Sky & Telescope - September 2023 - 38

TEAM PHOTO: THE AUTHOR; POLYMELE PROFILE SERIES
AND MAP: LEAH TISCIONE / S&T, SOURCE: THE AUTHOR
Solar System Occultations
We have given the moon the informal name of Shaun
(an homage to a sheep in the Wallace and Gromit universe).
Based on the discovery observations, Shaun is about 5 to
6 km across compared to the dimension of 13 by 26 km for
Polymele, and it orbits about 200 km away.
Since this discovery, we've focused our efforts on trying
THE TEAM The author's team in Oviedo, Spain, for a 2021 Polymele
occultation.
Recent Results
NASA's Lucy mission to the Jovian system is a voyage meant
to unlock the secrets of Jupiter's Trojan asteroids. To improve
Lucy's observations, we're using occultations to conduct an
initial reconnaissance of the mission targets. The five primary
targets, Polymele, Leucus, Orus, Eurybates, and Patroclus,
ranging from 20 to 120 km (12 to 75 miles) in diameter,
represent a diverse sample for study. The sizes, shapes, and
astrometry we collect will assist in the planning of the spacecraft
flyby.
The most exciting results have come from Polymele, the
smallest of the five. Due to its smaller size and fainter brightness,
it is also the least observed of the targets. Photometry
of Polymele shows a very slight variation in brightness as
it rotates, which many assumed meant that the asteroid is
nearly spherical. Our two first occultation results in 2021 and
2022, combined with a spherical assumption, disagreed with
each other about the inferred size. For them to agree, the light
curve would have to have a much larger amplitude.
The resolution to Polymele's contested size and shape came
from our successful campaign in March 2022. For this campaign,
I deployed 21 stations spaced 1.8 km apart along with
14 additional stations sprinkled in among the teams, all scattered
along the shadow path from Kansas to North Carolina.
Of these stations, 28 collected useful data. The resulting 12
chords revealed an elongated object twice as long as it is high.
Guided by these new data, we now see that Polymele has a
shape more like a flat disk, similar in size and aspect ratio to
the larger lobe of Arrokoth.
Along with solving the mystery of Polymele's shape, we
also snagged a big surprise. Two observers noticed a brief
occultation in their data that couldn't have been Polymele.
We quickly realized that we had seen an object in orbit
around the asteroid: a Trojan moon. This event was definitely
a lucky occurrence, and it makes Polymele the third of Lucy's
targets to have a companion - Eurybates has a small moon,
and Patroclus is a binary.
38 SEPTEMBER 2023 * SKY & TELESCOPE
to find the moon again. To see Shaun, the observing stations
must be spaced closer than the moon's size, otherwise it could
slip between observers. For our February 2023 campaign, I
chose to target a spacing of 2 km to give us a chance to obtain
multiple chords, but more importantly to make sure that
there was redundancy in the observations in order to minimize
the chance of completely missing Shaun due to equipment
failures. The spacing constraint required 100 stations to
cover the 200 km zone of interest. I don't think there's ever
been a case of a coordinated deployment of telescopes on a
single object with this many stations.
We began preparations in the middle of November 2022,
at a time when we were still busy coordinating campaigns in
France involving Eurybates and Orus. The first step was to
buy 29 new telescopes and all the accessories. That brought
the total number of telescopes I could deploy up to 90. Half
of these are 11-inch Celestron CPC 1100 Schmidt-Cassegrain
telescopes, and the rest are Sky-Watcher's 8-inch 200P Dobsonians
or 16-inch Flextube 400P Dobs. By adding additional
IOTA observers, we reached a final tally of 95 telescopes in
the U.S., while additional observers in Spain and Portugal
added extra coverage on Polymele.
Nothing went smoothly. Supply-chain issues made it hard
to obtain the equipment we needed in time. Also, in midJanuary
we only had about half of the observers we needed.
We reached out again to universities where we had already
been recruiting: University of Michigan; Northern Arizona
University; University of Colorado, Boulder; University of
Central Florida; and University of Virginia. One week before
uSHAUN THE MOON Observers tracking Polymele not only caught
the asteroid itself but also a small moon, orbiting about 200 km from
Polymele. The center and right panels show the asteroid and moon profiles
in more detail, with possible outlines fit to the data. Blue chords are
observers who detected an occultation, red are observers who didn't,
and gray are unsuccessful attempts to collect data.
Polymele system
100 km
Sky & Telescope - September 2023

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