Cosmic Diary Marchis

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The Triple Asteroid (87) Sylvia

June 15, 2016

Another beautiful simulation of the triple asteroid system (87) Sylvia and its two moons Romulus and Remus made in collaboration with the California Academy of Sciences.

(87) Sylvia was discovered in 1866 by N.R. Pogson, a British astronomer located in Madras, India. This main-belt asteroid is large with a diameter of ~150 km. That’s all we knew until recently.

In 2005, we discovered two moons around the asteroid that we baptized Romulus and Remus, sons of the Rhea Sylvia and founder of Rome. Both moons are very small with a diameter estimated to ~20 and ~7 km. The primary is irregular with a diameter of 193 x 129 x 124 km. Both moons orbit around the primary describing a coplanar and almost circular orbit.

From a long term study of their orbit we concluded that the primary is possibly differentiated, made of a dense core of ice. How the moons have formed is still unclear. They could be the product of a catastrophic disruption of a large asteroid early in the history of the  solar system.

The triple asteroid (87) Sylvia and its two moons Romulus and Remus from Franck Marchis on Vimeo.

(87) Sylvia is the first triple asteroid ever discovered. I have a fond memory of the day, I presented this discovery at the ACM conference in August 2005 at Buzios, Brazil, simultaneously with the publication in Nature. It was a very special moment in my career.

Today we know six  triple asteroids in the asteroid main-belt (45 Eugenia, 87 Sylvia, 93 Minerva, 130 Elektra, 216 Kleopatra, 3749 Balam)and there are probably more of them waiting to be discovered with the next generation of space-based and ground-based telescopes.

Clear Skies,

Franck M.


Flying through the binary trojan asteroid system (617) Patroclus

May 24, 2016

Another day, another video!

This time I am posting a video of the binary L5 Trojan Asteroid (617) Patroclus-Menoetius. In collaboration, with the team at the California Academy of Sciences, we have created a model of this interesting binary asteroid system which shares its orbit with Jupiter.

In 2001, a group of astronomer discovered that the L5 Trojan asteroid (617) Patroclus is in fact made of two components. In 2006, using Laser Guide Star Adaptive Optics System at W.M. Keck Observatory, we showed that those two components orbit around the center of mass of the system in ~4 days at 680 km describing a circular orbit. We named the second component Menoetius, the argonaut father of Patroclus in the greek mythology.

From the estimate of the size (derived from various techniques including Spitzer observations of mutual events taken in 2010 and stellar occultation on October 2013), we found out that the components are less dense than icy water, with a grain density very close to satellites of giant planets (like Amalthea, moon of Jupiter).

The Binary Trojan Asteroid (617) Patroclus-Menoetius from Franck Marchis on Vimeo.

Because Patroclus has a different color and density than (624) Hektor, we speculated that it could be a captured Jupiter-Saturn asteroid which ended up in the gravitational well of the Sun-Jupiter system during the migration of the giant planets 3.7 Billion years ago. Its binary nature could be the result of tidal disruption when primitive asteroid had a close encounter with Jupiter before the capture.

Ultimately, we will need to send a spacecraft there to really understand this system. NASA has pre-selected the LUCY new discovery mission which could flyby this binary Trojan asteroid in 2033.

Clear Skies,

Franck M.

Visiting the L4 Trojan Asteroid (624) Hektor

May 23, 2016

I finally started uploading some of the animations of the talk that I gave last month at the California Academy of Sciences. Today let’s watch (624) Hektor, the binary and bilobed largest Jupiter-Trojan asteroids. This is a puzzling multiple asteroid system with a lot of mysteries (eccentric and inclined orbit of the moon, complex shape and structure for the primary, …). 

Our study based on AO observations collected over 8 years was published in 2014. The conclusion of our work is that 624 Hektor is probably a captured Kuiper-belt object and the moon formed a long time ago from the slow velocity encounter of the components.

The Largest Jupiter Trojan: 624 Hektor and its moon from Franck Marchis on Vimeo.

We will probably need to send a spacecraft over there to really understand this complex mini-geological world. The good news is that several space agencies, including JAXA and NASA, are thinking about that.


I would like to thank to my colleague Josef Durech,  Matija Ćuk, Julie Castillo, Frederic Vachier, Jerome Berthier and numerous more for their long-term contribution to this project. I also should include my sister Helene Marchis for making the first drawing of this system. Thanks as well to the California Academy of Sciences for making those great CGI videos and the director Ryan Wyatt for inviting me.

Clear skies,

Franck M.