude difference in brightness is 0.2 in both filters, suggesting a similar surface composition.

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Posts: 131433
RE: Patroclus and Menoetius

When Patroclus encountered the planet Jupiter several billion years ago, it may have gotten a little too close to the mighty planet. The tremendous gravitational forces of Jupiter, which are three times stronger than that of Earth, may have split the small and porous body in half through an effect known as “tidal splitting.”

An illustration of "Lagrangian points" in the Sun-Jupiter system. Lagrangian points are five positions in interplanetary space where small objects, affected only by gravity, are stationary relative to two larger objects (such as the Sun and Jupiter).
Purple lines represent the L4 and L5 Lagrangian points. More than a thousand asteroids have been discovered in these positions, indicated by white dots.

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Posts: 131433

Astronomers using the Keck II Laser Telescope in Hawaii, have found that the two Trojan asteroids, 617 Patroclus and its companion, Menoetius, have a density of only 0.8 grams per cubic centimetre— much less dense than normal asteroids should be.
This leads the researchers to think that Patroclus and Menoetius are comets, which are typically composed mainly of water ice .
With a density of only a third of solid rock, Patroclus and Menoetius are estimated to be about 122 kilometres and 112 kilometres wide, respectively.

It’s our suspicion that the Trojans are small Kuiper Belt objects” - Franck Marchis, University of California, Berkeley.

The researchers think Patroclus and Menoetius formed about 650 million years after the formation of the solar system.
The two objects are not the first to be mistaken for asteroids: in 1999, astronomers determined that C/199 J3 was also a comet.
Most comets are thought to form in the Kuiper Belt, a distant region of the solar system outside the orbit of Neptune.

The two Trojans are the only known binary objects around Jupiter. The two bodies orbit around each other while trailing 750 million kilometres from Jupiter in one of gas planet’s two so-called Lagrange points. At these points, the gravitational field of Jupiter and the sun are perfectly balanced, and objects can be captured and brought to relative rest. Jupiter has two Lagrange points, one in front and the other behind as the planet orbits the sun.

The two bodies had been considered rocky asteroids, but scientists now say they appear to be captured comets.

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