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Saturday, October 24 2020 @ 10:26 am EDT

Congratulations, It’s Twins!

Lunar & PlanetaryThe European Southern Observatory Very Large Telescope has discovered an asteroid with TWO orbiting moonlets, a first for any known asteroid. The Yepun instrument of the VLT facility made the historic find by utilizing the adaptive optics system instrument known as NACO. Results were published in the August 11 issue of the Journal Nature.

Discovered in 1866, asteroid 87 Sylvia, named for the mythological mother of the founders of Rome, now reveals her twin sons, Romulus and Remus.
As though they were technicians conducting an ultrasound examination of a fetus, a surprise awaited observers using the 8.2 meter Yepun telescope at the European Southern Observatory in Cerro Paranal, Chile. A team led by Franck Marchis of University of California Berkeley and co-discoverers Pascal Descamps, Daniel Hestroffer, and Jerome Berthier of the Observatoire de Paris, France, had been using the telescope to observe asteroid 87 Sylvia to check for moonlets circling about the main asteroid. Measurements had previously indicated that 87 Sylvia was an asteroid with one small companion in 2001 after work done by Mike Brown and Jean-Luc Margot at the Keck telescope in Hawaii.

Marchis and colleagues made 27 observations with the huge 8.2 meter Yepun telescope during a two-month period. On every image the known companion moonlet was seen allowing for that object's precise orbital elements to be determined, but 12 of those images revealed a separate, smaller companion orbiting closer in towards the main asteroid. What was once a single moonlet, is now twins.

The 87th asteroid discovery, 87 Sylvia has been determined to be a huge pile of rubble loosely joined together by a compilation of debris giving the overall appearance of a potato shaped asteroid. It would seem that at some time in Sylvia’s ancient past, something struck her with enough force to completely shatter her into a pile of debris, of which the attendant moonlets may simply be left-over material that didn't rejoin the main mass. This asteroid has a density estimated to be only 20% that of liquid water, and could be up to 60% empty space.

Artists conception of the Minor Planet 87 Sylvia and her attendant "twins", Romulus and Remus. The twins names were suggested by the discoverers and approved by the International Astronomical Union.
Asteroid 87 is named for the mythological mother of the founders of Rome, Rhea Sylvia. How appropriate that a second “child” was discovered after these most recent observations. The twin companions to 87 Sylvia have now been named Romulus and Remus for those mythological twin sons. This observation is also the first ever which gives proof that asteroids can and do have multiple moonlets and designates 87 Sylvia as the first triple system!

Companion Romulus was known since 2001, but it’s size and orbit were not as well known before these recent observations. It is now known that Romulus orbits 87 Sylvia at a distance of 1360 km with an orbital period of 87.6 hours, and is estimated to be 18 km across. Newfound Remus, on the other hand, is estimated to be only 7 km across, orbiting at 710 km from Sylvia in only 33 hours time. Asteroid 87 Sylvia is estimated to spin once in an approximate time of 5 hours and 11 minutes. Marchis believes that more such “rubble-pile” asteroids are expected to be seen in the future due to the way in which this triple system formed. Other such shattered and re-aggregated asteroids are indeed suspected and this new discovery may well touch off the search for more multiple "offspring".

For the full article and a link to ESO

All information and images are courtesy ESO/UC Berkeley and are used by permission 081805
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